DEPARTMENT OF PHYSICS IN-HOUSE SYMPOSIUM 2008

INDIAN INSTITUTE OF SCIENCE BANGALORE 560012 (NOVEMBER 24-25, 2008)

FORWARD

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 selected faculty members and students, and poster presentations by all those who would like to present their recent results. This year we have 10 talks by faculty, 2 by post-doctoral fellows, 17 by the senior students and as many as 28 posters by the rest. I hope this package would be as 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 Drs. Prabal Maiti, Arindam Ghosh and Banibrata Mukhopadhyay 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. Chandan Dasgupta Chairman November 24-25, 2008

IN-HOUSE SYMPOSIUM

DEPARTMENT OF PHYSICS

Indian Institute of Science

November 24-25, 2008

TECHNICAL PROGRAMME (Venue: Physics Lecture Hall- I)

Day 1 (24/11/2008)

08.55-09:00 Welcome Chandan Dasgupta

09:00-10:25 SESSION I Chair: Vasant Natarajan

09:00-09:20 F1 Chanda J. Jog The surprisingly flattened dark matter halo of the Andromeda Galaxy 09:20-09:40 F2 H. R. Krishnamurthy Novel Physics in Hetero-structures of Strongly Correlated Materials 09:40-09:55 S1 S. Srivastava Polymer nanocomposites near glass transition: Role of interface 09:55-10:10 S2 Naresh Babu Pendya Observation of low temperature Persistent Photoconductivity in flower shaped dendritic PbS structures 10:10-10:25 S3 T. Phanindra Sai Fabrication and Electrical Characterization of Charge Transfer Molecular Wires

10:25-11:00 TEA

11:00-12:55 SESSION II Chair: Prabal K Maiti

11:00-11:20 F3 Chandan Dasgupta Growing length and time scales in glass forming liquids 11:20-11:40 F4 Jaydeep Basu Nanophotonics with Quantum Dot Arrays and Optical Near Fields

11:40-11:55 S4 S. K. Biswas Diffuse optical tomographic non-invasive breast and brain imager using a single laser light source 11:55-12:10 S5 Murthy O. V. S. N High pulsed field magnetotransport and hole mobility in HgCdTe 12:10-12:25 S6 K. G Padmalekha EMR Studies of LSMO nanoparticles dispersed in PVA 12:25-12:40 S7 Mogurampelly Santosh Force Induced DNA Melting 12:40-12:55 S8 N. Kamaraju Ultrafast electron dynamics and cubic optical nonlinearity of single and double walled carbon nanotubes

12:55-14:00 LUNCH

14:00-15:30 POSTER + TEA

15:30-17:15 SESSION III Chair: Rahul Pandit

15:30-15:50 F5 A. K. Sood Nonequilibrium Fluctuations in Sheared Jammed States 15:50-16:10 F6 K. P. Sinha Extended Einstein-Hilbert (E-H) action and the emergence of Riccions as cosmic dark matter and consequent cosmic acceleration 16:10-16:25 S9 Sunil Kumar Low frequency vibrational modes in carbon nanotubes: terahertz time domain spectroscopy 16:25-16:45 P1 Rema Krishnaswamy Jamming and shear banding in two dimensional suspensions 16:45-17:00 S10 Surajit Saha Phonon anomaly in pyrochlore spin-ice Dy2Ti2O7 and non- magnetic Lu2Ti2O7: A temperature dependent Raman and x-ray study 17:00-17:15 S11 K Vijay Kumar Brownian Inchworm Model of a Self-propelled particle

Day 2 (25/11/2008)

09:00-10:25 SESSION IV Chair: V. Venkataraman

09:00-09:20 F7 Ramesh Chandra Mallik Thermoelectric Properties of Co4Sb12 Skutterudite Materials with In Addition 09:20-09:40 F8 Diptiman Sen Scattering of electrons from an interacting region 09:40-09:55 S12 Prasad Perlekar Statistically Steady Turbulence in Soap Films: Direct Numerical Simulations with Ekman Friction 09:55-10:10 S13 Samriddhi Sankar Ray Hyperviscosity, Bottlenecks and New Surprises in the Galerkin- truncated Burgers Equation 10:10-10:25 S14 Avatar Tulsi General framework for quantum search algorithms

10:25-11:00 TEA

11:00-12:40 SESSION V Chair: S. V. Bhat

11:00-11:20 F9 S. Ramakumar Crystal Structures of native and substrate complex of an alkali thermostable xylanase from an alkalophilic Bacillus sp. NG-27. Structural insights into alkalophilicity. 11:20-11:40 F10 Arindam Ghosh Intrinsic magnetism in high-mobility semiconductor – a bottom-up route to spin-electronics? 11:40-11:55 S15 D. Samal The incongruous observation of phase separation in La0.85 Sr0.15CoO3 spin –glass system 11:55-12:10 S16 Amrita Singh New electrochemistry based route to Materials Engineering in Nanoscale Systems 12:10-12:25 S17 M Suheshkumar Singh Ultrasound-modulated Diffuse Optical Tomographic Imaging System for Screening Breast Cancer 12:25-12:45 P2 Norio Kikuchi A filament in an active medium: buckling, stiffening and negative dissipation

VOTE OF THANKS BY CHAIRMAN

POSTERS

No Name Title

P1 Arjun Joshua and V. Venkataraman Enhancement of sensitivity of detection of Kerr rotation by time averaging P2 Vikram Rathee, Sajal Kumar Ghosh, Rema Krishnaswamy, V. A. Raghunathan and A. K. Sood Reentrant phase behavior of a mixed surfactant system with strongly binding counter-ions P3 Geetanjali, D. Banerjee and S V Bhat Structural, magnetic and EMR studies of CaMnO3 nanoparticles P4 Sayak Ghoshal and P.S.Anil Kumar Magnetic and Magnetotransport studies of ZnO films with and without intrinsic magnetic moment P5 Sumilan Banerjee, Chandan Dasgupta, and T. V. Ramakrishnan Ginzburg Landau Theory for Cuprate Superconductivity P6 Suropriya Saha and Sriram Ramaswamy Non Equilibrium noise in electrophoresis: the micro-ion wind P7 Sakshath. S. and P. S. Anil Kumar Magnetic Anisotropy in Epitaxial Fe/MgO films grown by Pulsed Laser Deposition P8 Debangsu Roy, and P.S. Anil Kumar Observation of the exchange spring behavior in hard-soft- ferrite nanocomposite P9 Biswanath Chakraborty, Anindya Das and A.K. Sood Probing the local stress in Graphene by Raman Spectroscopy P10 Chinkhanlun Guite and V. Venkataraman Electrical detection of spin polarized electrons in semiconductors using a radio- frequency pick-up coil P11 Anupam Gupta, Prasad Perlekar, and Rahul Pandit Polymer Addtives in Decaying Two-Dimensional Turbulence P12 Smarajit Karmakar, Chandan Dasgupta and Srikanth Sastry Numerical study of a glass forming liquid in two dimensions to test the dimension dependence of the Adam-Gibbs relation P13 Ramakanta Naik,R.Ganesan,K.S.Sangunni. Compositional dependence of the optical changes in (As2S3)1-xSbx chalcogenide film. P14 Nitin P. Lobo and K.V. Ramanathan Measurement of Heteronuclear dipolar couplings- A systematic study of cross polarization dynamics for Separated Local Field (SLF) experiments P15 S.M. Mohanasundaram and Arindam Ghosh Detection of microcantilever deflection using electron transport measurements P16 Chandni U and Arindam Ghosh A fluctuation-based probe to criticality in structural transitions

P17 Jayakrishna Khatei and K.S.R. Koteswara Rao Temperature dependent photoluminescence study in Hg1-xCdxTe (x~0.8) nano and microcrystals P18 Alok Ranjan Nayak, T.K. Shajahan, and Rahul Pandit The Dependance of Spiral-Wave Dynamics on Inhomogeneities in the TNNP Model of Cardiac Tissue P19 Nitin Kumar, Sayantan Majumdar, Aditya Sood, Rama Govindarajan, Sriram Ramaswamy and A.K. Sood Motion of falling spheres and rising bubbles in a viscoelastic gel: spontaneous oscillations and bursting P20 Sayantan Majumdar and A.K. Sood Negative viscosity fluctuations and their universality in a driven repulsive colloidal glass P21 Aditya A. Wagh, H.L.Bhat, P. S. Anil Kumar and Suja Elizabeth MAGNETOTRANSPORT STUDIES OF FLOAT ZONE-GROWN Gd1- xSrxMnO3 SINGLE CRYSTAL P22 Paramita Kar Choudhury, Debjani Bagchi, and Reghu Menon Conformational modification by conjugation length and solvent in rigid-rod organic semiconductor P23 K Jugeshwar Singh, G Clark, K P Ramesh and Reghu Menon H-NMR and charge transport in metallic polypyrrole at ultra-low temperatures and high magnetic fields P24 Atindra Nath Pal and Arindam Ghosh Resistance noise in electrically biased bilayer graphene P25 Meghana Dharmik and Prabal Maiti Structure and stability of carbon and boronnitride nanorings P26 R.Koushik and Arindam Ghosh Conductivity noise in strongly correlated systems P27 Manas Khan and A. K. Sood Out-of-Equilibrium Microrheology to Probe Directional Viscoelastic Properties under Shear P28 Biswaroop Mukherjee, Prabal K. Maiti, Chandan Dasgupta and A. K. Sood Jump reorientation of water molecules confined in narrow carbon nanotubes

TALK ABSTRACTS

The surprisingly flattened dark matter halo of the Andromeda Galaxy

Chanda J. Jog

The rotation curve of a spiral galaxy is normally used to deduce its dark matter halo content. Another complementary constraint is provided by the vertical scale- heights of the interstellar gas, but this has not been exploited much in the literature. We use both these for the nearby Andromeda galaxy to determine its halo density distribution (Banerjee & Jog 2008, Astrophysical Journal, 685, 254). We model the galaxy as a gravitationally coupled star-gas system responding to the external force-field of the dark matter halo. We show that a highly flattened oblate halo with an axis ratio of 0.4, and an isothermal density profile, provides the best fit to the observed data. The flattened shape of the dark matter halo is a robust result, and is strikingly different from the spherical shape generally assumed. This has important implications for galaxy formation which will be discussed. A similar approach has been applied to the low surface brightness galaxy UGC 7321 for which the scaleheight information is available. Here the best-fit result from our model implies high gas dispersion, this could explain the low star formation seen in this galaxy (Banerjee, Matthews & Jog 2008, Mon. Not. Roy. Astron. Soc., submitted).

1 Novel Physics in Hetero-structures of Strongly Correlated Materials

H. R. Krishnamurthy Department of Physics, Indian Institute of Science

The study of hetero-structures constructed out of strongly correlated materials, such as oxides, has been attracting a lot of attention recently. One of the reasons for interest in these systems is that new physical phenomena emerge in these hetero- structures that are absent in the bulk materials they are composed from. I will review a few recent examples, like the appearance of two-dimensional electron gases at the interfaces between band and Mott insulators, and their low-temperature superconductivity, and discuss the physics involved. I will end with a brief discussion of our recent research [1], showing the possibility of another such striking, quantum emergent phenomenon, namely that at sufficiently low temperatures, a finite width Mott insulator sandwiched between two metallic leads could display perfect dc conductance.

[1] H. Zenia , J.K. Freericks, H. R. Krishnamurthy,Th. Pruschke : arXiv:0809.4993v1 Polymer nanocomposites near glass transition : Role of interface

S.Srivastava and J.K.Basu Department of Physics

The unique optical, thermo mechanical, electronic properties of polymer nanocomposite provide opportunities for better understanding of certain fundamental aspects of polymer nanocomposite. Recent experiments show that such drastic change in properties results due to possibility of the existence of a region with modified property at the nanoparticle- polymer interface as well as due to confinement .Thermal property, importantly, Glass transition is effected significantly which is an important property in determining the suitability of a polymer nanocomposite for many application. Thus to understand the dynamics of the polymer segments at interface needs to be probed with high resolution technique to relate this to macroscopic properties like visco-elasticity, glass transition (Tg), etc. To address this issue we have performed highly sensitive modulated differential scanning calorimetry (MDSC) measurements of glass transition of polymethylmethacrylate (PMMA) capped gold nanoparticles dispersed in same matrix. Experiments have shown that Tg decreases for non-interactive metal-polymer interface and increases for the attractive interaction, however we have shown for the first time the cross over in the sign of the shift of Tg for a neutral polymer-metal interface[1]. The dynamics of polymer segments at the metal - polymer interface can be controlled by the variation of the interfacial width, σ, which is an important parameter to study confinement effect on the polymer glass transition.. We have used a new parameter, nanoparticle-polymer interfacial width, σ, in elucidating role of surface effects on glass transition of polymers. Our results [2] show that interfacial width less than characteristic length, ξ, results in reduction and more than it gives rise to enhancement in the glass transition temperature of the polymer. To study the effect of interface on the dynamics of such systems we have performed X- ray photon correlation spectroscopy measurements . Measurements performed at various temperatures around the glass transition temperature of the polymer nanocomposite powders revealed intriguing differences in the respective wave vector dependent intermediate scattering function elucidating the role of interface morphology [3] in determining the relaxation dynamics, especially a gel - to –attractive glass transition,

References:

1). S.Srivastava and J.K. Basu, J. Nanoscience and Nano Technology 7, 2101, 2007. 2). S.Srivastava and J.K. Basu, Physical Review Letter 98, 165701, 2007. 3). S. Srivastava , A. K. Kandar, and J. K. Basu, M.K. Mukhopadhyay, and S. Narayanan (Manuscript under review).

Observation of low temperature Persistent Photoconductivity in flower shaped dendritic PbS structures

Naresh Babu Pendya, K.S.R.K.Rao Semiconductor Lab, Department of Physics, I.I.Sc. Bangalore-560012

Abstract

Persistent Photoconductivity (PPC) has been observed in flower shaped PbS dendrite structures upto 220o K for first time. Potential variation due to the presence of various sized particles might be the cause for the PPC. PPC relaxation behavior can be well β described by a stretched-exponential function ItIppc( )=− ppc (0)exp[ ( tτβ ) ](≺ 1) where τ is the relaxation time constant and β the decay exponent. The photocurrent build-up curves can be described by an equivalent stretched-exponential function β I ppc (tI )=−−max [1 exp( (α t ) ]. Along with the PPC, we also observed Negative Persistent Photoconductivity (NPPC) below 40o K, and above which it is Positive Persistent Photoconductivity (PPPC). A metastable state at 40 K might be the reason for the above observations. The exact nature of the metastable state is not known at present. These primary results certainly encourage us to do more measurements on this system as well on other dendrite structures. Fabrication and Electrical Characterization of Charge Transfer Molecular Wires T.Phanindra Sai*, 1 and A.K.Raychaudhuri 1, 2 1 Department of Physics, Indian Institute of Science, Bangalore-560012 2 Unit for Nanoscience, S.N.Bose National Center for Basic Sciences, Salt Lake Kolkata-700 098 ------The aim of the present work was to grow isolated molecular wires of charge transfer complex and study their electrical and transport properties. The molecular wires were grown using thermal co- evaporation technique, where the donor and acceptor charge transfer molecules are simultaneously evaporated form two different crucibles in a vacuum chamber on to a lithographically patterned substrates. An electric field is also applied between anode and cathode to enhance the growth of the molecular wires. The donor materials used were TMTTF and TTF and the acceptor was TCNQ. The substrate is also maintained at a constant temperature during evaporation. SEM images of the molecular wires grown across wider and larger electrode gaps showed that each molecular wire consists of bunches of smaller wires of diameter 200 nm in the case of TMTTF-TCNQ samples and even 50 nm in the case of TTF-TCNQ wires. We are interested in studying the properties of the single molecular wire of these charge transfer molecules of different widths ranging from 500 nm down to 50 nm. To fabricate single molecular wires e beam lithography was used to write contact pads with reduced electrode gaps and electrode widths onto silicon oxide substrates. A resistance vs temperature measurement of the grown molecular wires shows a semiconductor like behavior whereas previous studies on single crystals of TTF-TCNQ molecules have shown metallic behavior. This happens because the wires grown by thermal evaporation does not have the 1:1 stiochiometry due to different vapor pressures of TTF and TCNQ molecules.

*Tetrathiafulvalene (TTF), Tetracayanoquinodimethane (TCNQ), Trimethyltetrathiafulvalene (TMTTF). Growing length and time scales in glass forming liquids Smarajit Karmakar1, Chandan Dasgupta1,2, Srikanth Sastry2 1 CCMT, Department of Physics, Indian Institute of Science, Bangalore 2 Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore

Explaining the enormous increase in the viscosity and relaxation time of a liquid upon supercooling is essential for an understanding of the structural glass transition. Although the notion of a growing length scale of “cooperatively rearranging regions” is often invoked to explain dynamical slow down, the role of length scales relevant to glassy dynamics is not well established. Recent studies of spatial heterogene- ity in the local dynamics provide fresh impetus in this direction. Using finite-size scaling for the first time for a realistic glass former, we establish that the growth of dynamical heterogeneity with decreasing temperature is governed by a growing dynamical length scale. However, the dependence of the simultaneously growing relaxation time on system size does not exhibit the same scaling behaviour as the dynamical heterogeneity. We show that the relaxation time is instead determined, for all studied system sizes and temperatures, by the configurational entropy, in accordance with the Adam-Gibbs relation, but in disagreement with the prevailing belief that the configurational entropy is not relevant above the critical temperature of mode coupling theory. These results provide new insights into the dynamics of glass-forming liquids and pose serious challenges to existing theoretical descriptions. Nanophotonics with Quantum Dot Arrays and Optical Near Fields.

Haridas M and Jaydeep Basu

Nanophotonics, which involves local electromagnetic interactions between nanometer- scale materials and an optical near field (ONF), promises to open the doors towards next generation technology for quantum information processing using light and quantum devices. ONF's are characterised by the presence of optical fields with high lateral spatial frequencies able to excite surface states not accessible by means of conventional far-field optical excitations including possibilities of exciting dipole-forbidden states. Scanning Near field Optical Microscopy (SNOM) is recently developed experimental technique used to produce such ONF and combines the advantages of the relatively new scanning probe based microscopy, with the rich and well developed field optical spectroscopy. SNOM has been widely used to study single and coupled quantum dots as well as metallic nanostructures. Most of the experimental and theoretical studies limited to studying very small number of ideal quantum dots which are homogeneous and are arranged an in an ordered manner without disorder. However, for any potential device applications, the real system would involve a large array of QDs in which both inhomogenity and disorder would be present. Studying the coupling of ONF with such a QD array would lead to a understanding of not only the differences with the far- field spectra but also the role of disorder and inhomogenity in the near field spectra of large scale QD arrays.

Here we will discuss the use of SNOM to study coupling between ONF and excitons in large scale arrays of CdSe QD. The PL spectra collected in SNOM mode and in confocal mode (far field) from the CdSe based QD arrays shows striking differences. We also compare the spatially resolved PL spectra collected in SNOM mode from different region of the QD array and find coupling fine structure in such spectra. We discus the possible sources of such fine structures and point to the need for a unified theory to treat the ONF and excitons in QD arrays as a single coupled quantum system.

Reference:

1. Haridas M and J. K. Basu (submitted) 2. Haridas M and J. K. Basu (manuscript in preparation) 3. Haridas M and J. K. Basu (manuscript in preparation) Diffuse optical tomographic non-invasive breast and brain imager using a single laser light source

S. K. Biswas1, K. Rajan1, and R. M. Vasu2 Department of Physics1 Department of Instrumentation2 Indian Institute of Science, Bangalore, 560012 India.

Near-Infrared (NIR) diffuse optical tomography technique has the capability of providing good quantitative reconstruction of distribution of tissue absorption and scattering parameters with ad- ditional inputs such as input and output modulation depth and photon leakage correction through the measurement process. The light transport through a tissue is diffusive in nature and can be modeled using diffusion equation (DE). The optical parameters of the inhomogeneity can be recov- ered if they are close to the optical properties of the background. Diffusion equation for photon transport is solved using Finite Element Method (FEM) and Jacobian is modeled for reconstructing the optical parameters of the object. We study the development and performance of DOT system using 100MHz modulated single laser source and multiple detectors. The theory has been evolving continuously over the past 15 years but very few has come forward to implement and handover a product to human society. Diffuse Optical Tomography (DOT) is a promising tool for non-invasive imaging of deep tissue. To reduce the gap between the theory and the experiment we have calcu- lated the 2-D input modulation depth from 3-D diffusion to model the 2-D diffusion of photons. The photon leakage when light traverses from fiber tip to the phantom is estimated using a solid angle model. We study the development and performance of DOT system using 100MHz modulated single laser source and multiple detectors. The experiment is carried out with 5mm, 6mm 8mm (diame- ter) inhomogeneities with absorption coefficient almost three times as that of the background. The experimental result shows that our single source system with additional inputs such as separated 2-D input/output modulation depth from 3-D diffusion and weighted air fiber interface correction is capable of detecting 5mm 6mm and 8mm separately and can identify the size difference of multiple inhomogeneities with zero localization error.

FIG. 1: Reconstructed results from Experimental data for single as well as multiple inhomogeneities with different sizes. The size of inhomogeneities are 5mm shown in Fig.1:a and 6mm , 8mm shown in Fig.1b . The reconstructed image shows the exact positions of inhomogeneities as it were in the phantoms that we used in our experiment. High pulsed field magnetotransport and hole mobility in HgCdTe Murthy O.V.S.N., V. Venkataraman, R.K. Sharma

Mercury Cadmium Telluride, despite competition, is still a material of choice for manufacturers of infrared detectors especially in the 10 μm region due to its high efficiency. Being a narrow gap semiconductor, its physics is much different from the Si/GaAs genus. A study on magnetotransport in bulk as well as epilayers of HgCdTe at both low and high temperatures is presented.

We have designed, constructed and calibrated high performance pulsed magnets operating upto 17T from 77K-300K and 12T down to 4K for magnetotransport measurements [1]. Data is obtained using a fast A/D card and analyzed using digital lock- in method.

The composition of the samples under study is determined using optical absorption measurements. High magnetic fields are required to probe low mobility holes owing to the electronic domination of conductivity at low B even in p-type samples. Multicarrier analysis is used to extract mobilities and densities from the measured mixed conductivity tensor upto 15T. Hole mobility at higher temperatures from 200K-300K in magnetic fields is investigated using a hybrid method by various scattering mechanisms. We have solved Boltzmann transport equation numerically without relaxation time approximation [2] to include polar optical phonon scattering. Other scattering mechanisms taken into account with or without using relaxation time approximation [3] are: ionized impurity, nonpolar optical, acoustic, dislocation, alloy disorder and piezoelectric. Results are presented comparing our data with the calculated mobility.

[1] Murthy O.V.S.N., Venkataraman V., Rev. Sci. Instrum. 78, 113905 (2007) [2] Rode, D.L., Phys. Rev. B 2, 1012 (1970) [3] Yadava, R.D.S. et al, J. Electron Mater. 23, 1359 (1994)

EMR studies of LSMO nanoparticles dispersed in PVA

K G Padmalekha and S V Bhat

We report low temperature EMR (Electron Magnetic Resonance) data for La 0.67 Sr 0.33 MnO 3 nanoparticles dispersed in Poly Vinyl Alcohol. The system under study had 30% LSMO by weight dispersed into self-standing thin films of PVA. We observe that the EMR signals show asymmetry which is a function of temperature. At low temperatures, we observe that the low field signal is stronger than the high field signal. As the temperature increases the high field peak becomes more and more intense. The low temperature data fits to a Dysonian with asymmetry parameter ranging from 0.47 (at 3.8K) to 0.18(at 30K). At a relatively high temperature (~230K) the shape undergoes a crossover and the signal becomes symmetric and fits to a Lorentzian.

The resonance field variation and line width variation with temperature is strongly suggestive of ferromagnetic nature in the entire temperature range (3.8K to 300K). The ratio of peak heights A/B equals 1 at around 230K. The crossover suggests some kind of transition happening in the system. The conductivity studies suggest that the system is not metallic at room temperature. And, the VSM data suggest a broadened out T c in the system. Though the nanoparticles are ferromagnetic at room temperature, their T MI may be below 300K and this could be responsible for the change in the EMR data. If the microwave skin depth of the sample is less than the sample dimensions, we get asymmetric Dysonian lines in the EMR spectra. The average size of the LSMO nanoparticles used in this system is 25nm and the skin depth at high frequencies is known to be less than particle sizes of this order.

Another explanation could be that there are two competing signals moving with respect to each other as the temperature varies. However, there was no resolution of these lines with lowering of temperature. Further experiments are underway to investigate the line shapes in detail.

Acknowledgements:

The authors would like to thank Debamalya Banerjee for EMR measurements. Force Induced DNA Melting

Mogurampelly Santosh† and Prabal K Maiti* Center for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore­12

Abstract

When pulled along the axis, double­strand DNA undergoes a large conformational change and elongates roughly twice its initial contour length at a pulling force about 70 pN. The transition to this highly overstretched form of DNA is very cooperative. Applying force perpendicular to the DNA axis (unzipping), double­strand DNA can also be separated into two single­stranded DNA which is a fundamental process in DNA replication. We study the DNA overstretching and unzipping transition using fully atomistic molecular dynamics (MD) simulations and argue that the conformational changes of double strand DNA associated with either of the above mentioned processes can be viewed as force induced DNA melting. As the force at one end of the DNA is increased the DNA start melting abruptly/smoothly after a critical force depending on the pulling direction. The critical force fm, at which DNA melts completely decreases as the temperature of the system is increased. The melting force in case of unzipping is smaller compared to the melting force when the DNA is pulled along the helical axis. In the cases of melting through unzipping, the double­ strand separation has jumps which correspond to the different energy minima arising due to different base pair sequence. The fraction of Watson­Crick base pair hydrogen bond breaking as a function of force does not show smooth and continuous behavior and consists of plateaus followed by sharp jumps.

† [email protected] * [email protected] Ultrafast electron dynamics and cubic optical nonlinearity of single and double walled carbon nanotubes.

N. Kamaraju, Sunil Kumar, B. Karthikeyan and A. K. Sood Department of Physics, Centre for Ultrafast Laser Applications, Indian Institute of Science, Bangalore-560012.

We have studied cubic optical nonlinearities of single walled carbon nanotube (SWNT) suspensions and a free standing thin film of double walled carbon nanotubes (DWNT) using 790 nm, 80 femtosecond light pulses in open and closed aperture z-scan experiments. The nanotube suspensions were prepared in water using sodium dodecyl sulphate [1] and by microwave treatment with amide functionalization [2]. We have developed a modified theoretical expression to analyze the z-dependent transmission (both open and closed aperture) in the saturable limit. This gives a value of Im (c(3)) ~ 4.9 x 10-10 (0.45 x10-10) esu and Re (c(3)) ~ 5.4 x 10-10 (2.2 x10-9) esu for SWNT (DWNT). Degenerate femtosecond pump-probe experiments (790nm, 80 fs) to investigate [3] the optical switching time and the underlying ultrafast electron dynamics in DWNT show ultrafast (97 fs) photo bleaching followed by a photo-induced absorption with a slow relaxation of 1.8 ps, whereas microwave treated SWNT suspension shows a bi-exponential ultrafast photo-bleaching with time constants of 160 fs (130 fs) and 920 fs (300 fs) for water soluble (amide functionalized) nanotubes.

We thank Department of Science and Technology for financial support. This work was done in collaboration with Bhalchandra Kakade, Vijayamohanan K. Pillai, Alexander Moravsky, R. O. Loutfy, Srini Krishnamoorthy, Shekhar Guha and C. N. R. Rao.

Reference:

1. “Large nonlinear absorption and refraction coefficients of carbon nanotubes estimated from femtosecond z-scan measurements”, N. Kamaraju, Sunil Kumar, Srinivasan Krishnamurthy, Shekhar Guha, A. K. Sood, C. N. R. Rao. Appl. Phys. Lett. 91, 251103 (2007).

2. “Ultrafast electron dynamics and cubic optical nonlinearity of free standing thin film of double walled carbon nanotubes”, N. Kamaraju, Sunil Kumar, B. Karthikeyan, Alexander Moravsky, R. O. Loutfy and A. K. Sood. Appl. Phys. Lett. 93, 091903 (2008).

3. “Ultrafast switching time and third order nonlinear coefficients of microwave treated single walled carbon nanotube suspensions”, N Kamaraju, Sunil Kumar, B. Karthikeyan, Bhalchandra Kakade, Vijayamohanan K. Pillai and A. K. Sood, J. Nanoscience and Nanotech. (2008).

Nonequilibrium Fluctuations in Sheared Jammed States

A.K. Sood Department of Physics, Indian Institute of Science Bangalore 560 012, India.

Recently we have shown that the shear rate at a fixed shear stress in a micellar hexagonal phase exhibits large fluctuations, including several negative values [1]. The probability distribution functions (PDF’s) of the global power flux to the system derived from the shear rate fluctuations are Gaussian or non- Gaussian, depending on the external drive (applied stress) on the system. In both cases, the PDF is consistent with the Gallavotti-Cohen steady state fluctuation theorem. We show that an effective temperature of the jammed state can be measured using fluctuation theorem. Interestingly, the non-Gaussian PDF of the global power flux to the system matches exactly with one of the universal extreme value distribution.

1. S. Mazumdar and A.K. Sood, Phys. Rev. Lett. 101, 78301 (2008).

Extended Einstein-Hilbert (E-H) action and the emergence of Riccions as cosmic dark matter and consequent cosmic acceleration

K.P.Sinha INSA Honorary Scientist Department of Physics Indian Institute of Science Bangalore 560 012.

Abstract:

When higher-order curvature terms are added to E-H action Ricci scalars behave as a massive physical field and obey Klein-Gordon type equation. These particles are called Riccions. The role of riccions as cosmic dark matter is explored. It is found that the universe will accelerate when appropriate conditions are satisfied particularly in the later period of the evolution of the universe.

The dual nature of the Ricci scalar in the present model solve many problems such as the removal of the gravitation singularity1, creation of spinless particles2 , the creation of spin particles and antiparticles3 and the dominance of baryonic matter besides being the source of dark matter4

References:

1. S.K.Srivastava and K.P.Sinha, Phys. Lett. B 307, 40 (1993) 2. K.P.Sinha and S.K.Srivastava, Pramana 44, 333 (1995) 3. S.K.Srivastava and K.P.Sinha, Mod. Phys. Lett A 12, 2933 (1997) 4. S.K.Srivastava and K.P.Sinha, Mod. Phys. Lett A. (in press)

Low frequency vibrational modes in carbon nanotubes: terahertz time domain spectroscopy

Sunil Kumar1, N. Kamaraju1, Marc Tondusson2, E. Freysz2 and A. K. Sood1,* 1Department of Physics and Center for Ultrafast Laser Applications, Indian Institute of Science, Bangalore 560 012, India 2CPMOH, Universite de Bordeaux I, 351, Cours de la liberation, 33405 Talence cedex, Fance * Electronic address: [email protected]

Ultrafast terahertz time domain spectroscopy (THz-TDS) has been used to study the far infrared (FIR) optical properties of double walled carbon nanotubes (DWNTs) in the 0.1- 3.0 THz frequency range. The 200 nm freely standing DWNT film, used in our experiments works as a THz filter in the time domain as the input THz power is cut by almost half while the spectral transmission remains almost constant in the 0.1-2.0 THz range. This feature, usually expected for metallic substrates with patterned whole arrays [1] or metamaterials with artificial structures [2], suggests a possible application of such films of carbon nanotubes (CNTs) for real time control and modulation of THz radiation. The spectral dielectric response of the film shows small but distinct resonant dispersive features at frequencies, 0.46 and 0.76 THz related to the very low frequency flexural modes of vibration around the tube axis. Such low frequency phonon modes in CNTs had been predicted theoretically [3, 4] but so far could not be detected experimentally. In addition we have observed a broad absorption band at around 1.45 THz similar to that seen in single walled CNTs (SWNTs) attributed to the electronic excitation across the FIR band gap induced by intertube coupling and curvature in bundles of metallic nanotubes [5]. The optical conductivity was measured to be very high, of the order of 2x1014 sec-1 which confirms the metallic nature of the film (optical conductivity of metals ~ 1016 sec-1).

References 1. C. L. Pan, C. F. Hsieh, R. P. Pan, M. Tanaka, F. Miyamaru, M. Tani, M. Hangyo, Opt. Exp. 13, 3921 (2005). 2. H. T. Chen, W. J. Padilla, J. M. O. Zide, A. C. Gossard, A. J. Taylor, Nature 444, 597 (2006). 3. H. Suzuura, T. Ando, Phys. Rev. B 65, 235412 (2002). 4. G. D. Mahan and Gun Sang Jeon, Phys. Rev. B 70, 075405 (2004). 5. M. Ouyang, J. L. Huang, C. L. Cheung, C. M. Lieber, Science 292, 702 (2001). Jamming and shear banding in two dimensional suspensions

Sayantan Majumdar, Rema Krishnaswamy and A. K. Sood

The flow behaviour of jammed systems which exhibit a solid-like elastic behaviour at low shear stress and flow above a critical yield stress is often described by phenomenological models, where a homogenous flow is predicted at a fixed shear stress. Recent studies however, indicate that most jammed systems unjam through the formation of shear bands where only part of the system flows. Two dimensional (2D) colloidal suspensions where the flow behaviour can be visualized relatively easily are eminently suited for understanding the role of the microstructure in determining the jamming or yielding behaviour. I will discuss some of our recent studies where we investigate the viscoelastic and flow behaviour of dense monodisperse and bidisperse 2D colloidal suspensions formed at fluid interfaces by combining surface rheology with optical microscopy.

In densely packed colloidal monolayers at packing fractions > 0.8, the shear rate at a fixed shear stress exhibits large fluctuations with positive and negative values. The validity of Steady State Fluctuation Relation in these sheared two dimensional suspensions will also be examined.

Phonon anomaly in pyrochlore spin-ice Dy2Ti2O7 and non-magnetic Lu2Ti2O7: A temperature dependent Raman and x-ray study

Surajit Saha,1 Surjeet Singh,2 B. Dkhil,3 S. Dhar,4 R. Suryanarayanan,2 G. Dhalenne,2 A. Revcolevschi,2 and A. K. Sood1

1Department of Physics, Indian Institute of Science, Bangalore, India. 2Laboratoire de Physico-Chimie de l’Etat Solide,Universite Paris-Sud, Orsay, France. 3Laboratoire Structures, Ecole Centrale Paris, Chatenay-Malabry Cedex, France and 4Tata Institute of Fundamental Research, Mumbai, India

Abstract

We present here temperature-dependent Raman, x-ray diffraction and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore

compound Dy2Ti2O7 and its non-magnetic analogue Lu2Ti2O7. Raman data show a “new” band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in Dy2Ti2O7, appearing below a temperature of Tc =110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction 18 analysis. Low temperature Raman experiments on O -isotope substituted Dy2Ti2O7 confirm the phonon origin of the “new” mode. These findings, absent in Lu2Ti2O7, suggest that the room temperature cubic lattice of the pyrochlore Dy2Ti2O7 undergoes a

“subtle” structural transformation near Tc. We find anomalous red-shift of some of the

phonon modes in both the Dy2Ti2O7 and the Lu2Ti2O7 as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.

Brownian Inchworm Model of a Self-propelled particle

K Vijay Kumar,1, ∗ Adrian Baule,2, † Sriram Ramaswamy,1, ‡ and Madan Rao3, §

1CCMT, Department of Physics, Indian Institute of Science, Bangalore 560012, India. 2The Rockefeller University, 1230 York Avenue, New York, NY 10065, USA. 3Raman Research Institute, Bangalore 560080, India.

Directed motion without an imposed external gradient is seen not only in living systems but also in agitated granular matter. The essential ingredients are an external energy input and an inherent asymmetry. Unlike traditional “Brownian ratchet models”of directed motion, the asymmetry of interest in the above systems is internal to the motile objects, and does not lie in an external periodic potential. In this work, we present a Brownian inchworm model of a self-propelled elastic dimer in the absence of an external potential. Nonequilibrium noise together with a stretch- dependent damping form the propulsion mechanism. The crucial asymmetry is in the stretch-dependence of the damping coefficients. Our model connects three key nonequi- librium features – position-velocity correlations, a nonzero mean internal force, and a drift velocity. Our analytical results, both perturbative calculations and an exact solution, dis- play striking current reversals and compare very well with numerical simulations [1, 2]. The model unifies the propulsion mechanisms of DNA helicases, polar rods on a vibrated surface, crawling keratocytes and Myosin VI.

∗ Electronic address: [email protected] † Electronic address: [email protected] ‡ Electronic address: [email protected]; Also at CMTU, JNCASR, Bangalore 560064, India. § Electronic address: [email protected]; Also at NCBS (TIFR), Bangalore 560065, India. [1] K Vijay Kumar, S. Ramaswamy and M. Rao, Phys. Rev. E, 77, 020102(R), (2008). [2] Adrian Baule, K Vijay Kumar and Sriram Ramaswamy, J. Stat. Mech., P11008 (2008). Thermoelectric Properties of Co4Sb12 Skutterudite Materials with In Addition

Ramesh Chandra Mallik*, Christian Stiewe, Gabriele Karpinski, Ralf Hassdorf, Eckhard Müller

German Aerospace Center (DLR), Institute of Materials Research,

D-51170 Cologne, Germany

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

Abstract

The properties of Co4Sb12 with various In additions were studied. X-ray diffraction technique used for structural and phase analysis. The homogeneity and morphology of the samples were observed by the Seebeck microprobe and scanning electron microscopy, respectively. The temperature dependence of the Seebeck coefficient, electrical and thermal conductivity was measured from room temperature up to 673 K.

The Seebeck coefficient of all In-added Co4Sb12 materials was negative. When the In concentration increases, then the Seebeck coefficient decreases. The samples with In additions above the limit (x= 0.22) show an even lower Seebeck coefficient due to the

formation of secondary phases: InSb and CoSb2. The temperature variation of the electrical conductivity is semiconductor-like. The thermal conductivity of all the samples

decreases with temperature. The central region of the In0.4Co4Sb12 ingot shows the lowest thermal conductivity probably due to the combined effect of (a) rattling due to maximum filling and (b) the presence of a small amount of fine -dispersed secondary phases at the grain boundaries. Thus, regardless of the non-single-phase morphology, a promising ZT value of 0.96 at 673 K has been obtained with an In addition above the filling limit.

Email: [email protected]

Scattering of electrons from an interacting region

Diptiman Sen (CHEP)

We study the problem of transmission of electrons through a region where they interact with each other. The two-particle problem turns out to completely solvable. We show that if there is no other source of scattering, the interactions always reduce the conductance from the value of 2e^2/h. But if the region itself causes scattering, interactions can either increase or decrease the conductance. We discuss the possibility of two-particle resonance, in which two particles with a particular value of the total energy can transmit through the region even though they cannot individually do so.

Collaborators: Abhishek Dhar and Dibyendu Roy (RRI) and Abhiram Soori Statistically Steady Turbulence in Soap Films: Direct Numerical Simulations with Ekman Friction

Prasad Perlekar1, ∗ and Rahul Pandit1, † 1Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, India. We present a detailed direct numerical simulation (DNS) designed to investigate the combined effects of walls and Ekman friction on turbulence in forced soap films. We concentrate on the forward-cascade regime and show how to extract the isotropic parts of velocity and vorticity structure functions and thence the ratios of multiscaling exponents. We find that velocity structure functions display simple scaling whereas their vorticity counterparts show multiscaling; and the probability distribution function of the Weiss parameter Λ, which distinguishes between regions with centers and saddles, is in quantitative agreement with experiments.

∗Electronic address: [email protected] †Electronic address: [email protected]; also at Jawaharlal Nehru Centre For Advanced Scientific Research, Jakkur, Bangalore, India Hyperviscosity, Bottlenecks and New Surprises in the Galerkin-truncated Burgers Equation1

Samriddhi Sankar Ray

It is shown that the replacement in hydrodynamical equations of the usual dissipation by a high power α of the Laplacian leads asymptotically to truncating the dynamics to a a finite number of Fourier modes. When this number is large, a range of thermalized modes appear through the mechanism discovered by Cichowlas et al. The dynamics at small and intermediate wavenumbers is governed by the ordinary Navier–Stokes equations but a huge bottleneck in thermal equilibrium with Gaussian statistics is present at large wavenumbers. The usual (α = 1) bottleneck can be viewed as an aborted thermalization. Practical implications for turbulence modelling are discussed. We then discuss surprises in the Galerkin-truncated Burgers Equation.

1. Hyperviscosity, Galerkin truncation and bottlenecks in turbulence, U. Frisch, S. Kurien, R. Pandit, W. Pauls, Samriddhi Sankar Ray, A. Wirth, and J-Z Zhu. Phys. Rev. Lett. 101, 144501. General framework for quantum search algorithms

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

Grover’s quantum search algorithm drives a quantum computer from a prepared initial state to a desired final state by using selective transforma- tions of these states. We present a framework when one of the selective transformations is replaced by a more general unitary transformation. Our framework encapsulates several previous generalizations of Grover’s algo- rithm like Kato’s algorithm, Ambainis’ algorithm for element distinctness problem, spatial search algorithms, and quantum random walk search algo- rithms. Our framework also explains the phase-matching condition.

We discuss two applications of our framework. The first one is related to the problem of two-dimensional spatial search. The previous algorithms take √N ln N steps to solve this problem, and it was an open question whether the performance can be improved. Our framework allows us to design a faster algorithm that solves the problem in √N ln N steps. The second application is a better quantitative understanding of adiabatic search algorithms.

References

[1] General framework for quantum search algortihms Avatar Tulsi, quant-ph/0806.1257, Accepted for publication in Phys. Rev. A.

[2] Faster quantum walk algorithm for the two-dimensional spatial search Avatar Tulsi, Phys. Rev. A 78, 012310 (2008).

[3] Adiabatic quantum computation with a one-dimensional projector Hamilto- nian. Avatar Tulsi, quant-ph/0806.0385, Accepted for publication in Phys. Rev. A.

1 Crystal Structures of native and substrate complex of an alkali thermostable xylanase from an alkalophilic Bacillus sp. NG-27. Structural insights into alkalophilicity.

Ramakumar, S. Department of Physics Indian Institute of Science, Bangalore – 560012, INDIA

Crystal structures are known for several glycosyl hydrolase family 10 (GH10) xylanases. However, none of them is from an alkalophilic organism that can grow in alkaline conditions. We have determined the crystal structures at 2.2 Å of a GH10 extracellular endoxylanase (BSX) from an alkalophilic Bacillus sp. NG-27, for the native and the complex enzyme with xylosaccharides. The industrially important enzyme is optimally active and stable at 343 K and at a pH of 8.4. Comparison of the structure of BSX with those of other thermostable GH10 xylanases optimally active at acidic or close to neutral pH showed that the solvent exposed acidic amino acids. Asp and Glu, are markedly enhanced in BSX, while solvent- exposed Asn was noticeably depleted. The BSX crystal structure when compared with putative three-dimensional homology models of other extracellular alkalophilic GH10 xylanases from alkalophilic organisms suggests that a protein surface rich in acidic residues may be an important feature common to these alkali thermostable enzymes. A comparison of the surface features of BSX and of halophilic proteins allowed us to predict the activity of BSX at high salt concentrations, which we verified through experiments. This offered us important lessons in the polyextremophilicity of proteins, where understanding the structural features of a protein stable in one set of extreme conditions provided clues about the activity of the protein in other extreme conditions. The work brings to the fore the role of the nature and composition of solvent-exposed residues in the adaptation of enzymes to polyextreme conditions, as in BSX. Intrinsic magnetism in high-mobility semiconductor – a bottom-up route to spin- electronics?

Arindam Ghosh

Ultra high-mobility GaAs/AlGaAs heterostructures are excellent systems for simulating elementary magnetic interactions with great control. The usual methods of generating magnetism in these semiconducting systems are two-fold: (1) defining quantum dot assemblies where each dot contains an odd number of electrons so that they play the role of d-electrons embedded in a metallic matrix, or (2) doping GaAs with magnetic atoms such as Mn. Here we propose a new route towards achieving a long-range magnetism in GaAs/AlGaAs heterostructures that does not require either formation of quantum dots or external doping of magnetic atoms. The idea is based on a two-component nature of background potential fluctuations in modulation-doped GaAs/AlGaAs systems which naturally trap localized spins at specific values of conduction electron density. A surface gate can modulate the mutual interaction between the spins, making the magnetic state tunable electrically. Although the present devices required the experiments to be carried out at sub-Kelvin temperature range, our experiments suggest that such a model can be utilized in realizing spin field-effect transistors in low-dimensional semiconductors over a wide parameter range.

Ref: [1] A. Ghosh et al., Phys. Rev. Lett. 92, 116601 (2004); [2] A. Ghosh et al., Phys. Rev. Lett. 95, 066603 (2005). [3] C. Siegert, A. Ghosh, M. Pepper, I. Farrer and D. A. Ritchie, Nature Physics 3, 315 (2007). [4] C. Siegert, A. Ghosh, M. Pepper, I. Farrer, D. A. Ritchie, D. Anderson and G. A. C. Jones, Phys. Rev. B (2008).

The incongruous observation of phase separation in La0.85 Sr0.15CoO3 spin –glass system D. Samala, C. Shivakumarab and P.S. Anil Kumara

aDepartment of Physics, Indian Institute of Science, Bangalore, 560012, India bSolid State and Structural Chemistry Unit, Indian Institute of Science

Phase separation in hole-doped Cobaltites (La1-xSrxCoxO3) is drawing renewed interest recently.

In particular, the magnetic behavior of La0.85Sr0.15CoO3 has been subjected to a controversial debate for the last several years; while some groups show evidence for phase separation, others show spin glass behavior. An attempt is made to resolve the controversy related to “phase separation versus spin glass” behavior in this compound. Here, we present the results of comprehensive investigation of dc magnetization, ac susceptibility, and magnetotransport properties of La0.85Sr0.15CoO3 samples. We contemplate that the phase separation in

La0.85Sr0.15CoO3 is neither intrinsic nor inherent but it is a consequence of the preparation conditions. It is realized that low temperature annealed sample (LTA) shows phase separation where as the high temperature annealed (HTA) sample shows pure spin glass behavior. The Brillouin like behavior of field cooled dc magnetization with temperature and apparently no frequency dependent peak shift in ac susceptibility for LTA sample characterizes it to be of ferromagnetic like where as a kink in field cooled dc magnetization and considerable amount (~3K) of frequency dependent peak shift in ac susceptibility for HTA sample characterizes it to be a pure spin glass state. The magneto transport properties show that the HTA sample is more semiconducting as compared to LTA sample. This is interpreted in terms of presence of isolated as well as coalescing metallic ferromagnetic clusters in case of LTA sample. Both LTA as well as HTA samples exhibit glassy transport phenomena i.e. glassy response to application or removal of application of magnetic field with respect to time. The magnetoresistance at 10K for HTA sample exhibits a huge value (~65%) as compared to the LTA sample and it monotonically decreases with rise in temperature. Such a high value of MR in case of HTA sample is strongly believed due to spin dependent part of random potential distribution. Further, the slow decay of remnant magnetization with progress of time and existence of hysteresis at higher temperatures (up to 200K) in case of LTA sample as compared to HTA sample clearly unveils different magnetic states associated with them. e-mail: [email protected]

New electrochemistry based route to Materials Engineering in Nanoscale Systems.

Amrita Singh and Arindam Ghosh Department of Physics, Indian Institute of Science, Bangalore, 560012

By appropriate engineering of electro-deposition technique, we have realized silver nanowires (AgNWs) in both hcp and fcc phase at room temperature. The nanowires stabilized in high energy 4H phase [1], exhibit some exotic properties. The measured noise level is heavily suppressed in 4H AgNWs as compared to fcc silver nanowires which is attributed to restricted dislocation dynamics in hexagonal crystal structure [2]. The time resolved transmission electron microscopy (TEM) shows a kinemetically new growth mechanism during electro-deposition. We observe a temperature independent noise behavior in hcp AgNWs, which is uncommon for metals. Moreover, these nanowires also exhibit a temperature induced phase transformation and acquire an unconventional fcc phase at low temperatures, which is supported by temperature dependent TEM measurement, higher order resistance fluctuation spectroscopy and Differential Scanning Calorimetry. Thus, the electron transport results pave a new understanding of the crystal structure dependent microscopic origin of noise in single crystal nanowires and our growth technique promises to produce high quality nano-materials with unprecedented control on its electrical and structural properties.

[1] Amrita Singh and Arindam Ghosh, J. Phys. Chem. C, 112, 3460-3463, 2008. [2]Amrita Singh, T. Phanindra Sai and Arindam Ghosh, Applied Physics Letters 93, 102107 (2008).

Ultrasound-modulated Diffuse Optical Tomographic Imaging System for Screening Breast Cancer

M Suheshkumar Singh and K Rajan Department of Physics, Indian Institute of Science Bangalore-560012, India.

R M Vasu Department of Instrumentation, Indian Institute of Science Bangalore-560012, India.

Near-Infrared (NIR) diffuse optical tomography (DOT) has been a promising tool for non-invasive imaging of optical parameters of tissues inside body over the past 15 years. Unfortunately, due to diffusive nature of light, sophisticated reconstruction algorithms are required to achieve good imaging depth and a reasonable resolution. Ultrasound (US), on the otherhand, is less scattered by soft tissues and it has been in use for imaging in biomedical ultrasound systems. Combination of the contrast sensitive property of light and good localization of ultrasound provides a challenging technique to reconstuct images of thicker biological tissues deep inside the body non-invasively. The US modulated optical signals which traverse the tissue are detected by using a CCD camera as detector array and the pixel map formed on the CCD is used to characterise the embedded inhomogeneity. The use of CCD camera improves the signal-noise-ratio (SNR) by averaging the signals from all of the CCD pixels. The conventional filtered backprojection algorithm is used for reconstruction. The pixel map is a measure of the projection data. A sequence of two to four images are captured on CCD camera with phase of US modulation signal shifted by 0◦, 90◦, 180◦ and 270◦ w.r.t. the signal of the laser source. From these images, the modulation depth (ac/dc) and the correlation co-efficient of the signals which are the signatures of the pathological changes can be extracted. It is possible to reconstruct 2-D or 3-D images of a tissue object parameters using this method. For pre-clinical study of validity of the system, an inanimate object (phantom) is used as sample tissues. The phantom used is made of poly vinyl alcohol (PVA) and its mechanical, optical, and acoustic properties are tailored by subjecting an acquous solution of PVA stock and water to a suitable number of freeze-thaw cycles and by varying the degree of hydrolysis in the PVA stock.

Keywords: Insonified, Tagged, Inhomogeneity, Backprojection A filament in an active medium: buckling, stiffening and negative dissipation

Norio Kikuchi1, Allen Ehrlicher2, Daniel Koch2, Josef A. K¨as2, Sriram Ramaswamy1, and Madan Rao3,4 1 Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2 Institute of Soft Matter Physics, Linn´estraße 5, 04103 Leipzig, Germany 3 Raman Research Institute, C.V. Raman Avenue, Bangalore 560 080, India 4 National Centre for Biological Sciences (TIFR), Bellary Road, Bangalore 560 065, India

We present a generic theory for the dynamics of a stiff filament in an active medium with orientational correlations, such as a microtubule in contractile actin. In sharp contrast to the case of a passive medium, we find the filament can stiffen, and possibly oscillate, or buckle, depending on the contractile or tensile nature of the activity and the filament-medium anchoring interaction. We present experiments on the behaviour of microtubules in the growth cone of a neuron, which provide evidence for these apparently opposing behaviours. We also demonstrate a strong violation of the fluctuation-dissipation (FD) relation in the effective dynamics of the filament, including a negative FD ratio. Our theory also applies to the dynamics of axons and auditory hair cells. Detailed tests of our predictions can be made using a single filament in actomyosin extracts or bacterial suspensions.

POSTER ABSTRACTS

Enhancement of sensitivity of detection of Kerr rotation by time averaging

Arjun Joshua and V. Venkataraman Abstract Experiments in spintronics necessarily involve the detection of spin polarization. The sensitivity of this detection becomes an important factor to consider when extending the low temperature studies on semiconductor spintronic devices to room temperature, where the spin signal is weaker. In pump-probe experiments which optically inject and detect spins, the sensitivity is often improved by using a photoelastic modulator (PEM) for lock-in detection. However, spurious signals can arise if diode lasers are used as optical sources in such experiments, along with a PEM. In this work, we eliminated the spurious electromagnetic coupling of the PEM onto the probe diode laser, by the double modulation technique. We also developed a test for spurious modulated interference in the pump-probe signal, due to the PEM. Besides, an order of magnitude enhancement in the sensitivity of detection of spin polarization by Kerr rotation, to 3 × 10−8 rad, was obtained by using the concept of Allan variance to optimally average the time series data over a period of 416 s. With these improvements, we are able to experimentally demonstrate at room temperature, photoinduced steady-state spin polarization in bulk GaAs. Thus, the advances reported here facilitate the use of diode lasers with a PEM for sensitive pump-probe experiments. They also constitute a step towards detection of spin-injection in Si at room temperature. Reentrant phase behavior of a mixed surfactant system with strongly binding counter-ions

Vikram Rathee1, Sajal Kumar Ghosh2, Rema Krishnaswamy1, V. A. Raghunathan2 and A. K. Sood1

1Department of Physics, Indian Institute of Science, Bangalore 2Raman Research Institute, Bangalore

The role of strongly binding counter ion in a mixed surfactant system consisting of oppositely charged amphiphiles, is to decrease the spontaneous curvature of the micellar aggregates giving rise to a change in their morphology from Spheres Rods Disks Vesicles , on varying the surfactant composition. Hence a rich phase behavior is observed in the concentrated micellar solutions of such systems with a variety of liquid crystalline phases. We will present our recent studies on the microstructure and dynamics of micellar solutions consisting of an anionic surfactant sodium dodecyl sulphate (SDS) and a strongly binding counter ion paratoluene hydrochloride (PTHC).

Small angle x-ray scattering studies reveal a novel route from hexagonal to lamellar phase through a reentrant phase transition rod like Nematic Isotropic disk like nematic, on varying α= [PTHC]/[SDS], molar ratio at a fixed surfactant concentration (1). We will present our studies on SDS+PTHC system with higher surfactant concentration. A distinct viscoelastic and flow behavior is observed for the two nematic phases. The results are analyzed in conjunction with the observation from polarizing microscope and x-ray scattering measurements.

(1) S.K.Ghosh, Ph.D. Thesis; Raman Research Institute, Bangalore (unpublished). Structural, magnetic and EMR studies of CaMnO3 nanoparticles

Geetanjali, D. Banerjee and S V Bhat

Calcium manganite, CaMnO3 is the end member of the much studied doped rare earth pervoskite RE1-xCaxMnO3 (x=1) and is a G-type antiferromagnetic insulator, with a Neel temperature of 120 K. Recently we have shown that [1] many rare earth manganites loose their antiferromagnetic ground state and become ferromagnetic when prepared in nano scale (nanoparticles and nanowires). In this work, we investigate the behavior of CaMnO3 nanoparticles.

Sol-gel method was used to prepare the calcium manganite nanoparticles (D~50 nm). XRD, TEM and SEM were used to characterize the sample. X-band EMR studies were carried out between room temperature and 4 K. At room temperature, a relatively narrow (ΔHpp ≈ 900 Oe) and strong signal was observed in contrast with the broad (ΔHpp ≈ 1300 Oe), weak signal observed in the bulk CaMnO3 [2]. The line-width shows a further reduction around 270 K indicating a possible ferromagnetic transition and a sharp increase around 120 K characteristic of an antiferromagnetic transition. The details of ac-susceptibility measurements and the analysis of EMR parameters will be presented in an attempt to understand the magnetism in nanoparticles of CaMnO3.

References:

[1] S.S.Rao, K.N. Anuradha, S.Sarangi, and S.V. Bhat; APL., 87, 182503 (2005) S.S.Rao, S. Tripathi, D.Pandey and S.V. Bhat, Phys. Rev. B 74, 144416 (2006) [2] S.B. Oseroff, M. Torikachvili, J. Singley, S. Ali, S.W. Cheong, S.Schultz, Phys. Rev. B 53, 6521 (1996)

Acknowledgments: The authors would like to thank M.M. Borgohain, K.G. Padmalekha and K.S. Bhagyashree for help in experiments. Dr. P.S. Anil Kumar’s help in ac-susceptibility measurements is gratefully acknowledged. Magnetic and Magnetotransport studies of ZnO films with and without intrinsic magnetic moment

Sayak Ghoshal and P.S.Anil Kumar Department of Physics, Indian Institute of Science, Bangalore-560012, INDIA

Transition metal doped ZnO was perceived to be an important material in view of the applications in Spintronic devices. Recently, we have shown that under suitable conditions of preparation, pure ZnO also can exhibit intrinsic magnetic moment and we were able to tune this moment by tuning the oxygen content in the pure ZnO films. We have also shown that this intrinsic moment is suppressed by an order of magnitude by doping with transition metal, in our case Cobalt. In addition, the magnetic properties of the bulk Co doped ZnO is also process dependent. Transport studies on the pure ZnO films reveal the correlation between the carrier concentration and the magnetic property of these films, which in turn helps us to exclude the extrinsic origin of the moment. We observed negative magnetoresistance (MR) for the ZnO films with higher conductivity and with intrinsic moment and positive MR for the less conducting diamagnetic ZnO films at 4.6K. For the doped sample we observe competition between the positive and negative MR and their behavior at different temperatures and fields. In this presentation, magnetic and magneto-transport properties of pure as well as Co doped ZnO films will be presented. Ginzburg Landau Theory for Cuprate Superconductivity

Sumilan Banerjee, Chandan Dasgupta, and T. V. Ramakrishnan∗ Dept. of Physics, Indian Institute of Science, Bangalore 560012. ∗Also Banaras Hindu University, Varanasi 221005, India. Abstract We propose and develop the consequences of a theory in which the free energy F of a cuprate is expressed as a functional of the complex nearest neighbour spin singlet bond pair order parameter 2 4 ∆ij exp (iφij ). F is a sum of two terms P (a∆ +b∆ ), and F1 = c P ∆mn cos (φm φn); here m m m mn − m is the site corresponding to ij on the dual lattice (also square) and m,n are nearest neighbours. The doping x and temperature T dependences of a, b and c are rationalized ( eg, c x for small x). ∝ The pseudogap ( due to incoherent bond pairs) and the parabolic x dependence of Tc ( AF ordering of the 2d-XY spin ∆m exp (iφm) leading to d wave superconductivity) are described. The observed

Cv(T ) behaviour is shown to be due to order parameter fluctuations. Detailed calculations of the vortex structure show a crossover from a Josephson like to a BCS like form with increasing doping, mirroring a similar change in superconductivity.

1 Non Equilibrium noise in electrophoresis: the micro-ion wind Suropriya Saha and Sriram Ramaswamy

We have obtained the extended nonequilibrium Langevin eqs for a single colloidal particle in externally applied electric field. The eq. for the system has been obtained by Squires who has assumed that only source of noise is thermal and satisfies Fluctuation-dissipation. We have looked into the manner in which the fluctuations of the counter-ion concentration lead, in the presence of an electric field to additional sources of noise in the colloid motion. We have shown that the particle shows enhanced brownian motion (the ‘extra-noise’ being proportional to field strength), i.e. systems are in steady non-equillibrium states. We have studied two systems- SysI: A neutrally buoyant particle drifting uniformly under the field in an unbounded fluid; SysII: A particle with density higher than the fluid, stably levitated by the balance between gravity sedimenting it towards a wall and the electric field driving it away from the wall. A qualitative measure of the extra-noise has been obtained for the two cases. In the first case the noise is found to be comparable or two to three orders of magnitude more than the thermal noise. In the second case the noise is found to be at least comparable to the thermal noise. In both cases the extra noise is anisotropic- strength in the electric field direction is nearly an order of magnitude more than that in the transverse T ωS(ω) plane. An effective temperature ( eff ) is defined as the ratio 2Imχ(ω) , which would be the temperature in a thermal equilibrium system. Teff is obtained as a function of frequency ( =0) ω T ω Teff ω ω ‘ ’ and is shown that eff has strong ‘ ’ dependence; T changes by a factor of 2 as changes from 0 to 5.5Dκ2 (D = diffusivity of the micro-ion s, κ−1 = debye screening length). Nonequilibrium effects are clearly very significant in these systems.

Abstarct for Poster

1 Magnetic Anisotropy in Epitaxial Fe/MgO films grown by Pulsed Laser Deposition

Sakshath. S. and P. S. Anil Kumar Department of Physics, Indian Institute of Science, Bangalore 560012 Email: [email protected]

Abstract We report the observation of uniaxial in-plane magnetic anisotropy using MOKE studies on the top ferromagnetic layer of the Fe/MgO/Fe single crystalline Magnetic tunnel junctions grown epitaxially on GaAs(100) substrates. Squid measurements show that independent magnetic switching of the ferromagnetic layers is present at sufficiently thick MgO layers. Observation of the exchange spring behavior in hard-soft- ferrite nanocomposite

Debangsu Roya, and P.S. Anil Kumara*

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

Bangalore, 560012, India

Nanocomposite of hard and soft ferrite are prepared by the mixing of the individual

ferrite components at appropriate ratio and subsequent heat treatment. Initially the

microstructure of the individual phases is controlled by suitable processing. We have

observed the exchange spring behavior in the soft-hard ferrite composite for the first time

by tailoring the particle size of the individual phases and by suitable thermal treatment of

the composite. It is found that the exchange interaction dominates over the dipolar

interaction for smaller particle sizes of the soft ferrite. The magnetization of the

composite showed hysteresis loop that is characteristic of the exchange spring system.

With the following method of preparation we were successful enough to get an enhancement in the Fe3O4 and BaCa2Fe16O27 nanocomposite. An enhancement of

(BH)max of ~13% for a particular mixture compared to the parent hard Ferrite. The

presence of the exchange interaction between the hard and the soft grains was confirmed

by the Henkel plot.

Probing the local stress in Graphene by Raman Spectroscopy

Biswanath Chakraborty, Anindya Das and A.K. Sood

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

We observe the strain induced softening of G and 2D Raman mode of monolayer graphene, on Si/SiO2. The strain in the graphene flake is largely due to the fabrication process. During mechanical exfoliation, the graphene layer is stretched as a result of friction between the HOPG adhered to scotch tape and the oxide surface. We show a softening up to ~ 32 cm-1 for G peak along with a 2D softening of ~ 80 cm-1. Both these modes exhibit splitting into two peaks, which evolve as we vary the incident polarization direction. This evolution serves as an easy method to determine the local stress axis along the flake. We also report a non uniform distribution of strain on the graphene sample. A tensile strain of ~0.4% is reported here.

Electrical detection of spin polarized electrons in semiconductors using a radio-frequency pick-up coil

Chinkhanlun Guite and V. Venkataraman Department of Physics, IISc Bangalore

We present here preliminary results from an experiment which involves the measurement of electronic spin polarization in bulk semiconductors using electromagnetic induction. The main idea is to excite the electrons from the valence band to the conduction band using a circularly polarized laser. Due to the strong spin-orbit coupling in the valence band, a part of the angular momentum of the circularly polarized light is transferred to the electron which allows the excited electron to be spin polarized to certain degree of efficiency, for e.g. 50% for GaAs. The spin of the excited electron decays with a lifetime of about 50 ps at 300 K. In steady state, for a pump intensity of about 200mW at 808nm, a spin polarized density of ~109 cm-3 can be achieved in GaAs. The weak magnetization generated by this population can be detected by sensitive modulation techniques.

The laser light was phase modulated using an electro-optic modulator from left to right circular polarization at a high frequency of 2.5 MHz. This creates an alternating spin polarization in the sample, which generates varying magnetic field at the frequency of the modulator. Using a sensitive inductive pick-up coil with a Q~40 at 2.5MHz and an RF Lock- in Amplifier, the induced voltage can be detected and studied. We have measured a signal of ~10nV in our experimental setup against an expected signal of ~2nV. We are currently involved in eliminating the spurious intensity modulation background, which is ~5-10 times larger than the signal, by suitably choosing the phase-modulation scheme. We believe this measurement technique will give an insight into the behavior of electronic spins inside GaAs and can be extended to other materials, particularly indirect semiconductors such as Ge. Polymer Addtives in Decaying Two-Dimensional Turbulence

Anupam Gupta, Prasad Perlekar, and Rahul Pandit Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, INDIA

We present direct numerical simulations of decaying, two-dimensional isotropic turbulence with polymer additives. We find that, on the addition of polymers to the turbulent fluid, the enstrophy- dissipation rate reduces. This is the two-dimensional analogue of energy-dissipation-rate reduction in three-dimensional turbulence. We relate the reduction of the enstrophy-dissipation rate to changes in the fluid energy spectrum, in the presence of polymers, especially at small length scales. Numerical study of a glass forming liquid in two dimensions to test the dimension dependence of the Adam-Gibbs relation

Smarajit Karmakar1,∗ Chandan Dasgupta1,2,,† and Srikanth Sastry2,‡ 1Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore, India 2 Condensed Matter Theory Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India.

In a recent study [1], we confirmed the existence of a dynamic correlation length that grows as the liquid is supercooled, by performing a detailed finite-size scaling analysis of dynamic heterogeneity in a model glass-forming liquid. The observed behaviour of this correlation length as a function of temperature is qualitatively consistent with recent predictions of inhomogeneous mode coupling theory. However, our study also showed that it is necessary to take into account properties of the underlying potential energy landscape (in particular, the configurational entropy) to understand the dynamics of supercooled liquids even above the dynamic transition temperature of mode coupling theory. Contrary to expectations, we found that the empirical Adam-Gibbs relation [2] between the relaxation time and the configurational entropy describes our simulation results very well. Motivated by this observation, we have examined whether the same Adam-Gibbs relation also holds good in any arbitrary dimension. We have tried to determine whether the Adam-Gibbs relation has any dimension dependence by performing molecular dynamics simulations of the Kob-Anderson binary mixture [5] in two dimensions. This question arises partly because of the recent success of the Random First Order Transition Theory (RFOT) [3, 4] where configurational entropy plays a major role, leading to a dimension dependent Adam-Gibbs-type relation[2]. We have also performed a detailed finite-size scaling analysis of a four-point density-density correlation function [6] to extract the underlying correlation length and calculated the configurational entropy to test whether one can explain the results using RFOT. We found that indeed the Adam-Gibbs relation has dimension dependence, but our results are not fully consistent with the predictions of either mode coupling theory or RFOT.

[1] Growing length and time scales in glass forming liquids - Smarajit Karmakar, Chandan Dasgupta, and Srikanth Sastry, http://arxiv.org/abs/0805.3104 [2] Adam, G and Gibbs, J H (1965),J. Chem. Phys. 43:139–146. [3] T. R. Kirkpatrick and P. G. Wolynes, Physical Review A. 35, 3072 (1987) [4] T. R. Kirkpatrick and D. Thirumalai, Physical Review A. 40, 1045 (1989) [5] Kob, W and Andersen, H C (1995), Phys. Rev. E 51:4626–4641. [6] C Dasgupta, A V Indrani, S Ramaswamy and M K Phani, Europhys. Lett. 15, 307(1991)

∗Electronic address: [email protected] †Electronic address: [email protected] ‡Electronic address: [email protected] Compositional dependence of the optical changes in (As2S3)1-xSbx chalcogenide film. Ramakanta Naik,R.Ganesan,K.S.Sangunni. Department of Physics, Indian Institute of Science, Bangalore, 560012,India.

Arsenic trisulfide (As2S3) is the most studied chalcogenide glass and deserves applications in infrared optics and optical coatings because of its excellent IR transmission, large glass forming tendency, and resistance to moisture and chemicals. Studies on thin films of Sb2S3 are attracting wide attention, for its special applications as a target material for television cameras, microwave devices, switching devices and various opto electronic devices. Even though As and Sb belongs to the same group of the periodic table, As2S3 and Sb2S3 do not display the same glass forming tendency. Glassy Sb2S3 is very difficult to form and its preparation requires high cooling rates. However, addition of As2S3 to Sb2S3 enhances greatly the glass forming ability of the later and glasses in the mixed As-Sb-S system can be formed. Recently we prepared the ternary As-S-Sb chalcogenide glass composition (As2S3)1-xSbx by keeping As/S ratio same with 2%,7%,10% and 15% of the Sb content. The purpose of the work is two fold; first to study the local glass structure as a function of composition and second to explore the possibility of mixed As-S-Sb bridges. A number of works exist which trace the influence of composition and preparation conditions of the films on the physiochemical properties. These glasses are sensitive to the absorption of electromagnetic radiation and show a variety of photo induced effects as a result of illumination. These changes are accompanied by changes in the optical constants, i.e., changes in the optical band gap, refractive index and optical absorption coefficient. These light-induced changes are favoured in chalcogenide glasses due to their structural flexibility (low coordination of chalcogens) and also due to their high-lying lone-pair p states in their valence bands. Annealing of chalcogenide glasses can affect the photoinduced changes, in particular irreversible effects occur in as-deposited films, while reversible effects occur in well-annealed films as well as bulk glasses. Changes in local atomic structure are observed on illumination with light having photon energy near the optical band gap of the chalcogenide. The film composition was checked by EDAX. The compositional dependence of the (As2S3)1-x Sbx films were studied through FTIR, XPS and Raman experiments. Addition of Sb results in the increase of density of the bulk (As2S3)1-x Sbx alloy. The optical band gap of the (As2S3)1-x Sbx films decreases while the width of localized states increases with the increase in Sb content. The peak shift and intensity variation of the core level spectra were resulted due to the addition of Sb. It can be suggested that the photo and thermally induced changes are caused by chemical bond redistribution,according to the equation 2M-S=M-M+S-S (1) Where M stands for As or Sb, S is for sulfer and dash denotes a chemical bond. The equilibrium of Eq. (1) can be moved in both directions, depending on the initial state of the film and the conditions of light exposure or annealing. Photo and thermally induced changes can be assigned to chemical bond redistribution.Bleaching of the asprepared film due to annealing is assigned to an increase of heteropolar bond density. Photodarkening of the illuminated film and annealed illuminated film is due to the increase in homo polar bond density. Measurement of Heteronuclear dipolar couplings- A systematic study of cross polarization dynamics for Separated Local Field (SLF) experiments

Nitin P. Loboa and K.V. Ramanathanb aDepartment of Physics and bNMR Research Centre Indian Institute of Science, Bangalore-560012

Separated Local Field (SLF) spectroscopy based on the transient oscillations observed during cross polarization represents a class of two dimensional solid state NMR experiments in which the coherent transfer of energy between dipolar coupled spin systems provides a means of resolving heteronuclear dipolar couplings on the basis of chemical shifts and characterizing them. It has become one of the important methods of structural elucidation of oriented molecules. Nematic liquid crystals, which orient in magnetic field have been extensively studied by this technique. In this method spin evolution during polarization transfer gives rise to the dipolar cross peaks, which for a two spin system can be understood to arise from evolution under mutually commuting zero and double- quantum subspaces. Under the conditions of exact Hartman-Hahn match, the evolution in the zero-quantum sub space alone contributes to the dipolar cross peak while the evolution in the double-quantum sub-space contributes only to the axial-peak. Increasing the cross peak intensity enhances the sensitivity of the experiment, while suppression of the axial-peak enables resolution of cross-peaks arising from small dipolar couplings. This requires that the initial density matrix is essentially of zero-quantum in nature which can, for example, be achieved by Polarization Inversion (PI). Subsequently, the dipolar oscillations are monitored with the removal of the homonuclear dipolar couplings by a suitable decoupling scheme. Experiments on simple oriented systems has been carried out to estimate the relative cross-peak to axial-peak intensities in 2D-SLF experiments based on the dipolar oscillations during cross polarization. Along with CP pulse sequence other pulse sequences namely CP with Polarization Inversion (PI-CP), another novel variation of the standard CP experiment i.e. Equilibrium X-nuclear- polarization enhanced cross-polarization (EXE-CP) and PISEMA experiment have been considered. In combination with Frequency Switched Lee- Goldburg (FSLG) homonuclear decoupling sequences, experiments on the oriented liquid crystalline samples carried out and the performance of the pulse schemes compared. Since Dipolar couplings provide useful structural and dynamics information the present study is aimed at optimizing and improving methods used for the measurement of heteronuclear dipolar couplings for static oriented samples using solid state NMR methods. Detection of microcantilever deflection using electron transport measurements S.M. Mohanasundaram and Arindam Ghosh Department of Physics, IISc

Abstract:

Using Silicon bulk micromachining technique, we have fabricated Silicon dioxide micro- cantilevers. On top of it, we have deposited and patterned Gold thin films using lift-off process. When we apply a force on the micro-cantilever, it bends and applies an elastic strain on the metal film. For our initial experiments, force is applied by using the surface tension of an evaporating liquid (propan-2-ol). Electrical resistance is measured during the process. We have found that the mechanical strain clearly translates into a change in the electrical resistance. We are also planning to measure the resistance fluctuations as function of mechanical strain. This kind of micro-cantilevers can be used for applications such as Atomic Force Microscopy (AFM), bio-molecular sensing, etc.

A fluctuation-based probe to criticality in structural transitions Chandni U and Arindam Ghosh Department of Physics, Indian Institute of Science, Bangalore- 12.

Many natural phenomena, extending from geophysics, biology to material science, involve slowly driven dissipative systems that are far from thermal equilibrium. The universal critical dynamics in these systems defines a class of athermal phase transitions that are triggered only by an external field that act as the tuning parameter and to which the systems respond through scale‐free avalanches in physical observables. In spite of decades of research, the relevance of the tuning parameter in this scale‐invariant dynamics at criticality is still an open question. Experiments are inconclusive whether these systems self organize to the critical state over a broad range of external field, or if there exists a unique critical point that is smudged by a wide critical zone as postulated by the concept of ‘plain‐old criticality’. Here, through experimental determination of time dependent avalanches, or noise, in electrical resistivity during temperature‐driven martensite transformation in thin nickel‐titanium films, we demonstrate that higher order statistics of avalanches can serve as an excellent kinetic detector of criticality in continuously driven non‐equilibrium systems. We show, for the first time, the existence of a singular `global instability' or divergence of the correlation length as a function of temperature in martensite transition indicating, (i) mapping of non‐equilibrium first order phase transition and equilibrium critical phenomena, and (ii) conventional nature of critical behavior, even though many previous experimental results as well as theoretical models predict a self‐ organized criticality in such systems. These results not only establish an independent experimental probe to critical phenomena in a wide variety of complex natural systems, but perhaps also call for a re‐evaluation of existing experimental data in various cases.

References: 1. "Signature of martensitic transformation on conductivity noise in thin films of NiTi shape memory alloys", Chandni U., Arindam Ghosh, H. S. Vijaya and S. Mohan, Applied Physics Letters 92, 112110 (2008). (Also in arXiv:0811.0101 (2008)) 2. “Criticality of tuning in athermal phase transitions”, U. Chandni, Arindam Ghosh, H. S. Vijaya and S. Mohan, arXiv:0811.0102 (2008).

Temperature dependent photoluminescence study in Hg1-xCdxTe

(x~0.8) nano and microcrystals

Jayakrishna Khatei and K.S.R. Koteswara Rao

Abstract

Hg1-xCdxTe (MCT) nanoparticles have been prepared by solvothermal method which is a facile, low-cost, solution phase approach to the large scale preparation of MCT at 180oC. Different compositions of MCT (x~0.1, 0.3, 0.4, 0.5 & 0.8) have been prepared by varying the proportion of Hg and Cd. EDAX result revealed that the compositions are very much close to the intended one. The photoluminescence (PL) measurements of nanocrystalline Hg1-x Cdx Te (x~0.1, 0.3, 0.4, 0.5 & 0.8) have been performed at room temperature as well as low temperature. The temperature dependent PL characterization has been done over the temperature range 10-300 K on higher composition Hg1-xCdxTe

(x~0.8) nanocrystals. The detailed characteristics of the PL emission which shows a broad emission from 0.6eV to 1eV consisting of 5 prominent peaks (0.92, 0.76, 0.68, 0.61 and 0.57eV) and the peak intensity at different temperature were studied. With increasing temperature, these peaks show different temperature dependences. The intensity of most of the peaks increases with temperature from 10 K to 50 K and then the intensity decreases with increase in the temperature. The position of the emission peaks remains constant with change in temperature. These anomalous intensity behaviors of the PL spectra are explained on the basis of defect levels.

* The results will be presented as poster. The Dependance of Spiral-Wave Dynamics on Inhomogeneities in the TNNP Model of Cardiac Tissue

Alok Ranjan Nayak1, T.K. Shajahan2, and Rahul Pandit1 1Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560012, INDIA and 2Indian Institute of Science Education and Research (IISER), Thiruvananthapuram 695016, INDIA

Regular electrical activation waves in cardiac tissue lead to the rhythmic contraction and expan- sion of the heart that ensures blood supply to the whole body. Irregularities in the propagation of these activation waves can result in cardiac arrhythmias, like ventricular tachycardia (VT) and ventricular fibrillation (VF), which are major causes of death in the industrialised world. Indeed there is growing consensus that spiral or scroll waves of electrical activation in cardiac tissue are associated with VT, whereas, when these waves break to yield spiral- or scroll-wave turbulence, VT develops into life-threatening VF: in the absence of medical intervention, this makes the heart incapable of pumping blood and a patient dies in roughly two-and-a-half minutes after the initiation of VF. Thus studies of spiral- and scroll-wave dynamics in cardiac tissue pose important challenges for in vivo and in vitro experimental studies and for in silico numerical studies of mathematical models for cardiac tissue. We present a detailed and systematic study of spiral-wave turbulence in a mathematical model for ventricular myocytes proposed recently by ten Tusscher et al [1] In particular, we use extensive numerical simulations [2] to elucidate the interaction of spiral-wave in it with conduction and ionic inhomogeneities that occur commonly in cardiac tissue. We also examine a control scheme that has been suggested for the control of spiral turbulence, via low-amplitude current pulses, in this model.

[1] ten Tusscher, K.H.W.J., Noble, D., Noble, P.J., Panfilov, A.V. A model for human ventricular tissue.Am. J. Physiol. Heart Circ. Physiol. 286, H1573−H1589, 2004. [2] T.K. Shajahan, Alok Ranjan Nayak, and Rahul Pandit. Spiral-Wave Turbulence and its Control in the Presence of Inho- mogeneities in Four Mathematical Models of Cardiac Tissue. Submitted for publication, 2008. Motion of falling spheres and rising bubbles in a viscoelastic gel: spontaneous oscillations and bursting

Nitin Kumar1, Sayantan Majumdar1, Aditya Sood2, Rama Govindarajan3, Sriram Ramaswamy1 and A.K. Sood1

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

2Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur-208 016, India

3 Engineering Mechanics Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560 064, India

Abstract

The motion of a spherical ball falling under gravity through a simple Newtonian fluid is governed by the well-known Stokes’ law where the ball attains a steady terminal velocity after a certain time when the net force on the ball becomes zero. However, in non-Newtonian fluids, the velocity of the ball does not achieve a steady state and shows large fluctuations [1].

We will report our recent experiments done in surfactant CTAT in water, which forms a viscoelastic wormlike micellar gel. We show experimentally that a ball falling in the viscoelastic gel undergoes a transition from a steady state velocity to oscillatory behaviour as the weight of the ball is increased. The oscillations in velocity of the ball are non-sinusoidal, consisting of high-frequency bursts occurring periodically at intervals long compared to the period within the bursts. We offer a simple theoretical model based on the interplay of the mobility tensor of the particle and the orientational order parameter of the micelle solution. For the case of bubbles, the situation is interesting as both shape and size can vary [2]. We show that an air bubble rising in the viscoelastic gel shows a discontinuous jump in the velocity at a critical volume (Vc)jump, of the bubble, followed by a drastic change in its shape from a "teardrop" to almost spherical. We also observe shape oscillations for bigger bubbles with the tail swapping in and out periodically.

References:

[1] A. Jayaraman and A. Belmonte, Phys. Rev. E (67), 065301(R) (2003)

[2] Nestor Z. Handzy and Andrew Belmonte, Phys. Rev. Lett. 92, 124501 (2004)

Negative viscosity fluctuations and their universality in a driven repulsive colloidal glass

Sayantan Majumdar and A.K. Sood Department of Physics, Indian Institute of Science, Bangalore 560012, India

Abstract There are a lot of recent interests in the statistical properties of non-equilibrium fluctuations in driven jammed systems. Recently proposed Fluctuation Relations (FR) [1] predict the symmetry properties of the Probability Distribution Functions (PDF) of these fluctuations arbitrary far from equilibrium. Also it has been observed that the PDF of a global measure in a large class of highly correlated equilibrium and non-equilibrium systems have the same functional form [2].

In this context we have studied the statistical properties of non-equilibrium fluctuations in a driven repulsive colloidal glass of Laponite. Laponite particles are disk shaped with a diameter of ~30 nm and thickness of ~1 nm. When these particles are suspended in water they acquire more and more surface charge with time because of gradual ion dissociation. Due to strong long range electrostatic interaction they can form a colloidal glass even at very low volume fraction (~ 3 wt %). We study the viscosity of 3 wt % Laponite solution in water as a function of time by applying a constant shear stress on the sample. With time the particles form a highly jammed state (a colloidal glass). The viscosity (and shear rate) shows large fluctuations including many negative values. The PDF of these fluctuations can be Gaussian or non-Gaussian depending on the applied stress but they satisfy the Gallavotti-Cohen Steady State Fluctuation Relation (SSFR) for both Gaussian and non-trivial case of non-Gaussian PDF. The PDF of global power fluctuations for very high applied stresses become strongly non-Gaussian and follow exactly the same Universal functional form predicted in [2]. We attribute this remarkable observation to the presence of high degree of spatial and temporal correlation in a jammed state.

References

[1] G. Gallavotti and E. D. G. Cohen, Phys. Rev. Lett. 74, 2694 (1995); G. Gallavotti and E. D. G. Cohen, J.Stat.Phys. 80, 931 (1995).

[2] S. T. Bramwell et al. Nature, vol 396, 10 December 1998; S. T. Bramwell et al. Phys. Rev. Lett. 84, 3744 (2000).

MAGNETOTRANSPORT STUDIES OF FLOAT ZONE-GROWN Gd1-xSrxMnO3 SINGLE CRYSTAL

Aditya A. Wagh, H.L.Bhat, P. S. Anil Kumar and Suja Elizabeth Department of Physics, Indian Institute of Science, Bangalore-560012, India

The multiferroic behavior, observed in manganites containing small rare-earth cations, has attracted considerable attention in recent times because of the interesting physics involved as well as their high application potential (1). Colossal Magnetoresistance exhibited by mixed valence perovskites has motivated us to study the effect of strontium substitution in multiferroic gadolinium manganite crystals. Single crystal of Gd1-xSrxMnO3 (x=0.5) was grown by Float Zone (FZ) technique. Initially, Gd1-xSrxMnO3 was synthesized by solid state reaction. Crystals were grown using four mirror optical image furnace with a typical growth rate of 1 mm/hr. Crystallinity was ascertained by X-ray diffraction. Chemical composition was estimated by ICPAES analysis. Transport properties of grown crystals were studied in the temperature range 40 to 300 K. Temperature evolution of electrical resistivity of all the crystals showed semiconducting behaviour as expected (2). Magnetoresistance measurements were performed by applying a magnetic field perpendicular to the direction of current. Drastic reduction in the resistivity was observed in the presence of magnetic field. At relatively high magnetic field a metal-insulator transition was observed and this transition temperature was field dependent. The temperature and field dependence of the magnetoresistance was also studied in detail. The magnetization of the samples was measured by vibrating sample magnetometer in the temperature range 5 to 300 K. Gd0.5Sr0.5MnO3 crystal exhibited a bifurcation in the zero-field cooled (ZFC) and field cooled (FC) plots. The electrical and the magnetic properties of these crystals will be discussed in detail. References: (1) Kadomtseva A. M. et. al. JETP Letters, Vol. 82, No. 9, pp. 590–593 (2005) (2) Garcia-Landa B. et. al. Journal of Applied Physics, Vol. 83, No.12,7664 (1998) Conformational modification by conjugation length and solvent in rigid-rod organic semiconductor

* Paramita Kar Choudhury , Debjani Bagchi, and Reghu Menon

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

The conducting polymer MEH-PPV poly [2-methoxy-5-2’ –ethyl-hexyloxy)-1,4- phenylene vinylene] is an important optoelectronic material for thin film devices, as it exhibits fluorescence in the visible region of the spectrum. By studying the polymer chain conformations with small angle X-ray scattering (SAXS), we have explored the factors such as conjugation length and effect of solvents, which can remarkably alter the fluorescence and conductivity of MEH-PPV films. The studies reveal that increase in effective conjugation in organic polymers changes their conformation by making them more rigid-rod like in the nano-scale regime. At a slightly larger length scale we find oriented stacking and long range ordering of the worm-like chains even in a very dilute solution. Aromatic non-polar solvent xylene solvates the backbone leaving the chains more rigid and stacked, while non-aromatic polar solvent tetrahydrofuran (THF) solvates the lateral groups and leads to lesser, and more randomly oriented coil structures.

1H-NMR and charge transport in metallic polypyrrole at ultra-low temperatures and high magnetic fields

K Jugeshwar Singh, G Clark, K P Ramesh and Reghu Menon

The temperature dependence of conductivity, proton spin relaxation time (T1) and magnetoconductance (MC) in metallic polypyrrole (PPy) doped with PF6. have been carried out at mK temperatures and high magnetic fields. At T < K both electron.electron interaction (EEI) and hopping contributes to conductivity. The temperature dependence of a proton T1 is classified in three regimes: (a) for T<6 K.relaxation mechanism follows a modified Korringa relation due to EEI and disorder, (b) for 6 K< T< 50 K.relaxation mechanism is via spin diffusion to the paramagnetic centers and (c) for T > 50 K, relaxation is due to the dipolar interaction modulated by the reorientation of the symmetric PF6 groups following the Bloembergen, Purcell and Pound (BPP) model. The data analysis shows that the Korringa ratio is enhanced by an order of magnitude. The positive and negative MC at T < 250 mK is due to the contributions from weak localization and Coulomb-correlated hopping transport, respectively. The role of EEI is observed to be consistent in conductivity, T1 and MC data, especially at T<1 K. Resistance noise in electrically biased bilayer graphene

Atindra Nath Pal and Arindam Ghosh Department of Physics, Indian Institute of Science, Bangalore 560 012, India

Abstract:

The growing interest in bilayer graphene (BLG) is fueled by the ability to control the energy gap between its valence and conduction bands through external means. We demonstrate that the low‐frequency resistance fluctuations, or noise, in bilayer graphene is strongly connected to its band structure, and displays a minimum when the gap between the conduction and valence band is zero. Using double‐gated bilayer graphene devices we have tuned the zero gap and charge neutrality points independently, which offers a unique mechanism to investigate the low‐energy band structure, charge localization and screening properties of bilayer graphene. We show: (1) the noise to be minimum when band gap (Δg) = 0 even if it corresponds to a nonzero carrier density (n), (2) the evidence of localized states near the band tails even at Δg = 0, with a mobility edge that strongly depends on the external electric field E, and finally, (3) a method to directly determine the dielectric properties of BLG in both electron and hole‐doped regimes.

References: 1. Oostinga, J. B.; Heersche, H. B.; Liu, X; Morpurgo, A. F.; Vandersypen, L. M. K. Nat. Mater., 7, 151, 2007. 2. T. Ohta, A. Bostwick, T. Seyller, K. Horn, E. Rotenberg, Science 313 (2006) 951. 3. Yu‐Ming Lin and Phaedon Avouris, NANO LETTERS 8 : 2119 DOI 10.1021/nl0802411 2008. 4. E. McCann, Phys. Rev. B 74 (2006) 161403.

Structure and stability of carbon and boron­nitride nanorings

Meghana Dharmik and Prabal Maiti Center for Condensed Matter Theory,Department of Physics,Indian Institute of Science.

We have studied the structure and stability of Carbon and Boron­Nitride nanorings using Molecular mechanics calculations.We have used conjugate gradient method to minimize the nanorings using Dreiding force field to get an estimate of the critical diameter of the nanoring.Below the critical diameter the ring structure is unstable with formation of kinks.Above the critical diameter kinks disappear and the ring structure is stable.The critical diameter of the nanoring strongly depends on the chirality of the tube and increases with increase in tube diameter .We identify the kink location with the high strain energy region .Our calculation suggests that for a given chirality Boron – Nitride nanoring has larger critical diameter than the corresponding Carbon nanoring. This might be due to the strong coulombic interaction arising due to the polar interaction between the B and N atoms.

Conductivity noise in strongly correlated systems

R.Koushik and Arindam Ghosh Department of Physics, Indian Institute of science, Bangalore- 560012, India

Abstract: Modulation doped GaAs/AlGaAs heterostructures are ideal candidates to study the effects of disorder on electron-electron interaction in a systematic manner. In these systems the strength of the disorder is determined by an undoped AlGaAs spacer, which separates the charged silicon dopants from the two dimensional electron gas, and can be tuned to atomic precision in molecular beam epitaxy. Several disorder-stabilized strongly interacting many-body phases have been predicted in numerous theoretical studies in these systems, in particular when the effect of interaction is non-perturbative. Conventional techniques, based on time averaged current-voltage characteristics, are however often inconclusive in this regard. In this work, we report the measurement of low-frequency resistivity fluctuations in mesoscopic 2D electron systems in delta-doped GaAs/AlGaAs heterostructures in the strongly interacting regime. Our measurements at low temperatures (267 mK) reveals a spectacular increase over six orders of magnitude in resistivity noise with a decrease in applied bias, which cannot be explained using hopping (variable-range or nearest-neighbor) conduction mechanisms. Our measurement technique is quite unique in the sense we have kept the AC bias across the sample constant at 30uV and varied the DC bias across the sample from -800uV to +800uV. We attribute the results to the manifestation of a new many-body quantum state of the electrons that has been recently claimed in the context of metallic transport in 2D.

References:

1. Modulation origin of 1/f noise in two-dimensional hopping, V.Ya.Pokrovskii, A.V. Savchenko, W.R. Tribe and E.N. Linfield, Phys Rev B 64, 201318 (2001) 2. Onset of glassy dynamics in a Two-Dimensional Electron system in Silicon, Snezana Bogdanovich and Dragana Popovic, PRL 88, 236401 (2002) 3. Low temperature collapse of electron localization in two dimensions, Matthias Baenninger, Arindam Ghosh, Michael Pepper, Harvey E.Beere, Ian Farrer, and David A. Ritchie, PRL 100, 016805 (2008) 4. Conductivity noise in strongly correlated systems, R. Koushik, Baenninger et al. under preparation Out-of-Equilibrium Microrheology to Probe Directional Viscoelastic Properties under Shear

Manas Khan and A. K. Sood Department of Physics, Indian Institute of Science, Bangalore - 560012, India

Many wormlike micellar systems exhibit appreciable shear thinning due to shear induced alignment. As the micelles get aligned, introducing directionality in the system, the viscoelastic properties no longer remain isotropic. An optical tweezers based technique enables us to probe the out-of-equilibrium rheological properties of CTAT system simultaneously along two orthogonal directions - parallel to the applied shear, as well as perpendicular to it. While the displacements of a trapped bead - in response to active drag force - carry signature of conventional shear thinning, its spontaneous position fluctuations along the perpendicular direction manifest an orthogonal shear thickening. Jump reorientation of water molecules confined in narrow carbon nanotubes

Biswaroop Mukherjee1,3,,∗ Prabal K. Maiti1,∗,† Chandan Dasgupta1,3,,‡ and A. K. Sood2,§ 1Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2 Department of Physics, Indian Institute of Science, Bangalore 560 012, India 3 Condensed Matter Theory Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India. Abstract We use molecular dynamics (MD) simulations to study the reorientational dynamics of water molecules confined inside narrow carbon nanotubes immersed in a bath of water. Our simulations show that the confined water molecules exhibit bistability in their reorientational relaxation, which proceeds by angular jumps between the two stable states. The energy barrier between these two states is about 2kBT . These jumps cause the ratio of the timescales of the first and second order reorientational correlation functions to exceed the value in the diffusive limit. The analytic solution of a simplified model, which qualitatively explains this “unusual” relaxation, is also presented. These results will have important implications in understanding proton conduction in water-filled ion channels.

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

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