5

FOREWORD

As a periodic review of its activities, the Department of Physics has been organizing In-house Symposium on annual basis during recent years. This one-day symposium usually consists of oral presentations by faculty members, post-docs and students, and poster presentations by all those who would like to present their recent results. This year we have a total of 20 talks and 60 posters. I hope this package would be a reasonable representation of the ongoing research activities in the department. This event is also particularly useful to freshers (including senior undergraduates) to familiarize themselves with the current research activity in our Department in various branches of Physics.

I would like to thank Arindam Ghosh, Prabal Maiti and Prateek Sharma of our department who have shouldered the responsibility to organize this In-house Symposium. I urge all of you to actively participate in this important scientific activity. I hope you will all have an enjoyable and fruitful day.

Prof. V. Venkataraman Chairman November 27, 2015

Department of Physics, IISc Bangalore

Inhouse Symposium 2015

November 27, 2015 Auditorium, New Physical Sciences Building Programme

Session I 9:00-10:30 Chair: Anindya Das

T01 9:00-9:15 Arnab Rai Choudhuri Magnetic Cycles of Sun-like Stars and their Theoretical Modelling Naveen Yadav T02 9:15-9:30

Dynamics of supernova driven superbubbles

T03 9:30-9:45 Sudeep Kumar Ghosh Squished Baryons in Synthetic Dimensions

T04 9:45-10:00 Sudeesh K

Active Micrometer Sized Heat Engine

T05 10:00-10:15 S R K Chaitanya Indukuri Enhancement of fluorescence and anisotropic decay dynamics from CdSe quantum dots by hybrid photonic-plasmonic templates Prasad Vishnubhotla T06 10:15-10:30 Electron field emission from carbon nanostructures

10:30-11:00 Tea Break Session II 11:00-1:00 Chair: Vijay Shenoy

T07 11:00-11:15 Subroto Mukerjee Many-body localization in the presence of a single particle mobility edge T08 11:15-11:30 Ravi Kiran Saripalli Optical and Dielectric studies on single crystals of organic Glucuronic acid γ-Lactone

T09 11:30-11:45 Pramod Verma Phonon anharmonicity in pyrochlores T10 11:45-12:00 A. V. Pradeep Spin polarized low energy electron diffraction from Ir(100)-(5×1)-H, Ir(100)- (2×1+1×2)-O and Fe/Ir(100) surfaces.

T11 12:00-12:15 Arvind Kumar DLTS analysis of amphoteric interface defects in high-k TiO2 MOS structures prepared by sol-gel spin-coating

T12 12:15-12:30 Md. Ali Aamir Anomalous conductivity noise in gapped bilayer graphene heterostructure

T13 12:30-12:45 Vineeth Mohanan P Current induced deterministic switching of magnetization in Pt/Co/Pt films

T14 12:45-1:00 Ramesh Chandra Mallik Synthetic Teterahedrites for Thermoelectric Applications

1:00-2:00 Lunch Break

Session 2:00-4:30 Poster Session III

Session 4:30-5:30 Chair: Ambarish Ghosh IV

T15 4:30-4:45 Tanmoy Das Assembling topological insulators with lasers

Rakesh Manjappa T16 4:45-5:00 Ray tracing based refraction correction in Optical tomography of polymer gel dosimeters T17 5:00-5:15 M. Praveena Plasmon-mediated emergence of collective emission and enhanced quantum efficiency in quantum dot films T18 5:15-5:30 Kiran Lakhchaura Entropy profiles of Galaxy Clusters using MCMC T19 5:30-5:45 Kallol Roy Dual gated graphene-MoS2 field effect transistors (FET) with extremely high optical detectivity

5:45-6:15 High Tea Session V 6:15-7:45 Chair: V. Venkataraman

Anil Kumar Memorial 6:15-7:15 B N Dev, IACS, Kolkata Lecture Atomic Scale and Nano scale Quantum Structures

7:15-7:45 Best Poster Award Viswamitra Memorial Prize Best write-up Award in PH300 (Seminar Course) Concluding Remarks

7:45-9:00 Dinner Sl. Poster Title Presenter's name no. no. 1 P01 Cold gas in cluster cores: Global stability analysis and non-linear Prakriti Pal Choudhury simulations of thermal instability in spherical and plane-parallel atmospheres

2 P02 Probing Photoexcited Carriers in a few layer MoS2 Laminate by Time- Srabani Kar Resolved Optical Pump-Terahertz Probe Spectroscopy 3 P03 Detection of Localized Surface Plasmons in Tellurium Nanowires Mithun K. P. using Time-Resolved Optical Pump-Terahertz Probe Spectroscopy

4 P04 Enhancing photoresponsivity using MoTe2-graphene vertical Manabendra Kuiri heterostructures 5 P05 Probing 2D black phosphorus by quantum capacitance measurements Manabendra Kuiri 6 P06 Noise measurement in Moire and Bilayer Graphene Chandan Kumar 7 P07 Conductance oscillation under high magnetic filed in Graphene Chandan Kumar 8 P08 Roughening transition in colloidal membranes Lachit Saikia

9 P09 A Three-Dimensional Babcock-Leighton Solar Dynamo Model with Non- Gopal Hazra Axisymmetric Subsurface Flow 10 P10 Squished Baryons in Synthetic Dimensions Umesh K. Yadav 11 P11 Pressure-dependent Semiconductor to Semimetal and Lifshitz transitions Achintya Bera in α-MoTe2 : Raman and Density functional studies

12 P12 Electrical characterization of MoS2-MoTe2 vertical heterojunction Arup Paul 13 P13 Quantum Impurities develop Fractional Local Moments in Spin-Orbit Adhip Agarwala Coupled Systems

14 P14 Quantum transport at graphene NbSe2 junction Manas Ranjan Sahu 15 P15 Fluctuation theory of Rashba Fermi gases: Gaussian and beyond Vijay B. Shenoy

16 P16 Study of thermoelectric properties of the pseudo-binary Cu2Te-Sb2Te3 Shriparna Mukherjee system

17 P17 Raman Signatures of Strong Kitaev Exchange Correlations in (Na1- Satyendra Nath Gupta xLix)2IrO3 : Experiments and Theory

18 P18 Magnetocaloric Effect in Double Perovskite Ho2NiMnO6 Tirthankar Chakraborty 19 P19 87Rb-133Cs molecule formation probability in an optical lattice using Manjari Gupta strong-coupling expansion 20 P20 Percolative switching in transition metal dichalcogenide field-effect Tathagata Paul transistors at room temperature

21 P21 Local and non-local transport in the Topological Kondo Insulator SmB6 Sangram Biswas

22 P22 Enhancement of spin accumulation near Verwey transition in Fe3O4 based Shwetha G. Bhat spin injection devices 23 P23 Design and construction of a continuous-wave Titanium-Sapphire Laser Debrup Bhattacharya

24 P24 In situ Raman spectroscopic measurement of top gated MoTe2 Field Effect Subhadip Das Transistor 25 P25 Low-frequency noise in edge-contacted graphene Paritosh Karnatak 26 P26 Pulsed laser deposition of topological insulator thin films Abhishek Banerjee 27 P27 Yielding of a Langmuir monolayer under periodic shear Pradip K. Bera 28 P28 Magnitude and origin of electrical noise at individual grain boundaries in Kimberly Hsieh graphene 29 P29 Crucial Role of Internal Collective Modes in Underdoped Cuprates Aabhaas V. Mallik 30 P30 Boosted one dimensional superconductors on a lattice Sayonee Ray

31 P31 Pressure Induced Metal to Metal transition in Eu2Ir2O7: Raman and X-ray Anoop Thomas Studies 32 P32 Phase Change Properties of Chalcogenide Glasses - Some Interesting K. Ramesh Observations 33 P33 Pressure dependence of glass transition in As-Te and Ge-Te glasses from K. Ramesh electrical resistivity measurements 34 P34 Observation of fractional conductance quantization in graphenic edge Amogh Kinikar states 35 P35 Photo-response in hBN encapsulated Graphene-TMDC heterostructures Avradip Pradhan 36 P36 Exploring Utilities of Reverse Cross-Polarization-Spectral Editing in P. Lokeswara Rao Proton NMR spectroscopy

37 P37 Spectral response of single layer Graphene/MoS2 photodetector Ranjit V. Kashid 38 P38 Tailoring Curie temperature and magnetic anisotropy in ultrathin Pt/Co/Pt Vineeth Mohanan P films 39 P39 Defect Assisted Carrier Relaxation in Graphene Nanoribbons Gyan Prakash 40 P40 Structural aspects and interaction profiles of PAMAM dendrimers on a Mounika Gosika graphene sheet

41 P41 Antiphase boundary induced Exchange bias in LuMn0.5Fe0.5O3 Tanushree Sarkar

42 P42 Structure driven multiferroic properties of LuFeO3 nanoparticles Pittala Suresh 43 P43 Deterministic switching of magnetization states in Cobalt nano rings Manohar Lal

44 P44 Substrate screening effects on quasiparticle excitations of pristine and Mit H Naik vacancy-defected MoS2 45 P45 Bulk-Induced 1/f Noise at the Surface-states of Three-Dimensional Semonti Bhattacharyya Topological Insulators

46 P46 High detectivity in Photosensitive Graphene-hBN-MoS2 multilayer Tanweer Ahmed heretostructures 47 P47 Molecular mechanism of water permeation in helium impermeable Sudip Chakraborty graphene and graphene oxide membrane 48 P48 Dendrimers as blocking reagent for the toxic protein pores Subbarao Kanchi 49 P49 Gate Controlled Seebeck effect in twisted bilayer graphene PBS Mahapatra 50 P50 The SPL7013 Dendrimer Destabilizes the HIV-1 gp120-CD4 Complex Anil Kumar Sahoo 51 P51 Differential functional role of the three subunits of gp41 trimer on HIV- Satyabrata Das entry: a study based on homology modelling and MD simulation 52 P52 The study of electroporation phenomenon using an optically transparent Amit Kumar Majhi polymer device and molecular dynamics simulations 53 P53 Multifractal universal conductance fluctuation in graphene Kazi Rafsanjani Amin 54 P54 Cooling a Band Insulator with a Metal: Fermionic Superfluid in a Arijit Haldar Dimerized Holographic Lattice

55 P55 Unconventional 1/f noise in Graphene on SrTiO3 substrate Anindita Sahoo 56 P56 Spatiotemporal heterogeneity of lipid interaction in solid supported lipid N. K. Sarangi bilayers induced by binding of pore forming proteins 57 P57 Molecular structure of the Discotic Liquid Crystalline Phase of Hexa-peri Saientan Bag Hexabenzocoronene / Oligothiophene Hybrid and their Charge Transport properties 58 P58 Micromagnetic Simulations: A method to model the Magnonics for spin Venkateswarlu Dasari wave band structures 59 P59 Binary Fluid Turbulence : Signatures of Multifractal Droplet Dynamics Nairita Pal and Dissipation Reduction 60 P60 Splitting, Coalescence and Shape fluctuations of Multielectron bubbles in V. Vadakkumbatt liquid helium

TALK ABSTRACTS

Title: Magnetic Cycles of Sun-like Stars and their Theoretical Modelling

Speaker: Arnab Rai Choudhuri Department of Physics Indian Institute of Science, Bangalore

Abstract: Our Sun has a magnetic cycle (the sunspot cycle) with a period of 11 years, which is explained with the help of dynamo theory. Over the last few decades, astronomers have gathered quite a lot of observational data about similar magnetic cycles in many Sun-like stars. After summarizing the relevant observational data, I shall discuss how we extrapolate models of the solar dynamo to these stars and explain many aspects of the salient observational data. Dynamics of supernova driven superbubbles Authors:

Naveen Yadav (Indian Institute of Science, Bangalore, India), Dipanjan Mukherjee (Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, ACT 2611, Australia), Prateek Sharma (Indian Institute of Science, Bangalore, India) and Biman B. Nath (Raman Research Institute, Bangalore, India)

Abstract: Energy injection by supernovae is believed to be one of the primary sources which powers the expansion of supershells. There is a qualitative difference between isolated supernovae (SNe) and overlapping SNe. For typical interstellar medium (ISM) conditions an isolated supernova loses most of the injected mechanical energy by 1 Myr. In contrast, for SNe going off in bubbles the radiative losses are much smaller. While the -3 outer shock going off in the dense ISM (~1 cm )​ becomes radiative well before 1 Myr, ​ there is a strong non-radiative termination shock that keeps the bubble over-pressured till the lifetime of the OB association (10s of Myr; Sharma et al. 2014). This has relevance for supernova feedback in galaxy formation simulations. In our previous 1-D treatment all the SNe were assumed to occur at the same location in space. It was found that a steady wind inside the bubble (Chevalier & Clegg 1985) can occur only if the number of 4 SNe is large (>~10 )​ and a supernova going off inside the bubble can thermalize within ​ the termination shock. In the present work we study the effect of SNe separated in both space and time using 3-D hydrodynamic simulations with radiative cooling. If the separation between SNe is larger than the remnant’s radius at the time it becomes radiative, SNe are in the isolated regime. The explosion energy is deposited as thermal 4 energy in a uniform, static interstellar medium (ISM) with temperature 10 K​ , ​ corresponding to the warm neutral medium. The key parameters of our idealized setup are the ISM density (ngas), the number of SNe (N★) and the spatial separation between ​ ​ ​ ​ SNe (Rcl). The shock radius when it becomes radiative depends on the ISM density and ​ ​ number of SNe. We obtain the critical values of the key parameters (ngas, N★, Rcl) which ​ ​ ​ ​ ​ ​ lead to the formation of a superbubble. e.g., at least 103 SNe are required to maintain an -3 ​ 2 over-pressured bubble at 20 Myr in an ISM with 1 cm ;​ similarly 10 SNe going off within -3 ​ ​ a region of 100 pc in an ISM with 1 cm a​ re effectively in the isolated regime. We also ​ compare our simulation results with observational data on superbubbles. Squished Baryons in Synthetic Dimensions

Sudeep Kumar Ghosh, Umesh K. Yadav, and Vijay B. Shenoy Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India

The idea of “synthetic dimension” is to use internal states of atoms moving in a 1d optical lattice, to realize a hopping Hamiltonian equivalent to the Hofstadter model on a square lattice strip. We investigate the physics of SU(M) symmetric interactions in this synthetic dimension system. We show that this system is equivalent to particles (with SU(M) symmetric interactions) experiencing an SU(M) Zeeman field at each lattice site and a non-Abelian SU(M) gauge potential that af- fects their hopping. This equivalence brings out the possibility of generating non-local interactions between particles at different sites of the optical lattice. In addition, the gauge field induces a flavor-orbital coupling, which mitigates the “baryon breaking” effect of the Zeeman field. For M particles, concomitantly, the SU(M) singlet baryon which is site localized in the usual 1d optical lattice, is deformed to a non-local object (“squished baryon”). We conclusively demonstrate this effect by analytical arguments and exact (numerical) diagonalization studies. Our study promises a rich many-body phase diagram for this system. The study of the few body physics of the system uncovers a rich collection of many body phases. We construct the full many body phase diagram of the system using bosonization and DMRG by looking at different correlation functions. We demon- strate that there are interesting transitions between quasi-condensates of squished baryons induced by the gauge field. There are, also, ample possibilities of generating novel phases like the FFLO state. Active Micrometer Sized Heat Engine

Sudeesh Ka, Subho Ghoshb, Dipankar Chatterjib, Rajesh Ganapathyc, A K Sooda,c

a Department of Physics, Indian Institute of Science b Molecular Biophysics unit, Indian Institute of Science c International Centre for Material Sciences, Jawaharlal Nehru Centre for advanced scientific research

Abstract: Macroscopic heat engines have been successfully described by the laws of thermodynamics for the last two hundred years. Despite their conceptual simplicity, development of microscopic engines and a complete description under the framework of stochastic thermodynamics has only been recently possible. These developments present us with an intriguing possibility to operate engines with active fluctuations, hitherto impossible in the macroscopic regime. Here, we present such a realization of an active microscopic engine driven by active flutuations from a bacterial reservoir, while being subject to time dependent potentials in an optical trap. We interpret our results under the framework of stochastic thermodynamics and elucidate the reasons for the high performance of the system. Our results present the inability of the existing effective temperature descriptions to capture the performance of the system and motivate the extension of the framework of stochastic thermodynamics to describe active engines. Enhancement of fluorescence and anisotropic decay dynamics from CdSe quantum dots by hybrid photonic-plasmonic templates.

S.R.K.Chaitanya Indukuri* Deepika Chaturvedi and Jaydeep.K.Basu Department of physics, Indian Institute of science, Bangalore, 560012, India. Email: [email protected]. Tailoring light-matter interactions, especially the ability to control spontaneous emission of quantum emitters (QE), has been actively pursued since the suggestion by Purcell of a strategy to enhance spontaneous emission rate, through the Fermi’s golden rule, by controlling the local density of photonic states (LDOS), “ ρ “ . Various methods are used to control LDOS at the scale of the wavelength of light, like photonic crystals (PC) and photonic resonators. High values of Purcell factor, F=Q/V, have been achieved in several such resonator systems. Here, “Q” is the quality factor of the cavity or resonator, “V” is electromagnetic mode volume. For deep sub-wavelength scale, plasmonic nano-antennas appear to be the most promising candidates for achieving large “F” due to the extremely small “V” values. Unfortunately strong dissipative losses in these systems reduce the “F” value. Hence, alternative strategies using hierarchical assembly methods are beginning to be developed to alleviate this problem. Here we report the first study of LDOS tuning in a functional, flexible and hierarchical self- assembled plasmonic templates using extremely small gold (Au) nano-particles of diameter 5 nm which by themselves are believed to be unable to produce any significant enhancement of emitter emission intensity [1]. Using these Au nano-antenna elements in a two dimensional (2D) polymer template a plasmonic template has been developed which enables continuous tuning of the photoluminescence (PL) of cadmium selenide (CdSe) semi-conductor quantum dots embedded in this template, as a function of the composition of Au nanoparticles. Experimentally, maximum enhancement of 5 has been demonstrated while finite difference time domain (FDTD) calculation of enhancement for maximal “ ΦAu“ values in such templates leads to a predicted enhancement of 10. Coupled with such unexpectedly large enhancements we also observe reasonably strong polarization of emission decay of the quantum dots embedded in such templates which are otherwise isotropic. Polarization dependent emission decay dynamics can be attributed to the polarization dependent LDOS in these templates. Our work opens up a completely new method to engineer large enhancements of quantum emitters using assemblies of extremely small metal nanoparticles as nanoantenna elements which could help overcome one of the key challenges in nano-photonics [1,2].

Figure 1: (A) Shows the photoluminescence (PL) spectra from the templates. (B) LDOS as a function of frequency for (a) Different geometrical parameters of template and (b) Polarization dependence LDOS as a function of “ΦAu “ .

Reference:

[1] Chaitanya Indukuri, Arnab Mukherjee, and J. K. Basu . Applied Physics Letters 106, 131111 (2015);

[2] Chaitanya Indukuri, Deepika Chaturvedi and Jaydeep. K. Basu, Plasmonics, (2015). Prasad Vishnubhotla Electron field emission from carbon nanostructures

Prasad V, Department of Physics, Indian Institute of Science, Bangalore-12 Abstract

In this talk I will present the recent studies from our laboratory on electron field emission from multiwall carbon nanotubes, carbon nanotube- polymer composites, zinc oxide (core) /graphite (shell) nanowires and reduced graphene oxide on polymer films. High emission currents and low threshold fields in CNT-polymer composites in the vertical configuration are observed. Field emission properties also depend on the weight fraction of the filler in a polymer matrix. The results are analyzed by Fowler-Nordheim tunnelling mechanism. Many-body localization in the presence of a single particle mobility edge

Ranjan Modak and Subroto Mukerjee

Many body localization is a phenomenon associated with quantum systems in which the system does not reach thermal equilibrium at any energy despite the presence of interactions among the constituent particles. Many-body localized systems are thus not describable in terms of conventional statistical mechanics. The phenomenon of localization in these systems is analogous to that of Anderson localization in non-interacting systems. Here, we will show that a 1D non-interacting system with both localized and delocalized states (and hence a mobility edge separating them) can undergo many-body localization when interactions are switched on. This happens despite the presence of the delocalized states, which might have been expected to resist the tendency to localize. Optical and Dielectric studies on single crystals of organic Glucuronic acid γ-Lactone

Ravi Kiran Saripalli *, H.L.Bhat, and Suja Elizabeth *[email protected]

Abstract

Organic nonlinear optical (NLO) material namely, Glucuronic acid γ-Lactone (Glucuronolactone) was crystallized from aqueous solution. Crystals of large dimension and full morphology were obtained by slow-cooling method in a custom-built solution growth setup. CHN analysis and X-ray diffraction confirmed the phase formation of the crystal. High resolution XRD studies followed by Rietveld refinement yielded accurate lattice parameters which compared well with the reported values. Thermal properties evaluated through TGA/DSC measurements revealed melting point of 171 oC.

Optical spectrum recorded for a b-plate of 2 mm thickness revealed low UV-cutoff at 250 nm. The principal refractive indices were obtained by using Brewster’s angle method. The crystallographic and the principal dielectric axes of Glucuronolactone crystal were identified through optical conoscopy. Preliminary test using Kurtz and Perry powder method revealed a high second harmonic generation conversion efficiency, 3.5 times that of standard KDP. Theoretically generated type 1 and type 2 second order phase matching curves match very well with the experimental results. Conic sections due to spontaneous noncollinear phase matching were also observed. Laser damage studies carried out on (101) plate of the sample yielded high threshold values.

Dielectric constant and dielectric loss were monitored as a function of frequency. Piezoelectric resonance peaks were observed in the range 0.2 to 1.5 MHz which are dependent on the plate thickness. The temperature dependence of the resonance peak frequencies was studied. Piezoelectric coefficients were estimated by resonance- antiresonance method.

Owing to its NLO activity and piezoelectricity, glucuronolactone shows prospect for resonance enhanced electro-optic applications. Phonon anharmonicity in pyrochlores

Pramod Verma ∗, U V Waghmare †, A K Sood ‡, H R Krishnamurthy § Deparment of Physics, Indian Institute of Science, Bangalore, India Jawaharlal Nehru Center for Advanced Scientific Research, Bangalore, India

Pyrochlores with the general formula A2B2O7 are quite interesting from many points of view and have a wide varity of compositions due to its structural flexibility. Their properties vary from highly insulating through semiconducting to metallic behavior depending on the chemical composition. It was experimentally recently found by Saha et al. that some of the

phonon modes in A2Ti2O7 (A = Sm, Gd, Tb, Dy, Ho, Er, Yb, Lu, and Y) show anoma- lous temperature dependence (softening of the phonon modes, i.e. decrease of the phonon frequencies with cooling) and suggestion was made by the authors that phonon-phonon an- harmonic effects may be the main cause of these anomalies. Motivated by these experiments, we have studied the structural, electronic and phonon properties of two titanate pyrochlores

namely Y2Ti2O7 and Dy2Ti2O7, which are insulating and nonmagnetic, from first-principles using Density Functiona Theory. Both these compounds show instabilities with respect to some optical distortions, in that some of the frequencies are found to be imaginary. This is likely to underlie the anomalous temperature dependence of the phonons that have been seen in other titanate pyrochlores. A small distortion of the atomic positions in the unit cell, which breaks the local symmetry of the structure while preserving the global cubic symme- try, stabilizes the structure in the sense that all the frequencies become real. The distortions are so small that it may not be easy to observe experimentally. Using the phonon frequencies of the stable structures from density functional pertubation theory, we have studied phonon- phonon anharmonic effects in titanate pyrochlores. Our results are in good agreement with the experimental results.

∗ Electronin address: [email protected] † Electronin address: [email protected] ‡ Electronin address: [email protected] § Electronin address: [email protected]

Spin polarized low energy electron diffraction from Ir(100)-(5×1)-H, Ir(100)- (2×1+1×2)-O and Fe/Ir(100) surfaces.

A. V. Pradeep and P. S. Anil Kumar

Department of Physics, Indian Institute of Science, Bangalore 560012, India

J. Kirschner

Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle/Saale, Germany

In electron spectroscopy, it is important to analyze spin of electrons to understand surface magnetism and other electron correlation phenomena. As electrons are insensitive to the Stern-Gerlach type spin filters, the analysis of the spin angular momentum of electron beam involves spin dependent scattering from crystalline surfaces. Spin polarized low energy electron diffraction (SPLEED) based spin detectors promises higher efficiency and longer lifetime compared to the conventionally used Mott detectors. Still the efficiency and life time of such spin filter is too small compared to the polarization filters of light. So there is an intense search for a spin filter which is efficient, stable and easy to prepare. Here we report the spin polarized low energy electron diffraction from hydrogen adsorbed Ir(100)- (5×1)-H surface, oxygen adsorbed Ir(100)-(2×1+1×2)-O surface and a few mono layers(ML) of Fe(100) grown on Ir(100). In our experiment, electron beam with transverse spin polarization, from a strained GaAs photocathode, is focused on to these surfaces at an angle of incidence of 45°. The Asymmetry, that is the normalized difference in intensities between the specularly reflected electron beams polarized parallel (I↑) and anti parallel (I↓) to the scattering plane, is recorded as a function of the energy of the incident electron beam, at room temperature. This asymmetry can be due spin-orbit interaction and/or exchange interaction. In the case of Ir(100)-(5×1)-H and Ir(100)-(2×1+1×2)-O surfaces, asymmetry is due to spin orbit interaction. These surfaces are easy to prepare and highly stable in ultra high vacuum (UHV). We have observed large asymmetry and reasonable figure of merit for both the surfaces. This suggests that the Ir(100)-(5×1)-H and Ir(100)-(2×1+1×2)-O surfaces are good candidates for SPLEED spin polarimeter based on spin-orbit interaction. The growth of Fe on Ir(100) is pseudomorphic up to 11 atomic layers and is layer by layer up to 20 atomic layers, at room temperature. Asymmetry due to spin orbit interaction and exchange interactions are evaluated for Fe(100)/Ir(100). The thickness of Fe is varied from 1-10ML. Large spin-orbit asymmetry is observed for 2ML Fe where as it is negligible for 3ML and above. Nonzero exchange asymmetries are observed for Fe(100) films thicker than 4 ML, whereas no exchange asymmetries are observed for 1-4 ML thick Fe(100) at room temperature. Exchange asymmetry data shows no thickness dependence in the strained BCT regime. This is a good criterion for spin filter since the thinner films avoids stray magnetic field, moreover there is no need for precise thickness measurement.

DLTS analysis of amphoteric interface defects in high-k TiO2 MOS structures prepared by sol-gel spin-coating

Arvind Kumar, Sandip Mondal and K.S.R. Koteswara Rao Department of Physics, Indian Institute of Science, Bangalore, 560012, India Email: [email protected]

In this work, high-k TiO2 thin films are fabricated from a facile, combined sol – gel spin – coating technique on p and n type silicon substrate. The defect parameters observed at the interface of Si/TiO2 were studied by capacitance – voltage (C – V) and deep level transient spectroscopy (DLTS). The flat – band voltage (VFB) and the density of slow interface states estimated are – 0.9, – 0.44 V and 5.24×1010, 1.03×1011 cm-2; for the NMOS and PMOS capacitors, respectively. The activation energies, interface state densities and capture cross – sections 11 11 -1 -2 -23 -23 2 measured by DLTS are EV + 0.30, EC – 0.21 eV; 8.73×10 , 6.41×10 eV cm and 5.8×10 , 8.11×10 cm for the NMOS and PMOS structures, respectively. A low value of interface state density in both P- and N-MOS structures makes it a suitable alternate dielectric layer for CMOS applications. And also very low value of capture cross section for both the carriers due to the amphoteric nature of defect indicates that the traps are not aggressive recombination centers and possibly cannot contribute to the device operation to a large extent.

Figure: DLTS spectra of Al/TiO2/Si (100) MOS structure for the p and n type Si substrate.

References: [1] G.D. Wilk, R.M. Wallace, and J.M. Anthony, J. Appl. Phys. 89, 5243 (2001). [2] A. Kumar, S. Mondal, and K.S.R.K. Rao, AIP Advances 5, 117122 (2015). [3] A. Kumar, S. Mondal, S.G. Kumar, and K.S.R. Koteswara Rao, Mater. Sci .Semi. Proc. 40, 77 (2015).

Anomalous conductivity noise in gapped bilayer graphene heterostructure

Md. Ali Aamir, Paritosh Karnatak, T. Phanindra Sai and Arindam Ghosh

Bilayer graphene has unique electronic properties – it has a tunable band gap and also, valley symmetry and pseudospin degree of freedom like its single layer counterpart. In this work, we present a study of conductance fluctuations in dual gated bilayer graphene heterostructures by varying the Fermi energy and the band gap independently. At a fixed band gap, we find that the conductance fluctuations obtained by Fermi energy ensemble sampling increase rapidly as the Fermi energy is tuned to charge neutrality point (CNP) whereas the time-dependent conductance fluctuations diminish rapidly. This discrepancy is completely absent at higher number densities, where the transport is expected in the 2D bulk of the bilayer system. This observation indicates that near the CNP, electrical transport is highly sensitive to Fermi energy, but becomes progressively immune to time-varying disorder. A possible explanation may involve transport via edge states which becomes the dominant conduction mechanism when the bilayer graphene is gapped and Fermi energy is situated close to the CNP, thereby causing a dimensional crossover from 2D to 1D transport. Our experiment outlines a possible experimental protocol to probe intrinsic topological states in gapped bilayer graphene. Current induced deterministic switching of magnetization in Pt/Co/Pt films

Vineeth Mohanan P and K.R. Ganesh

The recent surge of interest in perpendicularly magnetized layered structures of ferromagnets and heavy metals (like Pt, Ta etc.) owes to the observation of interesting phenomenon like spin orbit torque (SOT) arising out of spin Hall effect from the heavy metals. SOT can be used to easily switch the magnetization at lower current densities than that required for the conventional spin transfer torque. The main challenge here is to break the symmetry of clockwise or anticlockwise rotation of magnetization which can lead to deterministic switching. This was facilitated by applying an external in plane magnetic field along the current direction. Advances in this field led to growing of layered structures having gradients of anisotropy in the plane of the films which compensated for the in-plane external field.

In this work, we report the current induced magnetization reversal in full metallic Pt/Co/Pt trilayers without the need for an in plane magnetic field for deterministic switching. Pt4(seed)/Co0.43/Pt1(thickness in nm) were grown on thermally oxidized Si wafers using DC magnetron sputter deposition. The films were patterned to narrow wires of 3-5 µm width by electron beam lithography and subsequent argon ion etching. Kerr microscope imaging was used to monitor the magnetization states of the device as a function of current. Well defined switching at current density as low as 2.5x1011A/m2 was seen without applying any in-plane field. To further understand this, we investigated the interaction of two types of domain walls (namely the up-down and down-up) with applied current which showed a high level of asymmetry. The up-down domain walls moved effortlessly in the direction of current when compared to their down-up counterparts, which did not move even at current densities as high as 6 x1011A/m2. These observations of deterministic switching and asymmetry in the dynamics of two DW types confirm the presence of an intrinsic symmetry breaking field in the sample.

Synthetic Teterahedrites for Thermoelectric Applications Raju Chetty and Ramesh Chandra Mallik Thermoelectric materials and devices laboratory, Department of Physics, Indian Institute of Science, Bangalore 560012, India, Email: [email protected]

Abstract Thermoelectricity is the conversion of thermal energy into electrical energy and vice versa. Recently there is a lot of focus on finding the new materials to achieve good thermoelectric performance. Tetrahedrites are newly emerging materials for thermoelectric study because of various advantages compared to conventional TE materials (Bi2Te3, PbTe etc.). Tetrahedrites are belongs to the family of sulfosalts which are naturally earth abundant minerals. These materials are consists of light, non-toxic and low cost elements of Cu and S whereas conventional TE materials possess toxic elements such as Pb and Te. Tetrahedrite materials have beneficial TE properties such as complex crystal structure with large number of atoms per unit cell (say 58 atoms per unit cell) which facilitates to existence of intrinsically low thermal conductivity and it has the highly symmetric crystal structure (cubic). The band structure of tetrahedrite describes that the Fermi level exists inside the valence bond shows p-type conducting with degenerate semiconducting behavior. Optimization of TE properties can be done by doping with transition metals atoms (Mn, Fe, Co, Ni. Zn) on the Cu site of Cu12Sb4S13. We have studied the TE properties of synthetic tetrahedrite materials by substituting Mn and Co on the Cu site by varying the doping content for optimization. All the samples were prepared solid state synthesis method. Powder XRD was done for the phase identification and confirmed that tetrahedrite as main phase with a trace of impurity phases. It was further confirmed by Rietveld refinement and a systematic trend of lattice parameter with doping content was observed. Elemental composition and phase purity was analyzed by Electron Probe Micro Analysis which is in agreement with the XRD results. Temperature dependent transport properties were measure studied for all the samples. In both the Mn and Co doping electrical resistivity systematically increased with increase of doping through increase of carrier concentration resulted from the substitution of higher valence element on the lower valence atom. A similar trend like resistivity was observed for the Seebeck coefficient and it shows positive values in the entire temperature range measured indicates that dominant charge carriers are holes. A systematic decrease of total thermal conductivity with doping content was observed due to decrease of carrier contribution. A combined results of power factor and thermal conductivity showed an adverse effect on thermoelectric figure of merit (zT) for the Mn doped compounds whereas an increase of zT was observed for Co doped compound i.e. zT=0.98 at 673 K for Cu11.5Co0.5Sb4S13. Assembling topological insulators with lasers

Sayonee Roy, Kallol Sen, Tanmoy Das

Topological insulator phase arises when chiral states are localized in the bulk, with non-trivial bulk band topology and robust edge states. In solid state field, topological insulators are recently realized in various materials. But such state has yet not been obtained in optical lattice counterparts . More recently, spin-orbit coupling (SOC) is realized in optical lattices by carefully aligning orthogonal lasers with different frequencies. But the SOC is found to be one- dimensional (1D). 1D SOC cannot create a closed chiral orbit or a cyclotron -like orbit which thereby can become localized in the bulk to give rise to topological phases.

Here we propose a new and easily obtainable setup to engineer topological insulator in optical lattice. Two antiparallel SOC wires are assembled next to each other such that electron can tunnel between them. Due to SOC, in one wire, say, up spin particle moves along the right hand direction. Then the spin up particle will be moving along the left hand in the next SOC wire since it has SOC reversed. Now as the hopping amplitude is increased between the wires, it may become more favorable for up spin particle to hop to the next wire, in which it moves in the reversed direction. Finally it hops back to the original wire. By this process, the up spin electron encloses an area which allows integer Chern number. This can be understood in the language of TKNN invariant, which says, for a given magnetic field, the cyclotron orbit adjust its area such that it can only allow integer multiple of the flux quanta (which is the Chern number). In our case, the chiral orbit makes an area which can allow only integer Chern number, as if there is a magnetic field in the system. However, due to time reversal invariance, the down spin possess opposite Chern number to yield the total Chern number to vanish. We have quantified our result with the calculation of Z2 invariant and edge state calculation. Our proposal can open an unprecedented era for topological insulators in optical lattices.

Ray tracing based refraction correction in Optical tomography of polymer gel dosimeters

Rakesh Manjappa1, Sharath Makki S1, Rajesh Kumar2, Ram Mohan Vasu3, and Rajan Kanhirodan1,∗ 1Department of Physics, Indian Institute of Science, Bangalore, India - 560012 2Radiological Physics and Advisory Division, BARC, Mumbai, India - 400094 3Department of Instrumentation and Physics, Indian Institute of Science, Bangalore, India - 560012 E-mail: [email protected]

Abstract. Gel Dosimetry uses the principle of change in attenuation coefficient due to radiation exposure and is used in radiotherapy planning and verification [1]. Optical tomography setup using laser light to estimate dose information in polymer gels is described. Dose information conformal to treatment planning is required for complex radiotherapy plans. Light-matter interaction and changes in optical properties that lead to dose readout is explained. It is found that refractive index (RI) and optical density (OD) changes with irradiated dose[2]. This causes radiation induced refraction artifacts in optical CT of polymer gel dosimeters[3]. RTAP (Resolution- Time-Accuracy-Precision) criteria is defined as ≤ 1 mm3 spatial resolution, ≤ 1 hour imaging time, accurate to within 3%, and within 1% precision. Our prototype design is able to acheive all of these criteria. A model based on Ray tracing is proposed for dose reconstruction in 2D[4]. For this we have adopted the Jacob’s algorithm for fast ray tracing in a pixel/voxel basis. The proposed model corrects for refraction at the boundary and interior dose regions. For complex dose profiles, we have considered the voxel based ray tracing method. Some preliminary results by using polarized light are presented.

[1] C. Baldock Y. de Deene, S. Doran, G. Ibbott, A. Jirasek, M. Lepage, K. B. McAuley, M. Oldham, and L. J. Schreiner, “Polymer gel dosimetry,” Phys. Med. Biol. 55(5), R1R63 (2010). [2] M. J. Maryanski, Y.Z. Zastavker, and J. C. Gore, “Radiation dose distributions in three dimensions from tomographic optical density scanning of polymer gels: II. Optical properties of the BANG polymer gel,” Phys. Med. Biol. 41, 2705-2717 (1996). [3] Warren G. Campbell, Derek M. Wells, and Andrew Jirasek, ”Radiation-induced refraction artifacts in the optical CT readout of polymer gel dosimeters” Med. Phys. 41, 112102 (2014); [4] R. Manjappa, S. Makki, R. Kumar and R. Kanhirodan“Effects of refractive index mismatch in optical CT imaging of polymer gel dosimeters” Med. Phys. 42, 750-759 (2015). 1 Plasmon-mediated emergence of collective emission and enhanced quantum efficiency in quantum dot films M. Praveena 1, Arnab Mukherjee1, Murugesan Venkatapathi2, J. K. Basu1 1Department of Physics, Indian Institute of Science, Bangalore, India 2Computational photonics laboratory, SERC, Indian Institute of Science, Bangalore, India We present experimental and theoretical results on monolayer colloidal cadmium selenide quantum dot films embedded with tiny gold nano parti- cles. By varying the density of the embedded gold nano particles, we were able to engineer a plasmon mediated cross-over from emission quenching to enhancement regime, at interparticle distances for which only quench- ing of emission is expected. This cross-over and a unique non monotonic variation of photoluminescence intensity and decay rate, in experiments, is explained in terms of a new model for plasmon mediated collective emis- sion of quantum emitters which points to the emergence of a new regime in plasmon-exciton interactions. The addition of such metal particles in low fractions induces a strong evolution of the super-radiant modes of emission among quantum dots and aids their survival of thermal fluctuations; ex- hibiting a quantum phase transition. Whereas, an increase of size of metal particles results in an increase of local thermal fluctuations to revert to the behavior of apparently independent emitters. The presented methodology to achieve enhancement in optical quantum efficiency for optimal doping of gold nano particles in such ultra-thin high density quantum dot films can be beneficial for new generation displays and photo-detectors. Entropy profiles of Galaxy Clusters using MCMC

Kiran Lakhchaura, Prateek Sharma, Tarun Deep Saini

November 20, 2015

Abstract Clusters of galaxies, the largest gravitationally bound structures in the Universe, form through hierarchical merging. Purely gravity based models of structure formation predict self- 2/3 similarity in the clusters leading to tight mass-observable scaling relations (T ∝ M , LX ∝ M4/3 etc.). Observations, however, show large departures and dispersions in these relations. Non- gravitational (NG) processes (e.g., AGNs, SNe, Radiative Cooling etc.) have been invoked for explaining the observed scaling relations and, therefore, are very important for studying cluster evolution. The entropy of a cluster can be used as an ideal probe for studying the impact of NG processes on the clusters. Numerical simulations based on gravitational models predict self-similar power-law profiles for the cluster entropy (K ∝ R1.1). Various studies have been carried out in the literature making use of different analysis techniques, some of which have led to completely different results for the observed entropy profile shapes, specially near the centers. In this work, we have tried to apply the technique of Markov Chain Monte Carlo deprojection on the X-ray data of a sample of clusters. The method marginalizes over the less important outer regions of a cluster to focus on the more important inner regions, and takes into account the correlation between the count errors. In this talk, I will present some of the results from this study.

1 Dual gated graphene-MoS2 field effect transistors (FET) with extremely high optical detectivity

Kallol Roy†, Harshit Dubey†, T. Phanindra Sai†, Ranjit Kashid†, Shruti Maliakal†, Kimberly Hsieh†, and Arindam Ghosh†

†Department of Physics, Indian Institute of Science, Bangalore 560012, India

Atomically thin graphene-on-MoS2 FET structures proved to have high quality opto-electronic response [1-3]. The photoresponsivity of such devices can be as high as ~1010 A W-1 [1]. However the use of single layer graphene in such devices give high dark current restricting the ultimate detectivity of the device. In single layer graphene the limitation to control dark current using an external electric filed appears because of the absence of bandgap in it [4]. Nevertheless, a bandgap can be opened in a bi-layer graphene device by applying an external electric field [5].

In this work we show that the use of bi-layer graphene on MoS2 structure gives better control over dark current without compromising the photoresponsivity of the device. A dual gated structure with transparent top gate is used to control the opening-of-bandgap and the Fermi energy of the channel separately, giving full control over the channel resistance hence dark current. The noise equivalent power (NEP) of such photodetectors is as low as ~10 W/√Hz and specific decectivity (D*) is as high as ~10 cm √Hz/W (Jones) making these a promising candidate for single photon detection.

References:

1. Roy, K. et al. Graphene – MoS2 hybrid structures for multifunctional photoresponsive memory devices. Nat. Nanotechnol. 8, 826–830 (2013) 2. Zhang, W. et al. Ultrahigh-gain photodetectors based on atomically thin graphene-MoS2 heterostructures. Sci. Rep. 4, 3826 (2014). 3. Koppens, F. H. L. et al. Photodetectors based on graphene, other two-dimensional materials and hybrid systems. Nat. Nanotechnol. 9, 780–793 (2014). 4. Castro Neto, a. H., Peres, N. M. R., Novoselov, K. S. & Geim, a. K. The electronic properties of graphene. Rev. Mod. Phys. 81, 109–162 (2009). 5. McCann, E. & Koshino, M. The electronic properties of bilayer graphene. Reports Prog. Phys. 76, 056503 (2013). Atomic-Scale and Nano-Scale Quantum Structures B. N. Dev

Department of Materials Science

Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

Materials structures, grown by epitaxial growth techniques with atomic scale precision, have been used in many fundamental discoveries of new phenomena in the field of low dimensional quantum structures. In 2001 National Research Council, USA, published a vision document “Physics in a new era: An overview” to elucidate scientific priorities and opportunities and identified six areas of grand challenges. Two among them are (1) developing quantum technologies and (2) creating new materials. Activities in these areas have been growing around the world. Molecular beam epitaxy (MBE) has been an important materials growth technique that has contributed enormously to discoveries of quantum phenomena and advanced technologies.

After presenting an overall perspective, a brief description of our effort in this area of research will be given. We have been growing nanostructures by MBE, mostly via self- organization, and investigating the growth features and quantum phenomena in them by various in-situ [scanning tunneling microscopy (STM) and spectroscopy (STS), reflection high energy electron diffraction (RHEED)] and ex-situ [transmission electron microscopy (TEM)] experimental techniques. We also investigate these systems theoretically, mainly by density functional theory (DFT) calculations. A combined experimental and theoretical approach will be presented. Fabrication of nanostructures using a focused ion beam and determination of lateral diffusion coefficient in nanostructures using photoemission electron microscopy (PEEM) will be discussed.

POSTER ABSTRACTS

Cold gas in cluster cores: Global stability analysis and non-linear simulations of thermal instability in spherical and plane-parallel atmospheres

Prakriti Pal Choudhury, Prateek Sharma

Abstract We perform global linear stability analysis and numerical simulations to understand the condensation of cold gas from hot/virial atmospheres (coronae) in global thermal equilibrium. We pay particular attention to geometry (e.g., spherical versus plane-parallel) and the nature of the grav- itational potential. Global linear analysis gives a similar value for the fastest growing global thermal instability modes in spherical and Carte- sian geometries. Theory and observations suggest that the cooling in haloes critically depends on the ratio of the cooling time to the free-fall time (tcool/tff ). The interplay of cooling and gravity is conveniently en- capsulated in this dimensionless parameter. Previous works highlighted the difference between the nature of cold gas condensation in spherical and plane-parallel atmospheres; namely, cold gas condensation is easier in spherical atmospheres. This apparent difference due to geometry arises because the previous simulations focussed on in-situ condensation of mul- tiphase gas for plane-parallel atmospheres but condensation anywhere for spherical atmospheres. Unlike previous claims, our nonlinear simulations show that there are only minor differences in cold gas condensation, either local or global, for different geometries. The amount of cold gas condens- ing depends on the shape of the gravitational potential well; gas has more time to condense if gravitational acceleration decreases toward the center, irrespective of geometry.

1 Abstract for Poster

Probing Photoexcited Carriers in a few layer MoS2 Laminate by Time-Resolved Optical Pump-Terahertz Probe Spectroscopy Srabani Kar,†,‡ Y. Su,§ R. R. Nair,§ and A. K. Sood*,†,‡ † Department of Physics and ‡ Center for Ultrafast Laser Application, Indian Institute of Science, Bangalore 560 012, India §School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, U.K.

Abstract

We have studied the relaxation dynamics of the photoexcited carriers in a free-standing MoS2 laminate consisting of a few layers (1-6 layers) using time-resolved optical pump-terahertz probe spectroscopy. Photo-excited with 800 nm (50 fs) pump pulse results in absorption of the terahertz radiation by the photoinduced charge carriers. The non-equilibrium charge carriers show fast as well as slow decay channels, analyzed using rate equation model incorporating defect-assisted Auger scattering of the electrons, holes and the excitons. The fast relaxation time occurs through the capture of electrons and the holes by the defects whereas the slower one arises from the blocking of the defect states due to defect-assisted exciton capture via Auger scattering. The excitons bound to the defects relax vary slowly preventing the Auger scattering of the electrons and the holes. Our results provide a comprehensive understanding of the non-equilibrium carriers in a system of unscreened Coulomb interaction, where defect-assisted Auger process dominate. Abstract for Poster Detection of Localized Surface Plasmons in Tellurium Nanowires using Time-Resolved Optical Pump-Terahertz Probe Spectroscopy

Mithun K. P.1, Srabani Kar1, Ahin Roy2, N. Ravishankar2 and A. K. Sood1

1Department of Physics and Center for Ultrafast Laser Application, Indian Institute of Science, Bangalore 560012 2Material Research Center, Indian Institute of Science, Bangalore 560012

Abstract

Nanowires have interesting properties due to the enhanced surface-area to volume ratio and quantum confinement. Here we present the results of terahertz time- domain spectroscopy conducted on Tellurium nanowires with diameters within 2nm-5nm and length of ~ 1µm. The measured differential complex photo-induced conductivity is best described using Lorentzian model, showing fluence and delay dependent resonant frequency varying from 1.2 to 1.5 THz. The shift of the resonant frequency with respect to fluence and delay time is very similar to the behavior of localized surface plasmons observed in GaAs and InP nanowires [1,2]. Hence we attribute these Lorentzian peaks to the localized surface plasmon modes. After photo-excitation with 800 nm pump, we observe the decrease of the terahertz transmission due to absorption of the THz radiation by photo-excited charge carriers. The non-equilibrium carriers show a faster relaxation component of ~7ps and a slower relaxation component of ~230ps which are independent of pump fluence.

References

1. Parkinson P. et. al. Nano Lett. 2007, 7, 2162-2165, “Transient Terahertz Conductivity of GaAs Nanowires” 2. Joyce H. J. et. al. Nano Lett. 2012, 12, 5325-5330, “Ultralow surface recombination velocity in InP Nanowires probed by terahertz spectroscopy”

Enhancing photoresponsivity using MoTe2-graphene vertical heterostructures

Manabendra Kuiri1, Biswanath Chakraborty1, Arup Paul1, Subhadip Das1, A K Sood1 and Anindya Das1

1Department of Physics, Indian Institute of Science, Bangalore 560012, India

Abstract

MoTe2 with a narrow band-gap of ~1.1 eV is a promising candidate for optoelectronic applications, especially for the near-infrared photo detection. However, the photo responsivity of few layers -1 MoTe2 is very small (<1 mAW ). In this work we show that a few layer MoTe2-graphene vertical heterostructures has a much larger photo responsivity of ~ 20 mAW-1. The trans-conductance measurements with back gate voltage show on-off ratio of the vertical transistor to be 105. The rectification nature of the source-drain current with the back gate voltage reveals the presence of a stronger Schottky barrier at the MoTe2-metal contact as compared to the MoTe2-graphene interface. In order to quantify the barrier height, it is essential to measure the work function of a few layers MoTe2, not known so far. We demonstrate a new method to determine the work function by measuring the photo-response of the vertical transistor as a function of the Schottky barrier height at the MoTe2-graphene interface tuned by electrolytic top gating. Probing 2D black phosphorus by quantum capacitance measurements

Manabendra Kuiri1, Chandan Kumar1, Biswanath Chakraborty1, Satyendra Gupta1 , Mit H Naik1, Manish Jain1, A K Sood1 and Anindya Das1

1Department of Physics, Indian Institute of Science, Bangalore 560012, India

Abstract

Two-dimensional materials and their heterostructures have emerged as a new class of materials for not only fundamental physics but also for electronic and optoelectronic applications. Black phosphorus (BP) is a relatively new addition to this class of materials. Its strong in plane anisotropy makes BP a unique material to make conceptually new type of electronic devices. However, the global density of states (DOS) of BP in device geometry has not been measured experimentally. Here we report the quantum capacitance measurements together with conductance measurements on a hBN protected few layer BP (∼ 6 layer) in a dual gated field effect transistor (FET) geometry. The measured DOS from our quantum capacitance is compared with the density functional theory (DFT). Our results reveal that the transport gap for quantum capacitance is smaller than that in conductance measurements due to the presence of localized states near the band edge. The presence of localized states is confirmed by the variable range hopping seen in our temperature- dependence conductivity. A large asymmetry is observed between the electron and hole side. The asymmetric nature is attributed to the anisotropic band dispersion of BP. Our measurements establish the uniqueness of quantum capacitance in probing the localized states near the band edge, hitherto not seen in the conductance measurements.

Noise measurement in Moire and Bilayer Graphene

Chandan Kumar, Manabendra Kuiri, Jeil Jung, Tanmoy Das and Anindya Das

We have investigated low frequency 1/f noise in cloned and Bilayer graphene, encapsulated between hexagonal boron nitride. Noise measurement is done with top gate and true noise of the channel is extracted using the dual gated geometry. We observe that magnitude of 1/f noise at cloned Dirac (CD) is smaller than primary Dirac. The low noise level at the CD is explained using screening property, which is found to be different at CD and primary Dirac (PD). We also find that noise magnitude can be tuned by two orders of magnitude by changing charge density and by opening band gap. Unlike previous studies our result suggest that 1/f noise indeed follows Hooge's empirical relation.

Conductance oscillation under high magnetic field in Graphene

Chandan Kumar, Manabendra Kuiri, Anindya Das

We investigate the transport property of monolayer graphene in p-n junction geometry. We observe prominent oscillation in conductance under high magnetic field in the bipolar regime. At low magnetic field we observe a clear quantum hall plateaus in both unipolar and bipolar regime but as the field is increased QH plateau disappear and clear conductance oscillation is seen only in bipolar regime. The Criss cross lines are seen in between the landau levels which hints towards the role played by the disorder. Disorder leads to the mixing of landau level and a suitable disorder can lead to oscillation in conductance.

Roughening transition in colloidal membranes Lachit Saikia, Debasmita Mondal and Prerna Sharma Department of Physics, Indian Institute of Science, Bangalore-560012

Abstract: Flat liquid-like 2-D monolayers of aligned rod shaped viruses spontaneously assemble in presence of depletion attraction between the rods. We show that as the range of depletion attraction becomes comparable to the rod diameter, dynamics and arrangement of the rods within these membranes become crystalline. The crystalline membranes are no longer flat instead roughen and buckle macroscopically. We use thermo-responsive assembly conditions to show that the phase transition is driven by nucleation and growth. Local buckling of the solid membranes compensates for the excess strain energy due to defects and disclinations. On the other hand, macroscopic buckling arises as the growing solid-like domains hit the membrane edge of the host liquid membrane.

A Three­Dimensional Babcock­Leighton Solar Dynamo Model with Non­Axisymmetric Subsurface Flow

Gopal Hazra1,2 and Mark S. Miesch3 1 Indian Institute of Science, Bangalore, India 2 Indian Institute of Astrophysics, Bangalore, India 3 High Altitude Observatory, Colorado, USA

The study of photospheric magnetic field evolution is very important because it is the only observable part of the Babcock­Leighton dynamo models and it sets the boundary condition for the coronal and heliospheric magnetic fields. Observationally it is evident that the solar photospheric magnetic fields is mostly transported by the large scale and small scale surface flows i.e, Differential Rotation, Meridional Circulation and Convective flows ( granular and supergranular flows ). Presently most of the Babcock­ Leighton Models use only the axisymmetric flows of sun. We are now incorporating the non­axisymmetric convective flows in our model also which are based on the observation of doppler measurement of the solar photosphere. From observation we have only the velocity measurement of the photosphere which is mainly horizontal flows on the surface but our model is a 3D model so we have extrapolated the horizontal surface flows radially below the photosphere inside the convection zone assuming that the mass flux is divergenceless. Since we have incorporated large scale and small scale velocity field simultaneously, the evolution of radial magnetic field on the surface of the sun more closely matches the observation. Squished Baryons in Synthetic Dimensions

Sudeep Kumar Ghosh, Umesh K. Yadav, and Vijay B. Shenoy

The idea of “synthetic dimension” is to use internal states of atoms moving in a 1d optical lattice, to realize a hopping Hamiltonian equivalent to the Hofstadter model on a square lattice strip. We investigate the physics of SU(M) symmetric interactions in this synthetic dimension system. We show that this system is equivalent to particles (with SU(M) symmetric interactions) experiencing an SU(M) Zeeman field at each lattice site and a non-Abelian SU(M) gauge potential that af- fects their hopping. This equivalence brings out the possibility of generating non-local interactions between particles at different sites of the optical lattice. In addition, the gauge field induces a flavor-orbital coupling, which mitigates the “baryon breaking” effect of the Zeeman field. For M particles, concomitantly, the SU(M) singlet baryon which is site localized in the usual 1d optical lattice, is deformed to a non-local object (“squished baryon”). We conclusively demonstrate this effect by analytical arguments and exact (numerical) diagonalization studies. Our study promises a rich many-body phase diagram for this system. The study of the few body physics of the system uncovers a rich collection of many body phases. We construct the full many body phase diagram of the system using bosonization and DMRG by looking at different correlation functions. We demon- strate that there are interesting transitions between quasi-condensates of squished baryons induced by the gauge field. There are, also, ample possibilities of generating novel phases like the FFLO state. Pressure-dependent Semiconductor to Semimetal and Lifshitz transitions in α-MoTe2 : Raman and Density functional studies

Achintya Bera1, Anjali Singh2, D V S Muthu1, U V Waghmare2 and A K Sood1

1Department of Physics, Indian Institute of Science, Bangalore 560 012, India and 2 Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India

In recent years the 2D transition metal dichalcogenide (TMD) materials have attracted a lot of attention from the viewpoint of band gap engineering, transistor on-off ratio, high carrier mobilities, effect of spin-orbit interactions, and spin-valleytronic devices. One of the unique properties of the group VI TMDs is that they can exist in different structural forms (polytypes) with different electronic properties. Recently, enormous research activities focus on the novel properties of the 1T or 1T' phases of TMDs. The 1T-phases of MoS2 has been synthesized to produce the transparent electrodes and energy storage devices. A proposal has been made to study the Z2-topological quantum devices in few layer 1T′-MoTe2 with a bandgap of ∼ 60 meV. Chemical pressure induced different polytypes including 3R-phase have been observed on MoTe2. Mechanical strain or deformation techniques is one of the routes for the TMDs to switch between different thermodynamically stable structural polytypes without introducing impurities.

We present high pressure Raman studies of bulk MoTe2 upto ∼ 30 GPa with support from first- principles density functional theoretical (DFT) calculations to determine its electronic structure and 1 vibrational properties. The change in pressure coefficients of E 2g and A1g Raman mode frequencies and in their relative frequencies bear the signature of the semiconductor to metal transition at P ∼ 6 GPa as revealed by the calculated electronic band gap. Also the integrated area ratio of the A1g mode 1 to the E 2g mode shows a maximum around this transition. Another transition is observed at P ∼ 24 GPa reflected in the pressure dependence of the A1g mode. Topology of the Fermi-surface evolves as a function of pressure with the appearance of electron and hole pockets at P ∼ 24 GPa marking a Lifshitz transition. In contrast to MoS2, MoTe2 does not exhibit a first order layer-sliding isostructural transition from 2Hc to 2Ha. Our first-principles DFT calculations confirms that there is no structural transition throughout the whole pressure range and hence we neglect the possibility of observing β-phase or 1T′-MoTe2 phase in the metallic region, otherwise we would have seen the new Raman modes because of symmetry lowering in the pressure range of 6 < P ≤ 30 Gpa.

Our findings will stimulate further study of high pressure and low temperature resistivity experiments to capture the anomalies near the Lifshitz transition.

References :

[1] Z.-H. Chi et al., Phys. Rev. Lett. 113, 036802 (2014) [2] A. P. Nayak et al., Nat Commun 5, 3731 (2014) [3] M. Riflikova, R. Martonak, and E. Tosatti, Phys. Rev. B 90, 035108 (2014) [4] M. Kan, H. G. Nam, Y. H. Lee, and Q. Sun, Phys. Chem. Chem. Phys. 17, 14866 (2015) Electrical characterization of MoS2-MoTe2 vertical heterojunction

Arup Paul, Manabendra Kuiri, Biswanath Chakraborty, Subhadip Das, A K Sood, Anindya Das

Abstract for poster

The discovery of 2D layered material has paved the way to understand device physics on atomic scale. TMDs are layered Van-der Waals materials with exceptional electrical and mechanical properties, which holds promises for next generation electronic, electro-mechanic and opto- electronic devices. Using the recent methodology of stacking different layered materials, atomically thin pn junction, has been realized. It has been shown that compared to only TMDs, TMD based heterostructures shows improved electrical and optical responses. Also it has been predicted that the charge transport mechanism in atomically thin junctions are completely different form that of a conventional bulk semiconductor pn junctions. Hence TMDs and TMD heterostructures are under extensive study to understand various mechanisms of transport.MoS2 and its vertical heterostructures with graphene and other TMDs have been extensively studied in recent years. MoTe2 is relatively new entrance to the above class of 2d materials having direct band-gap of 1.1ev for monolayer and indirect band-gap of 1.0eV for bulk. This makes it promising for near infrared optoelectronic devices. Its high spin-orbit coupling of 250meV is promising for spin based electronic devices. In this work we characterize the MoS2 – MoTe2 based pn junction. The devices fabricated consists of single layer n-type MoS2 and few layer thick p-type MoTe2. We observe gate-tunable diode like I-V response. We also observe an anomalous behavior in the transfer curves of the devices, which has not been observed in any other TMD based hetero-structure devices. Thus we try to understand the underlying physics of these devices and explore the suitability of these hetero-structure devices for future application. Quantum Impurities develop Fractional Local Moments in Spin-Orbit Coupled Systems

Adhip Agarwala∗ and Vijay B. Shenoy† Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India (Dated: November 20, 2015) Abstract Systems with spin-orbit coupling have the potential to realize exotic quantum states which are interesting both from fundamental and technological perspectives. We investigate the new physics that arises when a correlated spin-1/2 quantum impurity hybridizes with a spin-orbit coupled Fermi system. The intriguing aspect uncovered is that, in contrast to unit local moment in conventional systems, the impurity here develops a fractional local moment of 2/3. The concomitant Kondo effect has a high Kondo temperature (TK ). Our theory explains these novel features including the origins of the fractional local moment and provides a recipe to use spin-orbit coupling(λ) to

4/3 enhance Kondo temperature (TK ∼ λ ). These results will be useful in shedding light on a range of experiments, including those of magnetic impurities at oxide interfaces. Our predictions can also be directly tested in cold-atom systems where the spin-orbit coupling can be engendered via a uniform synthetic non-Abelian gauge field. In addition, this work opens up new directions of research in spin-orbit coupled Kondo lattice systems.

∗ [email protected] † [email protected]

1 Quantum transport at graphene NbSe2 junction Manas Ranjan Sahu, Anindya Das

Abstract for poster At a NS(Normal-Superconductor) junction electrons from the normal part can enter into the superconductor by a process called Andreev reflection(AR) in which the incident electron reflects back as a hole. For normal metals hole is reflected into the same band as of h e the incident electron retracing the same path(θ ref ≈ -θ inc). On the other hand, if the hole is reflected into a different band the reflection h e will be specular(θ ref ≈ θ inc). Graphene being a zero gap semiconductor is an ideal system where specular AR can be realized. Here we characterize graphene – NbSe2 junction supported by hBN. We observed suppression of conductance near dirac point which can be attributed to the reduction in allowed phase space for AR with decreasing Fermi energy. Fluctuation theory of Rashba Fermi gases: Gaussian and beyond

Jayantha P. Vyasanakere1 and Vijay B. Shenoy2∗ 1Department of Physics, University College of Science, Tumkur University, Tumkur 572 103, India and 2Department of Physics, Indian Institute of Science, Bangalore 560 012, India

(Dated: November 20, 2015) Abstract Fermi gases with generalized Rashba spin orbit coupling induced by a synthetic gauge field have the potential of realizing many interesting states such as rashbon condensates and topological phases. Here we address the key open problem of the fluctuation theory of such systems and demonstrate that beyond-Gaussian effects are essential to capture finite temperature physics of such systems. We obtain their phase diagram by constructing an approximate non-Gaussian theory. We conclusively establish that spin-orbit coupling can enhance the exponentially small transition temperature (Tc) of a weakly attracting superfluid to the order of Fermi temperature, paving a pathway towards high Tc superfluids.

1 Study of thermoelectric properties of the pseudo-binary Cu2Te-Sb2Te3 system

Shriparna Mukherjee1, Ch. Raju2, Olu Emmanuel Femi3, Kamanio Chattopadhyay1,3, Ramesh Chandra Mallik2

1Interdisciplinary Centre for Energy Research, Indian Institute of Science, Bangalore, 2Thermoelectric Materials and Devices Laboratory, Department of Physics, Indian Institute of Science, Bangalore, 3Department of Materials Engineering, Indian Institute of Science, Bangalore.

Abstract- Thermoelectric properties of the pseudo-binary system Cu2Te-Sb2Te3 have been studied in this work. Samples with different stoichiometric amounts of Cu, Sb and Te were prepared by solid state synthesis method. Powder X-ray Diffraction pattern confirmed the presence of both Cu2Te and Sb2Te3 phases. Scanning Electron Micrographs of the sample with eutectic composition (Cu 33 at.%) showed lamellar structure and Energy-Dispersive X- ray spectroscopy confirmed the phases to be Cu2Te and Sb2Te3. All other compositions showed the presence of dispersed phase of Cu2Te in the lamellar matrix of Cu2Te-Sb2Te3. Differential Scanning Calorimetry detected the phase transitions of the samples within the temperature range of 600 K to 800 K and it matched the literature values of phase transitions due to both Cu2Te and Sb2Te3. Thermogravimetric analysis confirmed the thermal stability of the pellets till 773 K. Seebeck coefficient (S) and electrical resistivity (ρ) were measured in the temperature range 300 K to 600 K. The absolute value of Seebeck coefficient was positive indicating that all the samples are of p-type conductivity throughout the measurement temperature range. Electrical resistivity of all the samples showed an increasing trend with increase in temperature from the range (0.88-1.53) µΩ-m at 350 K to the range (1.83-4.03) µΩ-m at 590 K. The highest power factor was found to be 0.7 mW/(m-K2) at 600 K for the sample with eutectic composition (Cu 33 at.%) where S= 53.35 µV/K and ρ= 4.03 µΩ-m. Further studies need to be done to improve the power factor and evaluate the thermoelectric figure of merit of this Cu2Te-Sb2Te3 system.

Raman Signatures of Strong Kitaev Exchange Correlations in

(Na1−xLix)2IrO3 : Experiments and Theory

Satyendra Nath Gupta1, P. V. Sriluckshmy2, Kavita Mehlawat3, Ashiwini Balodhi3, Dileep K Mishra1, D.V.S.Muthu1, S. R. Hassan2, Yogesh Singh3, T. V. Ramakrishnan1 and A. K. Sood1

1Department of Physics, Indian Institute of Science, Bangalore-560012, India 2The Institute of Mathematical Sciences, C.I.T. Campus, Chennai 600 113, India 3Indian Institute of Science Education and Research (IISER) Mohali, Knowledge City, Sector 81, Mohali 140306, India Abstract

Inelastic light scattering studies on single crystals of (Na1−xLix)2IrO3 (x = 0, 0.05 and 0.15) show a polarization independent broad band at ∼ 2750 cm−1 with a large band-width ∼ 1800 cm−1.

For Na2IrO3 the broad band is seen for temperatures ≤ 200 K and persists inside the magnetically ordered state. The intensity of this mode increases with Li content, increasing by a factor of ∼ 1.6 for x = 0.15, shifts to lower wave-numbers, and persists to much higher temperatures. Such a mode has recently been predicted theoretically for Na2IrO3 by Knolle et.al. as a signature of the gapless quantum spin liquid (QSL) in the Kitaev limit of the Kitaev-Heisenberg model. We assign the observation of the broad band to be a signature of strong Kitaev-exchange correlations. The fact that the broad band persists even inside the magnetically ordered state suggests that dynamically fluctuating moments survive even below TN . This is further supported by our mean field calculations. The Raman response calculated in mean field theory shows that the broad band predicted for the SL state survives in the magnetically ordered state near the zigzag-spin liquid phase boundary. A comparison with the theoretical model gives an estimate of the Kitaev exchange interaction parameter to be JK ≈ 57 meV.

1 Magnetocaloric Effect in Double Perovskite Ho2NiMnO6

Tirthankar Chakraborty*, Aditya Wagh, Suja Elizabeth Department of Physics, Indian Institute of Science, Bangalore-560012, India * Presenting author: [email protected]

ABSTRACT Utilization of the magnetic refrigeration based on magnetocaloric effect (MCE) is recently a great deal of interest for refrigeration technology. As it is more energy efficient and compact, magnetic refrigeration is much more advantageous than the existing vapor-cycle refrigeration technology. Specially, from environmental point of view it may be an alternative refrigeration technology to avoid the usage of harmful chlorofluorocarbons (CFCs). Materials with large MCE are always advantageous for energy efficient magnetic cooling. Since the discovery of giant MCE, Gd based materials are of great interest because of their large MCE [1]. Recently, perovskite and double perovskite materials are becoming interesting systems because of large MCE around their second order magnetic transition temperatures (TC) [2-3]. In the present work, we investigate the MCE of ferromagnetic double perovskite Ho2NiMnO6. The sample is prepared via nitrate route using highly pure precursors. Rietveld refinement of powder X-Ray diffraction pattern shows that all the samples crystalize in monoclinic P21/n space group similar to other double perovskite systems. The magnetic entropy change is calculated indirectly from the isothermal magnetization measurement around 86 K which is the ferromagnetic transition temperature. The magnetic entropy change with maximum value |∆SM| = 8.4 J Kg-1 K-1 obtained at 88K under an applied field 70 kOe is fairly comparable with that of many other perovskite and double perovskite systems [1]. The calculated relative cooling power (RCP) at an applied field 20 kOe is found to be 59.1 J Kg-1 which is good enough to be comparable or sometimes even higher than that of other reported perovskite and double perovskite systems [1]. Along with these, negligible magnetic hysteresis is observed within the measured temperature window which is another essential criteria for magnetocaloric materials. As a whole, with good magnetic entropy change and RCP values even at low field and negligible hysteresis, this system can be a potential magnetocaloric material for low temperature application (around its Curie temperatire).

References: 1. Manh-Huong Phan et al, Journal of Magnetism and Magnetic Materials 308 (2007) 325 2. M. H. Phan et al, Journal of Magnetism and Magnetic Materials 258–259 (2003) 309 3. Z.M. Wang et al, Journal of Magnetism and Magnetic Materials 234 (2001) 371 87Rb-133Cs molecule formation probability in an optical lattice using strong-coupling expansion

Manjari Gupta∗ and H. R. Krishnamurthy† Center For Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India

J. K. Freericks‡ Department of Physics, Georgetown University, Washington, D.C. 20057, USA (Dated: November 20, 2015) We have estimated the trap parameters for Rb and Cs atoms in a 3D optical lattice with an overall harmonic trap, in order to create single occupancy Mott phase for both species. These trap parameters will lead to the highest probability for creating single Rb-Cs Feshbach molecules at each lattice site by fine tuning the external magnetic field near Rb-Cs Feshbach resonance. We have used Falikov-Kimball model to describe the relevant system, and strong-coupling expansion about the mean-field solution to calculate the density profiles and ef- ficiency for single occupancy for both Rb and Cs up to second order in the perturbative expansion. We also calculate the entropy per particle which serves as an estimation of temperature for the experiments.

∗Electronic address: [email protected] †Electronic address: [email protected] ‡Electronic address: [email protected]

Percolative switching in transition metal dichalcogenide field- effect transistors at room temperature

Tathagata Paul, Subhamoy Ghatak, Arindam Ghosh

The discovery of graphene has led to a surge of interest in the field of two dimensional (2D) materials. 2D semiconductors such as transition metal dichalcogenides (TMDCs), black phosphorous etc have since received much attention from the low dimensional physics community. The unique properties of TMDCs such as direct band gap, valley degeneracy, large ON/OFF ratio make them suitable candidates for novel electronic and optoelectronic applications. In this work we address the microscopic transport mechanism at the switching or ON/OFF transition in TMDC FETs. Simultaneous measurement of channel conductivity and its slow time dependent fluctuations (noise) in ultrathin MoS2 and WSe2 FETs reveals that the switching in conventional back gated TMDC FETs is a classical percolation in a medium of inhomogeneous carrier density distribution. From the experimentally observed exponents in the scaling of noise magnitude with conductivity we notice unambiguous signatures of percolation in both WSe2 and MoS2 FETs close to switching. Our work shows how noise can be used as a powerful experimental technique to probe the microscopic nature of near threshold electronic transport in TMDC FETs which can be expanded to other 2D materials as well. ABSTRACT FOR POSTER

Title: Local and non-local transport in the Topological Kondo Insulator SmB6

Authors:

Sangram Biswas1, Ramya Nagarajan1 , Suman Sarkar1 , Kazi Rafsanjani Amin1, M. Ciomaga Hatnean2, S. Tewari3, G. Balakrishnan2, and Aveek Bid1,

1Department of Physics, Indian Institute of Science, Bangalore 560012, India

2Department of Physics, University of Warwick, Coventry, CV4 7AL, UK.

3Department of Physics and Astronomy, Clemson University, Clemson, SC 29634, USA

The rare earth hexaboride SmB6 is a strongly correlated heavy fermion material. Unlike other rare earth hexaborides, SmB6 behaves as insulator at lower temperature. Its metal to insulator transition is well understood in terms of kondo insulator theory. Samarium ions in SmB6 provide localized 4f state and form a kondo lattice. Electrons of these localized states hybridize with the conduction electron sea at low temperature. Such hybridization opens up a gap (reported so far ~3meV to 6meV). When temperature is decreased below 3K, resistivity starts to saturate. It has been predicted that the resistance saturation at low temperature (<3K) is due to presence of topologically protected conducting surface states. We have studied quantitatively the electrical transport through surface states in high-quality single crystals of

SmB6. We observe a large nonlocal surface signal at temperatures lower than the bulk Kondo gap scale. Measurements and finite-element simulations allow us to distinguish unambiguously between the contributions from different transport channels. In contrast to general expectations, the electrical transport properties of the surface channels were found to be insensitive to high magnetic fields. We propose possible scenarios that might explain this unexpected finding. Local and nonlocal magnetoresistance measurements allowed us to identify possible signatures of helical spin states and strong interband scattering at the surface. We have studied low frequency (0.01563Hz < f < 3.5Hz) noise in the temperature range 13mK< T < 300K and we observe huge deviation from 1/f nature in power spectrum which leads to a peak in noise signal at low temperature. Enhancement of spin accumulation near Verwey transition in Fe3O4 based spin injection devices Shwetha G. Bhat Department of Physics, Indian Institute of Science, Bangalore – 560012, INDIA.

Electrical spin injection and detection using three-terminal Hanle (3TH) technique in semiconductors have attracted a lot of attention in the past few years owing to the successful demonstration of spin injection at room temperature in various systems. Fe3O4 is shown to be a promising material for the efficient spin injection and detection due to its half-metallic properties.

We have performed temperature dependent electrical spin injection and detection using 3TH measurement with Fe3O4 spin injectors. Spin injection into GaAs and Si (both n and p-type) semiconductors using Fe3O4 are achieved with and without a tunnel barrier (MgO), and evaluated with respect to the spin injection experiments using Fe/MgO/GaAs devices. We observe a drastic enhancement of spin accumulation voltage (ΔV) in Fe3O4 based devices for temperature < 120K (TV, the Verwey transition) irrespective of the type of barrier (Schottky or tunnel barrier) and semiconductors (GaAs or Si). However, such an enhancement in ΔV is absent in the devices with Fe as spin source. These observations are in consensus with the existing magnetoresistance based studies, which show an enhancement of spin polarization of Fe3O4 near

TV. We also notice a higher ΔV in devices with (111) oriented Fe3O4 than (100) oriented Fe3O4 which is also in agreement with the spin-polarization measurement by photoemission spectroscopy. In addition, we observe one order of magnitude higher spin injection into GaAs using Fe3O4 in contrast to Fe. These observations from our elaborate investigations show that spin polarization of Fe3O4 has a direct influence on spin injection. Due to enhanced values of the spin accumulation, Fe3O4 seems to be very promising for the future semiconductor spintronics device applications.

Design and construction of a continuous-wave Titanium-Sapphire Laser

Debrup Bhattacharya

Our proposed experimental setup that probes how light interacts with electron-spin of semiconductors requires a coherent source of light with proper control on the various aspects of light: power, wavelength, beam modes, etc. A suitable light source for the experiment can be provided by a continuous wave tunable Ti-Sapphire laser cavity. The poster describes the underlying principles in building a laser, its basic mathematics and how the calculations are extended to an actual cavity on breadboard. We will discuss simulations that calculate the geometric layout of the cavity; how variations in the alignment affect the output; design issues that were faced during building the laser and how they were overcome; the importance of matching spot-sizes of pump beam to the lasing beam within the crystal; and how the power of the lasing light actually builds inside the Ti-Sapphire crystal. Finally, experimentally obtained data - lasing threshold power, output power graphs, wavelength and beam modes- obtained from such a cavity will be shown.

In situ Raman spectroscopic measurement of top gated

MoTe2 Field Effect Transistor

Subhadip Das, Biswanath Chakraborty, D. V. S. Muthu and A. K. Sood

Department of Physics, Indian Institute of Science, Bangalore-560012, India

We capture the electron-phonon coupling in few layer MoTe2 field effect transistor using in-situ Raman spectroscopic studies. With hole doping, in- 1 plane vibrational E 2g mode softens accompanied by broadening of linewidth. The A1g mode which involves out-of-plane displacement of chalcogen atoms only does not show any change with doping. The results are in stark contrast with that of electron doped MoS2 transistor, in which we have shown the changes of A1g mode only [1]. The difference in results can be attributed to the coupling of different electronic states with respective phonons. Our results bring out a detail understanding of electron-phonon interaction in few layer MoTe2 and establish that in-situ Raman scattering is a powerful probe of doping in MoTe2 devices.

Reference:

1. Biswanath Chakraborty, Achintya Bera, D. V. S. Muthu, Somnath Bhowmick, U. V. Waghmare, and A. K. Sood, Phys. Rev. B 85, 161403 (R), 2012

Low-frequency noise in edge-contacted graphene

Paritosh Karnatak, Aamir Ali, and Arindam Ghosh Department of Physics, Indian Institute of Science

Time dependent low-frequency fluctuations in the electrical resistance of mesoscopic devices can strongly depend on the type of contacts. Here we investigate noise in edge-contacted graphene, where metal forms one- dimensional end contacts to graphene. This system shows ultra-high mobility and graphene never comes into contact with a polymer and has been extensively utilized recently. We discover that despite mobility as high as 80,000 cm2/Vs the noise in these devices is high and does not depend on the gate voltage. The temperature dependence is also very weak down to 77K unlike that seen in side contacted graphene. End contacted bilayer graphene also shows similar behavior indicating the noise may not be intrinsic to graphene. We are exploring if this noise behavior is due to the contacts alone or can be attributed to reflection from graphene edges due to ballistic transport.

References [1] Vandamme, L. K. J. "Noise as a diagnostic tool for quality and reliability of electronic devices." Electron Devices, IEEE Transactions on 41.11 (1994): 2176-2187. [2] Wang, L., et al. "One-dimensional electrical contact to a two-dimensional material." Science 342.6158 (2013): 614-617. [3] Pal, Atindra Nath, and Arindam Ghosh. "Resistance noise in electrically biased bilayer graphene." Physical review letters 102.12 (2009): 126805. [4] Pal, Atindra Nath, et al. "Microscopic mechanism of 1/f noise in graphene: Role of energy band dispersion." ACS nano 5.3 (2011): 2075-2081. Abstract for Poster

Pulsed laser deposition of topological insulator thin films

Abhishek Banerjee and Kunjalata Majhi

High quality epitaxial thin films of Bi2Se3 are grown on Si(111) substrates using pulsed laser ablation. Different growth regimes are explored leading to the formation of nanoflakes, nano-ribbons and atomically flat thin films. However, all regimes are characterized by an underlying spiral growth mechanism. The puzzling presence of spiral growth in a Van Der Waals bonded material is investigated further using scanning tunneling microscopy (STM). STM in ultra high vacuum reveals atomically resolved steps and terraces and (1x1) low energy electron diffraction pattern is obtained even on ambient-exposed samples after mild sputtering and subsequent annealing. Transport measurements on our thin films reveal high electronic mobilities (~750cm2/V-s) and large phase coherent lengths (~650nm) at 2K. Growth morphology dependence of these parameters reveals that while phase coherence length does increase monotonically with decreasing bulk crystalline disorder, mobility shows an anomalous rise at intermediate disorder strength before saturating at a value of ~400cm2/V-s. Title: Yielding of a Langmuir monolayer under periodic shear

Authors: Pradip K Bera, Ajoy Kandar, Rema Krishnaswamy and A. K. Sood Department of Physics, Indian Institute of Science, Bangalore-560012

Abstract:

Langmuir monolayers of insoluble surfactants or proteins with the amphiphilic molecules confined to the fluid interface are inherently two dimensional (2D) systems that behave as soft glassy materials similar to that of bulk amorphous solids at high surface concentrations. Hence they serve as model systems to study the onset of yielding and plastic flow behaviour of amorphous soft solids. I will present our recent results those examine the onset of yielding and plastic flow behaviour of sorbitan tristearate monolayers under large amplitude oscillatory shear by combining interfacial rheometry with particle imaging velocimetry techniques through the tracking of Brownian particles embedded in the monolayer. The transient dynamics of the linear viscoelastic moduli as well as the higher order harmonics reveal a critical-like behaviour with a divergence of timescales near the fluidization strain when the monolayer was subjected to a periodic shear of varying amplitudes over a large number of shear cycles. To obtain a microscopic picture of this critical-like behaviour we examine the probability distribution of the particle displacements of Brownian spheres embedded in the monolayer from stroboscopic imaging over a large number of shear cycles. The observed probability distribution of particle trajectories beyond the onset of nonlinearity can be explained by an earlier proposed diffusion-convection model of a Brownian particle subjected to a 2D planar shear flow. In addition to providing a microscopic picture of the yielding under oscillatory shear in a Langmuir monolayer, as far as we are aware, this is the first experimental verification of 2D transport of a Brownian particle subjected to nonlinear flows.

Magnitude and origin of electrical noise at individual grain boundaries in graphene

Vidya Kochat1, Chandra Sekhar Tiwary2, Tathagata Biswas1, Gopalakrishnan Ramalingam3, Kimberly Hsieh1, Kamanio Chattopadhyay2, Srinivasan Raghavan3,4, Manish Jain1, Arindam Ghosh1

1 Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2 Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012, India 3 Materials Research Center, Indian Institute of Science, Bangalore 560 012, India and 4 Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560 012, India

Abstract:

Two dimensional materials and their Van der Waals heterostructures show novel and exotic properties that have exciting prospects in the field of nano-electronics. However, for industrial-scale application requiring large sheets of few layered material of consistent physical and electronic properties, chemical vapour deposition (CVD) is the preferred method for production. This method is not without its own drawbacks, namely polycrystalline films with line and point defects especially along the grain boundaries (GBs) between grains grown from adjacent nucleation sites. These defects greatly hamper the mobility of the samples leading to degraded device quality and electronic performance.

This work studies the effect of grain boundary disorder on the noise signature of CVD grown graphene and tries to determine possible causes and mechanisms of scattering. We find that noise arising from disorders in the GBs can be as large as 104 times that from a single layer crystalline channel of the same dimensions and is highly dependent on the nature and thickness of the GBs. Away from the Dirac point (high n), the noise varies inversely as the doping concentration (~1/n) indicating Hooge-type mobility fluctuations while at low densities near the Dirac point, it can be attributed to fluctuations in the transverse modes of propagation across the GBs.

(Under review in Nano Letters)

Crucial Role of Internal Collective Modes in Underdoped Cuprates

Aabhaas V. Mallik,1, ∗ Umesh K. Yadav,1, † Amal Medhi,2, ‡ H. R. Krishnamurthy,1, § and Vijay B. Shenoy1, ¶

1Indian Institute of Science, Bengaluru - 560012, India 2Indian Institute of Science Education and Research, Thiruvananthapuram - 695016, India Abstract The enigmatic cuprate superconductors have attracted resurgent interest with several recent reports and discussions of competing orders in the underdoped side. Motivated by this, here we address the natural question of frailty of the d-wave superconducting state in underdoped cuprates. Using a combination of theoretical approaches we study t-J like model, and discover an – as yet unexplored – instability that is brought about by an “internal” (anti-symmetric mode) fluctuation of the d-wave state. This new theoretical result is in good agreement with recent STM and ARPES studies of cuprates. We also suggest experimental directions to uncover this physics.

∗ [email protected] † [email protected] ‡ [email protected] § [email protected] ¶ [email protected]

1 Boosted one dimensional superconductors on a lattice

Sayonee Ray, Subroto Mukerjee, and Vijay B. Shenoy Department of Physics, Indian Institute of Science, Bangalore 560 012, India

We study the effect of a boost (that engenders a current-carrying state) on one dimensional systems of lattice fermions with short-ranged attractive interactions. In the absence of a boost such systems possess algebraic superconducting order. Naively, one might expect a boost to weaken and ultimately destroy superconductivity, as in higher dimensions. However, we show that for one dimensional systems its effect is to strengthen the algebraic superconducting order by making correlation functions fall off more slowly with distance. We explain the physical underpinnings of these findings. Pressure Induced Metal to Metal transition in Eu2Ir2O7: Raman and X-ray Studies

Anoop Thomas, D V S Muthu, Anand Pal, P S Anil Kumar, A K Sood Department of Physics, Indian Institute of Science

Pyrochlore Iridates (A2Ir2O7 A=Y or lanthanide) have interesting features because of geometrical frustration, strong spin-orbit interaction, electron-electron correlation among t2g

1 electrons and band topology . Amongst the materials in pyrochlore family, Eu2Ir2O7 comes under a new class of correlated systems, Weyl semi-metal. It is the three dimensional analogue of graphene, having linear electronic energy dispersion at the Dirac points. Weyl Fermionic states have been

2 observed in Eu2Ir2O7 using optical conductivity measurements . Weyl electrons behave like photons which can make the electronic devices work faster. Eu2Ir2O7 crystallizes in pyrochlore structure with space group Fd3m. It is a Mott insulator at low temperatures because of electron electron correlation and Mott transition takes place at 120K. The room temperature metallic behavior shows a deviation from Fermi Liquid. Electrical conductivity measurements show an incoherent-coherent metal crossover around 7.5 GPa 3. We have carried out high pressure synchrotron X-ray diffraction and Raman spectroscopic studies on Eu2Ir2O7 upto 22 GPa and 31.5 GPa respectively at room temperature. The pressure dependence of the frequency and full width half maxima (FWHM) of A1g phonon mode (510 cm-1) shows an anomaly around 7.5 GPa. Synchrotron based XRD measurement does not show any structural transition till 22 GPa. This indicates that the cross over is not associated with any structural change.

References 1.William Witczak-Krempa,Gang Chen,Yong Baek Kim,and Leon Balents,Annual Review of Condensed Matter Physics,Vol. 5: 57-82 (2013) 2. A.B. Sushkov, J. Hofmann, G.S. Jenkins, J. Ishikawa, S. Nakatsuji, S. Das Sarma, and H.D. Drew arXiv:1507.01038 (2015) 3. F. F. Tafti, J. J. Ishikawa, A. McCollam, S. Nakatsuji, and S. R. Julian, Phys. Rev. B 85, 205104 (2012). Phase Change Properties of Chalcogenide Glasses - Some Interesting Observations

K. Ramesh*, Pumalianmunga, Sharona Thomas Horta, E.S.R.Gopal

Department of Physics, Indian Institute of Science, Bangalore 560012, India.

Key words: Chalcogenide Glasses, Phase Change, Electrical Switching, Joule Heating, Crystallization.

Abstract

Chalcogenide glasses switches from a high-resistance (OFF) state to a low-resistance (ON) state at a threshold voltage (Vth) under high electric fields. This electrical switching is of two types: (i) Threshold switching and (i) memory switching. Threshold switching device revert back to the OFF state immediately upon the removal of the applied voltage, whereas a memory device retains the ON state even after the removal of the applied voltage. Due to Joule heating, a filament is formed between the electrodes and the current is confined within this filament and there is an increase in current density. This increases the temperature inside the filament and there is a transition from high resistive amorphous/glass phase to a low resistive crystalline phase in memory switching materials. In the threshold switching glasses electronic processes like space charge, Poole-Frenkel effect, etc., are responsible. The structural transitions are irreversible whereas the electronic processes are reversible and hence the threshold glasses regain their original state (OFF) and memory glasses remain in the ON state. Interestingly, differential scanning calorimetric studies (DSC) show that both the threshold and memory switching glasses exhibit crystallization (structural transition). Accordingly, glasses which crystallize upon heating should exhibit memory switching behaviour. But the switching experiments indicate that among the glasses which undergo structural transition (crystallization) some show threshold switching and some show memory switching. To understand this, Cu-As- Se, Al-As-Te, Ge-As-Se-Te, Al-As-Se-Te glasses were thermally crystallized under vacuum in two ways: (i) by annealing at their respective crystallization temperatures (Tc) and (ii) heated up to their melting temperatures (Tm) and cooled back to room temperature. Interestingly, most of the threshold switching glasses shows amorphous nature or a huge amorphous background with crystalline peaks when cooled from their melting temperatures. The memory switching glasses crystallize in both the cases. We propose that both threshold and memory glasses undergo phase change and the crystalline phases formed from the melt state are responsible for switching to occur. Hence, at the time of switching the sample in between the electrodes undergo phase change by glass → melt → crystal transformation and not by the direct glass → crystal transformation. Pressure dependence of glass transition in As-Te and Ge-Te glasses from electrical resistivity measurements

K. Ramesh*, Pumalianmunga, Sharona Thomas Horta, E.S.R.Gopal

Department of Physics, Indian Institute of Science, Bangalore 560012, India.

Keywords: Chalcogenide Glasses, Glass Transition, High Pressure, Electrical Resistivity

Abstract

Amorphous solids prepared from their melt state exhibit glass transition phenomena upon heating. Derivatives of volume like viscosity, specific heat and thermal expansion coefficient show rapid changes at the glass transition temperature (Tg). In general, application of high pressure increases the Tg (a positive dTg/dP). This positive dTg/dP has been well understood with the Free Volume and Entropy models. However, there are few exceptions where a negative dTg/dP has been observed. It has been proposed that the glasses which undergo negative thermal expansion can exhibit a negative dTg/dP. In this study, electrical resistivity of semiconducting

As40Te60 and Ge20Te80 glasses at high pressures as a function of temperature has been measured in a Bridgman anvil apparatus. Electrical resistivity showed a pronounced change at Tg. The pressure dependence of Tg (dTg/dP) shows a decreasing trend (-dTg/dP) for both the glasses.

Chalcogenide glasses like Se, As2Se3 and As30Se30Te40 show a positive dTg/dP in contradiction to the present observation of negative dTg/dP. A model proposed by deNeufville and Rockstad finds a linear relationship between Tg and the optical band gap (Eg) when they are grouped according to their connectivity (Zav). Application of high pressure decreases the interatomic distance which in turn decreases the separation between the valence and conduction bands (optical band gap). This reduction in optical band gap shifts the glass transition to lower values. It is also suggested that the sign of the pressure derivative of Tg can be negative (-dTg/dP) if the thermal expansion coefficient is negative. Inelastic neutron diffraction studies show a negative thermal expansion coefficient for most of the Te based chalcogenide glasses. Hence, As40Te60 and Ge20Te80 glasses are unique that their pressure dependence of Tg obeys both thermodynamic and the Tg-Eg-Zav models. Observation of fractional conductance quantization in graphenic edge states

Amogh Kinikar1, T. Phanindra Sai1, Adhip Agarwala1, Tathagata Biswas1, Sanjoy K. Sarker1,2, H. R. Krishnamurthy1, Manish Jain1, Vijay Shenoy1, and Arindam Ghosh1

1 Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2 Department of Physics and Astronomy, The University of Alabama, Tuscaloosa, AL 35487, USA

We report in situ measurement of electrical conductance in nano-scale constrictions in mechanical exfoliations of single or few-layer graphene. The point contacts are created by shearing the surface of a graphite crystal with a metal tip. We observe the quantization of the conductance to integer multiples of the universal quantum of conductance 2e2/h

(Go), which indicate ballistic quasi-one dimensional transport. We have also observed a series of unexpected rational fractions of Go as a function of the constriction width. Theoretical modeling suggests that these nearly fractional conductances arises from the scattering of current carrying edge states by the narrowing geometry of the ribbons, a result that is robust against chemical doping of the graphite surface. Our work provides the first report of a study of the in situ transport as the geometry of the ribbon changes on the atomic scale. We show that there is a zero bias peak in the differential conductance further supporting our edge state hypothesis. Photo-response in hBN encapsulated Graphene-TMDC heterostructures

Avradip Pradhan*, Jayanta K. Mishra* and Arindam Ghosh

Department of Physics, IISc, Bangalore

Transition metal dichalcogenides (TMDCs) play as very suitable candidates for digital electronics and opto-electronics because of their unique band gap tunability with layer number. However, low electrical conductivity in TMDCs degrades the quantum efficiency of their optical response. An alternative approach to overcome this issue could be by putting a graphene monolayer on the top of TMDCs. Graphene, by virtue of its high mobility, enhances the optical response of the combined graphene-TMDC system, by several orders of magnitude [1]. Here we discuss the photo response of the Graphene-TMDC heterostructure encapsulated between two hexagonal Boron Nitride (hBN) flakes. Initially the sole motivation of this work was to investigate the device performance (electrical/optical) which was supposed to be enhanced at ambient condition because of hBN encapsulation. It was believed that hBN encapsulation can protect the device from several environmental factors such as humidity, moisture, temperature etc. At room temperature we have not observed any significant photo-response, but at low temperature we have observed photo-response with a significant responsivity at visible as well as at near IR region. However, the nature of photo-response could not be explained with the conventional theory that people have reported yet. We believe that this response can be a possible signature of the defect/impurity states present inside the band gap of hBN (which people have reported earlier [2]). The transport mechanism of photogenerated carriers of such a complex heterostructure would provide a better insight in improving optoelectronic devices based on 2D-materials. Additionally, this technique can be an alternative way to probe the defect/impurity states by controlling the device geometry and layer numbers of the TMDCs.

Reference:

[1] K. Roy et al Nature Nanotechnology 8, (2013)

[2] L. Ju et al Nature Nanotechnology 9, (2014)

*equal contribution Exploring Utilities of Reverse Cross-Polarization - Spectral Editing in Proton NMR spectroscopy

P. Lokeswara Rao and K.V.Ramanathan

Polarization transfer from nuclei having large spin polarization such as protons to other nuclear spins such as carbons is known as cross-polarization (CP) and is used routinely in NMR for enhancing the signal of insensitive nuclear spins. The reverse process of transfer from carbon to proton is also utilized, particularly in solution NMR, since indirect detection of carbon resonances through protons at a higher frequency increases the signal to noise ratio. In addition to providing increased signal to noise, CP has a number of other uses such as (i) in spectral editing methods (ii) a tool for studying molecular dynamics and (iii) estimation of dipolar coupling through dipolar oscillations during CP. In this presentation, we further explore the use of CP for editing proton spectra by identifying protons attached exclusively to a nucleus such as carbon. With high speed MAS, the use of proton spectroscopy has become a promising tool. However, spectral overlap limits the application of the method. It is very often necessary to distinguish CH protons from NH, OH and SH protons. In this context we examine the use of the CP-Reverse CP approach as a spectral editing method. The idea is to transfer the polarization from 1H to 13C usingoptimum mixing time and then back transfer the polarization from 13C to 1H using very short mixing time (typically ~ 100 μs). In the resultant CP-Reverse-CP 1H spectra, proton peaks due to all other nuclear associations (e.g., N, O) are expected to be absent. Suitable 1H-1H homonuclear decoupling during acquisition can be used to improve the resolution. The utility of the proposed method has been tested in several compounds. Results on Alanine, L-

Histidine.HCl.H2O and GSH (Glutathione reduced) will be presented. The possibility of utilizing the technique for hydrogen bonding studies is also being explored. Spectral response of single layer Graphene/MoS2 photodetector

Ranjit V. Kashid1*, Pranav Mundada1, Jayanta Kumar Mishra1, Kallol Roy1, Preeti Deshpande2, Ambrish Ghosh2, Arindam Ghosh1

1Department of Physics, Indian Institute of Science, Bangalore-560012, India

2Centre for Nano Science and Engineering, Indian Institute of Science, Bangalore 560012, India

*e-mail: [email protected]

Atomically thin two-dimensional heterostructures have emerged as a promising candidate for ultrasensitive photodetection in recent years1, 2. Amongst these heterostructures, especially

8 Graphene/MoS2 has attracted a great deal of attention owing to ultrahigh responsivity (10

A/W) at room temperature and gate voltage tuneable photoconductivity3. For understanding the photoconductivity mechanism and spectral dependence (400-700nm), we have measured photoresponse of single layer Graphene/MoS2 phototransistor as a function of wavelength. The photoresponse is found to be maximum at excitonic energy (~593 nm) of single layer MoS2. We believe that the observed photoresponse is a consequence of exciton dissociation due to the interfacial electric field.

References:

1. M. Buscema, J. O. Island, D. J. Groenendijk, S. I. Blanter, G. A. Steele, H. S. van der Zant and A. Castellanos-Gomez, Chemical Society reviews 44 (11), 3691-3718 (2015). 2. L. Britnell, R. M. Ribeiro, A. Eckmann, R. Jalil, B. D. Belle, A. Mishchenko, Y. J. Kim, R. V. Gorbachev, T. Georgiou, S. V. Morozov, A. N. Grigorenko, A. K. Geim, C. Casiraghi, A. H. Castro Neto and K. S. Novoselov, Science 340 (6138), 1311-1314 (2013). 3. K. Roy, M. Padmanabhan, S. Goswami, T. P. Sai, G. Ramalingam, S. Raghavan and A. Ghosh, Nature nanotechnology 8 (11), 826-830 (2013).

Tailoring Curie temperature and magnetic anisotropy in ultrathin Pt/Co/Pt films

Vineeth Mohanan P and K.R.Ganesh

The role of seed layers of heavy metals like Pt, Ta, Pd etc. on the magnetism in ultra thin films (t<1nm) of Co, Fe and Ni is being extensively investigated in recent years due to the growing demand for perpendicular high density magnetic storage media and in current induced domain wall motion experiments. The challenge is to grow room temperature ferromagnetic structures of cobalt (thickness of Co<0.6nm) with perpendicular magnetic anisotropy, square hysteresis loop and large Kerr rotation. In this work, we have investigated the changes in ferromagnetic anisotropy and Curie temperature (Tc) of Pt/Co/Pt thin films as a function of the thicknesses of Pt (bottom) and presence of Ta seed layer. Pt/Co/Pt thin films were grown on thermally oxidized Si wafers using DC magnetron sputter deposition. The Pt(seed) and Co thicknesses were varied between 2 to 8 nm and 0.35 to

1.8 nm (across the spin reorientation transition thickness) respectively and the Tc was measured using SQUID magnetometer and magneto optic Kerr effect. The capping Pt layer thickness was kept constant at 2 nm. We have observed a systematic dependence of Tc on the thickness of Pt(seed). For 8nm thickness of Pt(seed) the Co layer of 0.35nm showed ferromagnetism with perpendicular anisotropy at room temperature. As the thickness of the Pt(seed) was decreased to

2nm, the Tc went down below 250K. XRD data indicated polycrystalline growth of Pt(seed) on

SiO2. On the contrary Ta underlayer promoted the growth of Pt(111). As a consequence Ta(5nm)/Pt(2nm)/Co(0.35nm)/Pt(2nm) had much higher Tc (above 300K) with perpendicular anisotropy. Thus we could tune the ferromagnetic Tc and anisotropy by varying the Pt(seed) thickness and also by introducing Ta underlayers. We attribute these observations to the growth modes of Pt(seed) layer which hosts the Co layer. Defect Assisted Carrier Relaxation in Graphene Nanoribbons

Gyan Prakash1, Yan -Sheng Li2 ,Wei-Hung Chiang 2 and A. K. Sood1

1Center for Ultrafast Laser Applications (CULA) and Department of Physics, Indian Institute of Science, Bangalore-560012. 2National Taiwan University of Science & Technology, Taipei City, Taiwan 10607

We report ultrafast time resolved free carrier and exciton dynamics in ~4nm wide Graphene Nanoribbons obtained by unzipping carbon nanotubes. Formation of graphene nanoribbons is confirmed by absence of radial breathing mode of carbon nanotubes in Raman shift and TEM . Femtosecond time resolved pump-probe absorption measurements are done using pump wavelength of 800 and 400 nm and the probe wavelength is varied between 460 to 1000 nm. We find that the relaxation of photo-excited free carriers and excitons can be described by defect assisted Auger scattering. Decay dynamics is analyzed using detailed rate equations to obtain scattering and capture rates of free carrier and excitons.

Email : [email protected] Structural aspects and interaction profiles of PAMAM dendrimers on a graphene sheet

Mounika Gosikaa, Taraknath Mandalb and Prabal K Maitia aCenter for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore – 560012 bDepartment of Chemical Engineering, University of Michigan, Ann Arbor – 48109

Abstract:

Improving the energy densities of capacitive energy storage devices has been of great practical interest in the recent years. The well-defined molecular structure and flexibility in the size and shape makes dendrimers the potential candidates as electrolytes in these devices1. Hence, it is crucial to understand the structure and dynamics of the dendrimers on an electrode surface. We have performed fully atomistic Molecular Dynamics simulations for a system of dendrimer on a graphene sheet, solvated in water. To understand the structural aspects we have calculated the radius of gyration, principal moments of inertia for the equilibrated configurations. We present the results for G3 and G4 PAMAM dendrimers at various pH conditions (high, low and neutral). We have also calculated the potential of mean force (PMF) profiles for the interaction of two G3 as well as G4 dendrimers, adsorbed on the graphene sheet and compare with those of the bulk case i.e., when no graphene sheet is present2. Our results suggest that the dendrimer adsorbs on the graphene sheet and its spherical structure becomes flat like a 2D object. From the PMF profiles, we could conclude that the interaction between the dendrimers on a graphene sheet is purely repulsive, irrespective of the protonation level in the system, as reported by previous analytic, simulation works3.

References:

1. Lin, Terri C. (2013). Poly(amido amine) Dendrimers in Supercapacitors. United States. Doi:10.2172/1091321 2. Taraknath Mandal, Chandan Dasgupta, and Prabal K.Maiti, J. Chem. Phys. 141, 144901 (2014). 3. P. M. Welch, C.F. Welch, and N. J. Henson, ACS Macro Lett. 3, 180 – 184 (2014). Antiphase boundary induced Exchange bias in LuMn0.5Fe0.5O3 Tanushree Sarkar Department of Physics, Indian Institute of Science, Bangalore – 560012, Karnataka, India

Abstract for Poster

LuMnO3(LMO)is a well-known multiferroic having antiferromagetism below TN = 90 K and ferroelectricity with TC = 900 K. Fe doping in Mn site of LMO is expected to give better magnetic properties preserving the ferroelectric nature. The Single phase polycrystalline samples of LuMn0.5Fe0.5O3(LMFO) were synthesized using wet chemical method. XRD studies reveal that LMFO samples exist in hexagonal structure with P63cm space group.Temperature dependent magnetization (M-T) measurements show an antiferromagnetic transition at 100K. A rise in the moment is observed in M-T curves below 30 K, indicating the presence of ferromagnetic (FM) interactions along with AFM. M-H loops measured below transition temperature show unsaturated loops with non-zero remanent moment

(Mr = 0.002emu/g) and coercive field (HC=100 Oe). This confirms the presence of weak FM along with strong AFM. Positive and negative field cooled M-H loops measured below TN shows a shift in the loops towards the negative and positive side of the field axis respectively. This confirms the exchange bias property of LMFO samples. 57Fe Mossbauer spectroscopy measurements reveal the presence of antiphase boundary (APB) having very small internal field value. Strong pinning of spins across APB is evident from the cooling field dependent exchange bias study.

For the first time in bulk system, we observe FM nature of the APB spins which is exchange coupled at the interface with AFM host matrix giving rise to exchange bias property.

Structure driven multiferroic properties of LuFeO3 nanoparticles.

Pittala Suresh and K. Vijaya Laxmi

Department of Physics, Indian Institute of Science, Bangalore – 560012, Karnataka, India

Abstract.

LuFeO3 (LFO) is a multiferroic material at room temperature. LuFeO3, can crystallize in both orthorhombic (o-LFO) and hexagonal (h-LFO) structures depending on the preparative conditions. Among these, o-LFO is a stable structure and the preparation of h - LFO is tedious in the bulk form due to its metastable nature. However it is possible to stabilize it in thin film form. The nanoparticles of LFO are synthesized in Hexagonal and orthorhombic phases using sol-gel method. Hexagonal LuFeO3 (h-LFO) nanoparticles were synthesized for the first time by optimizing the preparation conditions. XRD analysis of h-LFO samples reveals its structure in P63cm space group, which has polar symmetry. In contrast, o-LFO samples exist in nonpolar Pbnm symmetry. Microstructure shows the homogeneous particles with an average grain size of 30 nm and 60 nm for the hexagonal and orthorhombic samples respectively. Raman spectra shows 13 active phonon modes corresponding to the P63cm space group at a different wave number to the Pbnm symmetry that has 12 active Raman modes. M-H loops at room temperature show antiferromagnetism for both the samples. Temperature-dependent magnetization shows a transition around 120 K for hexagonal samples, which is absent in orthorhombic samples. This transition at 120 K is a spin reorientation transition from antiferromagnetic to ferromagnetic state.

Deterministic switching of magnetization states in Cobalt nano rings

Manohar Lal∗ Spintronics and Thinfilm Magnetism Lab, Department of Physics, Indian Institute of Science, Bangalore, India-560012.

Magnetic nano ring structures possess various stable states based on the possible magnetic domain configurations in them by controlling the external magnetic fields. This indicates that the ring structures can be considered as potential candidates for the memory applications either by field induced or by current induced domain wall motion. In my work, I carried out the magnetization reversal studies on Cobalt nano rings to understand the distinct stable states by magneto-resistance measurement (MR).

The Cobalt nano ring structures were fabricated by high resolution electron beam lithography and the patterns were transferred by the lift off technique. The 20nm thin film of Cobalt capped with 5nm Niobium, is deposited using molecular beam epitaxy in ultra-high vacuum conditions.

Fig.1 Schematic of (a) rings with arms-pads and (b) rings with arms only.

We classify the devices into two groups, (a) rings with arms-pads and (b) rings with arms only, as shown in Fig.1. The ring is connected with arms on both side and has the same width as arms. The idea of using the pad is to have controlled way of injecting domain wall into the arms of the ring and also the possibility of obtaining the spin polarised current injection into the arms for current induced magnetization switching. In this way, it is possible to switch the magnetization of the ring element and the arms selectively. A constant current is passed through the arms and the magneto resistance is measured across the arms of ring by lock-in technique. The sweeping magnetic field is in plane and parallel to arm of the ring. The measured resistance shows a changed value (The corresponding field called as switching field) whether a domain wall is trapped at arm and ring junction, or a vortex or an onion state is formed in the ring. Anisotropic Magneto-resistance (AMR) causes a low resistance state of ring which corresponds to the presence of multiple domains. A sudden variation in AMR indicates there exist a switching of magnetization from one state to other. We have studied the statistical behaviour of these switchings by varying the thickness, width and diameter of the ring structures. The switching fields for ring with arms were 0.85kOe and 1.87kOe indicating arms are switching at different field than that of rings. In the case of rings with arm-pads, we observed three switching fields: 0.42kOe, 1.70kOe and 1.85kOe. The smallest field corresponds to nucleation of domains in the pads. These values are observed for the arm width of 70nm and diameter of 2µm. We also studied the correlation of the switching fields in connection to the geometrical parameters of the arm widths and ring diameters. The Switching field value decreases with the increase of diameter or width of the ring. In order to get more insight into the domain configurations in the device, micromagnetic simulations (OOMMF) are used, which confirms the two district stable states in the ring with arms and ring with arms-pads. ∗ [email protected]

1 Substrate screening effects on quasiparticle excitations of pristine and vacancy-defected MoS2

Mit H Naik1 and Manish Jain1

1Department of physical sciences, Indian Institute of Science, Bangalore 560012

Layered transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional materials beyond graphene, with a wide range of applications. Their 2D nature leads to the electron-electron interactions in the material to be weakly screened. In the presence of substrates, this interaction can be effectively screened and can lead to a renormalisation of the band gap. We perform ab initio GW calculations to study the effect of substrates on a single layer MoS2. Experimental work on MoS2 has revealed a high density of sulphur vacancies on the surface of MoS2 which act as electron donors and induce localised states in the gap. We study the charge transition levels of sulphur vacancies in MoS2 using methods based on ab initio pseudopotential density functional theory (DFT). It is well known that Kohn Sham density functional theory underestimates the band gap of materials. In order to correct for this underestimation we use the GW formalism. We also study the effect of substrate screening on the quasiparticle defect levels. Bulk-Induced 1/f Noise at the Surface-states of Three-Dimensional Topological Insulators

Semonti Bhattacharyya, Mitali Banerjee, Hariharan Nhalil, Saurav Islam, Chandan Dasgupta, Suja Elizabeth, and Arindam Ghosh

The helical surface states of 3D Topological insulators (TI) have garnered widespread interest both in terms of fundamental science as well as electronic applications. Although these surface states are topologically protected from 1800 backscattering, static-scattering from disorders, impurities and even phonons, which limits the mobility (102-104 cm2V-1s-1 ) in these states have been studied extensively. Time dependent scattering processes, which induces slow intrinsic fluctuations of resistance, i.e. flicker noise or 1/f-noise, is, however poorly understood in the surface transport of strong topological insulators (TIs). Here, we have systematically explored the 1/f-noise in field- effect transistors (FET) of mechanically exfoliated Bi1.6Sb0.4Te2Se TI films in the surface-transport dominated regime. We have found that at lower thicknesses (~50 nm) the noise magnitude in this devices can be extremely small (Hooge -4 parameter, γH≈10 ), confirming its suitability as an extremely low noise electronic element. From the temperature (T)-dependence of noise, which displayed sharp peaks at characteristic values of T, we have identified a unique source of the mobility fluctuations in surface transport, i.e. generation- recombination processes from interband transitions within the insulating TI bulk. Our experiments, apart from establishing an intrinsic microscopic origin of noise in TI surface channels, also reveal a unique spectroscopic information on the impurity bands that can be useful in bulk TI systems in general.

I. S. Bhattacharyya, M. Banerjee, Hariharan. N., S. Islam, C. Dasgupta, S. Elizabeth, and A. Ghosh,article asap, ACS Nano, DOI: 10.1021/acsnano.5b06163. High detectivity in Photosensitive Graphene-hBN-MoS2 multilayer heretostructures

Tanweer Ahmed, Kallol Roy and Arindam Ghosh

Rich structural and electronic properties of Graphene and transition metal di-chalcogenides (TMDCs), make them candidates for optoelectronic applications. Graphene is suitable for ultrafast electronic applications due to its high carrier mobility ~ 105 cm2 V-1s-1[ 1]. But its low optical absorption coefficient (2.3%) and absence of a gain mechanism have restricted the responsivity of the pure Graphene photodetectors down to a very low value[2]. Monolayer crystals of TMDCs have direct band gap, and in its DoS (Density of States), it has Van Hoff singularities which give rise to a high absorption coefficient and efficient electron-hole generation under optical excitation[3]. Despite its rich optical properties, photoresponsivity of pure a [5] MoS2 photodetector is very low because of VRH (variable range hopping transport) limited slow carrier dynamics[4]. A material with high optical absorption coefficient, when coupled together with another having high conductivity, to form a composite, can give rise to a high sensitivity photo-detectors, and opens up possibilities of many other potential optoelectronics applications. Single layer TMDCs and Graphene have direct bandgap and huge carrier mobility respectively and so they are perfect candidates for such a composite. Recently it has been demonstrated that a MoS 2- Graphene heterostructures can show responcivity of ~ 1010 along with versatile applications [6]. In this work we show that insertion of an insulating layer of h-BN, which acts as a trap free substrate for Graphene, not only restores the properties of

Graphene-MoS2 hybrid, but also, empowers it with high detectivity due to reduction of Graphene’s noise. Our Graphene-hBN-MoS2 multilayer hetero-structure shows a room temperature detectivity of 2*1015 Jones. This calculated value of detectivity lies in the highest regime of reported values of detectivities of all kinds of photodetectors. Molecular mechanism of water permeation in helium impermeable graphene and graphene oxide membrane

Nallani Raghav, Sudip Chakraborty and Prabal K Maiti *

Center for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012

Abstract: The layers of graphene oxide (GO) are found to be good for permeation of water but not for helium (Science 2012 335 (6067): 442-444) suggesting that the GO layers are dynamic in the formation of permeation route depending on the environment they are in (i.e, water or helium). To probe the microscopic origin of this observation we calculate the potential of mean force (PMF) of GO sheets (oxidized and reduced parts), with inter-planar distance as reaction coordinate in helium and water. Our PMF calculation shows that equilibrium interlayer distance between oxidized part of GO sheets in helium is at 4.8 Å leaving no space for helium permeation. In contrast PMF of oxidized part of GO in water shows two minima one at 4.8 Å and another at 6.8 Å corresponding to no water and water filled region and thus giving rise to permeation path. The increased electrostatic interaction between water with the oxidized part of the sheet helps the sheet opening up and pushing water inside. Based on the entropy calculations for water trapped between graphene sheets and oxidized graphene sheets at different inter-sheet spacing we also show the thermodynamics of filling [1].

Reference:

1. Raghav, N.; Chakraborty, S.; Maiti, P. K. PCCP 2015, 17, 20557.

Dendrimers as blocking reagent for the toxic protein pores

Subbarao Kanchia,b, Tharaknath Mandala, K.G.Ayappab, J.K.Basua and Prabal K. Maitia aDepartment of Physics, Indian Institute of Science, Bangalore, 560 012, India. bDepartment of Chemical Engineering, Indian Institute of Science, Bangalore, 560 012, India.

We have used fully atomistic molecular dynamics (MD) simulations to characterize the Cytolysin A (clyA) protein pores modified with fifth generation (G5) PAMAM and sixth generation (G6) PETIM dendrimers. Our results show that the dendrimer, in either of its protonated or nonprotonated form can spontaneously enter the protein lumen to spatially block the protein pores. We observe that the protonated dendrimers strongly couple with the protein wall due to the strong electrostatic interaction between the positively charged dendrimer and negatively charged protein wall. Because of this strong coupling, protonated dendrimers open their branches which eventually block the protein pore more efficiently. On the other hand, charge neutral nonprotonated dendrimers are attached to one side of the pore, thus allowing a free space on the opposite side of the wall. To quantify the effective blockage of the protein pore, we have calculated the ion and water conductance through the protein channel by applying force on the ions/waters. We find that both the ion and water current through the protein pore are remarkably decreased in presence of the dendrimers. However, protonated dendrimer blocks the ionic current more efficiently than the nonprotonated dendrimers because of its specific characteristics. Our investigation shows that the bio- compatible PAMAM and PETIM dendrimers can be used as blocking reagent for the pore forming toxins.

References:

1. Mueller, M.; Grauschopf, U.; Maier, T.; Glockshuber, R.; Ban, N. The structure of a cytolytic alpha-helical toxin pore reveals its assembly mechanism. Nature 2009, 459, 726-U135. 2. Soskine, M.; Biesemans, A.; Moeyaert, B.; Cheley, S.; Bayley, H.; Maglia, G. An engineered ClyA nanopore detects folded target proteins by selective external association and pore entry. Nano letters 2012, 12, 4895-4900. 3. Maiti, P. K.; Bagchi, B. Structure and dynamics of DNA-dendrimer complexation: role of counterions, water, and base pair sequence. Nano letters 2006, 6, 2478-2485. 4. R. Bhattacharya, Subbarao Kanchi, Roobala C., A. Lakshminarayanan, Oliver H. Seeck, Prabal K. Maiti, K. G. Ayappa, N. Jayaraman and J. K. Basu. Soft Matter, 2014, 10, 7577

Abstract: Gate Controlled Seebeck effect in twisted bilayer graphene Authors: PBS Mahapatra, Kingshuk Sarkar, Subroto Mukherjee, Arindam Ghosh

The recent developments of van der Waals heterostructures made from two dimensional atomic layers have led to the observation of many rich and intriguing physics. Graphene, an ideal two dimensional electron gas, also exhibits rich electronic properties depending on how it is stacked on top of another graphene layer. Unlike bilayer graphene in Bernal stacking, twisted bilayer graphene with a random orientation of the layers has a massless electronic dispersion similar to that of single layer graphene. Recently several intriguing properties such as renormalization of Fermi velocity, van Hove singularities, electronic localization and incoherent interlayer electronic transport have been observed in twisted bilayer graphene. Careful alignment of the crystallographic orientation of two graphene layers can achieve resonant tunnelling with conservation of both electron energy and momentum and exhibits negative differential conductance between the two graphene layers. Even though the electronic properties of twisted bilayer graphene has gained much attention recently, its thermo-electric properties have never been studied in detail. In this work, we have looked into the thermal transport through a single van der Waal gap between the two graphene layers in the twisted bilayer graphene which is sandwiched between two hexagonal Boron Nitride layers. We report observation of strong gate dependent Seebeck co-efficient across the van der Waal gap with a temperature independent maximum Seebeck co-efficient of 60 µV/K, which is consistent with our theoretical calculations. The SPL7013 Dendrimer Destabilizes the HIV-1 gp120-CD4 Complex

Bidisha Nandy1, Suman Saurabh1, Anil Kumar Sahoo1, Narendra M. Dixit2,3 and Prabal K. Maiti1

1Center for Condensed Matter Theory, Department of Physics, Indian Institute of Science,

Bangalore

2Department of Chemical Engineering, and 3Centre for Biosystems Science and Engineering, Indian

Institute of Science, Bangalore

ABSTRACT

The poly (l-lysine)-based SPL7013 dendrimer with naphthalene disulphonate surface groups blocks the entry of HIV-1 into target cells and is in clinical trials for development as a topical microbicide. Its mechanism of action against R5 HIV-1, the HIV-1 variant implicated in transmission across individuals, remains poorly understood. Using docking and fully atomistic MD simulations, we find that SPL7013 binds tightly to R5 gp120 in the gp120-CD4 complex but weakly to gp120 alone. Further, the binding, although to multiple regions of gp120, does not occlude the CD4 binding site on gp120, suggesting that SPL7013 does not prevent the binding of R5 gp120 to CD4. Using MD simulations to compute binding energies of several docked structures, we find that SPL7013 binding to gp120 significantly weakens the gp120-CD4 complex. Finally, we use steered molecular dynamics (SMD) to study the kinetics of the dissociation of the gp120-CD4 complex in the absence of the dendrimer and with the dendrimer bound in each of the several stable configurations to gp120. We find that SPL7013 significantly lowers the force required to rupture the gp120-CD4 complex and accelerates its dissociation. Taken together, our findings suggest that SPL7013 compromises the stability of the R5 gp120-CD4 complex, potentially preventing the accrual of the requisite number of gp120-CD4 complexes across the virus-cell interface and thereby blocking virus entry. Differential functional role of the three subunits of gp41 trimer on HIV-entry: a study based on homology modelling and MD simulation

Satyabrata Das@, Narendra M. Dixit# and Prabal K. Maiti$ @MathBio, DST Centre for Mathematical Biology, Department of Physics, #Department of Chemical Engineering, $Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore – 560 012 (INDIA)

Human Immunodeficiency Virus type 1 (HIV-1) is an enveloped retrovirus which occurs in human via cross-species transmission from non-human primate lentiviruses. Infected individuals in long run develop an inexorably fatal disease named as Acquired Immuno-Deficiency Syndrome, AIDS in which progressive failure of immune system lead to clinical collapse; a major global health problem due to unavailability of complete cure. The design of effective anti-HIV vaccine proved to be an exorbitantly difficult task but the foremost priority to stem the global HIV-1 pandemic. Efforts are also underway to develop more effective drug against target such as transmembrane glycoprotein 41 (gp41) which plays active role on viral entry1. Recently a work on dendrimer based blocking mechanism of HIV-1 entry has been published from our group2. The glycoprotein gp41 (~ 345 amino acids) consists of N-terminal ecto-domain (~ 172 residues), helical 22 residue transmembrane domain (TMD) and a ~ 151 residue long cytoplasmic tail. N-terminal ecto- domain and TMD plays active role in membrane fusion. gp41 undergoes large conformational changes during its functional life cycle, three distinctly different conformations are: pre-fusion gp120 bound metastable state3, pre-hairpin fusion intermediate in which it anchors both virus and host cell membranes and thermodynamically stable post-fusion six helix bundle4 or “trimer-of-hairpins”. However the correlation between atomic details of membrane rearrangements which leads to fusion pore formation and expansion with that of the gp41 conformational variations is not fully understood. Detail understanding of fusion mechanism will help to design more effective drug to prevent HIV-1 entry which is first obligatory step of its replication cycle. Membranotropic nature of the large part of ecto-domain hindered the structural study by X-ray / NMR; full length ecto-domain structure is not available till date. However the structures of various sub- functional regions are known. Also the structure of homologous simian immunodeficiency virus (SIV) gp41 truncated ecto-domain (27-149) is available5. Currently a homology model of trimeric ecto-TMD domain of HIV-1 gp41 in post-fusion conformation has been generated by MODELLER and validated. Attempt has been made to constructs model of membrane / vesicle and ecto-TMD trimer complex to gain insight of protein-lipid interactions during fusion process. The NMR structure of truncated SIV gp41 ecto-domain has been simulated for 200 ns using GROMACS 5.0.4, visually inspected and analysed. Attempt has been made to quantify the protein motion by hierarchical clustering of difference distance matrix generated from the MD-conformational ensemble. Preliminary results obtained from these two studies indicate the differential functional role of the three subunits of ecto-TMD domain trimer during membrane fusion. Further simulations and more rigorous analyses are required to establish it firmly. References: 1. Blumenthal R., Durell S., Viard M. HIV entry and envelope glycoprotein-mediated fusion. J. Biol. Chem. (2012) 287, 40841-9. 2. Nandy B., Saurabh S., Sahoo A. K., Dixit N. M., Maiti P. K. The SPL7013 dendrimer destabilizes the HIV-1 gp120-CD4 complex. Nanoscale (2015) 7, 18628-41. 3. Pancera M., Zhou T., Druz A., Georgiev I. S., et al. and Kwong P. D. Structure and immune recognition of trimeric pre-fusion HIV-1 Env. Nature (2014) 514, 455-61. 4. Chan D. C., Fass D., Berger J. M., Kim P. S. Core structure of gp41 from the HIV envelope glycoprotein. Cell (1997) 89, 263-73. 5. Caffrey M., Cai M., Kaufman J., Stahl S. J., Wingfield P. T., Covell D. G., Gronenborn A. M., Clore G. M. Three-dimensional solution structure of the 44 kDa ectodomain of SIV gp41. EMBO J. (1998) 17, 4572-84. The study of electroporation phenomenon using an optically transparent polymer device and molecular dynamics simulations

Amit Kumar Majhi November 17, 2015

Abstract

In-situ electroporation experiments were performed using an optically transpar- ent polymer device with transparent indium tin oxide parallel plate electrodes in horizontal geometry. The fluorescence measurement of two cell lines (C2C12 mouse myoblast cells and yeast cells) were studied using propidium iodide (PI) as a marker dye to quantify the efficiency of the developed electroporation device in three different electroporation buffers (phosphate buffer saline, electropora- tion buffer and 10 % glycerol) as a function of applied voltage from 10 to 90 V in a series of ∼ 2-ms pulses across 0.5-mm electrode spacing and field effects on the cells. The voltage dependence of the number of electroporated cells could be explained using a stochastic model for the electroporation kinetics, and the free energy for pore formation was found to be 112.5 ± 1.25 kJ mol−1 at room temperature with zero field application. The molecular dynamics simulations of electroporation in POPC and DPPC lipid bilayers have been carried out at different temperatures ranging from 230 K to 360 K for varying electric fields. The dynamics of pore formation, including threshold field, pore initiation time, pore growth rate, and pore closure rate after the field is switched off, was stud- ied in both the gel and liquid crystalline Lα phases of the bilayers. Using an Arrhenius model of pore initiation kinetics, the activation energy for pore open- −1 −1 ing was estimated to be 25.6 kJ mol and 32.6 kJ mol in the Lα phase of POPC and DPPC lipids respectively at a field strength of 0.32 V nm−1 which are similar to the experimental value.

References

(1) Amit Kumar Majhi, Greeshma Thrivikraman, Bikramjit Basu and V. Venka taraman, Optically transparent polymer devices for in situ assessment of cell electroporation, Eur. Biophys. J., 2015 (44) 4457-4467. (2) Amit Kumar Majhi, Subbarao Kanchi, V. Venkataraman, K.G. Ayappa, and Prabal K. Maiti, Estimation of activation energy for electroporation and pore growth rate in liquid crystalline and gel phases of lipid bilayers using molecular dynamics simulations, Soft Matter, 2015 (11) 8632-8640.

1 Multifractal universal conductance fluctuation in graphene Kazi Rafsanjani Amin,1 Samriddhi Sankar Roy,2 and Aveek Bid1, a) 1)Indian Institute of Science, Bangalore 2)International Centre for Theoretical Sciences,Bangalore

Universal Conductance fluctuations (UCF) are aperiodic oscillations in conductance observed either with changing magnetic field or with changing number density. An essential criteria for observing UCF is phase coherence between electrons (holes) from source to drain electrodes. UCF are observed in low temperatures when the phase coherence length(Lφ) is comparable or larger than the device dimension and thermal scattering length(LT ) is larger than Lφ. We have studied UCF in multiple single layer graphene devices, in a wide temperature range from 20 mK to 10K, and with different back gate voltages. We have, furthermore computed the fractal dimensions of conductance-magnetic field plots using Ketzmeric-variance method. In low temperature regime, scaling dimensions were found to be fraction, indicating a fractal transport. Fractal dimensions evolves with temperature and the dynamics show a transition from fractal to non-fractal nature with increasing temperature. Further detailed analysis showed that the plots are not characterized by a single fractal dimension. We have studied the multifractal scaling spectrum with varying temperature. Electron trajectories trapped in hierarchy of boundary of normal and chaotic regions in the device gives rise to fractal transport. Fractal nature of magnetoresistance indicates existence of long lived states inside the mesoscopic devices.

REFERENCES

1. H. Hegger et.al., Fractal conductance fluctuations in gold nanowires, Physical Review Letters 77, 3885 (1996). 2. T. Gneiting and M. Schlather, Stochastic models that separate fractal dimension and the hurst effect, SIAM Review 46, 269V282 (2004), http://dx.doi.org/10.1137/S0036144501394387. 3. C. A. Marlow et.al., Unified model of fractal conductance fluctuations for diffusive and ballistic semiconductor devices, Physical Review B 73, 195318 (2006) 4. A. P. Micolich et.al., Evolution of Fractal Patterns during a Classical-Quantum Transition, Physical Review Letters 87, 36802 (2001).

a)Electronic mail: [email protected]; visit at:http://www.physics.iisc.ernet.in/˜aveek˙bid/aveek.html Title

Cooling a Band Insulator with a Metal: Fermionic Superfluid in a Dimerized Holographic Lattice

Authors Arijit Haldar & Vijay B. Shenoy

Abstract A cold atomic realization of a quantum correlated state of many fermions on a lattice, eg. superfluid, has eluded experimental realization due to the entropy problem. Here we propose a route to realize such a state using holographic lattice and confining potentials. The potentials are designed to produces a band insulating state (low heat capacity) at the trap center, and a metallic state (high heat capacity) at the periphery. The metal “cools” the central band insulator by extracting out the excess entropy. The central band insulator can be turned into a superfluid by tuning an attractive interaction between the fermions. Crucially, the holographic lattice allows the emergent superfluid to have a high transition temperature– even twice that of the effective trap temperature. The scheme provides a promising route to a laboratory realization of a fermionic lattice superfluid, even while being adaptable to simulate other many body states.

Manuscripts Scientific Reports 4, 6655 (2014) Unconventional 1/f noise in Graphene on SrTiO3 substrate

Anindita Sahoo and Arindam Ghosh

Department of Physics, Indian Institute of Science, Bangalore 560 012, India

Recently graphene has acquired a great interest in both fundamental and applied research. It is now established that the impact of the supporting substrate is critical to the operation of graphene field effect devices, which can seriously modify several transport parameters including the electrical mobility, ++ intrinsic doping and low frequency noise. In this context, replacing the usual SiO2/Si substrate with

SrTiO3 [STO] substrate having very high dielectric constant, has opened up new possibilities, leading to large doping densities, higher electrical mobility, and also hysteretic transfer characteristics for memory applications. While the conventional transport characteristics of graphene on STO substrate is now a well-investigated field, the origin and magnitude of low-frequency flicker noise, also known as 1/f noise, of graphene field effect transistors directly placed on STO substrate, are still not known. We have studied low-frequency 1/f noise in dual-gated single layer graphene (SLG) field effect transistors sandwiched between STO (substrate) and mechanically exfoliated hexagonal boron-nitride with the latter acting as the dielectric for the top gate. The magnitude of 1/f noise normalized to channel area 2 (A Sv/V ) of SLG on STO ( is similar to SLG on SiO2. The noise amplitude followed an unexpected ‘W’-shape dependence of gate-bias with the central peak at Dirac point. This observation is in conflict with the usual ‘V’, ‘M’ or ‘’- type dependence of SLG noise on SiO2 or other substrates. We have discussed possible microscopic mechanisms for such behavior, considering the role of puckering of oxygen atoms introducing inward dipole moments that can form a new source of electrostatically tunable scattering mechanism at the graphene-STO interface.

Spatiotemporal heterogeneity of lipid interaction in solid supported lipid bilayers induced by binding of pore forming proteins

N. K. Sarangi1, I. P. Ilanila1,2, K. G. Ayappa2, S. S. Visweswariah3, and J. K. Basu1* 1Department of Physical Science, 2Department of Chemical Engineering, 3Dept. of Molecular Reproduction, Development and Genetics, Indian Institute of Science Bangalore- 560012

Pore forming proteins (PFPs) are capable of pore formation which allows the passage of ions, small bio-inspired molecules, proteins and other constituents through a wide variety of target membranes, ranging from bacteria to humans.1 They often cause cell death, as PFPs disrupt the membrane permeability barrier required for maintaining cell homeostasis. Understanding the lateral organization and dynamics of lipids and proteins in membranes is critical to many cellular processes.2 However, the detailed mechanism, dynamics and the time scale for the association of PFPs in biological membrane and subsequent docking and penetration phenomena are poorly understood. In this study we used super-resolution stimulated emission depletion (STED)3 with spatial resolution and fluorescence correlation spectroscopy (STED-FCS4) to illustrate the heterogeneous lipid dynamics induced in supported bilayer platforms upon exposure to the cholesterol dependent PFP, Listeriolysin O (LLO). Using STED-FCS we are able to resolve differences in lipid dynamics within a spatial resolution of 100 nm around the vicinity of pores formed by LLO on supported bilayer membrane.

References: 1. I. Iacovache, M. Bischofberger, and F.G. van der Goot, Curr. Opin. Struct. Biol., 2010, 20, 241–246. 2. M.E. Pipkin, and J. Lieberman, Curr. Opin. Immunol., 2007, 19, 301–308. 3. S. W. Hell, and J. Wichmann, J. Opt. Lett., 1994, 19, 780–782. 4. C. Eggeling et al., Nature, 2009, 457, 1159-1162.  Molecular structure of the Discotic Liquid Crystalline Phase of Hexa-peri- Hexabenzocoronene/Oligothiophene Hybrid and their Charge Transport properties

Saientan Bag

Abstract— Using atomistic molecular dynamics simulation we study the discotic columnar liquid crystalline (LC) phases formed by a new organic compound having Hexa-peri-Hexabenzocoronene (HBC) core with six pendant oligothiophene units recently synthesized by Nan Hu et al. (N. Hu, R. Shao, Y. Shen, D. Chen, N. A. Clark and D. M. Walba, Adv. Mater. 26, 2066, 2014). This HBC core based LC phase was shown to have electric field responsive behavior and has important application in organic electronics. Our simulation results confirm the hexagonal arrangement of columnar LC phase with a lattice spacing consistent with that obtained from small angle X-ray diffraction data. We have also calculated various positional and orientational correlation functions to characterize the ordering of the molecules in the columnar arrangement. The molecules in a column are arranged with an average twist of 25 degrees having an average inter-molecular separation of ~5 Å. Interestingly, we find an overall tilt angle of 43 degrees between the columnar axis and HBC core. We also simulate the charge transport through this columnar phase and report the numerical value of charge carrier mobility for this liquid crystal phase. The charge carrier mobility is strongly influenced by the twist angle and average spacing of the molecules in the column.

Reference:

S. Bag, V. Maingi, P. K. Maiti, J. Yelk, M. A. Glaser, D. M. Walba & N. A. Clark, J. Chem. Phys. 2015, 143(14), 144505.

Micromagnetic Simulations: A method to model the Magnonics for spin wave band strctures

Venkateswarlu Dasari∗ Spintronics and Thinfilm Magnetism Lab, Department of Physics, Indian Institute of Science, Bangalore, India-560012.

Magnonics[1] is a relatively new field in condensed matter physics, which is analogous to photonics. A quanta of spin wave excitations is called magnon. Though, the ”theory of spin waves” is a very classic topic in condensed matter physics, their interest was limited only for the fundamental study. But in recent years, it has got more attention because of the possible applications that can be tried with the help of recent developments in the associated device fabrication and characterization processes. In photonics, one need to have periodic variation in optical refractive index because there the photons are the information carriers. In the case of magnonics, the information is carried out by spin waves. Spin waves are nothing but collective excitation of mag- netic moments (spins). Due to this reason, one can think of carrying the information with no need of electron transfer unlike in general transport mechanisms. So, magnonics based devices dissipates less heat when compared to that of electronic and spintronic de- vices. More over, they can be considered for the microwave applications as the spin wave resonance occurs around GHz frequency bands. Also, for a given frequency, spin waves possess low wavelengths when compared to that of electromagnetic waves (in the case of Photonics). This indicates that magnonics devices can be miniaturized to the better length scales than that of photonics. Since the magnetic materials possess many parameters like magnetization, anisotropy field, shape anisotropy etc, it is very easy to think of making a periodic modulation in any of these properties to satisfy the basic condition for magnonics. At the same time, it is very challenging task in order to aim at required band structures with particular band gaps and band shapes. Because all the above mentioned parameters can influence and alter the spin wave band structure simultaneously. So, it shows the importance of theory and modelling in the field of magnonics to find out the suitable conditions prior to the device fabrication. Continuum model based theories like plane wave method involves lot of difficulties in finding out the boundary conditions in structures like magnonics. As an alternate, micromagnetic simulations have been proved to be a suitable method due to their simple approach. I have used Object Oriented Micromagnetic Framework (OOMMF), a finite difference method to model my magnonic structures composed of NiFe nano wire networks that are fabricated in house. OOMMF is a public code developed by NIST. OOMMF will give only spatial information of magnetization data. In order to get the spin wave resonant modes and band structure calculations, one need to develop their own code for post processing the magnetization data. I will be discussing about the methods that we developed for spin wave band structure calculations and how we employed them in correlating our experimental results obtained by brillouin light scattering (BLS).

Ref: 1. V V Kruglyak et al Journal of Physics D: Applied Physics, 2010, 43, 26400.

∗ [email protected] Binary Fluid Turbulence : Signatures of Multifractal Droplet Dynamics and Dissipation Reduction

Nairita Pal1, Prasad Perlekar2 and Rahul Pandit1 1Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India. 2TIFR Centre for Interdisciplinary Sciences, 21 Brundavan Colony, Narsingi, Hyderabad 500075, India

We present a numerically efficient way to model the dynamics of a droplet of the minority phase inside a turbulent background of the majority phase by using the Cahn-Hilliard-Navier- Stokes equations. By calculating probability distribution functions of the components of acceleration of the center of mass of the droplet we show that the droplet dynamics is qualitatively similar to that of finite-sized rigid particles when the surface tension value is high, and also has multi-fractal surface fluctuations like a soft droplet when the surface tension is low. Our study also reveals that the presence of the droplet leads to a scale- dependent viscosity in the droplet-fluid mixture and enhances the energy spectrum E(k) when the wave number k is large; this enhancement leads to dissipation reduction.

Splitting, Coalescence and Shape fluctuations of Multielectron bubbles in liquid helium

V. Vadakkumbatta, E. M. Josephb, and A. Ghosha,b aDepartment of Physics, Indian Institute of Science, Bangalore, India bCentre for Nano Science and Engineering, Indian Institute of Science, Bangalore, India

Multielectron bubbles are micron sized cavities in liquid helium with electrons localized on the inner surfaces of the bubbles. They form a rich platform to study the properties of an interacting system of electrons on curved surfaces. In the presentation, I will describe a new technique to trap these objects for more than few seconds, which have allowed us to observe number of interesting phenomena, such as coalescence of similarly charged bubbles, as well as their splitting into secondary bubbles at high speeds. Most interestingly, we have imaged their dynamics in the presence of static, as well as oscillating electric fields, which may provide interesting insight on the state of electrons present inside.