DEPARTMENT OF IN-HOUSE SYMPOSIUM 2011

INDIAN INSTITUTE OF SCIENCE BANGALORE 560012 (NOVEMBER 26, 2011)

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 is the first Inhouse Symposium in the New Physical sciences building. This year we have a total of 19 talks and 51 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 to familiarize themselves with the current research activity in our Department in various branches of Physics.

I would like to thank Prabal Maiti, Arindam Ghosh, Tarun Deep Saini, and Vijay Shenoy 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. H. R. Krishnamurthy Chairman November 26, 2011

Department of Physics, IISc Bangalore In-house Symposium 2011 November 26, 2011 Auditorium, New Physical Sciences Building

Programme

Session I 9:00-10:30 Chair: H. R. Krishnamurthy T01 9:00-9:30 Motile, shaken, stuck and relaxed

T02 9:30-9:45 Aveek Bid Neutral modes in the fractional quantum hall regime T03 9:45-10:00 M. Haridas Optical properties of semiconducting QD- metal nanoparticle hybrid arrays T04 10:00-10:15 Mattaparthi Venkata Satish Kumar Structure of DNA-functionalized dendrimer nanoparticles T05 10:15-10:30 Anurag Misra Crystal structure of a peptide inhibitor of human islet amyloid polypeptide (hIAPP) fibrillization: Implications for the treatment of Type II diabetes

10:30-11:00 Tea

Session II 11:00-01:00PM Chair: Tarun Deep Saini T06 11:00-11:15 K. P. Ramesh Ultra high field and temperature dependence of NMR T and charge transport studies in organic 1 conductor (PF - doped p3MT) 6 T07 11:15-11:30 Amal Medhi Synchronous and asynchronous Mott transitions in topological insulator ribbons T08 11:30-11:45 Vinod E. M. A composition dependence study on (GeTe)1-xSex chalcogenide alloys T09 11:45-12:00 V. S. Manu Use of genetic algorithm for quantum information processing by NMR T10 12:00-12:15 Bidya Binay Karak Modelling Maunder minimum using a dynamo model T11 12:15-12:30 Sayantan Majumder Discontinuous shear thickening in confined dilute carbon nanotube suspensions T12 12:30-12:45 Sakshath S A new magnetic memory concept based on ferromagnetic nanorings T13 12:45-1:00 Biswanath Chakraborty Phonon softening in top gated single layer MoS2 field effect transistor: in-situ Raman scattering and density functional theory

1:00-2:00 Lunch

Session III 2:00-3:15 Poster Session

Session IV 3:15-4:00 Chair: Prabal K. Maiti T14 3:15-3:30 Using light and sound to study electrons in Helium T15 3:30-3:45 Saroj Kumar Nandi Jamming and large deviations in micellar gels: a model T16 3:45-4:00 Nitin P. Lobo Adiabatic cross-polarization applied to solid state NMR experiments

4:00-4:30 High Tea

Session V 4:30-5:30 Chair: Vijay B. Shenoy T17 4:30-4:45 Prateek Sharma Gas physics of galaxy clusters T18 4:45-5:00 Atindra Nath Pal 1/f noise as a probe to investigate the band structure of graphene T19 5:00-5:15 Mogurampelly Santosh Nucleic acid interaction with carbon nanotube and graphene Concluding Remarks, Best Poster Award 5:15-5:30 and Vote of Thanks List of Posters

No. Presenter Title Mechanical properties of ZnS nanowires P01 Taraknath Mandal and thin films: Microscopic origin of the dependence on size and growth direction Solid state NMR Methodological P02 R. V. Sudheer Kumar Development for Structural Characterization Validity of Dynamical Density Functional P03 S. M. Kamil Theory Symmetry properties of Large-Deviation P04 Nitin Kumar Functions of the Velocity of a Self- Propelled Granular Rod Electroporation of cells with very short P05 Amit Kumar Majhi electric pulses Opto-electronic properties of graphene- P06 Medini Padmanabhan semiconductor hybrids Measurement of Electron Spin Lifetime and Optical Orientation Effeiciency in P07 Chinkhanlun Guite Germanium Using Electrical Detection of Radio Frequency Modulated Spin Polarization P08 Bidisha Nandy DNA compaction by dendrimer Over 100-fold increase in strain sensitivity P09 S. M. Mohanasundaram of a metal based piezoresistive MEMS transducer through nanoscale inhomogenization Fermionic Superfluid State in an Optical P10 Yogeshwar P. Saraswat Lattice via Band-Insulator to Superconductor Transition A 10 Tesla Table-top Controlled P11 Aditya N. Roy Choudhury Waveform Magnet Unconventional noise in two dimensional P12 Saquib Shamim doped Silicon Real Space Distribution of Ultra-cold Bosons in an Optical Lattice at a Finite P13 Manjari Gupta Temperature Using Strong Coupling Expansion Tuning between an antidot lattice and P14 M. A. Aamir quantum dot lattice in a double-gated GaAs/AlGaAs heterostructure Ultracold Fermions with Artificial P15 Jayantha P. Vyasanakere Rashba Spin-Orbit Coupling Heteronuclear Correlation between P16 Y. Jayasubba Reddy Carbon and Double Quantum Proton Chemical shifts in Solids

P17 Semonti Bhattacharyya Topological Insulator: Basic concepts and preliminary results Trapped fermions in a synthetic non- P18 Sudeep K. Ghosh Abelian gauge field Microfluidic Devices For Measuring and P19 Siddharth Khare Exerting Micro-Newton Forces For Biological Applications

P20 Subhamoy Ghatak Nature of Electronic States in Ultrathin MoS2 Field Effect Transistor Design of a polymer based Infra-red P21 Gurucharan V. Karnad sensor 1/f noise as a probe to investigate the P22 Vidya Kochat band structure and quantum interference in graphene P23 Tapan Chandra Adhyapak Active smectics Raman signatures of pressure induced P24 Achintya Bera electronic topological and structural transitions in Bi2Te3 DNA and Dendrimer assisted dispersion P25 Debabrata Pramanik of nanotubes Optical and Electrical Investigation of P26 Sandip Mondal CdTe Qds/PDDA Bistable Devices Fabrication of tunable potential barrier in P27 T Phanindra Sai Bilayer graphene

P28 Vishal Maingi DBT: A versatile Dendrimer Building ToolKit Development of Methodologies in Ex-Situ P29 KowsalyaDevi Pavuluri NMR Spectroscopy Strongly correlated transport in ultrathin P30 Marsha M. Parmar gold nanowires Strongly magnetized cold electron P31 Upasana Das degenerate gas: Mass-radius relation of the collapsed star P32 Ananyo Maitra Membrane coupled to Active Fluid Temperature Dependent Sign Reversal of P33 K. S. Bhagyashree Magnetocrystalline Anisotropy in Nanoparticles of La0.875Sr0.125MnO3 Thermoelectric properties of PbTe with P34 Ashoka Bali Bi precipitates Evidence for shift of rigidity percolation P35 M. Prashantha to higher coordination numbers Topological Insulator on the Edge: P36 Amal Medhi Boundary Conditions Revisited Structural and magnetic transition in P37 Dona Cherian Fe1+yTe single crystals EPR Study of Electron-Hole Asymmetry P38 Geetanjali Singh in Bulk and Nanoparticles of Bi1- xCaxMnO3(x = 0.4, 0.6): A Comparison P39 Vaisakh V Multi Electron Bubbles in Liquid Helium Numerical studies of dynamo action in a P40 Naveen Jingade linear shear flow with turbulence P41 Himanshu Joshi Structure of DNA Nanotubes Exploration of the statistical properties of P42 Vishwanath Shukla Gross-Pitaevskii turbulence in two dimensions. Confinement Induced Density Modulation P43 Shibu Saw and Spatially Resolved Dynamics of Confined Liquids

P44 Ruchika Yadav Structural and magnetic properties of Nd1-xYxMnO3 (0.1 ≤ x ≤ 0.6) Structure of Cytolysin A (ClyA) pore in P45 Swarna M Patra Lipid Bilayer Conversion between electromagnetically P46 Sapam Ranjita Chanu induced transparency and absorption in a degenerate lambda system Real-space manifestations of Energy- P47 Debarghya Banerjee spectra Bottlenecks: Insights from Hyperviscous Hydrodynamical Equations In-plane magnetic anisotropy in epitaxial P48 Sakshath S ultrathin Fe films Scroll-wave dynamics in human cardiac tissue: lessons from a mathematical model P49 Rupamanjari Majumder with inhomogeneities and fiber architecture Density functional studies of electrons P50 Pramod Kumar Verma and phonons in pyrochlores “Upper Branch” Fermi Gas and Tan's P51 Vijay B. Shenoy theorem

TALK ABSTRACTS

Motile, shaken, stuck and relaxed

Sriram Ramaswamy

I will give a quick summary of work done mainly with my students, concentrating on just­published or unpublished results. The topics: (i) ­­ living organisms, their motorized components, and inanimate imitations in granular or colloidal matter; (ii) confined liquids and the glass transition; (iii) other current directions in brief: quantum thermalization, phase­transition models in ecology. Neutral modes in the fractional quantum hall regime

Aveek Bid, N. Ofek, H. Inoue, M. Heiblum, D. Mahalu, V, Umansky and C. Kane

Current propagates in the fractional quantum Hall effect (FQHE) regime along the edges of the two- dimensional-electron gas (2DEG) via chiral edge modes with a chirality dictated by the applied magnetic field. For some fractional states so called, holes-conjugate states), such as ½

Controlling the emission properties of materials has been a challenging task for the material physicist over the past several decades. Recent interests in this field have been mainly focused on the study of interaction between different types of nano particles. A system consisting of metal nanoparticle (MNP) and semiconducting quantum dot(QD) is an example for such system. The interaction between surface plasmons of metal nanoparticles and excitons in semiconductors generates unique optical properties which is not achievable with a single type of nano structure. Optical properties can be tuned by various parameters like interparticle distance, density and spectral positions of QDs with MNPs. However the main challenge is to prepare the ordered arrays with well-defined periodicities. Self-assembly process emerges as a simple and versatile method because of their simplicity and reliability over other methods for the preparation of such arrays. We have prepared QD arrays and hybrid arrays using self assembly technique with di block copolymer as template. Chemically synthesised cadmium selenide (CdSe) QDs were dispersed inside the particular block by tuning the chemical property of the capping legends. Hybrid arrays were also prepared using the same template with polymer capped gold nanoparticles (Au NPs) dispersed inside the other block. Density and dispersion of Au NPs were varied by controlling the grafting density of the capping polymer. This method provides better control over the density and interparticle distance for the arrays. Emission properties of the ordered arrays were studied spatially resolved and time resolved spectroscopy techniques.

Photoluminescence (PL) spectra collected from the hybrid arrays shows enhancement/quenching with respect to the density and dispersion of Au NPs in the PS block compared to the corresponding QD films. Hybrid films with lower grafting density of Au NPs shows systematic enhancement in the PL emission, while the PL spectra from the samples with higher polymer grafting density Au NPs is quenched with respect to increase in Au NP density. Exciton lifetime for the hybrid arrays were estimated from the photoluminescence decay measurements, by fitting a double exponential to the decay profile. The longer time corresponds to radiative electron hole recombination in isolated CdSe QD, while the shorter time seems to originate from collective interaction amongst QDs. The lifetime data shows systematic reduction in t1 and t2 with increasing volume fraction of Au NPs. Similarly, the amplitude (a1, a2) of the two relaxations switch over with increase in volume fraction of Au NP. It is clear that the shorter time t1, becomes the dominant relaxation mode with increasing φAu, and is affected much more than t2, due to incorporation of Au NP. However samples with higher polymer grafting density of Au NPs shows slower decay compared to the samples with same volume fraction of Au NPs with lower polymer grafting density and same volume fraction of CdSe QDs. Variation of lifetime with respect to the volume fraction of Au NPs is less compared to the films with Au NPs of lower grafting density. The observed variation in the optical properties has been explained in terms of the energy transfer between CdSe QDs and Au NPs.

References M. Haridas and J. K. Basu, Nanotechnology, 21, 415202 (2010). M. Haridas, J. K. Basu, D. J. Gosztola and G. P. Wiederrecht, Appl. Phys. Lett, 97, 083307 (2010).

Structure of DNA-Functionalized Dendrimer Nanoparticles Mattaparthi Venkata Satish Kumar and Prabal K Maiti Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012

Abstract

Atomistic molecular dynamics simulations have been carried out to reveal the characteristic features of ethylenediamine (EDA) cored protonated (corresponding to neutral pH) poly amido amide (PAMAM) dendrimers of generation 3 (G3) and 4 (G4) that are functionalized with single stranded DNAs (ssDNAs). The four ssDNA strands that are attached via alkythiolate

[-S (CH2)6-] linker molecule to the free amine groups on the surface of the PAMAM dendrimers observed to undergo a rapid conformational change during the 25 ns long simulation period. From the RMSD values of ssDNAs, we find relative stability in the case of purine rich (having more adenine and guanine) ssDNA strands than pyrimidine rich (thymine and cytosine) ssDNA strands. The degree of wrapping of ssDNA strands on the dendrimer molecule was found to be influenced by the charge ratio of DNA and the dendrimer. As G4 dendrimer contains relatively more positive charge than G3 dendrimer, we observe extensive wrapping of ssDNAs on the G4 dendrimer than G3 dendrimer. This might indicate that DNA functionalized G3 dendrimer is more suitable to construct higher order nanostructure. The ssDNA strands along with the linkers are seen to penetrate the surface of the dendrimer molecule and approach closer to the center of the dendrimer indicating the soft sphere nature of the dendrimer molecule. The effective radius of DNA-functionalized dendrimer nanoparticle was found to be independent of base composition of ssDNAs and was observed to be around 19.5 Å and 22.4 Å when we used G3 and G4 PAMAM dendrimer as the core of the nanoparticle respectively. The observed effective radius of DNA-functionalized dendrimer molecule apparently indicates the significant shrinkage in the structure that has taken place in dendrimer, linker and DNA strands. As a whole our results describe the characteristic features of DNA-functionalized dendrimer nanoparticle and can be used as strong inputs to design effectively the DNA-dendrimer nanoparticle self-assembly for their active biological applications. Crystal structure of a peptide inhibitor of human islet amyloid polypeptide (hIAPP) fibrillization: Implications for the treatment of Type II diabetes

Anurag Misraa, Aseem Mishrab, T. Vaishnavi Murthyb, Madhavi Guptab, Virander Singh Chauhanb Suryanarayanarao Ramakumara, aDepartment of Physics, Indian Institute of Science, Bangalore-560012, India. bInternational Centre for Genetic Engineering and Biotechnology, New Delhi-110067, India. E-mail: [email protected]

Type-2 diabetes mellitus (T2DM), an endocrine metabolic disorder affects more than 100 million people worldwide and the prevalence is increasing dramatically in both the developed and developing countries. Amyloid deposits, observed in a vast majority of the T2DM patients are primarily on account of misfolding and aggregation into fibrils of human islet amyloid polypeptide (hIAPP), a 37 residue endocrine hormone secreted by pancreatic β-cells. It has been suggested that intermediates produced in the process of fibrillization are cytotoxic to insulin producing β-cells. Hence, the inhibition of misfolding/fibrillization of hIAPP which involves structural transition from its native state to β-sheet conformation could be a possible strategy to mitigate T2DM. We approached to target hIAPP fibrillization by designing short peptides containing the helix inducing α,β-dehydrophenylalanine (ΔPhe or ΔF) amino acid and the fibrillization inhibition was monitored by thioflavin T assay and electron microscopy. We found that the short peptide, FGAΔFL is the most effective inhibitor of hIAPP fibrillization in vitro and also did not show any cytotoxic effect on RIN4fm pancreatic cell line. We successfully crystallized the penta-peptide and solved its 3D structure using X-ray diffraction method. Molecular conformation of the peptide shows the occurrence of a nest-motif and a type-I β-turn. To gain structural understanding and to visualize the probable interactions of the hIAPP with FGA∆FL, molecular docking studies were performed using AutoDock4. We propose, on the basis of FGA∆FL crystal structure and molecular docking analysis, that the peptide binds to the helical conformation of hIAPP which is considered as transient in nature and/or preferred in membrane environment. Here, the penta-peptide binds at the C-terminal of hIAPP and stabilizes its transient helical conformation and makes the transition from alpha to beta structure unfavourable and thereby inhibits the fibrillization process. Thus, the crystal structure of the penta-peptide inhibitor together with computational docking studies provides an atomic level picture of the possible mechanism by which the peptide manifests its fibrillization inhibition activity. Further studies are underway in our laboratories to develop even more potent inhibitors of hIAPP fibrillization and the results will be presented.

Keywords: diabetes, amyloid, inhibitor, peptide Ultra high field and temperature dependence NMR T1 and charge PF− * transport studies in Organic Conductor ( 6 doped p3MT). K. Jugeshwar Singh1, W.G. Clark2 G. Gaidos2, W.G. Moulton3, A.P. Reyes3, P. Kuhns3, J. D. Thompson4, R. Menon1 and K.P. Ramesh1 1 Department of Physics, Indian Institute of Science, Bangalore-560012, India. 2Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547, USA 3National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA 4 Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Email: [email protected]; [email protected]

Ultra high field and ultra low temperature charge transport phenomenon is studied in doped organic conductor, poly 3 methylthiophene (p3MT) using NMR relaxation rates (1/T1), magnetic susceptibility and electrical conductivity techniques. The magnetic susceptibility data show contributions from both Pauli and Curie spins, with the size of the Pauli term depending strongly 1 on the doping level. Proton NMR relaxation ( H-T1) measurements are carried out over a wide temperature (T) range 3-300 K at magnetic fields, (B): 0.9, 9.0, 16.4 and 23.4 T, and fluorine 19 NMR relaxation ( F-T1) measurements were done at B = 9 T over the range 3 < T < 300 K. The doping level has been varied to enable to investigate the role of carrier density and electron- electron interactions (EEI) on the relaxation mechanism. Three dominant mechanisms, (i) due to conduction electron, (ii) via spin diffusion to the paramagnetic centers (SDPC) and (iii) due to reorientation motion of symmetric subgroups, are observed and each mechanism is found to dominate differently at different temperature regions. Frequency dependence of T1 shows that system behaves as a quasi 1-Dimensional (q1-D) at high temperatures and 3-D at low

19 − temperatures. F-T1 shows Intra chain diffusion of charge carriers other than PF6 reorientation. The cross relaxation among the 1H and 19F nuclei give the more insight into the role of both inter-chain and intra-chain conduction mechanisms. Further, a good correlation between electrical conductivity, magnetic susceptibility and NMR T1 data has been observed.

* This work is funded by the DST and NSF.

1 Synchronous and Asynchronous Mott Transitions in Topological Insulator Ribbons

Amal Medhi,∗ Vijay B. Shenoy,† and H. R. Krishnamurthy‡ Center for Condensed Matter Theory, Indian Institute of Science, Bangalore 560012, India

We address how the nature of linearly dispersing edge states of two dimensional (2D) topological insulators evolves with increasing electron-electron correlation engendered by a Hubbard like on-site repulsion U. We consider finite ribbons of two systems of topological band insulators with local electronic interactions incorporated. Using an inhomogeneous cluster slave rotor mean-field method developed here, we show that electronic correlations drive the topologically nontrivial phase into a Mott insulating phase via two different routes. In a synchronous transition, the entire ribbon attains a Mott insulating state at one critical U. In the second, asynchronous route, Mott localization first occurs on the edge layers at a smaller critical value of electronic interaction which then propagates into the bulk as U is futher increased until all layers of the ribbon become Mott localized.

PACS numbers: 71.10.Fd, 71.30.+h, 71.70.Ej, 73.20.At

† [email protected] ‡ [email protected]

∗ [email protected] A composition dependence study on (GeTe) 1-xSe x chalcogenide alloys

Vinod E. M, A.K. Singh, R. Ganesan, K. S. Sangunni*

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

Abstract:

In the age of technology revolution human modern living standard requires to efficient and cheaper memory materials for the devices. Chalcogenide based memory materials can full fill this requirement. Therefore, this study demonstrate the synthesis and characterizations of bulk (GeTe) 1-xSe x (X = 0.02, 0.10, 0.20 and 0.50) chalcogenide glasses. Obtained outcomes from X-ray Diffraction (XRD), Differential Scanning

Calorimetry (DSC), Raman spectroscopy, X-ray photoelectron Spectroscopy and

Scanning Electron Microscopy (SEM) measurements reveal that alloys structures changes with additive element (Se) concentration. Immiscibility of Ge-Te and Ge-Se phases have been verified at lower concentration of Se (X = 0.02, 0.10, 0.20) while a miscible single phase GeTeSe homogeneous amorphous structure is obtained for optimum composition (X = 0.50) alloy. Structural changes in described materials could be explained with help of bond theory of solids.

Use of Genetic Algorithm for Quantum Information Processing by NMR

V. S. Manu and Anil Kumar

Abstract

Genetic Algorithms (GAs) are well known global optimization methods inspired by nature’s evolutionary process. It finds application in biology, bioinformatics, computational science, engineering, economics, chemistry, manufacturing, mathematics, physics and other fields. We use its optimization power to find the decomposition of a general unitary operator in terms of experimentally preferable unitary operators in NMR. We demonstrate its use, for efficient decomposition of unitary operators and their NMR pulse sequences needed for Quantum Information Processing. Using this we demonstrate a probabilistic way of doing two qubit NMR homonulear quantum computations by using non-selective (hard) pulses. We also demonstrate efficient creation of Bell states directly from equilibrium state by using only global (hard) pulses in homonuclear systems. In three qubit NMR QIP, we will show the best known pulse sequences for preparing GHZ and W states using only nearest neighbor interactions.

Modelling Maunder minimum using a dynamo model

Bidya Binay Karak* and Arnab Rai Choudhuri

The Maunder Minimum is the period from 1645 to 1715 during which a few sunspots were appeared on the Sun. There are several indications that the Sun had passed through several grand minima like the Maunder one in past. Our motivation is to model such kind of Maunder minimum using a dynamo model known as flux transport dynamo model. Basically in this model the toroidal field is generated near the base of the convection zone by the stretching of the poloidal field through the differential rotation whereas the poloidal field is generated near the solar surface by the decay of tilted bipolar sunspots. These two source regions are connected to each other by two important transport agents -- turbulent diffusivity and the meridional circulation. There are several evidences that the poloidal field generation process and the meridional circulation are the processes in this model which involve randomness. With a suitable fluctuations in these two processes of a flux transport dynamo model we are able to reproduce a Maunder minimum remarkably well.

______

*electronic mail: [email protected] Discontinuous shear thickening in confined dilute carbon nanotube suspensions

Sayantan Majumdara, Rema Krishnaswamyb, and A. K. Sooda, b

a Department of Physics, Indian Institute of Science, Bangalore 560012, India; and b Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India.

Abstract A monotonic decrease in with increasing shear stress is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. Here we demonstrate a discontinuous shear-thickening transition on varying shear stress where the viscosity jumps sharply by four to six orders of magnitude in flocculated suspensions of multiwalled carbon nanotubes (MWNT) at very low weight fractions (approximately 0.5%). Rheo-optical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear- thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear-thickening phenomena in confined geometries is pertinent for flow-controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues.

References

1. Sayantan Majumdar, Rema Krishnaswamy, and A.K. Sood, PNAS, 108, 8996-9001 (2011).

A new magnetic memory concept based on Ferromagnetic nanorings Sakshath S and P S Anil Kumar Department of Physics Indian Institute of Science Bangalore 560012; India

Ferromagnetic rings and wires, on a mesoscopic scale show different stable domain configurations when subjected to external magnetic fields. These different configurations can be used to implement digital memory states. In addition, magnetic effects induced by the spin torque of a spin polarised electric current can be used to switch between different magnetic configurations. Resistance of a nanoscale ferromanetic wire or ring depends on the presence or absence of a domain wall. In materials where the AMR effect is higher, the presence of a domain wall decreases the resistance whereas in low AMR materials, the effect of domain wall is opposite. We have made Cobalt and NiFe nano-rings with ferromagnetic contacts and demonstrated a new memory device with well defined resistance states that can be electrically controlled. We also propose that three dimensional integration is possible. Phonon softening in top gated single layer MoS2 field effect transistor: in-situ Raman scattering and density functional theory

Biswanath Chakraborty1∗,AchintyaBera1,D.V.S.Muthu1,Somnath Bhowmick2,U.V.Waghmare2, A. K. Sood1, and C. N. R. Rao2 1Department of Physics, Indian Institute of Science, Bangalore - 560012, India and 2 Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India Abstract Phonon dynamics, particularly electron-phonon coupling plays an important role in transport properties of semiconductors where the mobility is largely affected by scattering from optical phonons. Raman scattering has already been established to probe electron-phonon interaction in doped materials ranging from bulk semiconductors to one dimensional carbon nanotube through two dimensional graphene. Here we report an in-situ Raman scattering study of an electrochemi- cally doped single layer of MoS2. The zone center A1g phonon shows a softening with simultaneous 1 broadening as the doping increases while the other Raman active zone center E2g mode is almost inert to increasing carrier concentration. We use first-principles density functional theory based calculations to develop understanding of why the A1g mode specifically exhibits a strong electron phonon coupling, and hence the sensitivity to electron doping. Electrical characterization of a similar top gated single layer device shows an high on-off ratio ∼ 105.

∗ electronic mail:[email protected]

1 Using light and sound to study electrons in Helium

Ambarish Ghosh Jamming and large deviations in micellar gels: a model

Saroj Kumar Nandi,1, ∗ Bulbul Chakraborty,2, † Sriram Ramaswamy,1, ‡ and A. K. Sood3, §

1Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India 2Martin Fisher School of Physics, Brandeis University, Mail Stop 057, Waltham, Massachusetts 02454-9110, USA 3Department of Physics, Indian Institute of Science, Bangalore 560012, India Abstract We present a simple model for the rheological behavior observed in some recent experiments performed on micellar gels. The model combines attachment-detachment kinetics with stretching due to shear, and shows well-defined jammed and flowing states. The large deviation function (LDF) for the coarse-grained velocity becomes increasingly non-Gaussian with larger applied forces. The power fluctuations are found to obey a steady-state fluctuation relation. We argue that a wider class of physical problems with attachment-detachment kinetics can be understood in terms of the present theory.

∗Electronic address: [email protected] †Electronic address: [email protected] ‡Electronic address: [email protected]; Also at: JNCASR, Bangalore 560 064 India §Electronic address: [email protected]

1 Adiabatic Cross-Polarization Applied to Solid State NMR Experiments

Nitin P. Lobo1 and K. V. Ramanathan2

1Dept.of Physics and 2NMR Research Centre, Indian Institute of Science, Bangalore-560012.

Abstract

Cross-Polarization (CP) is a technique used in NMR for enhancing sensitivity of nuclei with low gyro-magnetic ratio by transferring polarization from another nucleus with a large polarization. There are many different techniques of polarization transfer, of which, Hartmann-Hahn cross-polarization [1, 2] is the one commonly used in the solid state. Though it is well-known that adiabatic cross-polarization [3, 4] is the most effective way of polarization transfer for static solid samples, it has not been effectively exploited for many NMR experiments. In this presentation, we consider the use of adiabatic cross-polarization for a class of 2-dimensional NMR experiments that are used for the measurement of dipolar couplings. Measurement of dipolar couplings using separated local field (SLF)[5] NMR experiment is a powerful tool for structural and dynamics studies of oriented molecules such as liquid-crystals and membrane proteins in aligned lipid bilayers. SLF technique provides site-specific information, as a 2-dimensional plot between chemical shifts of nuclei such as 13C or 15N and the heteronuclear dipolar coupling of the nuclei to the neighbouring protons. Enhancing the sensitivity of such SLF techniques is of significant importance in present day solid state NMR methodology. The present study considers the use adiabatic cross- polarization for this purpose, which is applied for the first time to one of the well-known SLF techniques, viz., Polarization Inversion Spin Exchange at the Magic Angle (PISEMA) [6]. The experiments have been carried out on a single crystal of a model peptide and a dramatic enhancement in signal to noise up to 90% has been demonstrated [7]. We also implemented adiabatic cross-polarization technique in a 2-dimensional Proton Detected Local Field (PDLF) [8] pulse sequence for polarization transfer. The long-range dipolar couplings which corresponds to indirectly connected 13C-1H pairs can be easily determined unlike CP based SLF schemes where it is truncated by strong short-range dipolar couplings. Experimental results from static oriented nematic liquid crystal system are presented. The advantages of new schemes over regular experiments in terms of sensitivity enhancement in dipolar dimension and greater reduction of radio frequency power into system are discussed.

References: (1) S. R. Hartmann and E. L. Hahn, Phys. Rev. 128, 2042 (1962). (2) A. Pines, M. G. Gibby and J. S. Waugh, J. Chem. Phys. 59, 569 (1973). (3) C. Slichter and W. C. Holton, Phys. Rev. 122, 1701(1961). (4) A. Anderson and S. Hartmann, Phys. Rev. 128, 2023 (1962). (5) R. K. Hester, J. L. Ackerman, V. R. Cross and J. S. Waugh, Phys. Rev. Lett. 34, 993 (1975). (6) C. H. Wu, A. Ramamoorthy and S. J. Opella, J. Magn. Reson. Ser. A. 109, 270 (1994). (7) N. P. Lobo and K. V. Ramanathan, J. Phys. Chem. Lett. 2, 1183 (2011). (8) S. Caldarelli, M. Hong, L. Emsley and A. Pines, J. Phys. Chem. 100, 18696 (1996). Gas Physics of Galaxy Clusters Prateek Sharma

I will describe the main area of my current research, the baryonic physics of galaxy clusters. Structures in the universe form because of gravitational instability in an expanding universe. Gravitationally bound structures, dom- inated by dark matter, form in a bottom up fashion with the smallest struc- tures forming the earliest. Larger and larger structures form due to mergers of smaller halos and filaments. Galaxy clusters are the most massive grav- itationally bound objects in the universe, reaching up to 1015 solar masses. The gravitational force (and mass) in galaxy clusters is dominated by dark matter, and baryons are only 15% of the total mass. In addition, most of the baryons are in the form of diffuse, hot plasma called the intracluster medium (ICM); stars in constituent galaxies have less than 10% of baryons. Galaxy clusters serve as independent probes of cosmological parameters such as dark energy (accelerated expansion of the universe). However, the observed propertied such as X-ray luminosity, temperature, etc. have to be calibrated with the cluster mass dominated by dark matter. In addition to being cosmological probes, galaxy clusters are also home to most massive galaxies and black holes. The hot gas in cluster cores has a short cooling time and is expected to cool and form stars at a tremendous rate. However, this is not observed; the star formation and cooling rates are 10-100 times smaller than expected. It is now believed that massive black holes in cluster centers heat the cooling cluster cores in a feedback loop. More the cooling, more is the amount of cooling gas that can feed the black hole, and more is the core heating. I will describe an idealized model of this interplay of heating and cooling in cluster cores and show that a lot of observations can be explained in a physical framework. I will also describe some of the research projects that I am involved in and intend to carry out in near future. Most of my work involves numerical simulations of physical processes, such as turbulence, cooling, heating, winds, magnetic fields, etc. in the context of astrophysics. Studying such processes in idealized setups has a far-reaching impact on understanding wide range of astrophysical phenomena. 1/f noise as a probe to investigate the band structure of graphene Atindra Nath Pal* and Arindam Ghosh Department of Physics, Indian Institute of Science, Bangalore-560012, India. The flicker noise or low frequency resistance fluctuations in graphene depends explicitly on its ability to screen external potential fluctuations and more sensitive compared to the conventional time average transport. Here we show that the flicker noise is a powerful probe to the band structure of graphene that vary differently with the carrier density for the linear and parabolic bands. We have used different types of graphene field effect devices in our experiments which include exfoliated single and multilayer graphene on oxide substrate, freely suspended single layer graphene, and chemical vapor deposition (CVD)-grown graphene on SiO2. We find this difference to be robust against disorder or existence of a substrate. Also, an analytical model has been developed to understand the mechanism of graphene field effect transistors. Our results reveal the microscopic mechanism of noise in Graphene Field Effect Transistors (GraFET), and outline a simple portable method to separate the single from multi layered graphene devices.

References

1. Atindra Nath Pal and Arindam Ghosh, Physical Review Letters 102, 126805 (2009). 2. Atindra Nath Pal and Arindam Ghosh, Appl. Phys. Lett., 95, 082105 (2009). 3. Atindra Nath Pal, Ageeth A. Bol, and Arindam Ghosh, Appl. Phys. Lett. 97, 133504 (2010). 4. Atindra Nath Pal et al., ACS Nano 5, 2075 - 2081 (2011).

Nucleic Acid Interaction with Carbon Nanotube and Graphene

Mogurampelly Santosh1, Swati Panigrahi2, Dhananjay Bhattacharyya2, A. K. Sood1 and Prabal K. Maiti1,*

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

2Biophysics Division, Saha Institute of Nuclear Physics, Kolkata, India 700064.

Abstract

(Dated: November 17, 2011)

In an effort to design efficient platform for siRNA delivery, we combine all atom classical and quantum simulations to study the binding of small interfering RNA (siRNA) by pristine single wall carbon nanotube (SWCNT) and graphene. Our results show that siRNA strongly binds to SWCNT and graphene via unzipping its base-pairs and the propensity of unzipping increases with the increase in the diameter of the SWCNTs and maximum for graphene. The unzipping and subsequent wrapping events are initiated and driven by van der Waals interactions between the aromatic rings of siRNA nucleobases and the SWCNT/graphene surface. However, MD simulations of double strand DNA

(dsDNA) of the same sequence show that the dsDNA undergoes much less unzipping and wrapping on the SWCNT/graphene in the simulation time scale of 70 ns. This interesting difference is due to smaller interaction energy of thymidine of dsDNA with the SWCNT/graphene compared to that of uridine of siRNA, as calculated by dispersion corrected DFT methods. After the optimal binding of siRNA to

SWCNT/graphene, the complex is very stable which serves as one of the major mechanisms of siRNA delivery for biomedical applications.

POSTER ABSTRACTS

Mechanical properties of ZnS nanowires and thin films: Microscopic origin of the dependence on size and growth direction

Taraknath Mandal, Prabal K Maiti and Chandan Dasgupta Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012

Abstract

Mechanical properties of ZnS nanowires and thin films are studied as a function of size and growth direction using all-atom molecular dynamics simulations. Using the stress-strain relationship we extract the Young's moduli of nanowires and thin films at room temperature. Our results show that the Young's modulus of [0001] nanowires has a strong size dependence. On the other hand, [0110] nanowires do not exhibit a strong size dependence of the Young's modulus in the size range we have investigated. We provide a microscopic understanding of this behaviour on the basis of bond stretching and expansion due to the rearrangement of atoms in the surface layers. The ultimate tensile strengths (UTS) of the nanowires do not show much size dependence. To investigate the mechanical behaviour of ZnS in two dimensions, we calculate the Young's modulus of thin films under tensile strain along the [0001] direction. The Young’s modulus of thin films converges to the bulk value more rapidly than that of the [0001] nanowire.

Solid state NMR Methodological Development for Structural Characterization

R.V.Sudheer Kumar

NMR Research Centre, Department of Physics

Indian Institute of Science, Bangalore-560012, India.

Solid state NMR has developed into one of the most informative and direct experimental approaches for structural characterization, particularly of peptides and proteins. There are several techniques such as REDOR, DRAWS, 13C MQ NMR, fp-RFDR-CT etc are available for carrying out such studies. Based on these techniques complete high resolution three dimensional structures can be determined which forms the way for further biomedical applications. With this goal, we have started producing simulated powder patterns in solids using Matlab by considering chemical shift anisotropy, dipolar coupling and dipolar dephasing curve of REDOR pulse sequence etc. Experimental skills required for carrying out the above studies are being acquired. Such experiments are proposed to be carried out on systems such as peptides and later on amyloid fibrils and discussion with various groups interested in the area is in progress.

The second part of the study is to obtain proton chemical shifts in solid state indirectly. For this we propose to use Deuterium NMR. However deuterium is a spin 1 nucleus and gives rise to quadrupolar broadening. The use of magic angle spinning gives rise to centre band and side bands. If the spinning speed is fast enough, then side bands occur far away and do not interfere with chemical shift range of deuterium. Studies on the methods for polarization transfer between heteronuclei for static oriented systems are also started. Then we can establish correlation between deuterium and its attached carbon in a deuterated liquid crystal. Experimental results and further plan of work are under progress. Validity of Dynamical Density Functional Theory

S. M. Kamil and Chandan Dasgupta

Abstract Dynamical Density Functional Theory (DDFT) aims to write the equa- tions for time evolution of the coarse grained density ρ(r, t) of particles, interacting via a pair potential. In DDFT one assume the local equilib- rium. In thermodynamical equilibrium exact sum rule connect the equi- librium two-body density distribution function ρ(2)(r, r0) to the gradient of the one-body direct correlation function c(1)(r). c(1)(r) is the effec- tive one-body potential due to interactions in the fluid, which is given by the functional derivative of Fex[ρ], the excess (over ideal) part of the Helmholtz free energy functional. In DDFT one assume both these re- sults of equilibrium theory are also valid for dynamical case. From the final equations it is easy to conclude that the statistical weight of equi- librium distribution goes as ∼ exp(−βF[ρ]) , where F[ρ] is the density functional being used in equilibrium DFT. We check the validity of these equations by comparing the results of Monte Carlo simulation using F[ρ] as the Hamiltonian and MD simulation of hard spheres. These simulations have been performed with and without external periodic field. We find that in both the cases results are not in favor of DDFT, while equilibrium DFT gives very satisfactory results.

1 Symmetry Properties of Large-Deviation Functions of the Velocity of a Self-Propelled Granular Rod

Nitin Kumar*, Sriram Ramaswamy and A.K. Sood

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

A geometrically polar granular rod confined in 2D geometry, subjected to a sinusoidal vertical oscillation, undergoes noisy self-propulsion in a direction determined by its polarity. When surrounded by a medium of crystalline spherical beads, it displays substantial negative fluctuations in its velocity. We find that the large-deviation function (LDF) for the normalized velocity is strongly non-Gaussian with a kink at zero velocity, and that the antisymmetric part of the LDF is linear, resembling the fluctuation-relation known for entropy production, even when the velocity distribution is clearly non-Gaussian. We extract an analogue of the phase-space contraction rate and find that it compares well with an independent estimate based on the persistence of forward and reverse velocities. We further show that the velocity vector of the particle in a plane obeys a recently proposed Isometric Fluctuation-Relation [PNAS 108, 7704 (2011)] which calculates the probability of the isometric pair of current vectors pointing in different directions on a d-dimensional hypersphere.

Reference :

Nitin Kumar, Sriram Ramaswamy and A. K. Sood, Phys. Rev. Lett. 106, 118001 (2011).

______

*electronic mail: [email protected]

Electroporation of cells with very short electric pulses

Amit Kumar Majhi and V. Venkataraman

The application of short electric field pulses makes it possible to render cell membranes temporarily permeable to substances that otherwise would not be able to effectively enter the cell interior. This technique known as Electroporation is widely used in genetic engineering. Typically, millisecond pulses of electric field strengths are applied such that a transmembrane potential exceeding the natural cellular transmembrane potential difference of approximately 1V is generated which results in permeabilization (poration) of cell membranes [1].

A simple electrical model for living cells predicts [2, 3] an increasing probability for electric field interactions with intercellular substances of both prokaryotic cells and eukaryotic cells when pulse duration is reduced into sub-microsecond range. To investigate this, we have carried out Electroporation on yeast cells using propidium iodide (PI) as a fluorescent marker. Preliminary experiments have been carried out with pulse widths ranging from 3 milliseconds to 100 microseconds. The effects on yeast cells due to the pulses are examined with inverted fluorescence microscope. It is known that PI moves into the cell during electroporation and emits fluorescence when it binds with nucleic acid. We have observed the rapid intake of PI immediately after the pulse is applied. In both cases e.g. millisecond and microsecond pulse, intake of PI happens immediately. Further experiments are in progress to quantify the effect and also to reduce the pulse widths to sub-microsecond regime.

References:

1. “Medical applications of electroporation”, IEEE Trans. Plasma Science. 28:206–223.

2. “Nanosecond Pulse Electroporation of Biological Cells: The Effect of Membrane Dielectric Relaxation”, Elham Salimi et. al.

3. “The Effects of Intense Submicrosecond Electrical Pulses on Cells”, Biophys J. 2003 April; 84(4): 2709–2714. Title: Opto-electronic properties of graphene-semiconductor hybrids

Authors: Medini Padmanabhan, Kallol Roy, Gopalakrishnan Ramalingam, Srinivasan Raghavan and Arindam Ghosh

In this study we explore avenues for the integration of graphene with various semiconductors for optoelectronic applications. First, we present an electrochemical route for the growth of light sensitive copper-based alloys on graphene. Graphene grown using chemical vapor deposition (CVD) transferred to glass is found to be a robust substrate on which photoconductive Cu_{x}S films of 1-2 micron thickness can be deposited. Second, we present studies which show that the two-point resistance of graphene is modified in the presence of another semiconductor, MoS2, on its surface. MoS2 is an interesting semiconductor, the nature and value of whose bandgap depends on the number of atomic layers in the system. We also present a systematic study of the optical response of the two-point resistance of MoS2 flakes as a function of number of layers.

Measurement of Electron Spin Lifetime and Optical Orientation Effeiciency in Germanium Using Electrical Detection of Radio Frequency Modulated Spin Polarization

Chinkhanlun Guite and V. Venkataraman Department of Physics, Indian Institute of Science, Bangalore-560012, India

We propose and demonstrate a technique for electrical detection of polarized spins in semiconductors in zero applied magnetic fields. Spin polarization is generated by optical injection using circularly polarized light which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time- varying magnetic field which is detected by a sensitive radio-frequency coil. Using a calibrated pick-up coil and amplification electronics, clear signals were obtained for bulk GaAs and Ge samples from which an optical spin orientation efficiency of 4.8% could be determined for Ge with 1342nm excitation wavelength at 127K. In the presence of a small external magnetic field, the signal decay according to the Hanle-effect, from which a spin lifetime of 4.6±1.0 ns for electrons in bulk Ge was extracted. Abstract for Poster Inhouse Symposium 2011

DNA compaction by dendrimer

Bidisha Nandy , Prabal K Maiti

At physiological pH, a PAMAM dendrimer is positively charged and can effectively bind negatively charged DNA. Currently there has been great interest in understanding this complexation reaction both for fundamental (as a model for complex biological reactions) as well as for practical (as a gene delivery material and probe for sensing DNA sequence) reasons. We have studied the complexation between double stranded DNA (dsDNA) and various generations of PAMAM dendrimers (G3-G5) through atomistic molecular dynamics simulations in presence of water and ions. We report the compaction of DNA on nanosecond time scale. This is remarkable given the fact that such a short DNA duplex of length close to 13 nm is otherwise thought to be a rigid rod. Using several nanoseconds long MD simulations we have observed various binding modes of dsDNA and dendrimer for various generations PAMAM dendrimers at varying charge ratio and confirms some of the binding modes proposed earlier. The binding is driven by the electrostatic interaction and larger the dendrimer charge stronger the binding affinity. As DNA wraps/binds to the dendrimer, counter ions originally condensed onto DNA (Na+) and dendrimer (Cl-) get released. We calculate the entropy of counter ions and show that there is gain in entropy due to counter ion release during the complexation. MD simulations demonstrate that when the charge ratio is greater than 1 (as in the case of G5 dendrimer) the optimal wrapping of DNA is observed. Calculated binding energies of the complexation follow the trend G5>G4>G3 in accordance with the experimental data. For lower generation dendrimer like G3 and to some extent for G4 also we see considerable deformation in the dendrimer structure due to their flexible nature. We have also calculated the various helicoidal parameters of DNA to study the effect of dendrimer binding on the structure of DNA. B form of the DNA is well preserved in the complex as is evident from various helical parameters justifying the use of PAMAM dendrimer as a suitable delivery vehicle. Over 100-fold increase in strain sensitivity of a metal based piezoresistive MEMS transducer through nanoscale inhomogenization

S. M. Mohanasundaram1, Rudra Pratap2 and Arindam Ghosh1

1Department of Physics, Indian Institute of Science 2Center for Nano Science and Engineering, Indian Institute of Science

Abstract:

Metal-based electromechanical sensing devices could find a much wider applicability if their sensitivity to mechanical strain could be substantially improved. In spite of direct integrability to suboptical scale structures, convenient interfacing with high frequency readout, and, above all, simplicity in device fabrication process, metals are unpopular in micro and nano-electromechanical systems (MEMS and NEMS) because their strain sensitivity is one to two orders of magnitude lower in comparison to common semiconductor-based piezoresistors. Here, we demonstrate a simple method to enhance the strain sensitivity of metal films integrated to specially designed micro-cantilevers by over two orders of magnitude. By locally inhomogenizing thin gold films using controlled electromigration, we have achieved a logarithmic divergence in the strain sensitivity with progressive microstructural modification. The enhancement in strain sensitivity could be attributed to a non-universal tunneling-percolation transport, which creates a robust platform to engineer low resistance, high gauge factor metallic piezoresistors that may have profound impact on nanoscale self-sensing technology. Fermionic Superfluid State in an Optical Lattice via Band-Insulator to Superconductor Transition

Yogeshwar P. Saraswat, Vijay B. Shenoy Abstract We propose a model to realize a fermionic superfluid state in an optical lattice, which is one of the central problems in the area of cold quantum gases due to cooling problem. The idea hinges on using a characteristically low entropy system, a band-insulator and tuning the interaction in such a system to realize a superfluid state. By performing a detailed mean field analysis of the model, we show that the superfluid state is indeed stable and other competing phases like charge density wave, supersolid, will not come into picture.

1 A 10 Tesla Table-top Controlled Waveform Magnet

Aditya N. Roy Choudhury and V. Venkataraman

Department of Physics, Indian Institute of Science, Bangalore 560012

ABSTRACT

High magnetic fields are indispensable for modern-day solid-state research and pulsed magnets have been an essential part of it. [1] However, one limitation of the pulsed magnet is that its field shape with time is fixed for a given set-up. Controlled Waveform Magnets (CWMs) are a class of pulsed magnets from which one can obtain longer magnetic field pulses whose shapes with time can also be varied. [2] In the past few decades, some CWMs across the world have been reported [3,4] and new horizons of research have opened up [5,6]. A CWM also provides additional advantages over a standard pulsed magnet in terms of: minimizing pick-up in the sample wirings and better averaging of sample points in a semi-continuous field, and pulse-to-pulse reproducibility. However, a handful of CWMs exist across the globe and this scarcity is owed to the complexity of the elaborate power supplies that these magnets use. In this work we present a capacitor-driven, table-top CWM that can produce user-defined magnetic field shapes up to 10 Tesla. A 60 mF / 450 V capacitor bank is discharged into the magnet coil (wound with copper wire) by Insulated Gate Bipolar Transistors (IGBTs) that act as high current switches. The IGBT and its driver allow a maximum switching frequency, given a particular set of conditions, so one is left with a finite ripple on the magnetic field waveform that result out of this regulation. The charging voltage value determines the pulse width of the user- defined pulse for a given pulse shape. Lower magnetic field pulse shapes can also be produced for longer periods of time at a given charging voltage. The table-top CWM can produce any user-defined magnetic field waveform as long as the peak value and the slope of the user- reference is lesser than the peak magnetic field obtainable from the coil and the maximum rate at which magnetic field can rise or fall through the coil respectively.

REFERENCES

1. D. B. Montgomery, “Pulse Magnets”, in Solenoid Magnet Design , Wiley-Interscience, 1969. 2. L.J. Campbell and J.B. Schillig, “Controlled Waveform Magnets”, in High Magnetic Fields Science and Technology Vol.1, edited by Fritz Herlach and Noboru Miura, World Scientific Publishing, 2003. 3. R. Grossinger et al, Physica B 294-295 , 555-561 (2001). 4. L.J. Campbell et al, Physica B 216 , 218-220 (1996). 5. G.S. Boebinger et al, Physica B 294-295 , 512-518 (2001). 6. J.J.M. Franse et al, Physica B 201 , 217-226 (1994). 7. http://www.irf.com/technical-info/appnotes/an-990.pdf Unconventional noise in two dimensional doped Silicon

Saquib Shamim1, Suddhasatta Mahapatra2, Craig Polley2, Michelle Y. Simmons,2 and Arindam Ghosh1

1Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2 Centre for Quantum Computer Technology, University of New South Wales, Sydney NSW 2052, Australia

As classical information processing technology approaches the sub-20nm node, it is becoming increasingly important to control the exact number and position of dopants in electronic devices. Recent progress in using scanning tunnelling microscopy (STM) as a lithographic tool allows positioning of dopants with atomic scale precision. Combined with molecular beam epitaxy, this technology has been employed to realize heavily δ-doped planar nanostructures, such as tunnel gaps, nanowires and quantum dots. The same approach can also be used to fabricate vertically-stacked, multiple electrically-active layers. The time averaged transport properties of Si:P δ-doped layers have now been studied in detail, but very little is known about its long term charge stability. The importance of this issue is paramount to the overall development of devices with controlled dopant positioning at the nano-scale and in particular for single dopant spin based qubits.

We perform low frequency noise measurements in degenerately doped Si:P δ-layers and find that the noise magnitude in these systems is suppressed by several orders of magnitude as compared to bulk doped metallic silicon and is one of the lowest values reported for doped semiconductors. Though magneto-conductivity data at low temperatures indicate weak localisation, the noise magnitude is nearly constant with magnetic field at low fields. The noise magnitude reduces by a factor of two at high fields (Zeeman field) in both parallel and perpendicular magnetic field. This might indicate that the low temperature Hamiltonian of these heavily doped systems has very different symmetry properties than conventional disordered conductors. Real Space Distribution of Ultra-cold Bosons in an Optical Lattice at a Finite Temperature 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: October 28, 2011)

Ultra-cold bosonic atoms trapped in deep optical lattices are well described by the Bose- Hubbard model, in which atoms can hop to nearest neighbour sites with amplitude t and have onsite repulsion U. In addition there is an overall harmonic trap present. This causes an effective chemical potential variation throughout the lattice, which results in coexistence of consecutive annular regions of Mott and superfluid phases. Recently, real space density profiles of atoms in optical lattices has been measured experimen- tally for finite temperature systems. To compare with the experimental data a perturbative strong coupling (t/U) expansion at finite temperature and finite U is carried out upto second order about the Mott phase including superfluidity at mean field level by using mean field decomposition of the kinetic energy term of the Bose Hubbard model. We have also calculated the entropy per particle, which is a relevent experimental quantity, upto second order and the dominant contribution to it from different parts of the overall trap at very low temperatures.

∗Electronic address: [email protected] †Electronic address: [email protected] ‡Electronic address: [email protected] Title: Tuning between an antidot lattice and quantum dot lattice in a double-gated GaAs/AlGaAs heterostructure

Author: M. A. Aamir, Srijit Goswami, Saquib Shamim, Christoph Siegert, Michael Pepper, Ian Farrer, David A. Ritchie, Arindam Ghosh

Abstract: A two-dimensional electron gas (2DEG) formed in a GaAs/AlGaAs heterostructure offers an outstanding platform to study a wide range of physical phenomena. By imposing the appropriate electric potential variation on a 2DEG via gating one can either make a periodic array of antidots or quantum dots. Antidots act as scatterers and therefore allow for a study of electron dynamics. On the other hand, a quantum dot lattice provides the opportunity to study correlated electron physics at accessible energy scales. Building on this, we have designed a double-gated structure which gives an unprecedented control over the potential landscape in the 2DEG. We can conveniently form an antidot lattice or a quantum dot lattice in the same device with the right choice of gate voltages. We use a variety of electrical measurements such as conductance, thermo-voltage and current-voltage characteristics to probe these two contrasting regimes. Ultracold Fermions with Artificial Rashba Spin-Orbit Coupling

Jayantha P. Vyasanakere and Vijay B. Shenoy

Cold atomic gases enjoy the prospect of shedding light on outstanding and longstanding puzzles of quantum . The recent gener- ation of synthetic Rashba spin-orbit coupling (SOC), which is equivalent to a non-Abelian gauge potential, in 87Rb atoms by the NIST group is a landmark in that respect. Motivated by these developments, we theoretically investigate interacting fermions with spin-orbit coupling. The clue that fermions with SOC admits novel physics stemmed up from the study of the two-fermion problem in three spatial dimensions. In absence of SOC the two-body problem permits a bound state only if the attraction between them exceeds a threshold value and the binding energy is independent of their center of mass momentum. Remarkably, SOC induces a two-body bound state, in the zero center of mass momentum sector, for any attraction, however small. In contrast to this, for center of mass momenta much larger than the SOC, any attraction, however large fails to produce the two-body bound state. The two-body bound state wave function has a triplet content and associated spin-nematic structure similar to those found in liquid 3He. In absence of SOC, it is known that the ground state of the weakly attracting many-body problem is a BCS superfluid with large overlapping pairs. As the attraction is increased, the system undergoes a crossover to a BEC of tightly bound fermion-pairs. Our study reveals that at a fixed attraction, however small, increasing SOC invokes a crossover from the just discussed BCS superfluid state to a new type of BEC state. The BEC state that emerges is a condensate of bosons which are tightly bound pairs of fermions. Remarkably, at large SOC, the nature of these bosons is determined solely by the Rashba SOC and is neither influenced by the attraction nor by the density of particles – hence these bosons are referred to as rashbons. For a general SOC, rashbons have exotic properties like anisotropic dispersion. Our study estimates the transition temperature of the BEC of rashbons and suggests a route to enhance the exponentially small transition temperature of the system with a fixed weak attraction to the order of the Fermi temperature by tuning the strength of SOC.

References : arXiv: 1101.0411, 1104.5633, 1108.4872.

1 Heteronuclear Correlation between Carbon and Double Quantum Proton Chemical shifts in Solids

Y.Jayasubba Reddy a, b and K.V.Ramanathan a c

a NMR Research Centre, b Department of Physics, Indian Institute of Science, Bangalore-560012, India

c Corresponding Author : [email protected]

In recent times, Solid State NMR (ssNMR) spectroscopy has dramatically overcome the severe limitations in resolution and sensitivity that had limited its application to biomolecules and has established itself as an important tool for studying large and poorly soluble systems such as membrane proteins and protein fibrils. NMR studies in the solid state have been hampered mainly by the presence of very large 1H-1H dipolar couplings. Currently available structures in solids mainly rely on measurements made for rare spins, though 1H chemical shifts and 1H-1H proximities are being increasingly used for the above purpose. However, direct detection of protons does not provide enough resolution even with the use of windowed sequences, which require additional time and effort for optimization. We are, therefore, exploring a method for obtaining correlation between double quantum proton resonances and single quantum carbon resonances in which one of the proton is coupled to a carbon. Correlations based on both scalar and dipolar couplings are being explored. Such experiments have the following advantages, viz., (a) provide more definite assignments; (b) uncertainties in proton chemical shifts can be reduced by making use of the redundant information available due to possibilities of several DQ cross peaks present for the same carbon; (c) provide chemical shifts of protons, bonded to some other atoms such as the amide protons; (d) enable 13C-13C correlation. The application of this technique to several amino acids and possible applications to small peptides with definite structures will be presented. Abstract for poster

Topological Insulator: Basic concepts and preliminary results

Mitali Banerjee 1 ,Semonti Bhattacharyya 1,Nethra C2,

N. Ravishankar 2, Arindam Ghosh 1

1Dept. of Physics, 2Material Research Centre,

IISc,Bangalore - 560012

Topological insulators, a new quantum state of matter, have conducting surface states apart from insulating bulk states and those surface states are quite robust to disorder. Such surface states have been predicted to host various exotic quantum phenomenon and quasi particle excitations. Angle Resolved Photo Emission Spectroscopy (ARPES) has been an efficient tool to find such surface states in quite a few materials but probing of such states by electrical transport studies has been pretty challenging so far. Preliminary electrical transport measurements are performed on Bi 2Se 3, BiSbTe 3 nanocrystals. These nanocrystals are prepared in microwave-stimulated solvo-thermal method and been characterized by X-ray diffraction(XRD), tunneling electron microscopy (TEM), atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) etc.

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com Trapped fermions in a synthetic non-Abelian gauge field

Sudeep Kumar Ghosh,∗ Jayantha P. Vyasanakere,† and Vijay B. Shenoy‡ Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India

On increasing the coupling strength (λ) of a non-Abelian gauge field that induces a generalized Rashba spin-orbit interaction, the topology of the Fermi surface of a homogeneous gas of noninter- 3 acting fermions of density ρ ∼ kF undergoes a change at a critical value, λT ≈ kF [Phys. Rev. B 84, 014512 (2011)]. In this paper we analyze how this phenomenon affects the size and shape of a cloud 1 of spin- 2 fermions trapped in a harmonic potential such as those used in cold atom experiments. We develop an adiabatic formulation, including the concomitant Pancharatnam-Berry phase effects, for the one particle states in the presence of a trapping potential and the gauge field, obtaining approx- imate analytical formulae for the energy levels for some high symmetry gauge field configurations of interest. An analysis based on the local density approximation reveals that, for a given number of particles, the cloud shrinks in a characteristic fashion with increasing λ. We explain the physical origins of this effect by a study of the stress tensor of the system. For an isotropic harmonic trap, the local density approximation predicts a spherical cloud even for anisotropic gauge field configura- tions. We show, via a calculation of the cloud shape using exact eigenstates, that for certain gauge field configurations there is systematic and observable anisotropy in the cloud shape that increases with increasing gauge coupling λ. The reasons for this anisotropy are explained using the analyt- ical energy levels obtained via the adiabatic approximation. These results should be useful in the design of cold atom experiments with fermions in non-Abelian gauge fields. An important spin-off of our adiabatic formulation is that it reveals exciting possibilities for the cold-atom realization of interesting condensed matter Hamiltonians by using a non-Abelian gauge field in conjunction with another potential. In particular, we show that the use of a spherical non-Abelian gauge field with a harmonic trapping potential produces a spherical geometry quantum hall like Hamiltonian in the momentum representation.

PACS numbers: 03.75.Ss, 05.30.Fk, 67.85.Lm

∗ Electronic address: [email protected] † Electronic address: [email protected] ‡ Electronic address: [email protected] Title: Microfluidic Devices For Measuring and Exerting Micro-Newton Forces For Biological Applications

Author: Siddharth Khare

Research Advisor: Prof. V. Venkataraman

Abstract:

Forces right from pico-newton to mega newton are observed in nature. Usually we do not feel the forces below a few tens of milli newton since they are too small compared to most of the forces to which we are subjected. But the microorganisms deal with much smaller forces and their activities are largely influenced by such forces. Therefore measuring and exerting these small forces is of interest to biologists.

The forces exerted by microorganisms of sizes around a few 100's of microns are typically in 10's of micronewton range. We are trying to develop sensors and actuators for measuring and exerting forces in this range.

We use photo-lithography on a photo-sensitive polymer SU-8 (from Micro-chem). The structures made in SU-8 are used for replica molding of PDMS, which is a transparent and bio-compatible polymer. The molded PDMS forms the final device. For actuation, we use Iron-PDMS mixture and external magnetic field. These devices are flexible pillars of dimensions 50µm X 50µm X 250µm.

For biology part, we are working in collaboration with Prof. Sandhya Kaushika (NCBS). We use the micro-force sensors and actuators to probe a nematode called Caenorhabditis Elegans in order to study the neuro-biological problems.

References:

1. Joseph C. Doll et. al., Lab Chip, 2009, 9, 1449–1454 2. F M Sasoglu et al., J. Micromech. Microeng. 17 (2007) 623–632 3. Jimmy le Digabel et. al., Lab Chip, 2011, 11, 2630 4. S. N. Khaderi et. al., Lab Chip, 2011, 11, 2002 Nature of Electronic States in Ultrathin MoS2 Field Effect Transistor

Subhamoy Ghatak, Atindra Nath Pal and Arindam Ghosh

Molybdenum disulphide (MoS2) is a transition metal dichalcogenide. It is a layered material with a Mo layer sandwiched between two S layers (S-Mo-S), which forms the basic unit of its trigonal prismatic structure. Though the intra-layer bonding in a unit is quiet strong, each S- Mo-S unit is attached to other S-Mo-S units only with weak Van der Waals force. This enables to make an atomically thin single layer of MoS2. It is an indirect bandgap (1.2 eV) semiconductor in bulk, and becomes a direct bandgap (1.9 eV) semiconductor in single layer form. The presence of bandgap has made it an interesting material in thin film transistors as channel material. It has been reported [1] recently that very high on/off ratio (~108) can be obtained in single layer MoS2 transistor due to the presence of this bandgap. But though the on/off ration is very high, mobility in these transistors is considerably law compared to those commercial Si transistors. Here we have investigated the origin of such low mobility. From our temperature dependent study we find that atomically thin MoS2 layer becomes highly disordered in the presence of the substrate and electron got localised in the traps created by the charge impurities at substrate-MoS2 interface. We propose that high mobility can be obtained in these transistors by removing the charge impurity background.

[1] Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nature Nanotechnology 2011, 6, 147–150.

Email: [email protected]

Design of a polymer based Infra-red sensor

Gurucharan V. Karnad and V. Venkataraman Department of Physics, Indian Institute of Science, Bangalore-560012, India

We propose to demonstrate the detection of Infra-red radiation using the principles used in the Golay

Cell (1). It is based on the detection of the deflection of a membrane caused due to the thermal expansion of the gas trapped in a cavity. The temperature in the cavity is elevated due to the radiations absorbed by the absorber material, which in turn heats the gas, thus causing it to expand. While this principle has been well demonstrated in the form of a single element Golay cell, there has not been research demonstrating an array of Golay cells due to the inherent thermal cross talk produced due to high thermal conductivity of silicon. The low thermal crosstalk in PDMS based arrayed devices has been demonstrated (2) in research involving design of polymer based micro-mirror devices.

We therefore propose to build an array of Golay Cells using Poly(dimethylsiloxane) (PDMS) to reduce the thermal crosstalk. We have fabricated a PDMS based sensor by replica molding of the SU-8 structures, which were used as masters. Carbon Nanotubes (2) have been used to act as the infra-red absorber material, while a 60µm PDMS membrane has been used as the detection membrane. Preliminary results, involving the deflection of the detection membrane with variation in external temperature have been observed and measured using a Coherence Correlation Interferometer.

References 1. Pneumatic Heat Detector . Golay, Harold A. Zahl AND Marcel J. E. 11, s.l. : The Review of Scientific Instruments , November,1946, Vol. 17. 2. Thermo-pneumatically actuated, membrane-based micro-mirror devices. Zappe, Armin Werber and Hans. s.l. : J. Micromech. Microeng, 2008, Vol. 16. 3. Multiwall carbon nanotube absorber on a thin-film lithium niobate pyroelectric detector. John H. Lehman, Katherine E. Hurst, Antonije M. Radojevic, Anne C. Dillon, Richard M. Osgood, Jr. 7, s.l. : Optics Letters, April, 2007, Vol. 32. 1/f noise as a probe to investigate the band structure and quantum interference in graphene

Vidya Kochat1, Atindra Nath Pal1, Subhamoy Ghatak1, Sneha E. S.1, Arjun B. S.2, Srinivasan Raghavan2 and Arindam Ghosh1

1Department of Physics, Indian Institute of Science, Bangalore-560012, India. 2Materials Research Centre, Indian Institute of Science, Bangalore-560012, India.

Abstract

The flicker noise or the 1/f noise arises from the slow fluctuations in channel conductivity which can impact the utility of field effect transistors and limit their performance. But here, we show that the 1/f noise can be used as a very sensitive and robust probe to study the coupling of disorder dynamics to electronic transport in graphene in the different temperature regimes. In the higher temperature regime ( > 77K ), we find that the noise originates mainly from a fluctuating charge distribution in the vicinity of graphene and is a very sensitive electrical transport-based probe to study the energy band dispersion of graphene, thus being able to distinguish between single and multi-layer graphene. At low temperatures, where Universal Conductance Fluctuations (UCF) is the dominant mechanism responsible for 1/f noise, we discuss the implications of 1/f noise in understanding the time reversal (TR) symmetry present in graphene. Active smectics

Tapan Chandra Adhyapak,1 R Aditi Simha,2 Sriram Ramaswamy,1 and John Toner3

1Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India 3Department of Physics and Institute of Theoretical Science, University of Oregon, Eugene, OR 97403, USA (Dated: October 28, 2011) We show that smectic liquid-crystalline order in systems of active particles is long-ranged in dimensions d = 3 and quasi-long-range in d = 2.This is in contrast to the situation for systems at thermal equilibrium with the same spatial symmetry. We find that strong enough active stresses can disrupt smectic order, through instabilities whose character is oscillatory if the stresses are extensile. Our results apply to any active system that spontaneously develops one-dimensional spatial periodicity, including driven monolayers of rods and the Rayleigh-B´enardinstability. 2T 3.

Gopal K Pradhan, Achintya Bera , Pradeep Kumar, D V S Muthu and A K Sood.

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

In recent years, high-pressure studies of technological important Bi 2Te 3 have revealed giant improvement of thermoelectric power factor, superconductivity and reconstruction of Fermi surface topology giving rise to an electronic topological transition ( ETT ). We report Raman signatures of electronic topological transition ( ETT ) at 3.6 GPa and rhombohedral ( α-Bi 2Te 3) to monoclinic ( β-Bi 2Te 3) structural transition at ~ 8 GPa. At the onset of ETT , a new Raman mode appears near 107 cm -1 which is dispersionless with pressure. The structural transition at ~ 8 GPa is marked by a change in pressure derivative of A1g and E g mode frequencies as well as by appearance of new modes near 115 cm -1and 135 cm -1. The mode Grüneisen parameters are determined in both α and β-phases. DNA and Dendrimer assisted dispersion of nanotubes

Debabrata Pramanik and Prabal K. Maiti.

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

560012, India.

DNA and dendrimers are emerging out to be strong dispersive agents for nanotubes from the bundle geometry. To understand the efficiency of DNA and dendrimer in reducing the binding strength of bare nanotube we have calculated potential of mean force (PMF) between two single walled nanotubes which are wrapped either by single stranded DNA or PAMAM dendrimer. The calculated

PMF between dendrimer nanotube is ~ 8 kcal/mol in contrast to the effective interaction of ~30 kcal/mol between two bare nanotube. We find that poly A DNA wrapped nanotube has almost no binding affinity as revealed by the PMF calculation. These results demonstrate that DNA is most effective in nanotube dispersion. Dendrimer can also be use as dispersing materials as it lowers the effective interaction between nanotubes significantly. Optical and Electrical Investigation of CdTe QDs/PDDA Bistable Devices

Sandip Mondal, V. Venkataraman Department of Physics, Indian Institute of Science, Bangalore 560 012

Electrical bistability is a phenomenon in which a device exhibits two states of different conductivities at the same applied voltage. This behaviour is ideal for switch-able memory applications and has been widely studied recently. Quantum dots are promising candidates for future memory devices due to their attractive features like miniaturized dimensions and the possibility for nano-scale design through chemical synthesis. In recent years, A. J. Pal and his group have observed electrical bistable phenomena in various types of semiconductor and metallic nano-particle films such as CdS, PbS, CdSe and Au. In case of CdSe nano-particles, they have shown that higher the quantum confinement more is the electrical bistability. On the basis of that, we have investigated the electrical bistable phenomena in CdTe QDs devices. We are trying to model the reason of bistability by transport phenomena in different device geometry. CdTe QDs are studied as a subclass of CdSe QDs, but in the former case one can achieve strong quantum confinement more easily due to their large Bohr exciton radius. CdTe QDs were synthesized via colloidal hydrothermal route. This is a one pot synthesis method carried out in a Teflon lined stainless steel autoclave. In a typical procedure, devices for electrical bistable measurements were fabricated by spin coating a cationic polymer (PDDA) and MPA-capped CdTe QDs (anionic nature) on ITO coated glass substrate via layer by layer (LbL) deposition using the principle of electrostatic attraction. We have performed photoluminescence and optical absorption spectroscopic measurements for the devices prepared with a different number of CdTe/PDDA bilayers in order to investigate the influence of the film thickness on the electronic and optical properties. The excitation wavelength was 400 nm for 15 different devices of (CdTe-PDDA)*n with n=10,20, 30, 40 and 50, respectively. An increase in photoluminescence and optical absorption intensity was observed for the 50-bilayer film compared to the 30-bilayer devices. However, the intensity was much lower for the sample of 10 bilayers. So an clear increment of the intensity has been found with increasing number of bilayers. From the first excitonic peak of the absorption spectra the size of the QDs were calculated. We recorded the current-voltage (I-V) characteristics of the CdTe QDs films using Pt/Ir tips of STM. A typical set of I-V plot for a voltage has been found. For the devices, I-V characteristic shows an electrical bistability with low conducting states when the voltage is swept from 0 to +2.5 V and high conducting states with voltage sweep from +2.5 to 0 V in forward bias direction and vice-versa in reverse bias direction. The switching is reversible and cyclic and the dependence of I-V characteristics with sweep direction results in two values of current at one voltage and is generally quantified as on/off ratio. An interesting phenomenon has been observation is that the bistability increases as the size of CdTe QDs decreases, similar to what has been reported for CdSe QDs.

Acknowledgements:

Jayakrishna Khatei and K. S. R. K. Rao, Department of Physics, Indian Institute of Physics, Bangalore 560 012

Reference:

B. Pradhan, S. K. Batabyal, and A. J. Pal, J. Phys. Chem. B 110, 8274 (2006). S. Sahu, S. K. Majee and A. J. Pal, Applied Physics Letters 91, 143108 (2007). B. C. Das and A. J. Pal, ACS Nano 2, 1930 (2008).

1 Fabrication of tunable potential barrier in Bilayer graphene

T Phanindra Sai, Arindam Ghosh

Department of Physics, Indian Institute of Science, Bangalore.

Bilayer graphene which shows a bandgap opening on application of dual electronic gating has emerged as a prospective candidate for device applications as well as for validating many theoretical predictions. It has been recently predicted that using quantum point contact (QPC) structure on a bilayer graphene, in the first conductance plateau the QPC produces a strong polarization of valleys, provided the constriction has zig-zag edges along the direction of current flow. Their findings signify that two valleys can be individually addressed as independent internal degrees of freedom of conduction electrons by controlling the gate voltage. There is also a theoretical prediction that the edge band dispersion of graphene can be continuously changed by tuning the onsite energies on the boundary of the system. And under certain values of boundary potential, the edge band can completely merge with the bulk band or become valley dependent gapless chiral modes. These chiral modes are interesting because they are analogues to edge states in quantum spin hall effect (QSHE), where the valley index playing the role of spin.

As first step to realise device structures with which these theoretical predictions can be validated, we fabricated bilayer graphene devices using state of the art lithographic technique, with top split gates as QPC structures over the bilayer as well as across the edge using cross-linked PMMA as dielectric between the top gate and bilayer graphene. Transport studies were performed on such devices at low temperatures which showed that the bandgap was tunable using both top and bottom gates, but the presence of cross-linked PMMA lead to low mobility in the devices. To overcome the low mobility issue we had fabricated elevated split gates over the bilayer graphene with air gap as dielectric. Preliminary transport measurements indicated considerable improvement in mobility. Experiments are underway to measure the transport properties at low temperatures of the elevated split gated devices.

DBT: A versatile Dendrimer Building ToolKit

Vishal Maingi 1, Vaibhav Jain 2, Mattaparthi V. Satish Kumar 1, Prasad V. Bharatam 2 and Prabal K. Maiti 1*

1Center for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India

2Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research, Sector 67, S.A.S. Nagar, Punjab 160 062, India

We have developed a graphical user interface (GUI) based Dendrimer Builder Toolkit (DBT) which can be used to generate the dendrimer configuration of desired generation for various chemical functionality. The validation of structures generated by this tool was carried out by studying the structural properties of polyamidoamine (PAMAM, G3-G6 generations) and poly(propyl) imine (PPI, G5 generation) dendrimers. We also studied the interaction and release pattern behaviour of ligands (two soluble drugs namely Salicylic acid; Sal, L-Alanine; Ala and two insoluble drugs namely Phenylbutazone; Pbz, Primidone; Prim) with G5 PAMAM dendrimer using atomistic MD simulations. We have computed the Potential of Mean Force (PMF) variation with G5-PAMAM dendrimer complexed with drug molecules. The order of ease of release pattern was found to be: Ala > Sal > Prim > Pbz. We find that PMF energy barrier to be less for the drug when it is bound to non protonated dendrimer compared to the case of protonated dendrimer. Our results suggest that encapsulation of drug molecule into the host PAMAM dendrimer should be carried out at higher pH values (near pH 10). Development of Methodologies in Ex-Situ NMR Spectroscopy

KowsalyaDevi Pavuluri and K.V.Ramanathan

NMR Research Centre, Department of Physics

Indian Institute of Science, Bangalore-560012, India.

Nuclear magnetic resonance is an extremely versatile form of spectroscopy because of its precision, selectivity and noninvasiveness. A major limitation of NMR for getting high resolution is the requirement of strong and extremely homogeneous magnets. Portable NMR systems have been built with open single sided probes for studying objects or samples whose size is limited to fit inside the bore of the magnet. But their use remained mainly for product and quality control since spectroscopic information could not be recovered due to very large inhomogenities. Many techniques have been proposed to regain high resolution spectral information in presence of inhomogeneous magnetic field. Nutation echo is one of the novel techniques in which RF field gradients and static field gradients are matched to refocus static inhomogenities where as the full chemical shift information is maintained. To increase resolution and sensitivity we need to develop new methodologies which are of potential use for NMR spectroscopy in an inhomogeneous magnetic field produced by Ex-Situ surface Spectrometer. We are developing methods based on RF pulse sequences which will be advantageous in some of the need for added design complexity, RF power and time. For that we started with producing Ex-Situ NMR conditions in our spectrometer. Computational approach based on Matlab also being done to study such methodologies theoretically.

References: 1. Carlos A. Meriles et.al Science 293, 82-85, (2001). 2. Henrike Heise et.al Journal of Magnetic Resonance 156, 146-151, (2002). 3. Shuhui Cai et.al Current Analytical Chemistry 5, 70-83, (2009). 4. Dimitris Sakellariou et.al C.R. Physique 5, 337-347, (2004). Abstract for poster Strongly correlated transport in ultrathin gold nanowires

U. Chandni*, Marsha M. Parmar*, Paromita Kundu#, Subhojit Kundu#, Abhishek K. Singh#,

N. Ravishankar# and Arindam Ghosh*

*Department of Physics and #Materials Research Center,

Indian Institute of Science, Bangalore 560012, INDIA.

The electrical properties of metallic nanostructures, which find a range of applications in nanoscale devices to circuits, are generally understood in terms of Fermi liquid theory. Here we demonstrate a complete breakdown of the Fermi liquid description in single crystalline ultra-thin gold nanowires. These nanowires were fabricated via an oriented attachment process whereby nanoparticles of appropriate dimensions join in a linear fashion to form long stable wires. Electrical transport measurements and electrostatic gating effects in these wires are strongly suggestive of a non-Fermi liquid behavior and the emergence of collective excitations belonging to the Tomonaga Luttinger liquid (TLL). The linear response electrical resistance exhibits a power-law dependence on temperature, and the variation of current over a wide range of temperature and voltage obeys a universal scaling relation that provide compelling evidence for a TLL behavior. Our experiments also show that, by simple variations in the substrate-supported growth process, either power law or variable range hopping (characteristic of localized states) can be achieved with excellent control without any significant change in the geometry and crystallinity of the wires. The results thus establish TLL and localization, as experimentally distinguishable ground states in appropriately designed strongly correlated quasi-1D systems. We attribute this tunability to the subtle substrate-nanowire interactions that change from one method of growth to another.

References:

 V. V. Deshpande, M. Bockrath, L. Glazman and A. Yacoby, Nature 464, 209 (2010).

 L. Venkataraman, Y. S. Hong and P. Kim, Phys. Rev. Lett. 96, 076601 (2006).

 U. Chandni, P. Kundu, A. K. Singh, N. Ravishankar and A. Ghosh, ACS Nano, 10.1021/nn2031935 (2011).

 U. Chandni, P. Kundu, N. Ravishankar and A. Ghosh (Under Review)

Strongly magnetized cold electron degenerate gas: Mass-radius relation of the collapsed star

Upasana Das, Banibrata Mukhopadhyay Department of Physics, Indian Institute of Science, Bangalore 560012, India E-mail: [email protected], [email protected]

Abstract. We consider a relativistic, degenerate electron gas at zero-temperature under the influence of a strong, uniform, static magnetic field, neglecting any form of interactions. Since the density of states for the electrons changes due to the presence of the magnetic field (which gives rise to Landau quantization), the corresponding equation of state also gets modified. In order to investigate the effect of very strong magnetic field we focus only on systems in which a maximum of either one, two or three Landau level(s) is/are occupied. This is important since, if a very large number of Landau levels are filled, it implies a very low magnetic field strength which yields back Chandrasekhar’s celebrated non-magnetic results. The maximum number of Landau levels occupied is fixed by the correct choice of two parameters, namely the magnetic field strength and the maximum Fermi energy of the system. We study the equations of state of these one-level, two-level and three-level systems and compare them by taking three different maximum Fermi energies. We also find the effect of the strong magnetic field on the mass-radius relation of the underlying star composed of the gas stated above. We obtain an interesting theoretical result that, it is possible to have an electron degenerate static star with a mass significantly greater than the Chandrasekhar limit, provided it has an appropriate magnetic field strength and central density.

Keywords: degenerate Fermi gases, stellar magnetic fields, Landau levels, equations of state of gases, white dwarfs PACS: 67.85.Lm, 97.10.Ld, 71.70.Di, 51.30.+i, 97.20.Rp Membrane coupled to Active Fluid

Ananyo Maitra, Sriram Ramaswamy, Jean­Francois Joanny

We study the fluctuations of a membrane coupled to a fluid with suspended orientable particles endowed with “active” non equilibrium stresses that can arise, for example, in the cytoskeleton. The particles are apolar, and have a “soft” anchoring condition to the membrane, imposed via a free energy coupling. This, through the non­equilibrium stress term in the hydrodynamics of the bulk fluid, endows an initially tension free membrane with a dynamic, active tension. If the suspended “active” particles are allowed to be polar, the bending modulus of the membrane is renormalised. If there are “pores” in the membrane (a scalar species living in the membrane) such that the orientable particles prefer to cluster near them the purely passive “pores” act as “active pumps”. Temperature Dependent Sign Reversal of Magnetocrystalline Anisotropy in Nanoparticles of La0.875Sr0.125MnO3

Bhagyashree, K. S., Bhat, S. V.

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

Many properties of materials drastically change when they are prepared in nanoscale. For example, it has been recently shown that the charge ordered, antiferromagnetic manganite Nd0.5Ca0.5MnO3 loses its charge order and becomes ferromagnetic when the size of the particles is reduced to ~ 20-30 nm. In this report we present a ferromagnetic resonance study of magnetocrystalline anisotropy in nanoparticles (dia ~ 20 nm) of

La0.875Sr0.125MnO3(LSMO) as a function of temperature in the range 4 K – 300 K. We find that the nano LSMO shows negative uniaxial anisotropy at low temperatures which switches over to positive anisotropy at ~ 250 K. While composition dependent sign reversal of anisotropy is known to occur, to the best of our knowledge this is the first report of temperature dependent sign reversal of magnetocrystalline anisotropy.

The nanoparticles of LSMO were prepared using the sol-gel technique. The composition was checked with inductively coupled plasma emission spectroscopy. XRD and Rietweld fitting showed that the structure of the nanoparticles is rhombohedral analyzed using hexagonal space group R-3cH. TEM was used to find the particle size which was ~ 20 nm. SQUID magnetometry showed that the sample underwent a ferromagnetic transition at 300 K. The ferromagnetic resonance signals were recorded using a commercial EPR spectrometer between the temperatures 4 K - 300 K. FMR signal lineshape is a definitive indicator of the nature and sign of magnetocrystalline anisotropy of the sample. The signal shape of the LSMO nanoparticles at the lowest temperature used (i.e.4 K) is indicative of negative anisotropy. As the temperature is increased the lineshape gradually changes and around 250 K a signal characteristic of positive anisotropy is observed. We believe that the co-operative Jahn Teller transition occurring around room temperature in LSMO drives the sign change in the magnetocrystalline anisotropy.

Thermoelectric properties of PbTe with Bi precipitates

Ashoka Bali and Ramesh Chandra Mallik

Thermoelectric Materials and Devices Laboratory,

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

Abstract

Lead telluride is an established thermoelectric material in the temperature range 350K - 800K. The maximum figure of merit (zT) obtained in doped bulk PbTe is 1.5 at 773K [1], which is too less for commercial applications. Embedding precipitates in the bulk of PbTe is one method of increasing this zT. By doing this, a larger number of interfaces are introduced in the bulk, which is expected to reduce thermal conductivity by increased phonon scattering. In addition, embedding metallic precipitates has been theoretically predicted to enhance the power factor. Bismuth, being a semi-metal, and an n-type dopant in PbTe, comes across as a good choice for the precipitate element. Here, PbTe with precipitates of Bi embedded in the bulk of the material has been prepared by matrix encapsulation technique, where powdered PbTe and Bi were heated above the melting point of PbTe, and rapidly quenched in water. XRD result showed crystalline PbTe, even though Bi was not detected. Scanning electron micrographs confirmed the presence of a secondary phase in the bulk. Seebeck coefficient, electrical conductivity and thermal conductivity were measured from room temperature to 750K. Degenerate semiconductor behavior was observed from electrical conductivity data, which also showed a decrease with increasing Bi concentration. Negative values of Seebeck coefficient indicated n-type nature of the samples. Seebeck coefficient also showed a decrease with increase in Bi concentration, leading to an overall decrease in the power factor. Thus, no improvement in zT as compared to undoped PbTe was observed, for which the maximum value calculated was 0.8 at 725K.

Keywords: lead telluride, thermoelectric, matrix encapsulation, transport properties

Reference: J. P Heremans et al, Science 321, 554-557 (2008) Evidence for shift of rigidity percolation to higher coordination numbers

M. Prashantha and K. Ramesh Department of Physics, Indian Institute of Science, Bangalore - 560 012, India.

Chalcogenide glasses based on tellurium are difficult glass formers as they require higher cooling rates. They cannot be prepared over a wide composition range like Se based chalcogenide glasses by the melt quenching method. For example, in Ge-As-Te system, some compositions are difficult to form glasses as the cooling rate (102 K/s) achieved in normal melt quenching is not sufficient. Hence, there are two glass forming regions in Ge-As-Te system separated by these compositions. To achieve higher cooling rates and form glasses over a wide composition range without any discontinuity we rapidly quenched the melts with the help of a home built twin roller melt spinning apparatus.

In covalent network glasses, the bond bending and bond stretching constraints are balanced against the number of degrees of freedom available. At an average coordination number (Zav) = 2.40, the constraints and the degrees of freedom are balanced. Various properties as a function of average coordination number exhibit distinct changes at this point and it is called rigidity percolation threshold (RPT). Another threshold usually occurs at higher coordination number which is purely chemical in origin is called chemical threshold (CT). RPT occurs at Zav = 2.40 and CT occurs around 2.67. For example, in Ge-As-Se glasses (which is considered to be a model system for Se based glasses) exhibit RPT at Zav = 2.4 and CT at Zav = 2.67. In the same way Ge-As-Te can be considered as a model system for Te based glasses. But the difficulty with Te systems is the preparation of glasses over a wide range covering both the thresholds.

We could overcome this difficulty by quenching the melt of the glasses rapidly by melt spinning method. Thin flakes of Ge-As-Te glasses have been prepared covering average coordination numbers (Zav) from 2.20 to 2.85 in a single composition tie line for the first time. The glass transition (Tg) vs. average coordination number plots do not show any signatures of RPT at the critical coordination number Zav = 2.40, instead a maximum in Tg is observed at Zav = 2.70. This shift has been understood based on the modified constraint theory taking into the account of the metallic nature of tellurium. The metallic nature of Te affects the degree of covalent nature of the Ge-As-Te network, which in turn affects the balance between the constraints and the number of degrees of freedom available to the atoms. The metallic nature of Te, the decrease in the degree of covalency and the imbalance between constraints and the degrees of freedom shifts the RPT to higher coordination number = 2.70.

Topological Insulator on the Edge: Boundary Conditions Revisited

Amal Medhi∗ and Vijay B. Shenoy† Center for Condensed Matter Theory, Indian Institute of Science, Bangalore 560012, India (Dated: November 16, 2011) We present an analytic formulation for derivation of the boundary conditions and edge states in topological insulators with finite boundaries. By considering model Hamiltonians of topological insulators in the continuum limit, we show that there are two types of boundary conditions possible- the usual fixed boundary condition and a free boundary condition. By comparing with results from the corresponding tight binding model calculations, we argue that the appropriate boundary condition for topological insulators is the latter. We also elucidate the nature of edge states of a particular model (BHZ model) of topological insulators in detail.

PACS numbers: 71.10.Fd, 71.30.+h, 71.70.Ej, 73.20.At

∗ [email protected] † [email protected] Structural and magnetic transition in Fe1+yTe single crystals.

Dona Cherian 1, S. Röβler2 , U. Schwarz 2 , S. Wirth2 , H. L. Bhat1 and Suja Elizabeth1 . 1Department of Physics, Indian Institute of Science, Bangalore , India. 2 Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straβe, Dresden, Germany. Email : [email protected]

The discovery of superconductivity in iron pnictides, which posses a TC up to 56 K give a new momentum for research in the field of high TC superconductivity [1]. The binary member of the Fe superconductor family - Iron chalcogenides, gained much attention because of its unique properties. Single crystals of Fe1+yTe (y = 0.11,0.13) have been grown in our laboratory by modified Horizontal Bridgman method. The parent compound of Iron chalcogenides, Fe1+yTe is an antiferromagnet. In these compounds excess Fe occupies the 2c site in chalcogenide layer. It possesses a local moment which can interact with the Fe2Te2 layer resulting in a complex magnetic order[2]. We have investigated structural and magnetic properties of Fe1+yTe single crystals.

Magnetization , specific heat and high resolution Xray powder diffraction measurements done on two crystals of slightly different compositions (y = 0.11 and 0.13) reveal the splitting of phase transition . In contrast to the observed trend in Fe Pnictides, the structural transition in Fe1+yTe occurs at a lower temperature than the magnetic transition. High resolution powder pattern from synchrotron radiation clearly indicate the evolution of structural transition below the magnetic transition. The reversal in the sequence of phase transitions suggests a different microscopic coupling mechanism for Fe1+yTe than the Fe pnictide parent compounds[3].

References

[1]Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc. 130, 3296 (2008). [2]W. Bao, Y. Qiu, Q. Huang, M. Green, P. Zajdel, M. Fitzsimmons, M. Zhernenkov, S. Chang, M. Fang, B. Qian, et al., Phys.Rev.Lett. 102, 247001 (2009). [3] S. Rößler, Dona Cherian, W. Lorenz, M. Doerr, C. Koz, C. Curfs, Yu. Prots, U. K. Rößler, U. Schwarz, Suja Elizabeth, and S. Wirth, Phys.Rev.B , 84 ,174506 (2011)

EPR Study of Electron-Hole Asymmetry in Bulk and Nanoparticles of Bi1-xCaxMnO3(x = 0.4, 0.6): A Comparison.

Geetanjali Singh and S V Bhat

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

Electron-hole asymmetry1, which refers to the asymmetry in the phase diagram across x = 2 0.5 in doped rare earth manganites such as Re1-xAxMnO3 is shown to vanish on reducing the size of the particles to a few nanometers. Bismuth based manganites provide an interesting model system for comparison: e.g. though the doped rare earth manganites show disappearance of charge order (CO) on size reduction to nanoscale3, CO in Bi manganites is found to be more robust. Here we study the effect of size reduction on EPR parameters of electron (x = 0. 6, BCMOE) and hole (x = 0.4, BCMOH) doped Bi1- xCaxMnO3. Nearly spherical nanoparticles (d ~ 18 nm) of BCMO were prepared by sol- gel synthesis and the bulk samples using the solid state reaction route. X-band EPR was carried out between 5 and 300 K. Lineshape fitting was carried out using double Lorentzian function accounting for clockwise and counterclockwise rotating components of the microwave field. The extracted EPR parameters, namely, the linewidth, intensity and the resonance field for the bulk and the nano samples indicate that the differences observed in the EPR parameters for the electron and hole doped bulk samples persist in 2 the nanosamples as well in contrast with the results on Pr1-xCaxMnO3 . We understand this in terms of the presence of the highly polarizable 6s2 lone pairs on bismuth which is understood to cause many interesting departures from the behavior of rare earth manganites.

References:

1. Janhavi P. Joshi, A.K. Sood, S.V. Bhat, Sarathy, Vijay, C.N.R. Rao, J.Phys. Cond. Matter. 16, 2869 (2004) 2. K.G. Padmalekha and S.V. Bhat, -cond-mat/1010.3556. 3. S. S. Rao, S. Tripathi, D. Pandey and S. V. Bhat, Phys. Rev, B 74, 144416 (2006)

Multi Electron Bubbles in Liquid Helium

Ambarish Ghosh, Vaisakh V, Emil M Joseph, Anustuv Pal

Abstract

Our research involves the experimental study of multielectron bubbles in liquid helium, which are micron sized cavities that contain a nanometer thick layer of electrons on the inner surface of the bubble wall. They present a rich platform to study the behavior of a two dimensional electron gas on a curved surface. In particular, we are interested in studying the stability of MEBs, which are predicted to be unstable when stationary, and stable if and only if moving at a speed beyond a critical value. To validate some of these theoretical predictions, we have built a cryogenic system for performing electrical transport and acousto-optical measurements of MEBs.

We present our experimental setup and few preliminary experimental results.

Numerical studies of dynamo action in a linear shear flow with turbulence

Nishant K. Singh1,2, Naveen Jingade2 & S. Sridhar1 1Raman Research Institute, Bangalore, India 2Indian Institute of Science, Bangalore, India

Abstract Large–scale cosmic magnetic fields are believed to originate from dynamo action in the electrically conducting fluids. Conversion of kinetic energy present in the fluid into the magnetic energy is re- ferred to as the dynamo action. The framework of mean–field electro- hydrodynamics which has been developed/evolved by using Maxwell’s equations together with Ohm’s law during the past few decades aims to address some of the outstanding puzzles related to magnetic fields in the universe. One particular challenge is to explain the generation of magnetic fields over length scales exceeding the length scale of tur- bulence. It is known that the helical turbulent flows on its own can amplify seed magnetic fields through the α–effect but there has been no evidence yet which has shown the dynamo action to produce large– scale mean magnetic field due to non–helical homogeneous isotropic turbulence alone. Thus it is legitimate to think what large–scale prop- erties such systems should possess in order to generate large–scale mean magnetic field if the turbulence is non–helical? The role of mean veloc- ity shear as the large–scale feature has received some attention. Direct numerical simulations now provide a strong support for such a shear dynamo. While the shear dynamo has been conclusively demonstrated to function, it is not yet clear what makes it to work.

We have studied the generation of large scale magnetic field due to non–helical turbulence in a background linear shear flow. This problem known as the shear dynamo problem was studied for various combina- tions of control parameters, viz., fluid Reynolds number (Re), magnetic Reynolds number (Rm), shear parameter (Sh) etc. We have analyzed a particular case where Re < 1, and Rm > 1. This case has the- oretical interest because governing MHD equation for incompressible flow becomes linear. We present all the results obtained in numerical simulations using PENCIL CODE and discuss the implications for dy- namo action which enables us to distinguish between various analytical models.

1

Structure of DNA Nanotubes Anjan Dwarknath1, Himanshu Joshi2, Prabal K. Maiti2

1 Indian Institute of Technology, Chennai

2 Center For Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore.

DNA Nanotubes are tubular structures created by self assembling DNA strands whose sequences are designed so that the resulting molecule takes the shape of a nanotube. Here we report for the first time the algorithm to construct variety of DNA nanotube topology like six helix, eight helix bundles as well as triangular nanotubes. We use state of the art all atom molecular dynamics (MD) simulations, to study microscopic picture of the DNA nanotubes. We comment on their thermodynamic stability as a function of sequence and number of DNA helix forming the nanotube. We also calculate the stretch modulus of these structures by stretching them in constant velocity simulation. The stretch moduli of the DNA nanotubes are of the order of 4000 pN in contrast to 1000 pN for a single DNA double helix.

Exploration of the statistical properties of Gross-Pitaevskii turbulence in two dimensions.

Vishwanath Shukla and Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India November 18, 2011

Abstract We carry out an extensive numerical study of turbulence in the two- dimensional Gross-Pitaevskii equation,

∂Ψ(r,t) 2 2 i = + g Ψ Ψ(r,t) (1) ∂t −∇ | |  where Ψ(r,t) is the complex classical field describing a weakly interacting Bose gas, and g is the effective interaction strength, that is used to describe zero-temperature superfluids and optical turbulence. In particular, we explore the time evolution of initial conditions in which

2 k 1 (k k0) Ψ( , 0) = exp − 2 exp iΘ, (2) √π1/2σ − 2σ  k e r 2 2 where Ψ( , 0) is the Fourier transform of Ψ( , 0), k = kx + ky, and Θ = Θ(kx,ky) are uniformly distributed random numbersp in the interval e 0, 2π . We study the time evolution of such an initial condition as a function of k0 = n0∆k and σ = β∆k where n0 is an integer, β is a real number and ∆k = 2π/L . We use a pseudospectral direct numerical simulation for our study. In any practical calculation we must truncate the number of Fourier modes that we retain in our pseudospectral study. Thus, we have a trun- cated Gross-Pitaevskii system, which is a finite-dimensional Hamiltonian system that thermalizes at long times. We obtain the time evolution of (a) spectra for compressible, incompressible, interaction, and quantum pressure parts of the energy, (b) occupation-number spectra, (c) the pop- ulation in the zero-wave-number mode, (d) probability distribution func- tions (PDFs) for the velocity, and (e) pseudocolour plots of the vorticity. Some universal features emerge as the system approaches thermalization but not before that.

1 Title: Confinement Induced Density Modulation and Spatially Resolved Dynamics of Confined Liquids

Authors: Shibu Saw and Chandan Dasgupta

Confinement enhances or slows down the dynamics of liquids depending on the nature of the wall-liquid interaction [1,2]. A structured wall modulates the density of confined liquids. The liquid density gets modulated by a structureless repulsive wall as well. Using extensive computer simulations of the Kob-Andersen binary (AB) Lennard-Jones mixture, we study the effect of such density modulation on the spatially resolved dynamics. Initial results suggest that the density modulation indeed affects the spatially resolved dynamics. The spatially resolved dynamics of confined liquids oscillates in phase with the density modulation near the walls and approaches that of bulk liquids far away from the walls. This result is in contrast to those of earlier studies [3]. We have also observed that confinement of the Kob-Andersen binary Lennard-Jones mixture forces majority-type particles to accumulate near the low-density region (‘vacuum’) created by a non-preferential repulsive wall. Initial investigation suggests that this phenomenon is similar to the accumulation of majority-type particles near the liquid-gas interface in bulk phase separation. This behaviour seems to arise from a lowering of the interfacial energy of the system. According to Gibb's adsorption rule for an ideal mixture, the liquid with lower surface tension should accumulate near the vacuum. In the presence of interactions between the hetero-particles, a smaller number of particles of the low surface tension liquid accumulate near the surface [4]. In the present work, we find that regardless of the surface tension, the majority-type particles accumulate near the vacuum in confinement and near the liquid-gas interface in the bulk. By accumulating majority-type particles near the vacuum in the case of confinement and near the liquid-gas interface in the case of bulk phase separation, the system is able to lower its interfacial energy and thus, its free energy. The short-range interaction of B-B particle plays a crucial role in the accumulation of majority-type particles near the vacuum.

1. P. Scheidler, W. Kob and K. Binder, Europhys. Lett. 52, 277 (2000). 2. P. Scheidler, W. Kob and K. Binder, J. Phys. Chem. B 108, 6673(2004). 3. K. Watanabe, T. Kawasaki and H. Tanaka, Nature Mater. 10, 512 (2011). 4. E. DiMasi, H. Tostmann, O. G. Shpyrko, P. Huber, B. M. Ocko, P. S. Pershan, M. Deutsch and L. E. Berman, Phys. Rev. Lett. 86, 1538 (2001). Structural and magnetic properties of Nd1-xYxMnO3 (0.1 ≤ ≤ 0.6)

Ruchika Yadav1, Harikrishnan S2, Shilpa Adiga2, H.L.Bhat1 and Suja Elizabeth1 1Indian Institute of Science, Bangalore-560012, India 2 Peter Grünberg Institute-4/Jülich Centre for Neutron Sciences-2, Forschungszentrum Jülich, 52425 Jülich, Germany. E-mail: [email protected]

The multiferroic behavior observed in RMnO3 manganites containing small rare-earth cations (R = Gd, Tb, Dy) has motivated the search for similar materials from same family with different rare-earth cation [1], combination of rare earths [2] and chemical substitution. Doping at rare-earth site to achieve non-collinear magnetic structure leading to multiferroicity has been effective in case of EuMnO3 [3] . Yttrium doping in EuMnO3 results in magnetic frustration which, in turn, leads to colossal magnetoelectric response [3]. In order to understand the effect of cationic size on the evolution of the A-type antiferromagnetic system to incommensurate multiferroics, Y substitution in NdMnO3 is undertaken. In the present study, Nd1-xYxMnO3 compounds of several Y-doping concentration (0.1 ≤ ≤ 0.6) have been synthesized by solid state reaction method. Phase purity was confirmed by powder X-Ray diffraction. Structural analysis of the powder pattern carried out by Rietveld method using FULLPROF suite established that the compounds with ≤ 0.5 crystallized in orthorhombic Pbnm space group. For > 0.5 hexagonal phase co-existed along with the orthorhombic phase. Chemical composition was determined by energy dispersive X-ray analysis (EDX) and electron probe micro analysis (EPMA). Magnetic measurements as a function of temperature and field were carried out in a commercial PPMS (Quantum Design) with vibrating sample magnetometer option. It is clear from the dc magnetization studies show that the transition temperature of Mn N lattice, TMn decreases with increasing . Data also show a similar trend in AC susceptibility measurements. For the sample with = 0.3, results of magnetic measurement indicate ferrimagnet like behavior. The insulating nature of the samples is evident from the studies in temperature evolution of electrical resistivity. Optimization of single crystal growth using Float Zone mirror furnace is underway, for direction dependent studies on oriented single crystals.

References: [1] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, Y. Tokura, Nature 426 (2003) 55– 58 [2] S. Issing, A. Pimenov, V. Y. Ivanov, A. A. Mukhin, J. Geurts, Phys. Rev. B 81(2010) 24304. [3] J. Hemberger, F. Schrettle, A. Pimenov, P. Lunkenheimer, V. Ivanov, A. Mukhin, A. Balbashov, A. Loidl, Physical Review B 75 (2007) 35118. Structure of Cytolysin A (ClyA) pore in Lipid Bilayer

Swarna M Patra*, K Ganapathy Ayappa** and Prabal K Maiti* *Centre for Condensed matter theory Department of Physics Indian Institute of Science, Bangalore 560 012, India **Department of Chemical Engineering Indian Institute of Science, Bangalore 560 012, India Abstract: Membrane proteins are subject to same consideration as soluble proteins, but structural study of membrane proteins appear to be harder. Simulations of membrane proteins are a booming field with great application. Cytolysin A (ClyA) is a cytolytic protein expressed by Escherichia coli and other enterobacteria. The crystal structure of ClyA, 400kDa dodecameric trans membrane pore in DDM detergent micelles is reported. The soluble monomer structure and the tertiary structure of ClyA protomers are significantly different. In this study through molecular dynamics we have studied the structural aspects of ClyA (2WCD pdb) pore forming toxin in homogeneous lipid bilayer. In the present study we have taken two systems with different salt concentration. The work is under progress; our preliminary result shows the structural stability of the pore in lipid bilayer and the selectivity of pore towards Na ions. This observation is further supported with the Adaptive Poisson-Boltzmann Solver (APBS) calculation, indicating inside of the pore has strong negative potential. Our present work gives a better understanding of the structural aspect at molecular level. This has a potential application for the pharmaceutical industry in new drug discovery.

Conversion between electromagnetically induced transparency and absorption in a degenerate lambda system

Sapam Ranjita Chanu and Vasant Natarajan Departmant of Physics,Indian Institute of Science Bangalore-560012, INDIA November 21, 2011

Abstract We show that it is possible to change from a subnatural electromag- netically induced transparency(EIT) feature to a subnatural electro- magnetically induced absorption (EIA) feature in a (degenerate)three- level Λ system. The change is effected by turning on a second control beam counter propagating with respect to the first beam. We observe this change in the D2 line of Rb in a room-temperature vapor cell. The observations are supported by density-matrix analysis of the complete sublevel structure including the effect of Doppler averaging.

1 Real-space manifestations of Energy-spectra Bottlenecks: Insights from Hyperviscous Hydrodynamical Equations

Uriel Frisch, Samriddhi Sankar Ray, Ganapati Sahoo, Debarghya Banerjee and Rahul Pandit November 17, 2011

Abstract

We obtain bottlenecks in energy spectra for (a) the deterministi- cally forced, one dimensional (1D) hyperviscous Burgers equation(DHB), (b) the stochastically forced 1D hyperviscous Burgers equation(SHB), and (c)the forced, three dimensional Navier-Stokes equation (3D HNS) by using a combination of analytical and numerical methods for case (a) and pseudospectral direct numerical simulations for cases (b) and (c). By increasing the order of the hyperviscosity we enhance the size of the bottleneck and, thereby, its real-space manifestations. We show that these real-space manifestations are exponentially damped oscil- lations in the velocity profile in the vicinity of a shock in case (a) and similar oscillations in the velocity correlation functions in cases (b) and (c). We show that the wavelength of these oscillations is related to the inverse of the wavenumber at which the bottleneck peak occurs; and the correlation length that characterises the exponential decay of the real-space oscillations is related inversely to the width of the peak of the bottleneck.

1 In-plane magnetic anisotropy in epitaxial ultrathin Fe films Sakshath S, Padmalekha K G, S V Bhat and P S Anil Kumar Department of Physics Indian Institute of Science Bangalore 560012; India

Epitaxially grown Fe films show a plethora of magnetic anisotropies depending on the environment. The control of magnetic anisotropy in thin ferromagnetic films is important for several device applications such as Spin torque oscillators, Magnetic memories etc as well as from the point of view of understanding fundamental physics. Factors such as the material of the capping layer, underlayer used in growth, deposition technique, geometry of deposition, substrate material etc are observed to strongly influence the growth of Fe films. An understanding of the causes of anisotropy, particularly the uniaxial anisotropy in these films has not been concrete. We try to shed some light into this aspect with studies of anisotropy in Fe films. Fe thin films were grown at room temperature in an Ultrahigh Vacuum chamber with base pressure less than 2x 10-10 Torr with the following confiigurations: 1. MgO/Fe/GaAs 2. Au/Fe/GaAs 3. MgO/Fe/MgO/GaAs Structural characterisation was done using Low Energy Electron diffraction . Magnetic characterisation has been done using Magnetooptic Kerr effect in the longitudinal configuration and Ferromagnetic Resonance. The initial growth of Fe during the first few layers as well as the the electronic structure of the interfaces seem to be the most important reasons behind the uniaxial Anisotropy. For a smooth well oriented substrate, the electronic band structure of the substrate gives rise to the unexpected Uniaxial anisotropy in Fe films. Scroll­Wave Dynamics in Human Cardiac Tissue: Lessons from a Mathematical Model with Inhomogeneities and Fiber Architecture

Rupamanjari Majumder1, Alok Ranjan Nayak1, Rahul Pandit1,2

Cardiac arrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF), are among the leading causes of death in the industrialized world. These are associated with the formation of spiral and scroll waves of electrical activation in cardiac tissue; single spiral and scroll waves are believed to be associated with VT whereas their turbulent analogs are associated with VF. Thus, the study of these waves is an important biophysical problem. We present a systematic study of the combined effects of muscle­fiber rotation and inhomogeneities on scroll­wave dynamics in the TNNP (ten Tusscher Noble Noble Panfilov) model for human cardiac tissue. In particular, we use the three­dimensional TNNP model with fiber rotation and consider both conduction and ionic inhomogeneities. We find that, in addition to displaying a sensitive dependence on the positions, sizes, and types of inhomogeneities, scroll­wave dynamics also depends delicately upon the degree of fiber rotation. We find that the tendency of scroll waves to anchor to cylindrical conduction inhomogeneities increases with the radius of the inhomogeneity. Furthermore, the filament of the scroll wave can exhibit drift or meandering, transmural bending, twisting, and break­ up. If the scroll­wave filament exhibits weak meandering, then there is a fine balance between the anchoring of this wave at the inhomogeneity and a disruption of wave­pinning by fiber rotation. If this filament displays strong meandering, then again the anchoring is suppressed by fiber rotation; also, the scroll wave can be eliminated from most of the layers only to be regenerated by a seed wave. Ionic inhomogeneities can also lead to an anchoring of the scroll wave; scroll waves can now enter the region inside an ionic inhomogeneity and can display a coexistence of spatiotemporal chaos and quasi­periodic behavior in different parts of the simulation domain.

1 Department of Physics, Centre for Condensed Matter Theory, Indian Institute of Science, Bangalore, India, 2 Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India Density Functional Theory Studies of electrons and phonons in pyrochlores

Pramod Kumar Verma Department of Physics Indian Institute of Science , Bangalore Research Supervisor : H R Krishnamurthy November 23, 2011

Abstract Density functional theory calculations have been performed to ob- tain the lattice constant and electronic properties of the ideal py- rochlores with the composition Y2B2O7 (where B = Ti, Zr and Ir). From the band structure and the density of states we infer that Y2Ti2O7 and Y2Zr2O7 are insulators while Y2Ir2O7 is a metal. The dynami- cal atomic charges (also called Born effective charges) are obtained to quantify the degree of covalency or ionicity. A large anomalous con- tribution to the dynamical charge is observed for both the Zr and Ti (specially for Ti). It is attributed to the hybridization between the occupied 2p states of the oxygen and unoccupied d states of the B cation. Density functional perturbation theory (DFPT) calculations have been performed at the Gamma point to obtain the phonon prop- erties of the system.While for Y2Zr2O7 and Y2Ir2O7 all the phonon frequencies are positive as one would expect. Y2Ti2O7 shows insta- bilities with respect to some optical phonon distortions is that 6 of the frequencies are imaginary. This is likely to underlie the anoma- lous temperature dependent of the phonons that have been seen in other titanate pyrochlores. A pressure of the order of 3.5GPa or a small distortion to the atomic positions in the unit cell stabilizes the system.

1 “Upper Branch” Fermi Gas and Tan’s Theorem

Vijay B. Shenoy Centre for Condensed Matter Theory, Indian Institute of Science, Bangalore 560 012, India

“Upper branch” Fermi gases show a peculiar non-monotonicity in their energy with increasing scattering length – this apparently violates Tan’s theorem. By generalizing the Nozi´eres-Schmitt- Rink (NSR) method to the upper branch, this violation of Tan’s theorem is shown arise from Pauli blocking which causes the bound states of fermion pairs of different momenta to disappear at different scattering lengths. Reference: cond-mat/1106.0960