FOREWORD

As a periodic review of its activities, the Department of Physics has been organizing In-house Symposium on annual basis during recent years. This one-day symposium usually consists of oral presentations by faculty members, post-docs and students, and poster presentations by all those who would like to present their recent results. This year we have a total of 21 talks and 41 posters. I hope this package would be a reasonable representation of the ongoing research activities in the department. This event is also particularly useful to freshers to familiarize themselves with the current research activity in our Department in various branches of Physics.

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

Prof. H. R. KrishnamurthyChairmanNovember 23, 2012

Department of Physics, IISc Bangalore

Inhouse Symposium 2012

November 23, 2012

Auditorium, New Physical Sciences Building

Programme

Session I 9:00-10:30 Chair: P. S. Anil Kumar

T01 9:00-9:15 Tarun Deep Saini

Spiral patterns and instabilities in astrophysical disks

T02 9:15-9:30 Subhamoy Ghatak

T03 9:30-9:45

T04 9:45-10:00 K. Sathya Narayanan

T05 10:00-10:15 Srijit Goswami

T06 10:15-10:30 Anbalagan Ramakrishnan

10:30-11:00 Tea

Possible nature of internal disorder in ultra-thin MoS2 FET devices

Rupamanjari Majumder

Wave Dynamics in a Mathematical Model for Human Cardiac Tissue with Randomly Distributed Fibroblasts.

A new model of flow induced voltage generation in cabon nanotubes based on van der Waals friction

Landau Level Spectroscopy of Broken Symmetry States in High Mobility Graphene on Boron Nitride

Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex Skutterudites

Session II 11:00-1:00

T07 11:00-11:15

T08 11:15-11:30

Evidence of gradient in dynamics of confined polymers

T09 11:30-11:45

Nuclear spin

T10 11:45-12:00

T11 12:00-12:15

T12 12:15-12:30 D. Venkateswarlu

Confinement of Spin Waves in Grids of Permalloy Nanowires

T13 12:30-12:45 Sujit Kumar Nath

T14 12:45-1:00

1:00-2:00 Lunch

Session III 2:00-4:30 Poster Session

4:30-5:00 High Tea

Chair: Aveek Bid

Jaydeep K. Basu

Signature of novel plasmonic Dicke effect in 2D quantum dot solids mediated by surface plasmons of embedded nanoparticles

Sivasurender Chandran

Ananyo Maitra

Vikram Rathee

A reversible shear-induced crystallization above equilibrium freezing temperature in mixed surfactant system

Kaustuv Manna

In Search for the Origin of Glassiness in La0.85Sr0.15CoO3

Magnetohydrodynamic stability of stochastically driven accretion flows

Vijay B. Shenoy

Flow enhanced pairing and other stories of fermions in synthetic gauge fields

Session IV 5:00-7:00

T15 5:00-5:15

T16 5:15-5:30

T17 5:30-5:45 Nitin Kumar

T18 5:45-6:00 R. Koushik

T19 6:00-6:15 Indrani Banerjee

T20 6:15-6:30 Chanchal Sow

T21 6:30-6:45 Suropriya Saha

Single-particle and collective behaviour of colloidal swimmers

6:45-7:00 Concluding Remarks, Best Poster Award

7:00-7:30 Vishwamitra Memorial Prize

7:30-8:30 Dinner

Chair: Ramesh Mallik

K. Ramesh

A unique electrical switching behaviour in Cu-As-Se glasses

Y. Jayasubba Reddy

Heteronuclear Double Quantum Correlation Experiments Involving Protons for the study of Partially Ordered and Rigid Systems

Self-propelled granular rod amid a noisy medium: An experimental test of Isometric Fluctuation-Relation

Probing superconductivity in the 2D limit using resistivity noise

Nucleosynthesis inside gamma-ray burst accretion disks and associated outflows

Freezing of the octahedral tilt near ferromagnetic transition and appearance of a glassy phase at low temperature driven by the tilt instabilities in SrRuO3

List of Posters

No. Presenter Title

P01 Arnab Roy

P02 Sudeesh K.

P03 Bidya Binay Karak Why does sun occasionally stop giving sunspots for several years?

P04 Jayantha P. Vyasanakere

P05 Anbalagan Ramakrishnan

P06 Paritosh Karnatak High mobility graphene devices

P07 R.V. Sudheer Kumar

P08 Kowsalya Devi Pavulur Ultrafast NMR Techniques in Inhomogeneous Magnetic Fields

P09 M. Prashantha

P10 Avradip Pradhan

P11 Quantum Simulation of Dzyaloshinsky-Moriya Interaction

P12 Siddharth Madhav Khare

P13 Semonti Bhattacharyya

P14 Pradeep Kumar

P15 Amit Kumar Majhi

P16 Anindita Sahoo Transport properties and noise in hydrazine reduced graphene oxide.

P17 Rajan Modak Thermalization threshold in models of 1D fermions

P18 Debayan Dey

P19 Shibu Saw

P20 Gajanan V. Honnavar

Study of switching field statistics of Permalloy (Ni80Fe20 ) Hall bars by planar Hall effect

Unzipping Force Analysis to determine binding specificity of RNA Polymerase to T7A1 promoter sequence

Can interaction between emergent excitations be independent of the constituent interactions?

Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex Skutterudites

Measurement of Proton-Carbon Dipolar Couplings using an improved DAPT pulse sequence

Low temperature electrical transport studies on carbon nitride films prepared by chemical vapour deposition

Investigating DNA hybridization through changes in conductance of ultrathin Au nanowires

V.S. Manu

Measurement of Forces applied by C. elegans moving on Agarose surfaces.

Low frequency noise in Topological Insulator Bi1.5Sb0.4Te1.7Se1.3

Superconducting fluctuations, Anomalous Phonons and Electronic excitations in iron-based superconductors

Molecular dynamics simulation of electroporation of lipid bilayer membrane

Random matrix theory and gene correlation coefficient statistics of DNA- Microarray data: Application in understanding the system biology of gene regulation

Violation of Guggenheim Adsorption Rule at Wall-Liquid Interface in Binary Lennard-Jones Mixture

Study of effect of alkali mixture on V - O bond length in Oxyfluoro Vanadate glasses using Raman spectroscopy.

P21

P22 Hariharan N.

P23 Nairita Pal

P24 Ch. Raju

P25 Baban Wagh

P26 Yogeshwar Prasad

P27 Arpita Roy Superbubble breakout and galactic winds from disk galaxies

P28 Hemant Kumar

P29 Mohammed Ali Aamir Large linear magnetoresistance in a GaAs/AlGaAs heterostructure

P30 Vidya Kochat

P31 Amal Medhi

P32 Sudeep Kumar Ghosh

P33 Medini Padmanabhan

P34 Mitali Banerjee

P35 Achintya Bera

P36 Sreetama Das

P37 Pramod K. Verma Study of Phonon anharmonic effects in pyrochlores

P38 Kallol Roy Optoelectronic properties of graphene-MoS2 hybrids

P39 K.S. Vasu

P40 Upasana Das

P41 Arijit Haldar Superfluidity in bricks!

Saquib Shamim Suppression of localization in two dimensionally doped semiconductors at half-filling

Magnetic, Dielectric and Transport Studies of Single Crystal Tb0.5Sr0.5MnO3

Direct Numerical Simulation of Turbulence in the Two-Dimensional Navier-Stokes-Cahn-Hilliard Equations

Thermoelectric properties of chalcogenide based Cu2+xZnSn1-xSe4

Thermal Conduction, Feedback and Multiphase Gas in Galaxy Clusters

Realization of Fermionic Superfluid State in an Optical Lattice via a Bilaryer Band Insulator

Flow Induced Alignment of water molecules conned inside carbon nanotube: Insight from MD Simulations

Universal Conductance Fluctuations as a direct probe to valley coherence and universality class of disordered graphene

Sensory organ like response determines the magnetism of zigzag-edged honeycomb nanoribbons

Evolution of fermionic superfluid across the crossover from three to two dimensions

Induced photoconductivity in large area graphene by electrochemical deposition of thin films

Microwave assisted synthesis of single crystalline ternary alloy-Bi2-xSbxTe3 for thermoelectric applications

Sharp Raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3

Using cis peptide containing fragments for functional annotation of proteins

Bio– sensors based on electrical and optical properties of Carbon nanotubes & Graphene oxide

Violation of Chandrasekhar mass limit: Strongly magnetized white dwarfs as progenitors of super-Chandrasekhar type Ia supernovae

TALK ABSTRACTS

Spiral Patterns and Instabilities in Astrophysical Disks

Tarun Deep SainiDepartment of Physics, IISc Bangalore

Possible nature of internal disorder in ultra-thin

MoS2 FET devices

Subhamoy Ghatak, Paromita Kndu, N. Ravishankar and Arindam Ghosh

We investigate possible existence of intrinsic disorder in ultrathin MoS2 thin film transistors.

For that we perform three kinds of independent electrical measurements on two classes of

MoS2-FET devices, on SiO2 and on single crystalline hexagonal boron nitride (hBN)

substrate. Using time averaged transport measurement, we find that the device quality in

terms of mobility and charge relaxation remains almost similar for both the classes. We also

study low frequency 1/f noise, sensitive to charge dynamics of system and observe that in

both classes number density (n) fluctuation is the dominant mechanism of 1/f noise. From

this, we predict that although external source of disorder can’t be ruled out, internal disorder

might be the dominant source of disorder in atomically thin MoS2 films.

Wave Dynamics in a Mathematical Model for Human Cardiac Tissue with Randomly Distributed Fibroblasts.

Rupamanjari Majumder1, Alok Ranjan Nayak1, and Rahul Pandit1,2.

1 Centre for Condensed Matter Theory, Department of Physics, Indian Institute of Science, Bangalore, India2 Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India

We present a comprehensive numerical study of spiral-and scroll-wave dynamics in a state-of-the-art mathematical model for human ventricular tissue with fiber rotation, transmural heterogeneity, myocytes, and fibroblasts. Our mathematical model introduces fibroblasts randomly, to mimic diffuse fibrosis, in the ten Tusscher-Noble-Noble-Panfilov (TNNP) model for human ventricular tissue; the passive fibroblasts in our model do not exhibit an action potential in the absence of coupling with myocytes; and we allow for a coupling between nearby myocytes and fibroblasts. Our study of a single myocyte-fibroblast (MF) composite, with a single myocyte coupled to Nf fibroblasts via a gap-junctional conductance Ggap , reveals five qualitatively different responses for this composite. Our investigations of two-dimensional domains with a random distribution of fibroblasts in a myocyte background reveal that, as the percentage Pf of fibroblasts increases, the conduction velocity of a plane wave decreases until there is conduction failure. If we consider spiral-wave dynamics in such a medium we find, in two dimensions, a variety of nonequilibrium states, temporally periodic, quasiperiodic, chaotic, and quiescent, and an intricate sequence of transitions between them; we also study the analogous sequence of transitions for three-dimensional scroll waves in a three-dimensional version of our mathematical model that includes both fiber rotation and transmural heterogeneity. We thus elucidate random-fibrosis-induced nonequilibrium transitions, which lead to conduction block for spiral waves in two dimensions and scroll waves in three dimensions. We explore possible experimental implications of our mathematical and numerical studies for plane-, spiral-, and scroll-wave dynamics in cardiac tissue with fibrosis.

Based on:

Majumder R, Nayak AR, Pandit R (2012) Nonequilibrium Arrhythmic States and Transitions in a Mathematical Model for Diffuse Fibrosis in Human Cardiac Tissue. PLoS ONE 7(10): e45040. doi:10.1371/journal.pone.0045040

A new model of flow induced voltage generation in

Carbon Nanotubes based on van der Waals friction

K. Sathya Narayanan and A.K. Sood

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

We present here a simple model to explain the plethora of experimental results

concerning the flow induced voltage generation in carbon nanotubes which, thus

far, have been only inadequately explained. To do this, we consider the zeroth

order picture of the alignment of water molecules confined inside the nanotubes

subject to the force field exerted by the imposed flow outside the tubes. These

aligned water molecules polarize the nanotubes containing them, thereby

inducing a potential difference across its length. Importantly, we model this force

field as due to the van der Waals (VdW) frictional stress that exists between any

two dissipating dielectric media in relative motion with each other, which, here, is

between the confined water molecules and the flowing liquid. This simple model,

surprisingly, captures the qualitative behavior observed in the experiments such

as: induced voltage as a function of flow velocity, gate voltage and electrolyte

concentration. Further, this also explains the apparently unrelated non-local

voltage generation observed in isolated carbon nanotubes when filled with water

vapor. We extend this even more to try and understand the intriguing

phenomenon of flow induced voltage generation in carbon nanotubes trapped in

Ice, wherein the flowing medium is well isolated from the nanotubes by a

sufficiently thick layer of ice. We believe that this understanding is crucial to

optimization of future energy harvesting devices.

Landau Level Spectroscopy of Broken Symmetry States in High Mobility

Graphene on Boron Nitride

Srijit Goswami, Paritosh Karnatak and Arindam Ghosh

The presence of spin and valley degeneracy in graphene gives rise to four-fold degenerate

Landau levels (LLs) in the presence of a magnetic field perpendicular to the plane of the

graphene. Graphene devices on silicon oxide substrates typically exhibit very low mobility,

resulting in significant broadening of LLs. As a result, any broken symmetry states are

difficult to observe at reasonable magnetic fields.

Placing graphene on atomically flat hexagonal Boron Nitride (BN) results in significantly

higher carrier mobility. We have fabricated such high mobility graphene-BN

heterostructures and show that the four-fold degeneracy of the LLs is completely lifted at

moderate magnetic fields. Such broken symmetry states have been observed in a few

studies previously, however their exact nature is still a matter of debate.

We also show that charge transfer between graphene and a nearby defect site gives rise to

discrete jumps in the resistance. In the presence of a magnetic field the size and nature of

these resistance jumps is particularly sensitive to the position of the Fermi level. We discuss

some preliminary results which indicate that such a graphene-defect system may allow for a

more sensitive spectroscopy of the LLs, as compared to time averaged transport.

Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex Skutterudites

Anbalagan Ramakrishnan1, Esmaeil Royanian2, Ernst Bauer2, Gerda Rogl3, Peter Rogl3 and Ramesh

Chandra Mallik1

1Thermoelectric Materials and Devices Laboratory, Department of Physics, Indian Institute of

Science, Bangalore – 560012

2Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria

3Institute of Physical Chemistry, University of Vienna, Austria

Abstract:

Skutterudites Fe0.2Co3.8Sb12-xTex (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were synthesized by

induction melting at 1273 K and followed by annealing at 923 K for 144 h. X-ray diffraction

confirmed the polycrystalline nature of the skutterudite phase as the main phase in all the

samples with a small amount of (Sb) as a secondary phase. Since Te exceeds the solubility

limit, additional secondary phases, CoSb2 and CoTe2, were observed in Co3.8Fe0.2Sb11.4Te0.6.

Lattice constants and atomic positions are verified by Rietveld refinement using the

composition from electron probe micro analysis (EPMA). The temperature dependence of

transport properties has been measured for all samples between 300 K and 818 K. Whereas

for the alloys up to x=0.2 (Co3.8Fe0.2Sb11.8Te0.2) the electrical resistivity initially increased by

charge compensation, for x>0.2 it decreased with increasing Te content due to an increasing

electron concentration. At room temperature, a positive Seebeck coefficient (holes are

majority carriers) was obtained in single element (Fe) substituted Co4Sb12, whilst a negative

Seebeck coefficient (electrons are majority carriers) was obtained in Fe0.2Co3.8Sb12-xTex.

Thermally excited carriers change from n-type to p-type in Co3.8Fe0.2Sb11.9Te0.1 at 570 K

while all other samples exhibit negative Seebeck coefficients in the entire temperature range

measured. Lattice thermal conductivity decreased with increasing Te content due to mass

fluctuation and point defect scattering. The maximum dimensionless figure of merit, zT =

1.04 at 818 K, was obtained with an optimized Te content for Co3.8Fe0.2Sb11.5Te0.5.

Signature of novel Plasmonic Dicke effect in 2D quantum dot solids

mediated by surface plasmons of embedded nanoparticles

Jaydeep Basu

Quantum dot ensembles and solids find numerous applications from solar cells to novel nano-lasers. Understanding their optical and electrical properties and the ability to tune them is thus of vital importance. Here, we will discuss our recent work on photoluminescence (PL) from two dimensional solids made of CdSe quantum dots lightly doped with metal nanoparticles. The emission from the quantum dot solid can be tuned by controlling the packing of the solid, spectral overlap between the quantum dots and the metal nanoparticles as well as through doping concentration. While the PL from the quantum dots show expected variation at low density and large doping of gold nanoparticles, unexpected strong enhancement in PL occurs at low and intermediate doping, especially when the quantum dots and metal nanoparticles are spectrally at resonance. A recently suggested model [3] of plasmon mediated superradiance, due to virtual plasmon exchange between proximal quantum dots, leading to such enhancements seems to be the likely cause of such an unusual effect. Acknowledgements: The work presented here has been done in collaboration with Laxminaryanan Tripathi, M. Praveena, M. Haridas. References:

1. L. N. Tripathi, M. Praveena, J. K. Basu, Plasmonics (2012). 2. M. Haridas, L. N. Tripathi and J. K. Basu, Applied. Physics. Letter, 98, 063305

(2011). 3. Vitaliy N. Pustovit and Tigran V. Shahbazyan, Phys Rev Lett. 102, 077401

(2009).

Evidence of gradient in dynamics of confined polymers

Sivasurender Chandran

1, N. Begam

1, J. K. Basu

1 and M. K. Mukhopadhyay

2

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

2Applied Materials Science Division, Saha Institute of Nuclear Physics, Kolkata – 700064

Particle segregation to surface/interface, surface mobility of the particles, interfacial

viscosity and thereby the gradient in the dynamics of polymer thin films is a matter of debate,

which generated lot of controversies over the last few years. We report [1] the evidence of

gradient in dynamics by probing the diffusion coefficient of polymer grafted nanoparticles

(PGNP) dispersed in polymer thin films of different thickness viz., 2.5Rg and 8Rg of the matrix

chains (Rg is the radius of gyration). Using surface x-ray scattering, we observe a systematic

vertical dispersion of PGNP from a pinned in substrate interface layer to the surface on thermal

annealing. Even after annealing at high temperature (T>>Tg) and longer times, a fraction of

PGNP pertain to stay at the substrate forming a stable interface layer. This hints about the low

mobility of particles at the substrate interface and also emphasizes the presence of high

viscous/gel-like interfacial layer. Real space microscopic images show the formation of lateral

domains of the particles at air surface suggesting the higher surface mobility. In addition, it is

also observed that the fraction of particles in the air surface is more in annealed thinner films

compared to the thicker ones. Thus, we have correlated the observed lateral and vertical

dispersion and its evolution with annealing, to the gradient in dynamics along the thickness of

the thin films.

[1] Sivasurender Chandran, J. K. Basu and M. K. Mukhopadhyay, in communication

Nuclear spin

Abhishek KumarMechanobiology Institute and Department of Biological Sciences, NUS, Singapore 117411 and

National Centre for Biological Sciences, TIFR, Bangalore 560065, India

Ananyo Maitra*Department of Physics, Indian Institute of Science, Bangalore 560012 , India

Madhuresh Sumit and G.V. ShivashankarMechanobiology Institute and Department of Biological Sciences, NUS, Singapore 117411

Sriram RamaswamyDepartment of Physics, Indian Institute of Science, Bangalore 560012 , India and

TIFR Centre for Interdisciplinary Science, 21 Brundavan Colony,Osman Sagar Road, Narsingi, Hyderabad 500 075, India

(Dated: November 15, 2012)

We study the nuclear dynamics of single fibroblast cells, with effects of cell-migration suppressedby plating onto fibronectin-coated micro-fabricated patterns. It is observed that on circles, squaresand equilateral triangles, the nucleus undergoes persistent rotational motion, while on high-aspect-ratio rectangles of the same area it moves only back and forth. We show that our observationscan be understood through a hydrodynamic approach in which the nucleus is treated as a highlyviscous inclusion residing in a less viscous fluid of orientable filaments endowed with active stresses.Lowering actin contractility selectively by introducing blebbistatin at low concentrations drasticallyreduced the speed and persistence time of the angular motion of the nucleus, lending credence toour ideas. Time-lapse imaging of actin also revealed a correlated hydrodynamic flow around thenucleus, with profile and magnitude consistent with the results of our theoretical approach. Coherentintracellular flows and consequent nuclear rotation thus appear to be a generic property that cellsmust balance by specific mechanisms in order to maintain nuclear homeostasis.

Keywords:

A reversible shear-induced crystallization above equilibrium freezing temperature in mixed surfactant system

Vikram Rathee1, Rema Krishnaswamy

2, Antara Pal

3, V. A. Raghunathan

3, Marianne Imp´eror

4, Brigitte

Pansu4 and A.K. Sood

1,2

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

2 Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India,

3Raman Research Institute, Bangalore-560080, India, and

4Laboratoire de Physique des Solides, UMR 8502 CNRS, Bat 510, Universite' Paris-Sud 11, 91405 Orsay

Cedex, France.

Shear driven crystallization is a well studied phenomenon in colloidal suspensions, polymer melts,

surfactant mesophases, molecular liquids and atomic systems. The characteristic feature of the

crystallization which occurs below the equilibrium freezing temperature is that the positional ordering

persists even after the shear is stopped. Here we demonstrate a novel shear-induced crystallization

phenomenon in a weakly swollen isotropic (Li ) and lamellar (Lα) mesophases with bilayers formed in a

cationic-anionic mixed surfactant system. Synchrotron rheological x-ray scattering reveals the transitions

to be 1st order and reversible under shear i.e., on stopping the shear, the crystalline phase Lc, reverts

back to the equilibrium mesophase. Rheo-optical observations show a shear-thickening which occurs

along with the preordering of Li phase induced by shear flow to Lα phase before the nucleation of Lc

phase. Shear diagram of the Li phase constructed in the parameter space of shear-rate (�� ) vs

temperature (T) exhibits Li Lc + Li and Li Lα + Li, transitions above the equilibrium crystallization

temperature (TK), in addition to the irreversible shear-driven nucleation of Lc in Li phase below TK.

Besides revealing a new class of non-equilibrium phase transitions, the present study urges a novel

approach towards understanding shear-induced phenomena in concentrated mesophases of mixed

amphiphilic systems.

Abstract

In Search for the Origin of Glassiness in La0.85Sr0.15CoO3

Kaustuv Manna

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

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 (PS), others show spin-glass (SG) behavior. Here, we present a

comprehensive investigation of the structural, ac susceptibility, dc magnetization and

neutron diffraction studies on two sets of La0.85Sr0.15CoO3 polycrystalline samples

prepared from the same initial mixture but subjected to different heat treatment

processes. The dc magnetization study, done on both the samples show a kink in the

field-cooled magnetization and a peak in the zero-field-cooled magnetization which

shifts to the lower temperature at modest dc fields following an AT line behavior. In

addition, the ac susceptibility study exhibits a frequency dependent peak shift (~ 4 K)

which follows Volgel-Fulcher law, time-dependent memory effect, and a spin

relaxation time τo~10-13 s; all of which strongly indicate the characteristics of the SG

behavior. The neutron depolarization measurement done on the conventionally

prepared La0.85Sr0.15CoO3 sample clearly shows the existence of ferromagnetic clusters

embedded in a non-ferromagnetic matrix. But, once the same sample is properly

homogenized by the repeated grinding and annealing process, these clusters

disappears and the sample turns to a pure SG phase with zero depolarization of the

transmitted neutron beam. Thus, our comparative study clearly reveals that the phase-

separated nature is not intrinsic to La0.85Sr0.15CoO3 system; in fact this is an outcome

of the compositional inhomogeneity. In essence, all the present experimental findings

evidence that the true ground state magnetic property of La0.85Sr0.15CoO3 is spin-glass

in nature.

References:

1. Kaustuv Manna, D. Samal, Suja Elizabeth, H. L. Bhat, and P.S. Anil Kumar

The Journal of Physical Chemistry C, (2011) 115, 13985-13990.

2. Kaustuv Manna, D. Samal, Suja Elizabeth, H. L. Bhat, and P.S. Anil Kumar

Journal of Superconductivity and Novel Magnetism, (2011) 24, 833–837.

FMR experiments were done in collaboration with Prof. S. V. Bhat D.Venkat@physics.iisc.ernet.in

Confinement of Spin Waves in Grids of Permalloy Nanowires

D. Venkateswarlu

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

Key words—electron beam lithography, shape anisotropy, nanowires, FMR, demagnetization fields, micromagnetics.

Recent advances in magnonics highlight its importance in microwave frequency applications[1]. This involves the use of spin waves for device applications. Here one needs to look for better control over the spin waves and their band structure[2-3]. The periodic variation in the effective magnetic field in ferromagnetic materials is the basis for the spin wave confinement and their propagation. Since the effective field is the combination of exchange, external, anisotropy and demagnetization (dipolar/ stray) fields; one can achieve the periodic condition in many ways. In our study, we employed geometry engineering by which the shape anisotropy was tailored in obtaining the periodicity in demagnetization fields. Soft magnetic material permalloy (Ni80Fe20) was made into grid like structure using a top down approach.

Ferromagnetic resonance (FMR) measurements give the information about the non travelling spin waves (k=0 modes). In general, one can see only single resonance mode in FMR spectra in the case of thin films of permalloy. This is due to its weak crystalline anisotropy even in epitaxial conditions unlike the Fe thin films[4]. When the permalloy film is made into nanowires with high aspect ratio, it still gives the single mode but resonance condition varies with applied field angles. This is due to the fact that the shape anisotropy governs the condition for resonance. This advantage of the shape anisotropy points towards the usefulness of engineering the network structures. Fundamentally one needs to understand the dynamics of these network structures in order to meet the required conditions for spin wave band structures in order to use them in magnonics devices.

The grid structures were fabricated using electron beam lithography followed by DC magnetron sputtering and lift-off technique. The permalloy deposited on Si wafers was 20nm thick and is capped with 4nm gold to avoid oxidation. The thickness of the deposited materials was confirmed with the help of pre calibrated digital thinness monitor. The lateral dimensions of the grid structures were obtained using Scanning Electron Microscopy (SEM). The permalloy wires in the grids found with widths about 140-160nm. To understand the spin wave confinement effect we varied the periodicities along horizontal and vertical directions in the grid: (i) 560x560 (G1), (ii) 800x400 (G2) and (iii) 1000x500 (G3) (all in nm units).

The FMR spectra on all the three grids were compared with a reference sample, a continuous thin film grown

under same conditions during the fabrication. The continuous film showed an uni-axial anisotropy. This was induced due to the in-situ applied field during the growth. But the magnitude is very small when compared to that of structured samples. The quantitative comparison of anisotropy constants was obtained with the help of fitting done with theoretical equations involved in FMR phenomena.

Two well resolved modes were observed in all three grids whereas the reference sample gave only one mode. These two modes corresponds to the spin waves confined in horizontal and vertical sections of the grids. Spin wave confinement was understood with the help of the mode dependency on the in-plane angle of the applied magnetic field with the grid. There were multiple peaks observed at some angles in the G2, G3 which is not seen in G1.

The origin for these multiple peaks is understood with the help of static micromagnetic simulations (MMS). Dynamics of G1 using MMS followed by Discrete Fast Fourier Transformations were used to correlate FMR spectra. Our simulations methodology gave more insight into the understanding of spin waves confinement. The implementation of 2D-PBC[5] in our MMS made it possible to study the real systems in realistic time.

Fig.1 Spin wave confinement in the horizontal and vertical

sections of the G1.

REFERENCES [1] V. V. Kruglyak et al., J. Phys. D: Appl. Phys., vol. 43, p.

264001, 2010. [2] Z. K. Wang et al., Acs Nano, vol. 4, p. 643, 2010. [3] K. S. Lee et al., Phys. Rev. Lett., v. 102, p. 127202, 2009. [4] S. Sakshath et al., J. Supercond. Nov. Magn.,

DOI:10.1007/s10948-011-1269-3 [5] D. Venkateswarlu et al., IEEE Trans. Magn., vol. 48, no.

11, Nov. 2012.

Magnetohydrodynamic stability of stochastically driven accretion

flows

Sujit Kumar Nath1, Banibrata Mukhopadhyay1, Amit K. Chattopadhyay2

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

sujitkumar@physics.iisc.ernet.in ; bm@physics.iisc.ernet.in

2. Aston University, Non-linearity and Complexity Research Group, Engineering

and Applied Science, Birmingham B4 7ET, UK; a.k.chattopadhyay@aston.ac.uk

Abstract

We investigate the origin of magnetohydrodynamic turbulence in rotating shear flows. The

particular emphasis is the flows whose angular velocity decreases but specific angular momentum

increases with increasing radial coordinate. Such flows, which are extensively seen in astrophysics,

are Rayleigh stable, but must be turbulent in order to explain observed data. The present work

explores the effect of stochastic noise on such magnetohydrodynamic flows. We essentially con-

centrate on a small section of such a flow which is nothing but a plane shear flow supplemented

by the Coriolis effect. This also mimics a small section of an astrophysical accretion disk. It is

found that such stochastically driven flows exhibit large temporal and spatial correlations of per-

turbation velocities, and hence large energy dissipations of perturbation increasing indefinitely with

time, which presumably generates instability. A range of specific angular momentum (λ) profiles,

as functions of radial coordinate of background flow, starting from Keplerian to constant specific

angular momentum is explored. However, all the background profiles exhibit identical growth and

roughness exponents with similar amplitude of perturbations of energy, revealing a unique univer-

sality class for the stochastically forced magnetohydrodynamics of rotating shear flows. This work,

is an attempt to understand origin of instability and turbulence in the three-dimensional Rayleigh

stable rotating shear flows. This has important implications to resolve the turbulence problem in

astrophysical magnetohydrodynamic flows such as accretion disks.

Flow Enhanced Pairing and Other Storiesof fermions in synthetic gauge fields

Vijay B. ShenoyCentre for Condensed Matter Theory

Indian Institute of Science, Bangalore 560 012shenoy@physics.iisc.ernet.in

In this talk, intended for a broad audience, I will describe our recent work on interacting fermions in synthetic non-Abelian gauge fields. A uniform non-Abelian gauge field, such as those proposed and realized in cold-atom systems, produces a generalized Rashba spin-orbit interaction that influences the motion of the fermions. I will show that this system contains rich and novel physics whose significance extends from high temperature superconductivity to quantum computing. A novel feature uncovered in our most recent work is the phenomenon of flow enhanced pairing apparently contradicting Landau who argues that flow is detrimental to pairing.

Reference on flow enhanced pairing: 1211.1831. I thank J. P. Vyasanakere for comments and criticisms, and DAE/DST for generous support of this work.

A Unique Electrical switching behavior in Cu-As-Se glasses

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

ABSTRACT

A unique electrical switching behaviour has been observed in CuxAs40Se60-x glasses over

a wide range of composition (0 ≤ x ≤ 32). The glasses with lower Cu concentrations (x < 15) do

not exhibit switching, whereas glasses in the range 15 ≤ x ≤ 25 show a threshold type switching.

The glasses in the range 25 ≤ x ≤ 28 exhibited an unusual switching from low resistance to high

resistance state. For x ≥ 30, the glasses are found to show a memory switching. This is a unique

observation and for the first time a system showing no switching→ threshold switching → low

resistance to high resistance switching memory switching has been observed. →

The thermal crystallization of CuxAs40Se60-x glasses at their respective crystallization

temperatures indicate that the structural network is mainly characterized by Cu3AsSe4 and

As2Se3 for x < 15 and by Cu3AsSe4 and Cu2As3 for x ≥ 25. The composition range 15 ≤ x ≤ 20 is

characterized only by Cu3AsSe4 structural units. The samples cooled from their melt show only

the ternary Cu3AsSe4 for x ≤ 20. For x > 20, precipitates of ‘As’ has also been observed along

with Cu3AsSe4 and Cu2As3 phases.

Normally, the memory switching is explained with the thermal model and threshold

switching is explained with the electronic model. The present studies provide a unique way to

understand the electrical switching exhibited by chalcogenide glasses based on the thermal mode

and filament formation. The proposal of the influence of cross-linking and rigidity of the

structural network in changing the switching type from memory to threshold and vice versa is

also ruled out by the present studies.

Heteronuclear Double Quantum Correlation Experiments

Involving Protons for the study of Partially Ordered and

Rigid Systems

Y. Jayasubba Reddya,b

and K.V. Ramanathana

aNMR research Center, Indian Institute of Science Bangalore-560012, India

bDepartmment of Physics

The current objective of our research is to develop and apply high resolution solid state NMR

methods to study the intermolecular interactions and 3D molecular structure of small molecules

(like drugs, tri and octa peptides and thermotropic liquid crystals) at isotopic natural abundance.

1H NMR chemical shifts and dipolar couplings are the powerful probe for intermolecular

interactions, more specifically H-bonding, cis-trans conformation, π-π interactions and

dimerization, which control the self assembly of molecules in the solid state.

While considering all these advantages we have utilized 1H based heteronuclear correlation

experiments. Multiple quantum correlation spectroscopy in the case of rigid and semi rigid

systems can provide useful proximity information, as the coherences can be generated between

dipolar coupled spin systems. Here we present our efforts in utilizing proton double quantum and

carbon single quantum correlation experiments for the case of static oriented liquid crystal

samples and rigid biological samples. Correlations based on both scalar and dipolar couplings are

being explored. Examples of studies with some proline based tripeptides which exhibit cis-trans

isomerism, thiophene based systems and a liquid crystalline material will be presented. To

confirm the resonance assignments, we have also used the GIPAW module contained in the open

source Quantum Esspresso (QE) code and chemical shifts with reasonable agreement with

experiment have been obtained.

Self-propelled granular rod amid a noisy medium: An experimental test of Isometric Fluctuation-Relation

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

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

Abstract

We provide an experiment to examine Isometric Fluctuation Relation (IFR) [1] which relates the relative probabilities of two isometric current vectors pointing in different directions in d-dimensional space. The experiment consists of a self-propelled granular rod moving amidst a noisy medium of spherical particles on a two dimensional surface. We analyse its two dimensional velocity vector field (V) and show that IFR is indeed valid but only at “higher” velocities whose magnitude is greater than transverse width of velocity distribution at V = 0. Using IFR, we confirm that Gallavotti-Cohen Fluctuation Relation (GCFR) can be obtained as a special case when velocity vectors are pointing in opposite directions [2]. Moreover using IFR we show that GCFR is valid even when current fluctuations are oblique to the propulsive force. We compute the Large Deviation Function (LDF) which is very different from paraboloid signifying that system obeys IFR even when the fluctuations are non-Gaussian. References:

[1] P. I. Hurtado et al., PNAS 108, 7704 (2011) [2] Nitin Kumar, Sriram Ramaswamy and A. K. Sood, Phys. Rev. Lett. 106,118001 (2011).

_____________ 

*electronic mail: nksharma@physics.iisc.ernet.in

Probing superconductivity in the 2D limit using resistivity noise

R.Koushik1, Mintu Mondal2, John Jesudasan2, Pratap Raychaudhuri2, Aveek Bid1 and Arindam Ghosh1

1Department of Physics, Indian Institute of Science, Bangalore 560012, India2Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India

Superconductivity in low dimensions has invoked lot of interest in recent times with the advent of new materials like topological superconductors, electron gas at oxide interface etc. In 2D, the superconducting transition is known to occur via Berezenskii Kosterlitz Thouless (BKT) phase transition. Conventional techniques like resistivity measurements, superfluid density measurements, IV characteristics are generally used to study the nature of transition (BKT or BCS). But their sensitivity is limited as inhomogeneity can smear out signatures of BKT transition. In this work, we use the higher order statistics of resistivity fluctuations to address the nature of superconducting transition in ultra thin films of Niobium Nitride. Our technique involves detecting non-Gaussian component (NGC) in fluctuations which are sensitive to long- range correlations in the system. The measure of NGC is given by second spectrum (equivalent to kurtosis). We find strong non-Gaussian fluctuations closer to the transition temperature (TKT) in films exhibiting BKT transition which monotonically decrease with increase in temperature and reduces to background level as the mean field temperature is approached (TBCS). The NGC is completely absent in bulk films. We attribute the NGC to the presence of long-range interaction between vortices which naturally occur in a BKT transition whereas the Ginzburg Landau (GL) fluctuations occuring in bulk films are Gaussian in nature.

Our experiments underline a new method to identify the characteristic temperature scales of phase fluctuations in the superconducting state, that can be useful to probe other low dimensional superconductors as well.

Key words: Non Gaussian, Kosterlitz-Thouless transition

References

1. R. Koushik et al., (under preparation)2. Mintu Mondal et al., Phys. Rev. Lett 107, 217003 (2011)

TITLE: NUCLEOSYNTHESIS INSIDE GAMMA­RAY BURST ACCRETION DISKS AND ASSOCIATED OUTFLOWS 

AUTHORS: Indrani Banerjee, Banibrata Mukhopadhyay   Department of Physics, Indian Institute of Science, Bangalore­560012 

 

ABSTRACTMost popular models of long duration gamma­ray bursts invoke the core collapse of rapidly   rotating   stars.   The  mass  of   the   stars  undergoing   core   collapse   is  usually greater than 20 solar mass in the main sequence. This core collapse results in the formation of black holes of 2­3 solar masses if it is of Schwarzschild type and 6­7 solar masses if it is of maximally spinning Kerr type with an accretion disk around them. Such black holes accrete at the rate of 0.001­10.0 solar masses per second. We investigate   nucleosynthesis   inside   such   gamma­ray   burst   accretion   disks   with accretion rate upto 0.1 solar mass per second since these disks are more likely to synthesize   heavy   elements.   We   show   that   varying   accretion   rate   changes   the nucleosynthesis  products.  We  also   report  how nucleosynthesis   is   sensitive   to   the variation of the initial abundance of elements in the accretion disk, namely whether it is Si rich or He rich. In addition to the formation of various isotopes of Fe, Co and Ni we report the synthesis of new elements like Ar35, F21, Mn53 and various isotopes of Cr which have not been reported earlier. Next, we investigate whether these elements survive in the outflows from the disk and we find that the result is sensitive to the fraction of mass ejected to the mass accreted and hence to the velocity of ejection. When the velocity of ejection is small we find that many new elements like isotopes of Ti, V, Cu and Zn are synthesized. We also give a rough estimate of the change in the mass fraction of  a particular species due to these core collapse events during the lifetime  of  a  galaxy,  which  affects   the  metallicity  of   the  universe.  Many  of   these elements   thus  synthesized  have  been  observed   in   the  X­ray  afterglows  of   several gamma­ray bursts. 

Freezing of the octahedral tilt near ferromagnetic transition and appearance of a glassy

phase at low temperature driven by the tilt instabilities in SrRuO3

Chanchal Sow

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

SrRuO3 is a well known itinerant ferromagnet with many intriguing characteristics. Here we

present a critical investigation on the structural, magnetic, and magnetotransport properties of

polycrystalline SrRu(1-x)O3 (0.07 < x < 0) samples with uniquely defined ferromagnetic transition

temperatures. The ac magnetic susceptibility study exhibits the remarkable memory effect, a

distinct characteristic of glassy behavior, at low temperatures. The transport study suggests a

crossover from Fermi-liquid to non-Fermi-liquid behavior. Most strikingly, the temperature-

dependent magnetoresistance reveals the possibility for an additional magnetic ordering (apart

from ferromagnetic) by demonstrating a peak in magnetoresistance at the low temperature side

as well. In addition, the temperature-dependent coercive field shows a plateau around 50 K. In

order to understand the genesis of such unusual low-temperature magnetic features, we have

undertaken a detailed temperature dependent (5-250 K) neutron diffraction study. We observe a

freezing of the octahedral tilt near the ferromagnetic transition and unusual changes in the

structural parameters (unit-cell lattice parameters, octahedral tilt etc.) near the onset of low

temperature spin glass like phase. A reduction of the ordered magnetic moment and a decline in

the total integrated magnetic intensity is also observed around the same temperature. Hence it is

believed that the low-temperature anomalous magnetic response is closely intertwined to the

lattice-parameter change.

Reference:

[1] C. Sow, D. Samal, P. S. A. Kumar, A. K. Bera, and S. M. Yusuf, Phys. Rev. B 85, 224426

(2012).

[2] C. Sow, D. Samal, and P. S. A. Kumar, J. Appl. Phys. 111, 07E121 (2012).

Single-particle and collective behaviour of

colloidal swimmers

Suropriya Saha, Sriram Ramaswamy, Ramin Golestanian

November 16, 2012

Colloidal particles with a catalytic region on their surface, when immersed ina reactant solution, generate a chemical gradient in their own vicinity [1], and usethe resulting anisotropic stresses to propel themselves through the mechanism ofdiffusiophoresis. We have have shown that polar active particles of this type canalso orient themselves along an imposed gradient of reactant concentration. Thisamounts to a theoretical demonstration of a phoretic analogue of chemotaxis,that is, the ability of a self-propelled particle to align with respect to, and henceto move up or down, a chemical gradient. The nature of the chemotaxis dependson the shape of the particle, on the distribution of enzymatic sites on its surface,and on the surface mobility. We have also shown that a collection of theseparticles when supplied with reactants at a steady rate can interact throughtheir long ranged diffusion fields to produce clumping or patterning instabilities.We have studied these instabilities by looking at the mode structure and thestructure factor in different limits – abundant or meagre supply of reactants andslow or fast reaction dynamics.

References

[1] R. Golestanian, T. B. Liverpool and A. Ajdari, Phys. Rev. Lett. 94, 220801(2005)

1

POSTER ABSTRACTS

Titile: Study of switching field statistics of Permalloy (Ni80Fe20 ) Hall bars by planar Hall effect

Author: Arnab Roy

Planar Hall effect was used to study the switching behaviour of 1mm*100µm*15nm permalloy (Ni80Fe20 ) Hall bars grown in (111) orientation on Si(100). Reversal model was Arrhenius type

activation over energy barriers, Tk

HE

BeHp)(

)(∆−

= . Statistical analyis of the field-driven magnetization

reversal process was carried out by studying the switching field for a large number of magnetic field cycles beween ±20Oe. A potrtion of a few typical hysteresis curves is shown in figure 1.

The model proposed by M.P.Sharrock[1] : m

H

HKVHE

−=∆

0

1)( with HO =2xK/M was used to find

the shape of the energy landscape for the bar undergoing reversal in an applied field. Multiple reversal paths were observed for a given wire under the same conditions(figure 2), each distribution in very good agreement with the above model, allowing the calculation of :1. Temperature dependence of the effective anisotropy constant for the bar; K(T)= (1/2)*HoM(T) (x=1 for applied field at 0O) 2. Energy barrier landscape: An exponent m= 1.5 to 2 is expected to leading order in the expansion of the energy barrier according to the Stoner Wolfarth model, however, our results give an exponent of 2.8 to 3 for all angles (out of plane) of the applied field for the principal reversal path, pointing to mechanisms other than coherent rotation at work. If domain wall propagation and pinning is the mechanism of reversal, this study determines the energy landscape around the pinning field.

Figure 1. Figure 2.

References:[1] M.P.Sharrock, J. Appl. Phys. 76, 6413 (1994);

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 5.0

0

20

40

60

80

100

120

No

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f R

eve

rsa

ls

H(Oe)3 4 5 6 7

-4.0x10-5

-3.5x10-5

-3.0x10-5

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-2.0x10-5

Hal

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)

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Unzipping Force Analysis to determine binding specificity of

RNA Polymerase to T7A1 promoter sequence

Sudeesh K†,1, Subho Ghosh* ,1, Dipankar Chatterji*, A K Sood†

† Department of Physics, Indian Institute of Science

* Molecular Biophysics unit, Indian Institute of Science

Abstract:- Transciption is one of the important steps in the central dogma of molecular biology. Using

RNA Polymerase of Escherichia coli and T7A1 promoter from T7 phage as the model system, we

probe the sequence dependence of the unzipping force of transcription at the single molecule level. We

unzip wild type and various mutated sequences of DNA with RNA-polymerase attached to each of

them and determine the change in the binding force at the beginning of the unzipping of the

transcription bubble. We thus determine the role of each individual base pair of the consensus

promoter sequence affecting the binding of RNA-Polymerase to the promoter.

1) S.K and S.G contributed equally for the work

Why does sun occasionally stop giving sunspots for several years?

Bidya Binay Karak & Arnab Rai Choudhuri

One of the most striking aspects of the 11-year sunspot cycle is that there have been times in the past when some cycles went missing. A most well-known example of this is the Maunder minimum in 17 th

century when sunspots disappeared for about 70 years. Analyses of cosmogenic isotopes (C14 and Be10) indicate that there were about 27 grand minima in the last 11,000 yr. This implies that about 2.7 of the solar cycles had conditions appropriate for forcing the Sun into grand minima. We address the question how grand minima are produced and specifically calculate the frequency of occurrence of grand minima from a theoretical model. With reasonable assumptions we show that a dynamo model leads to the conclusion that about 1-4% of the sunspot cycles may have conditions suitable for inducing grand minima.

Can interaction between emergent excitations be independent of

the constituent interactions?

Jayantha P. Vyasanakere∗ and Vijay B. Shenoy†

Centre for Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560 012, India

(Dated: November 16, 2012)

Abstract

Normally, in condensed matter systems, the interaction between emergent excitations will depend

on the interactions among their constituents. For example, magnon-magnon interaction depends

on the exchange interaction between the spins. Here we illustrate a novel system in which this is

not the case.

We had recently shown that a non-Abelian gauge field induces a BCS-BEC crossover in a weakly

interacting fermionic system. We also showed that the BEC thus obtained is a condensate of novel

kind of bosons called rashbons, which are fermion-dimers. Here we study this system by construct-

ing a Gaussian theory of quantum fluctuations and show that the rashbon-rashbon interaction is

independent of the interaction between the constituent fermions. In fact, the rashbon-rashbon in-

teraction depends solely on the rashba spin-orbit coupling induced by the non-Abelian gauge field,

which enters the kinetic energy term in the Hamiltonian.

This is, to the best of authors’ knowledge, a unique and an interesting state in any condensed

matter system.

Research funding: CSIR, DST, DAE.

References : arXiv: 1201.5332, 1108.4872, 1104.5633.

1

Thermoelectric Properties of Fe0.2Co3.8Sb12-xTex Skutterudites

Anbalagan Ramakrishnan1, Esmaeil Royanian2, Ernst Bauer2, Gerda Rogl3, Peter Rogl3 and Ramesh

Chandra Mallik1

1Thermoelectric Materials and Devices Laboratory, Department of Physics, Indian Institute of

Science, Bangalore – 560012

2Institute of Solid State Physics, Vienna University of Technology, Vienna, Austria

3Institute of Physical Chemistry, University of Vienna, Austria

Abstract:

Skutterudites Fe0.2Co3.8Sb12-xTex (x=0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6) were synthesized by

induction melting at 1273 K and followed by annealing at 923 K for 144 h. X-ray diffraction

confirmed the polycrystalline nature of the skutterudite phase as the main phase in all the

samples with a small amount of (Sb) as a secondary phase. Since Te exceeds the solubility

limit, additional secondary phases, CoSb2 and CoTe2, were observed in Co3.8Fe0.2Sb11.4Te0.6.

Lattice constants and atomic positions are verified by Rietveld refinement using the

composition from electron probe micro analysis (EPMA). The temperature dependence of

transport properties has been measured for all samples between 300 K and 818 K. Whereas

for the alloys up to x=0.2 (Co3.8Fe0.2Sb11.8Te0.2) the electrical resistivity initially increased by

charge compensation, for x>0.2 it decreased with increasing Te content due to an increasing

electron concentration. At room temperature, a positive Seebeck coefficient (holes are

majority carriers) was obtained in single element (Fe) substituted Co4Sb12, whilst a negative

Seebeck coefficient (electrons are majority carriers) was obtained in Fe0.2Co3.8Sb12-xTex.

Thermally excited carriers change from n-type to p-type in Co3.8Fe0.2Sb11.9Te0.1 at 570 K

while all other samples exhibit negative Seebeck coefficients in the entire temperature range

measured. Lattice thermal conductivity decreased with increasing Te content due to mass

fluctuation and point defect scattering. The maximum dimensionless figure of merit, zT =

1.04 at 818 K, was obtained with an optimized Te content for Co3.8Fe0.2Sb11.5Te0.5.

High mobility graphene devices

Paritosh Karnatak, T Phanindra Sai, Srijit Goswami, Subhamoy Ghatak, Arindam Ghosh

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

Graphene being a single atomic layer is significantly affected by the substrate, as the

intrinsic properties of graphene are often masked in the presence of disorder. It is well known

that the presence of substrate trap charges, polar phonons, roughness and ripples collectively

hinder studies on ultrahigh quality graphene. It was also predicted that if the effect of such

external disorder could be eliminated, graphene would show extremely high mobility.

Here we report the methods followed in making high mobility graphene devices. One of

the ways to achieve high mobility is by etching out the substrate underneath the graphene. In

such suspended graphene devices we have achieved mobilities of ~150,000 cm2/V-s. Such

devices are potential candidates to explore new physics and may find viable applications as

sensor devices. Another approach to produce high mobility graphene is to transfer graphene on

to thin flakes of hexagonal boron nitride, which has a similar arrangement of atoms as graphite.

Boron nitride flakes are found to be atomically flat, do not contain dangling bonds or traps. The

result is that graphene on boron nitride has significantly higher mobilities ~50,000 cm2/V-s,

which is significantly higher than the common graphene/SiO2/Si system. In addition, these

systems are mechanically more stable than suspended graphene structures, and open up the

possibilities of designing more intricate device structures.

Measurement of Proton-Carbon Dipolar Couplings using animproved DAPT pulse sequence

R.V. Sudheer Kumar$, # and K.V. Ramanathan#

Department of Physics$, NMR research centre#

Indian Institute of Science, Bangalore, India

Dipolar couplings provide valuable information on order and dynamics of liquid crystals.Experiments to measure heteronuclear dipolar couplings are very powerful and widelyimportant in solid state NMR, since it provides site specific dipolar couplings in alignedsamples. Interpretation of these heteronuclear dipolar couplings are hampered by thechemical shift anisotropy (CSA) and dipolar interactions among abundant spins (1H-1Hcouplings). Additionally, multiple 13C-1H dipolar couplings found in complicated systemsmake the spectra more difficult to use. For static samples the complications are resolvedpartially by separated local field (SLF) spectroscopy where the local field due toheteronuclear dipolar coupling and chemical shift interactions are separated into twofrequency domains in a 2D experiment. A method of separated local field experiment ispresented for measuring heteronuclear dipolar couplings in oriented systems. This method isbased on Dipolar assisted polarization transfer (DAPT). Compared to rotating frametechniques based on Hartmann-Hahn match, this approach is easy to implement and isindependent of any matching condition. DAPT can be utilized either as a proton encodedlocal field (PELF) technique or as a separated local field (SLF) technique, which means thatthe heteronuclear dipolar coupling can be obtained by following either evolution of theabundant spin like proton (PELF) or that of the rare that of carbon (SLF).The DAPT pulsesequence has been improved for efficient homonuclear decoupling by the interpretation ofBLEW-48 pulse sequence. The implementations of the modified DAPT experiment both as aPELF and as SLF on oriented liquid crystalline samples have been carried out. Theperformance of this experiment is compared with PISEMA.

Ultrafast NMR Techniques in Inhomogeneous Magnetic Fields

KowsalyaDevi Pavuluri$, # and K. V. Ramanathan#

Department of Physics$, NMR Research Centre#

Indian Institute of Science, Bangalore

Nuclear Magnetic Resonance (NMR) enables one to probe the structure and dynamics of matter in non-invasive manner. A major limitation of NMR for getting high resolution is the requirement of strong and extremely homogeneous magnets. Portable NMR systems have been built with open single sided probes for studying objects or samples whose size is limited to fit inside the bore of the magnet. But their use remained mainly for product and quality control since spectroscopic information cannot be recovered due to very large inhomogeneities. Nutation echo is one of the novel techniques where RF field gradients and static field gradients are matched to refocus static inhomogeneities but the full chemical shift information is maintained. Multidimensional NMR experiments can be designed based on nutation echo to get structural information in presence of inhomogeneous magnetic fields. But multidimensional NMR experiments are inherently multi scan in nature and rely on a series of independent acquisitions to sample the spin evolutions throughout the indirect time domains. Spatial encoding technique enables the collection of complete multidimensional NMR data sets in single scan. Designing experiments based on nutation echo and spatial encoding can provide high resolution NMR spectra in inhomogeneous magnetic fields with in fraction of a second. In this presentation, results of a 2D experiment obtained using spatial encoding is presented. Possibilities of using this approach in the presence of inhomogeneous magnetic fields will be discussed.

References: 1. Carlos A. Meriles et.al Science 293, 82-85 (2001). 2. Henrike Heise et.al J. Magn. Reson. 156, 146-151 (2002). 3. Y.Shrot, Lucio Frydman J. Chem. Phys. 128, 052209 (2008)

Low temperature electrical transport studies on carbon nitride films prepared by chemical vapour deposition

M. Prashantha, E.S.R. Gopal and K. Ramesh∗

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

The search for new materials for advanced application leads to the discovery of new materials with interesting electrical, physical, chemical and mechanical properties. The advancement of society and quality of human life also depends on these advanced materials. In this aspect carbon nitrides have been predicted to have high hardness, high wear resistance with low friction coefficient. These properties make them as a promising material for various applications such as organic semiconductors, fuel cells and photocatalysis, mechanical cutting tools, protective coatings, biomedical applications, electroluminescence devices, optical materials etc. The coating can reduce the wear, friction and corrosion, which can increase the life time and efficiency of the high speed moving parts. Carbon nitride coating is also found to be biocompatible. So, the successful synthesis of carbon nitride would have an enormous impact not only on the basic science but also on the technological development. In this work, we have attempted to prepare carbon nitride by chemical vapour deposition (CVD). We have used Azabenzimidazole (C6H5N3) as the precursor which has both carbon and nitrogen bonding in its structure. In a two zone furnace, the vapours of the precursor evaporated at 450 oC in Zone I are made to enter into Zone II which is kept at high temperature. The vapours get pyrolysed and deposit on quartz and silicon substrates. The samples were prepared at different pyrolysis temperatures of 725, 750, 775, 800 and 825 oC. As the C-N bond is significantly strong, even at high temperatures a considerable amount of C-N bonding is retained. This method is simple and enables one to have control over the amount of nitrogen in the system by controlling the pyrolysis temperature, volume of the liquid and the process time. The concentrations of N decreases gradually from 26 to 20 at % for the films prepared at pyrolysis temperatures of 725 to 825 oC. Electrical transport studies at low temperature (RT to 4.5K) show that the carbon nitride films exhibit Metal-Insulator (MI) transition. The incorporation of nitrogen into carbon introduces disorder in the structure. The disorder increases with the decrease of the nitrogen content, greatly influences the electrical transport properties. Disorder induced metal-insulator transitions are well known and can be studied by varying the temperature, pressure, doping level etc. In present study, metal insulator transition exhibited by carbon nitrides prepared at different pyrolysis temperatures has been studied. It is observed that the increase in pyrolysis temperature shifts the MI transition temperature to lower values. The transition temperatures for the samples prepared at 725 oC, 750 oC and 775 oC are 84.7 K, 67.7 K and 9.5 K respectively. The reduced activation energy indicates that the metallic regime of the samples prepared at pyrolysis temperatures > 800 oC lies at low temperatures. It is also observed that the activation energy decreases with the increase in pyrolysis temperature.

∗Corresponding author: kramesh@physics.iisc.ernet.in

1

Investigating DNA hybridization through changes in

conductance of ultrathin Au nanowires

Nidhi Lal1, Avradip Pradhan

2, Arindam Ghosh

2 and N. Ravishankar

1

1Materials Research Centre, Indian Institute of Science, Bangalore 560012, India

2Department of Physics, Indian Institute of Science, Bangalore 560012, India

DNA hybridization is a biological process which is unique in being extremely specific

where nucleotides in one strand bind to their counterpart in the other strand via

hydrogen bonds. DNA molecules also carry negative charge and thus have the potential

to change the conductivity of various nanostructures when bound to them. We have

used the very same property of these molecules to identify the process of DNA

hybridization by changing the conductivity of ultrathin Au nanowires. Label free electrical

detection of DNA hybridization has been studied using various nanostructures (CNTs,

silicon nanowires and graphene, for instance). In the present study, single stranded

DNA (ssDNA) molecules were immobilized over Au nanowires. The immobilization was

confirmed by AFM characterization done on ssDNA attached to Au nanowires. Upon

addition of target molecules, a significant change in the conductance of Au nanowires

was observed. These measurements were carried out by two probe resistance method

done at room temperature.

Quantum Simulation of Dzyaloshinsky-Moriya Interaction

V. S. Manu and Anil KumarCentre for Quantum Information and Quantum Computing,

Department of Physics and NMR Research Centre, Indian Institute of Science, Bangalore-560012

Quantum simulation of a Hamiltonian H requires unitary operator decomposition (UOD) of its evolution operator, (U = exp(−iHt) ) in terms of experimentally preferable unitaries. Here, using Genetic Algorithm optimization, we numerically evaluate the most generic UOD for the Hamiltonian, DM interaction in the presence of Heisenberg XY interaction, HDH . Using these decompositions, we studied the entanglement dynamics of Bell state in the Hamiltonian HDH and verified the entanglement preservation procedure by Hou et al.[1].

References:

1. Y.C. Hou, G.F. Zhang, Y. Chen, and H. Fan. Preservation of entanglement in a two-qubit-spincoupled system. Annals of Physics, 327:292296, 2012.

Title : Measurement of Forces applied by C. elegans moving on Agarose surfaces.

Authors: Siddharth Madhav Khare*, Prof. V. Venkataraman*, Prof. Sandhya P. Koushika#

* Department of Physics, IISc Bangalore #TIFR, Mumbai

Abstract:

The technique of using flexible micropillars as force sensors has already been used to probe a wide range of forces in the range of piconewtons to micronewtons. Micro-pillars made of SU8 have been used by Doll et. al.1 to measure force exerted by C. elegans moving on agar pieces. Force pattern of moving C. elegans using poly Dimethyl Siloxane(PDMS) pillar arrays has been recorded by A. Ghanbari et.al.2

In the present work we use a similar technique. We fabricate micropillars of PDMS using photolithography on SU8 followed by replica molding. Pillar height is maintained at 134(±10.78)µm and diameter is maintained at 48(±1.73) µm. Inter pillar distance is 50µm. Stiffness of the pillar is calculated to be 1.01µN/µm.2 Force patterns of C. elegans wild type and touch defective animals have been recorded. Our device geometry is simple to fabricate, easy to handle and easy to use. We are attempting to determine whether different mutants of C. elegans generate different forces during locomotion. C. elegans strains N2 have been shown to move faster than mec-4(e1339) and mec-10(e1515) in periodic agar structures by Park et.al.4

Our device is capable of combining force measurement with the artificial soil like environment.3

References:

1. Joseph C. Doll, Nahid Harjee, Nathan Klejwa, Ronald Kwon, Sarah M. Coulthard, Bryan Petzold, Miriam B. Goodman and Beth L. Pruitt, Lab Chip, 2009, 9, 1449–1454

2. Ali Ghanbari, Volker Nock, Shazlina Johari, Richard Blaikie, XiaoQi Chen and WenhuiWang, J. Micromech. Microeng. 22(2012) 095009

3. S. R. Lockery, K. J. Lawton, J. C. Doll, S. Faumont, S. M. Coulthard, T. R. Thiele, N. Chronis, K. E. McCormick, M. B. Goodman, and B. L. Pruitt, J Neurophysiol 99: 3136–3143, 2008.

4. Sungsu Park, Hyejin Hwang, Seong-Won Nam, Fernando Martinez, Robert H. Austin, William S. Ryu, PLoS ONE 3(6): e2550. doi:10.1371/journal.pone.0002550

Low frequency noise in Topological Insulator Bi1.5Sb0.4Te1.7Se1.3

Semonti Bhattacharyya, Mitali Banerjee, Hariharan N, Saurav Islam

Suja Elizabeth and Arindam Ghosh

Department of Physics, Indian Institute of Science, Bangalore 560012

ABSTRACT

Topological insulator is a new quantum state of matter which exhibits exotic metallic surface states in the bulk insulating band gap. These surface states are protected from back scattering by high spin-orbit coupling and time reversal symmetry. Probing these surface states using electrical transport measurement is a challenging task because of natural bulk doping present in the crystals caused by defects or imperfections (Se vacancies in case of Bi2Se3 and antisite defects in case of Bi2Te3). Bi1.5Sb0.4Te1.7Se1.3 has proved to be one of the best compositions to achieve maximal surface transport as donors and acceptors compensate each other in this material (1). We have studied electronic transport in mechanically exfoliated thick (~150 nm) and thin (>15 nm) flakes of Bi1.5Sb0.4Te1.7Se1.3. For thick crystals the temperature dependence of resistance shows activated-type behavior whereas for thin crystals it shows metallic behavior. It is also observed that in thick crystals the resistance starts saturating at 50 K indicating the onset of surface transport. The magnitude of “1/f” noise in these thick samples decreases in the temperature range between 200K to 80 K; then it starts increasing and finally reaches a maximum at ~18K.This data suggests that “1/f” noise can be used as an important tool to identify the contribution of surface states in electronic transport. The gate voltage dependence of noise shows that the noise magnitude also increases with decreasing carrier density. For thin samples the magnitude of 1/f noise increases at lower temperatures and with decreasing carrier densities.

1. PHYSICAL REVIEW B84, 165311 (2011)

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Superconducting fluctuations, Anomalous Phonons and

Electronic excitations in iron-based superconductors

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

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

We will present our recent Raman studies on iron-based superconductors (FeBS) as a

function of temperature. In Ca4Al2O5.7Fe2As2 superconductor, our spectroscopic studies

reveal that the phonon mode observed at ~ 230 cm-1 shows a jump in frequency by ~ 2 %

and linewidth increases by ~ 175 % at To ~ 60 K. Below To, anomalous softening of the

mode frequency and a large decreases by ~ 10 cm-1 in the linewidth are observed. These

precursor effects at T0 ( ~ 2Tc ), seen for the first time in FeBS, are attributed to

significant superconducting fluctuations possibly arising from a reduced coupling

between the two well separated ( ~ 15 Å ) Fe-As layers in the unit cell. Furthermore, a

large blue-shift of the mode frequency between 300K and 60K ( ~ 7% ) indicates strong

spin-phonon coupling [1].

In case of the 122 system i.e. Ca(Fe0.97Co0.03)2As2, all three observed phonon mode show

strong renormalization effects below the structural as well as magnetic transition

temperature (TSM ~ 160K ) attributed to the spin-phonon and electron-phonon coupling.

We observed four very weak modes in the range 400-1000 cm-1 attributed to the

electronic Raman scattering. In addition, we have observed a broad Raman band

centering at ~ 3200 cm-1 signaling the existence of coupled orbital and magnetic

excitations owing to its anomalous temperature dependence [2].

We thank all our coauthors mentioned in the references below.

Reference:

[1] Pradeep Kumar et al., Appl. Phys. Lett. 100, 222602 ( 2012).

[2] Pradeep Kumar et al., To be submitted.

Molecular dynamics simulation of electroporation of lipid bilayer membrane

Amit Kumar Majhi, V. Venkataraman, Prabal K Maiti

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

Abstract

MD Simulation [1] has been performed using NAMD and VMD [2-4] to determine the pore formation time scale for different applied fields and analyzed the movement of ions across the electroporated lipid bilayer membrane. In addition, we have also studied the closing dynamics of a pore when the field is switched off. The MD simulation has been performed for a total of 100 ns in presence of KCl solution of 0.2 M concentration in the water layer with different electric fields ranging from 0.2 V/nm to 1 V/nm to elucidate the pore formation mechanism. The field is always applied along the z-axis (perpendicular to the lipid layer) after 4 ns of equilibration. We find that a pore formation starts at different time depending on the strength of applied fields. For example, the pore formation starts 1ns after the application of an electric field of 0.4 V/nm but for a field of 1V/nm pore formation starts after 0.2ns. Once a pore appears on the membrane it expands quickly but if the field is switched off the pore reseals back but at slower speed. We have also calculated ionic current as a function of time for different applied fields. When the field is just turned on, the current does not flow at all but after some time it increases sharply. This sharp rise coincides with the formation of the pore. The magnitude of the ionic current through the membrane is found to be 10-20 nA. Pore formation mainly depends on the magnitude of the field which indicates the increase in conductivity of the membrane with application of electric field.

References:

1. Low voltage irreversible electroporation induced apoptosis in HeLa cells,. Wei Zhou, Zhengai Xiong, Ying Liu, Chenguo Yao, Chengxiang Li,. 2012, DOI:10.4103/0973-1482.95179.

2. Scalable molecular dynamics with NAMD. J. C. Phillips, R. Braun, W. Wang, J. Gumbart, E. Tajkhorshid, E. Villa, C. Chipot, R. D. Skeel, L. Kale and K. Schulten,. 2005, Journal of Computational Chemistry, Vol. 26, pp. 1781-1802.

3. VMD: Visual molecular. W. Humphrey, A. Dalke and K. Schulten,. 1996, Journal of molecular graphics,. Vol. 14, pp. 33-38.

4. Particle mesh Ewald: An N. log (N) method for Ewald sums in large systems. Darden T, York D, Pedersen L,. 1993, the Journal of Chemical Physics, Vol. 98, pp. 10089-10092.

Title of abstract: Transport properties and noise in hydrazine reduced graphene oxide.

Authors: Anindita Sahoo (1), Ryugo Tero (2), Tran Viet Thu (2), Yuji Tanizawa (3), Hiroshi Okada (2,3), Adarsh Sandhu (2,3), Arindam Ghosh (1)

Affiliation: (1) Department of Physics, Indian Institute of Science, Bangalore 560012, (2) Electronics Inspired Interdisciplinary Research Institute (EIIRIS), (3) Department of Electrical and Electronic Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku, Toyohashi 441-8580, JAPAN

Abstract:

Chemically reduced graphene oxide (GO) has attracted immense interest for large scale production of graphene. This makes electrical properties, in particular resistivity and flicker noise, in these systems extremely important for variety of electronic applications. Here we present a study of electrical resistivity, bias stressing and flicker noise (or 1/f noise) in hydrazine-reduced graphene oxide films as a function of the extent of reduction. The electrical resistance measurement on individual monolayer flakes of reduced graphene oxide (RGO) showed that its resistance decreases by four orders of magnitude when GO is reduced by hydrazine. However, this decrease was associated with an increasing D-peak in the Raman spectroscopy in comparison to shorter hydrazine treatments indicating that hydrazine treatment introduces strong localized disorder in RGO, possibly through crystal defects and impurities. This was further confirmed with bias stressing and the measurement of 1/f noise (low frequency resistance fluctuation) on the RGO-FET which showed that the presence of mobile defects in RGO with shorter hydrazine treatment leads to a faster relaxation of source-drain current and higher value of noise amplitude. Our experiments indicate that the nature as well as the kinetics of defects depends on the extent of hydrazine treatment in chemically reduced GO films.

Thermalization threshold in models of 1D fermions

Ranjan Modak∗ and Subroto Mukerjee

Centre for Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore

Sriram Ramaswamy

TIFR Centre for Interdisciplinary Sciences, Hyderabad

Abstract

The question of how isolated quantum systems thermalize is an interesting and open one. In

this study we equate thermalization with non-integrability to try to answer this question. In

particular, we study the effect of system size on the integrability of 1D systems of interacting

fermions on a lattice. We find that for a finite-sized system, a non-zero value of an integrability

breaking parameter is required to make an integrable system appear non-integrable. Using exact

diagonalization and diagnostics such as energy level statistics and the Drude weight, we find that

the threshold value of the integrability breaking parameter scales to zero as a power law with

system size. We find the exponent to be the same for different models with its value depending

on the random matrix ensemble describing the non-integrable system. We also study a simple

analytical model of a non-integrable system with an integrable limit to better understand how a

power law emerges.

∗Electronic address: modak@physics.iisc.ernet.in

1

Random matrix theory and gene correlation coefficient statistics of DNA- Microarray data: Application in understanding the system biology of gene regulation

Debayan Dey1, S. Ramakumar1

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

Abstract: The fundamental question in biology is to understand the mechanism by which a cell functions. The gene regulation of a cell and its interaction with environment & other cells makes a complex living organism. Gene regulation is the key process which dictates cell function and any imbalance in it results in disease. Understanding gene regulation using high throughput methods is pivotal to understand the holistic nature of gene regulatory network. But it suffers from large embedded noise within it; so a noise reduction method is very important to deduce sensible biological information which further can be experimentally tested. DNA-Microarray technique provides gene expression level data for the whole cell’s activity at a given time. The understanding of gene correlation matrix provided by the data is essential for biological elucidation of gene regulatory network.

In our study, we have used Random matrix theory (RMT) to separate non random and system specific features in the complex biological system from noisy data. We have analyzed various parameters that affect the threshold determination of Gene correlation network (GCN) e.g. size of the matrix, no. of variables (biological conditions) used to create the matrix, quality of microarray data etc. Here, we report the variations in statistical properties of gene correlation coefficient and its eigenvalue distribution in different biological scenarios. This property reflects the switching mechanism and dynamicity in the gene regulatory controls. We further discuss on the gene specific correlation coefficient distribution and its unique properties regarding global gene regulation. Gene correlations are not constant but change in response to physiological changes. The variations in the gene correlation and its effect in the gene regulatory network are analyzed. We have applied this method to understand the gene regulatory network of Mycobacterium tuberculosis and to understand its global regulatory circuit. We discuss a few more areas in computational biology where RMT can be used to separate noise from true correlations.

Violation of Guggenheim Adsorption Rule at Wall-Liquid Interface in Binary

Lennard-Jones Mixture

Shibu Saw1, Syed Mohammed Kamil1, and Chandan Dasgupta1,2

1 Centre for Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560012, India2 Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India

Guggenheim Adsorption rule states that the majority/minority component enrich the interfacefor negative/positive mixing energy of a binary mixture, if surface tension doesn’t play an importantrole. We perform molecular dynamics simulation of Binary Lennard-Jones liquid mixtures at theinterface created by a non-preferential wall. We show that the Guggenheim Adsorption rule is obeyedfor the negative mixing energy and violated for positive mixing energy at such interface which isvalidated by Density-Functional-Theory using Ramakrishnan-Yussouff functional. We argue thatsuch violation is due to the dominance of the entropy of the liquid mixture.

Study of effect of alkali mixture on V - O bond length in

Oxyfluoro Vanadate glasses using Raman spectroscopy.

Gajanan V Honnavar#* and K P Ramesh

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

# On leave of study from PES Institute of Technology, Bangalore South Campus (Formerly PES

School of Engineering), Near Electronic city, Hosur Road, Bangalore – 560 100, India.

*author for correspondence: gaju@physics.iisc.ernet.in

Abstract:

Raman spectroscopic study on Oxyfluoro Vanadate glasses containing various proportions of lithium

fluoride and rubidium fluoride were carried out to see an effect of mixture of alkali on vanadium -

oxygen (V – O) bond length. Glasses with general formula 40V2O5 - 30BaF2 - (30 - x) LiF - xRbF (x =

0 – 30) were prepared. Room temperature micro Raman spectra (Fig a and Fig b) of these glass

samples (VBL to VBR) were recorded in back scattering geometry. The data presented is in “reduced

Raman intensity” form with maximum peak scaled to 100[1]. We have used υ = A*exp(BR), where A

and B are fitting parameters (see Franklin D Hardcastle and Israel E Wachs, 1991[2]) to correlate the

bond length R with Raman scattering frequency υ. We observed that variation in bond length and its

distribution about a most probable value can be correlated to the alkali environment present in these

glasses. We also observed that the network forming unit is more homogenous when its first Co-

ordination is all Rubidium than all Lithium.

References: [1] Wim J Malfait and Werner E Halter, Phy.Rev.B 77, 014201(2008)

[2] Franklin D Hardcastle and Israel E Wachs, J. Phys. Chem. 95, 5031(1999)

Suppression of localization in two dimensionally doped

semiconductors at half-filling

Saquib Shamim1, Suddhasatta Mahapatra2, Giordano Scapucci2, W. M. Klesse2, P.B.S.

Mahapatra1, Michelle Y. Simmons2 and Arindam Ghosh1

1Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2 Centre for Quantum Computation and Communication Technology, University of New South Wales, Sydney

NSW 2052, Australia

The nature of quantum states, and transport of electrons, in a two dimensional (2D) disordered solid

change significantly in the presence of a background periodic potential, such as that arising from the

host crystal lattice. The interplay of the electron wavelength and lattice periodicity leads to a number

of exotic phenomena, in particular close to half filling, that range from collapse of scaling theory and

Anderson localization, hole-mediated (Nagaoka) ferromagnetism, to new modes of quantum

transport. Much of these phenomena remain unexplored experimentally, primarily due to the lack of a

suitable material platform. Advances in material engineering now allow the dopants, such as

phosphorus (P), confined within one atomic layer inside bulk crystals of silicon (Si) and germanium

(Ge), leading to a unique two-dimensional (2D) electron system whose Fermi energy exists naturally

at or very close to the center of band. Here we demonstrate that quantum interference effect in 2D

Si:P and Ge:P is drastically different from conventional weakly localized 2D electron systems. This

is manifested in two unexpected ways: First, the quantum correction to low temperature conductivity

shows a strong weak antilocalization (WAL), in addition to the conventional weak localization (WL)

behaviour, and second, a spontaneous factor of two suppression in the universal conductance

fluctuations at low areal density of the dopants. We attribute these observations to the emergence of

new modes, called the π-Cooperons and π-Diffusons, of quantum diffusion at half-filled bands, where

electrons propagate through ‘umklapp scattering’ in the presence of a nested of Fermi surface and the

resulting particle-hole symmetry.

Magnetic, Dielectric and Transport Studies of Single Crystal Tb0.5Sr0.5MnO3

Hariharan N, Aneesh C, H L Bhat and Suja Elizabeth

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

ABSTRACT

Half doped Tb0.5Sr0.5MnO3 single crystal is grown by four mirror Optical Float-Zone Furnace.

Temperature dependence of dc magnetic susceptibility is carried out in sample with arbitrary

orientation at different fields and a magnetic anomaly is observed around 42 K. Isothermal

magnetization measurements carried at 5 K shows a magnetic hysteresis which is not seen at 50 K, just

above the magnetic anomaly temperature. Dielectric properties are measured in the temperature range

15K-300K with different frequencies ranging from 400Hz to 4MHz .Both real and imaginary parts of the

dielectric constant show clear frequency dispersion which suggests that these materials could be relaxor

ferroelectric. Transport properties of the sample are measured in four probe geometry between 60-

300K. While cooling in this temperature range resistance of the sample steadily increases and was five

orders in magnitude higher at 60 K.

References

1. Appl.Phys.Lett. 96, 152103, (2008).

2. J.Appl.Phys. 83, 7664, (1998).

Direct Numerical Simulation of

Turbulence in the Two-Dimensional

Navier-Stokes-Cahn-Hilliard Equations

Nairita Pal, Anupam Gupta and Rahul PanditDepartment of Physics,

Indian Institute of Science,Bangalore

November 15, 2012

Abstract

We study a phase-field model for a mixture of two-dimensional(2D)incompressible fluids, both of which obey the Navier-Stokes Equa-tion.We elucidate their mixing by introducing a phase-field order pa-rameter which obeys the Cahn-Hilliard Equation. We have developedan MPI, parallel, pseudospectral code for the direct numerical simu-lation (DNS) of the coupled Navier-Stokes-Cahn-Hilliard Equations in2D. We present preliminary results from this DNS for decaying turbu-lence in this system. We concentrate on statistical properties such asthe energy and dissipation time-series, the enrgy and enstrophy spec-tra, dissipation-reduction-type phenomena and the motion of dropletsof the minority phase in this turbulent binary-fluid mixture.

1

Thermoelectric properties of chalcogenide based Cu2+xZnSn1-xSe4

Ch. Rajua, M. Falmbiglb, P. Roglb, X. Yanc, E. Bauerc, J. Horkyd, M. Zehetbauerd,

and Ramesh Chandra Mallika*

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

b Institute of Physical Chemistry, University of Vienna, Währingerstrasse 42, A-1090 Wien, Austria c Institute of Solid State Physics, TU Vienna, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria

d Research Group Physics of Nanostructures Materials, University of Vienna, Boltzmanngasse 5,

A-1090 Wien, Austria

Abstract:

Quaternary chalcogenide compounds Cu2+xZnSn1-xSe4 (x=0, 0.025, 0.05, 0.75, 0.1, 0.125, 0.15) were prepared by solid state synthesis. The structural and phase identification of these compounds were studied by Rietveld X-ray powder diffraction (XPD) refinement combined with Electron Probe Micro Analyzes (EPMA). The XPD patterns of all samples showed a stannite phase isotypic with the tetragonal Cu2FeSnS4-type including ZnSe as a secondary phase. The samples with the highest Cu-content (x=0.125, 0.15) contained CuSe and SnSe as secondary phases in addition to ZnSe. Raman spectroscopy was employed for the phase identification in order to resolve Cu2ZnSnSe4 and ZnSe phase contents, because XPD patterns of both the phases overlap. In all samples the presence of vibrational modes corresponding to Cu2ZnSnSe4 were observed and confirm the stannite phase. The electrical resistivity, Seebeck coefficient and thermal conductivity measurements were carried out as a function of temperature in the range 300-720K. The temperature dependent electrical resistivity data of the undoped sample confirmed semiconducting behavior in-between 4.2 and 250 K, whereas the samples with nominal compositions Cu2+xZnSn1-xSe4 (x=0.05, 0.15) were metallic in nature. The electrical resistivity is decreasing with increasing doping concentration except for the sample with the nominal composition Cu2.1ZnSn0.9Se4, probably due to the presence of a high content of the ZnSe impurity phase. All the samples show a positive Seebeck coefficient throughout the temperature range investigated, which indicates that the majority carriers are holes. The total thermal conductivity and phonon thermal conductivity of the undoped sample was significantly less as compared to the remaining doped samples and this may be due to the larger electronic contribution and the presence of a higher content of the ZnSe secondary phase in the doped samples. The maximum figure of merit zT = 0.3 at 720 K occurs for the sample with the nominal composition Cu2.05ZnSn0.95Se4, but the improvement in zT is rather small due to the presence of secondary phases. The attempt to improve thermoelectric properties employing a high-pressure torsion treatment resulted in an enhancement of zT by 30 % up to 625 K for Cu2.05ZnSn0.95Se4.

Thermal Conduction, Feedback and Multiphase Gas in GalaxyClusters

Baban Wagh, Prateek Sharma

Galaxy clusters are the largest gravitationally bound, relaxed astronomical objects in the universe, with totalmass around 1014 − 1015M�, where M� is a solar mass. Most of the cluster mass, about 85%, is in the darkmatter halo, while 15% is the baryonic mass. Of this 15%, around 90% is in the form of hot, X-ray emittingplasma (∼ 107 − 108K), called the intracluster medium (ICM). Due large gas density in the central regions,the ICM loses energy primarily by bremsstrahlung radiation, and cools. But there is a discrepancy betweenthe predicted and the observed cooling rate in galaxy clusters, and this is called the cooling flow problem. It isbelieved that the heat lost to cooling is replenished by feedback heating from the central supermassive black holeand via thermal conduction bringing in heat from larger radii. Sharma and others ([1]) have identified ratio ofthe thermal instability timescale and the free-fall time (tTI/tff) as an important factor in multiphase formation.If tTI/tff . 10, then cold gas condenses from the hot phase from the ICM in thermal balance. Condensationof cold gas is suppressed by thermal conduction, and thermal conduction in dilute ICM plasmas is along thelocal magnetic field direction. We study the role of thermal conduction in multiphase gas formation in galaxyclusters using idealized MHD simulations maintained in global thermal equilibrium. We find that while isotropicconductivity can effectively suppress cold gas formation, anisotropic thermal conduction does not change thetTI/tff criterion based on hydro simulations. We also quantify the relative role of feedback heating and heatingdue to thermal conduction in cluster cores.

References

[1] Sharma, P., McCourt, M., Quataert, E., & Parrish, I. J. 2012, MNRAS, 420, 3174

1

Realization of Fermionic Superfluid State in an Optical Lattice

via a Bilaryer Band Insulator

Yogeshwar Prasad, Amal Medhi, Vijay B. Shenoy

Centre for Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560012, India.

Abstract

We propose a model to realize a fermionic superfluid state in an optical lattice, which is one of

the central problems in the area of ultracold quantum gases due to cooling problem. The idea of

our model hinges on a characteristically low entropy state, a band-insulator in an optical bilayer

system and tuning the interaction in such a system to realize the superfluid state. The system is

designed such that the superfluid phase beats other competing phases such as charge density wave

as we show by detailed Monte Carlo calculations. Within the Gaussian approximation, we show

that the superfluid state has a high characteristic temperature scale of the order of hopping energy.

We suggest a route for the possible experimental realization of this state in an optical lattice.

Work supported by CSIR, DAE and DST.

Reference : arXiv:1206.2407v2

1

Superbubble breakout and galactic winds from

disk galaxies

Arpita Roy; Biman B. Nath; Prateek Sharma; Y. Shchekinov

We study the evolution of superbubbles driven by large star clus-ters in disk galaxies using Kompaneets approximation and numeri-cal simulations. We investigate the effect of radiative losses on thedynamics of superbubbles, in particular the conditions required forsuperbubbles to break through the disk material to throw the innerhot gas into the halo with sufficient speed so as to create a galacticoutow. We nd that our calculations and simulations may explainthe observed threshold star formation surface density required tocreate superwinds from disk galaxies.

1

Flow Induced Alignment of water molecules confined inside carbon nanotube: Insightfrom MD Simulations

Hemant Kumar,1, ∗ Prabal K. Maiti,1, † Chandan Dasgupta,1, ‡ and A. K. Sood2, §

1Centre for Condensed Matter Theory, Indian Institute of Science, Bangalore, India-5600122Department of Physics, Indian Institute of Science,Bangalore, India-560012

Molecular dynamics study for the flow induced structural change of water molecules flowingthrough carbon nanotubes (CNT) has been presented. We show that dipole moment of the confinedwater molecules gets aligned along the axis of nanotube under the effect of flow. With increasingflow velocities, net dipole moment first increases and eventually saturates to a constant value. Thisobservation is very similar to Langevin theory of paramagnet where flow velocity acts as an effectivealigning field. Preferential entry of entering water molecules with dipole pointing inward has beenshown to be responsible for this effect. This observation provides a way to control the dipolaralignment of water inside nano-channel which can be used for various nano-electrical devices andsupports the study by K.Sathya Naryanan and A.K. Sood.

∗ hemant@physics.iisc.ernet.in† maiti@physics.iisc.ernet.in‡ cdgupta@physics.iisc.ernet.in§ sood@physics.iisc.ernet.in

Large linear magnetoresistance in a GaAs/AlGaAs heterostructure

Mohammed Ali Aamir,1 Srijit Goswami,1 Matthias Baenninger,2 Vikram

Tripathi,3 Michael Pepper,4 Ian Farrer,2 David A. Ritchie,2 and Arindam Ghosh1

1Department of Physics, Indian Institute of Science, Bangalore 560 012, India.2Cavendish Laboratory, University of Cambridge,

J.J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom.3Department of Theoretical Physics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India

4Department of Electrical and Electronic Engineering,University College, London WC1E 7JE, United Kingdom

Large electrical response to magnetic field is one of the key requirements in materials engineeringtoday. It may seem that a magnetic nature of the material is vital for this [1], but some materialsexhibit it even without magnetism at any scale. Over the last decade, it has been shown thatmaterial inhomogeneity alone can provide a route to large magnetoresistance (MR) in non-magneticsemiconductors [2, 3]. Interestingly, this MR also has a linear characteristic. Here, we study the MRof a two-dimensional electron system (2DES) in a GaAs/AlGaAs heterostructure where a disorderedband-structure is induced by applying a large negative gate bias. We find that MR increases linearlywith magnetic field when the device is operated in the non-equilibrium regime with high source-drain bias [4]. Remarkably, the magnitude of MR is as large as 500% per Tesla, thus dwarfing mostnon-magnetic materials which exhibit this linearity. Its primary advantage over other materials isthat both linearity and the enormous magnitude are retained over a broad temperature range (0.3 Kto 10 K), thus making it an attractive candidate for on-chip magnetic field sensing.

[1] M. N. Baibich, J. M. Broto, A. Fert, F. N. Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, and J. Chazelas,Phys. Rev. Lett. 61, 2472 (1988).

[2] R. Xu, A. Husmann, T. F. Rosenbaum, M. Saboungi, J. E. Enderby, and P. B. Littlewood, Nature 390, 57 (1997).

[3] M. P. Delmo, S. Yamamoto, S. Kasai, T. Ono, and K. Kobayashi, Nature 457, 1112 (2009).

[4] M. A. Aamir, S. Goswami, M. Baenninger, V. Tripathi, M. Pepper, I. Farrer, D. A. Ritchie, and A. Ghosh, Phys. Rev. B86, 081203 (2012),

Universal Conductance Fluctuations as a direct probe to valley coherence and

universality class of disordered graphene

Vidya Kochat, Atindra Nath Pal and Arindam Ghosh

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

Quantum interference of carriers produces reproducible fluctuations of the order of e2/h in the

electrical conductance of mesoscopic semiconductors and metal films as the Fermi energy, magnetic field or

disorder configuration is varied. Both universal conductance fluctuations (UCF) and weak localization (WL)

effects are inevitable components of quantum transport in disordered metals at low temperatures as they

encode crucial information on phase coherence, nature of scattering and symmetry properties of the

Hamiltonian that govern the level statistics of the underlying disordered system. Unlike in conventional metal

films and doped semiconductors where UCF and WL are mainly due to inelastic scattering mechanisms, the

scenario in graphene is more complex due to the existence of two degenerate valleys (K and K’) in its

hexagonal Brillouin zone. The quantum correction to conductivity in graphene is determined by the elastic

scattering mechanisms involving the valleys, namely the inter-valley and intra-valley scattering, in addition to

the phase breaking inelastic scattering events and these competing mechanisms are reflected in the WL and

weak anti-localization of carriers.

Here we present the first unambiguous observation of the effect of valley symmetry on UCF in

monolayer disordered graphene. The UCF magnitude within a single phase coherent box in graphene is

suppressed by an exact factor of four as the carrier density is increased from close to the Dirac point, where

long range Coulomb potential fluctuations dominate, to the high electron or hole density regime, where

potential fluctuations are primarily short range in nature. This also implies a density dependent crossover of

the universality class of graphene from symplectic near the Dirac point to orthogonal at high densities. We also

find that in the presence of a magnetic field, the UCF magnitude decreases by an exact factor of two and this

corresponds to a transition from the symplectic / orthogonal ensemble to the unitary ensemble characterized

by the absence of time reversal symmetry. This work also examines the robustness of time reversal symmetry

in mesoscopic graphene which has been controversial owing to pseudo-magnetic fields arising from ripples,

local moments at edges etc.

The valleys which resemble a spin-like entity can be exploited to form the platform for a new

emerging field termed as valleytronics, having applications ranging from valley-based quantum computation,

to valley filters or polarizers. Our experiments underline a new method using UCF that can probe the valley

coherent states in graphene at low temperatures.

Reference:

Atindra Nath Pal, Vidya Kochat and Arindam Ghosh, Phys. Rev. Lett. 109, 196601 (2012).

Sensory organ like response determines the magnetism of zigzag-edged honeycombnanoribbons

Somnath Bhowmick1,∗ Amal Medhi2,† and Vijay B Shenoy2‡1Materials Research Center, Indian Institute of Science, Bangalore 560 012, India

2Centre for Condensed Matter Theory, Department of Physics,Indian Institute of Science, Bangalore 560 012, India

(Dated: November 12, 2012)

We present an analytical effective theory for the magnetic phase diagram for zigzag edge termi-nated honeycomb nanoribbons described by a Hubbard model with an interaction parameter U . Weshow that the edge magnetic moment varies as lnU and uncover its dependence on the width Wof the ribbon. The physics of this owes its origin to the sensory organ like response of the nanorib-bons, demonstrating that considerations beyond the usual Stoner-Landau theory are necessary tounderstand the magnetism of these systems. A first order magnetic transition from an anti-parallelorientation of the moments on opposite edges to a parallel orientation occurs upon doping with holesor electrons. The critical doping for this transition is shown to depend inversely on the width of theribbon. Using variational Monte-Carlo calculations, we show that magnetism is robust to fluctua-tions. Additionally, we show that the magnetic phase diagram is generic to zigzag edge terminatednanostructures such as nanodots. Furthermore, we perform first principles modeling to show howsuch magnetic transitions can be realized in substituted graphene nanoribbons.

PACS numbers: 75.75.-c, 73.20.-r, 75.70.-i, 73.22.Pr

∗ bsomnath@mrc.iisc.ernet.in

† amedhi@physics.iisc.ernet.in‡ shenoy@physics.iisc.ernet.in

Evolution of fermionic superfluid across the crossover from three to two dimensions

Sudeep Kumar Ghosh∗ and Vijay B. Shenoy†

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

Motivated by recent experiments on the evolution of fermionic superfluid pairing from three to twodimensions, we construct and study a Bogoliubov-de Gennes theory that accurately accounts for theperiodic potential that induces this dimensional crossover. We consider a system of spin- 1

2fermions

interacting in the singlet channel via a contact interaction confined by an optical lattice potentialin the z-direction and the motion in plane is free. With the increase in potential depth, the systemgets divided into stacks of two dimensional layers with gradually decreasing inter layer hopping.The mean field equations are solved numerically to obtain the Bloch bands. For small potentialdepth, the linear response of density and pairing gap of the system are obtained numerically andcompared with that of perturbation theory calculations. In deep lattice limit the system becomestwo dimensional and the binding energy is found to be in close agreement with the two dimensionalresult. The radio frequency spectrum of the system shows characteristic asymmetric dissociationpeak structure and a clear pairing gap emerges with increasing lattice depth as seen in experiments.

∗ Electronic address: sudeep@physics.iisc.ernet.in† Electronic address: shenoy@physics.iisc.ernet.in

Title: Induced photoconductivity in large area graphene by electrochemical deposition of thin films

Authors: Medini Padmanabhan, Uma Maheswari P, Shishir Kumar, Srinivasan Raghavan and Arindam Ghosh Abstract: In this work we study the optical response of large area graphene electrochemically integrated with light-sensitive semiconductors. Chemical vapor deposited (CVD) graphene is transferred onto a suitable substrate such as glass or silicon wafer. Two- point resistance of graphene is measured with and without a layer of electrodeposited CdS. We find that, in the presence of CdS, appreciable photoconductivity is induced in graphene. The hybrid-device is observed to respond to light at a much faster timescale compared to the characteristic photodesorption curves of bare graphene. We explore various avenues of integrating this growth technique with solar cell architectures.  

Microwave assisted synthesis of single crystalline ternary alloy-Bi2-xSbxTe3 for thermoelectric applications

Mitali Banerjee1, R.Venkatesh2, Arindam Ghosh1 and N. Ravishankar2

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

2 Materials Research Centre, Indian Institute of Science,Bangalore 560012, India

ABSTRACT

The discovery of colossal enhancement of thermoelectric figure of merit in bismuth-based chalcogenide nanostructures has identified them as potential candidates for thermoelectric applications. Especially one shot facile synthesis of p-type bismuth antimony telluride which has been reported to show a large thermoelectric power around the room temperature still remains as a challenge. We report a surfactant-assisted wet chemical synthesis of nanostructures of single crystalline ternary Bi2-xSbxTe3 (0.5<x<1.5) alloy by a microwave-assisted method. The powder X-ray diffraction confirmed formation of single phase ternary alloys. The atomic percentage of ternary compositions is also confirmed using energy-dispersive spectroscopy (XEDS). Transmission electron microscopy demonstrated single crystalline nature of the hexagonal flakes. The thicknesses of the flakes were determined using atomic force microscopy. Different morphologies have been observed in scanning electron microscope while varying the tellurium-source precursor from telluric acid to tellurium metal. The hexagonal flakes with cross section of around 1 μm, with thickness around 20 nm have formed using telluric acid while octahedral crystals of nearly 20 nm size have been found using the highly pure Te-metal with tri-octyl-phospine as the tellurium sources. The effect of temperature and microwave doses on morphologies is also investigated in this work. The difference in morphology observed in this two microwave assisted synthesis procedure is advantageous for tuning the thermal conductivity of nanostructure and thereby enhancing the thermoelectric figure of merit.

Sharp Raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3

Achintya Bera1, Koushik Pal2, 3, D. V. S. Muthu1, Somaditya Sen4, Prasenjit Guptasarma4, U. V. Waghmare3, and A. K. Sood1 1Department of Physics, Indian Institute of Science, Bangalore-560012, India 2Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India 3Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India and 4Department of Physics, University of Wisconsin-Milwaukee, Wisconsin-53211, USA Recently discovered three-dimensional Topological insulators (TI) have attracted a lot of attention of researchers from diverse backgrounds due to robust and exotic phenomena, such as quantum spin Hall states, arise from the geometric and topological properties of the manifold of their electronic states. However, experimental evidence for nontrivial topology of electronic states is so far mainly based on the time-reversal symmetry protected chiral surface states. Signatures of the nontrivial topology of a TI in its bulk behaviour are subtle, and not yet identified. Our work presented here establishes that (a) bulk signatures of the electronic topology become significant and detectable as anomalies in the phonon frequency and linewidth of Eg Raman mode at an electronic topological transition (specifically in single crystal Sb2Se3), (b) their origin lies in physical mechanisms that involve going beyond adiabatic approximation as a metallic (vanishing gap) state of the bulk appears at the transition, and (c) they involve electron-phonon couplings some of which are not captured by the standard methods to calculate electronic structure. References: [1] Zhang, H. et al. “Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface”. Nature Phys. 5, 438-442 (2009). [2] G.K. Pradhan et al. “Raman signatures of pressure induced electronic topological and structural transitions in Bi2Te3”. Solid State Commun. 152, 284–287 (2012)

Using cis peptide containing fragments for functional annotation of proteins

Sreetama Das1, Suryanarayanarao Ramakumar1 and Debnath Pal2

1Department of Physics, 2Supercomputer Education Research Centre, Indian Institute of Science, Bangalore 560012, India

BACKGROUND: Proteins are linear polymers of the twenty naturally occurring amino acids, and are essential for many cellular processes. The consecutive amino acids are covalently linked by a peptide bond, which may be in cis or trans conformation. There are a large number of proteins whose structures are known but their functions are not. Here from arises the need for protein function prediction methods.

Cis peptide bonds, and especially those involving non-Proline amino acids, although rare in occurrence in protein structures, are implicated to play an important role in their structure and/ or function. It is, therefore, pertinent to ask if singular protein segments containing cis peptides can provide functional annotation of proteins with known structure but unknown function.

RESULTS:

We used our own protein backbone geometry-based clustering algorithm to group cis peptide containing fragments of specific lengths. Of these we identified fragments of length 6 ─ 8, suitable for annotation studies. Grouped fragments of these lengths were subjected to analysis of enrichment of Gene Ontology (GO) molecular function terms. Fragments associated with statistically-enriched GO terms were identified as functionally important. These functionally “significant” fragments were thereafter searched in homologous proteins. Their utility as singular cis-containing peptide fragment for providing functional annotation was thereafter confirmed through a high area under the Receiver-Operator-Curve (ROC). Subsequently, we are attempting to locate these fragments in proteins of unknown function for annotation purposes.

CONCLUSIONS:

Fragments associated with enriched GO molecular function showing propensities ≥ 20 and p-value thresholds ≤ 0.05 point to cis peptide-containing fragments important for protein function. This fact was verified through a literature survey. When used for identifying similar fragments in a set of non-redundant entries from the PDB database, the fragments alone were sufficient to identify close homologues and related proteins. Currently, we are establishing the utility of these fragments for functionally annotating proteins with no known function.

Study of Phonon anharmonic effects in pyrochlores

P K Verma ∗, U V Waghmare †, A K Sood ‡, H R Krishnamurthy §

Deparment of Physics, Indian Institute of Science, Bangalore, India

Jawaharlal Nehru Center for Advanced Scientific Research, Bangalore, India

Structural and electronic properties of ideal pyrochlores with the composition Y2B2O7

(where B = Ti, Zr, and Ir) were studied using the first-principles calculations. Both Y2Ti2O7

and Y2Zr2O7 are insulators while Y2Ir2O7 is metal. A large anomalous Born effective charge

was observed for Ti, attributing to the hybridization between the occupied 2p states of the

oxygen and unoccupied d states of the B cation. Density functional perturbation theory

calculations were performed to obtain the phonon properties. While all the phonon frequen-

cies of Y2Zr2O7 and Y2Ir2O7 were found to be real valued as one would expect, Y2Ti2O7

showed instabilities with respect to some optical distortions, in that 6 of the frequencies

were found to be imaginary. This is likely to underlie the anomalous temperature dependent

of the phonons that have been seen in other titanate pyrochlores. In a study as to how to

stabilize Y2Ti2O7, we found that the system becomes stable at 7GPa at the Γ point, but

remains unstable at other k-points in the Brillouin Zone; it becomes stable at all k-points

at 12GPa. Small distortions of the atomic positions inside the unit cell can also stabilize

the structure at ambient pressure. Phonon-phonon anharmonic effects are very important in

titanate pyrochlores as have been experimentally seen in other titanates. We have treated

the effects of these upto third order anharmonic corrections. We found phonon anomalies

for some of the phonon modes of Y2Ti2O7, in qualitative agreement with experiments.

∗ Electronin address: pramod@physics.iisc.ernet.in† Electronin address: waghmare@jncasr.ac.in‡ Electronin address: asood@physics.iisc.ernet.in§ Electronin address: hrkrish@physics.iisc.ernet.in

Optoelectronic properties of graphene-MoS2 hybrids

Kallol Roy,1 Medini Padmanabhan,1 Srijit Goswami,1 T. Phanindra Sai,1

Gopalakrishnan Ramalingam,2 Srinivasan Raghavan,2 and Arindam Ghosh1

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

2Materials Research Center, Indian Institute of Science, Bangalore 560012, India

Abstract

Graphene is a very interesting layered material because of its high quality electronic character-

istics which can be tuned by application of a gate voltage. However optical absorption property of

single layer graphene is very poor, and hence most of the light (> 95%) gets transmitted. Molyb-

denum disulphide (MoS2) is another layered material which is optically active and its bandgap

changes with number of layers. In this report we have shown that the hybrid structure of graphene

and MoS2 can be made to integrate their electronic and optical characteristics, and gate tunable

optical response can be induced in graphene.

Bio – sensors based on electrical and optical properties of Carbon nanotubes & Graphene oxide

K. S. Vasua, S. Sridevib, S. Asokanb, N. Jayaramanc and A. K. Sooda a Department of Physics, Indian Institute of Science, Bangalore

b Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore c Department of Organic Chemistry, Indian Institute of Science, Bangalore.

We show that single walled carbon nanotubes (SWNTs) decorated with sugar functionalized

poly (propyl ether imine) (PETIM) dendrimer (DM) is a very sensitive platform to quantitatively

detect carbohydrate recognizing proteins, namely, lectins. The mannose attached PETIM

dendrimers undergo charge – transfer interactions with the SWNT. The changes in the

conductance of the dendritic sugar functionalized SWNT after addition of lectins in varying

concentrations were found to follow the Langmuir type isotherm, giving the concanavalin A

(Con A) – mannose affinity constant to be 8.5 x 106 M-1 [1].

We have recently shown that etched Fiber Bragg Gratings (eFBGs) coated with nano – carbon

materials can be used as potential biochemical sensors. The shift in the Bragg wavelength (∆λB)

with respect to the λB values of SWNT (or GO) – DM coated eFBG for various concentrations of

lectin follows Langmuir type adsorption isotherm and quantitatively analyzed to establish the

detection limit [2].

References: 1) K. S. Vasu, K. Naresh, R. S. Bagul, N. Jayaraman and A. K. Sood, Appl. Phys. Lett. 101,

053701 (2012).

2) S. Sridevi, K.S. Vasu, N. Jayaraman, S. Asokan and A.K. Sood (2013).

Violation of Chandrasekhar mass limit: Strongly

magnetized white dwarfs as progenitors of

super-Chandrasekhar type Ia supernovae

Upasana Das, Banibrata Mukhopadhyay

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

upasana@physics.iisc.ernet.in, bm@physics.iisc.ernet.in

November 15, 2012

Abstract

Recent observations of peculiar Type Ia supernovae - SN 2006gz, SN 2007if,SN 2009dc, SN 2003fg - seem to suggest super-Chandrasekhar-mass white dwarfswith masses up to 2.4 - 2.8 solar mass, as their most likely progenitors. We showthat strongly magnetized white dwarfs can violate the Chandrasekhar mass limit(which is 1.44 solar mass) significantly, owing to the Landau quantization of therelativistic electron degenerate gas. Interestingly, our results seem to lie withinthe above observational limits. We also establish that accretion on to commonlyobserved magnetized white dwarfs, coupled with the phenomenon of flux freezing,leads to the generation of very strong magnetic fields in the interiors of these whitedwarfs. This would in turn explain the super-Chandrasekhar masses according toour proposed theory.

1

Superfluidity in Bricks!

Arijit Haldar and Vijay B. ShenoyCentre for Condensed Matter Theory

Indian Institute of Science, Bangalore 560 012

An optical brick lattice has recently be realized by the ETH group. We show that this systems offers the possibility of realizing a superfluid state of fermions with a high transition temperature. The brick lattice defined by three hopping parameters, in a regime, has a band gap. Exploiting this, we propose to the band insulator obtained in this regime to beat the entropy problem, to obtain an optical lattice superfluid by tuning an attractive interaction between the fermions. By studying the quantum field theory of the system including Gaussian fluctuations, we estimate the Kosterlitz-Thouless transition temperature of the system. We find that in a regime of parameters the transition temperature is “ high” , i.e., of the order of the hopping scale.

See also: Related poster by Yogeshwar Prasad et al.