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 23 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 (including senior undergraduates) to familiarize themselves with the current research activity in our Department in various branches of Physics.

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

Prof. H. R. KrishnamurthyChairmanNovember 16, 2013

November 16, 2013

Auditorium, New Physical Sciences Building

Session I 9:00-10:30

T01 9:00-9:15

T02 9:15-9:30

T03 9:30-9:45

T04 9:45-10:00Collective order-disorder transition in active granular matter

T05 10:00-10:15

T06 10:15-10:30

10:30-11:00 Tea

Session II 11:00-1:00

T07 11:00-11:15Continuous beam of laser-cooled Yb atoms

T08 11:15-11:30

T09 11:30-11:45

T10 11:45-12:00

T11 12:00-12:15

T12 12:15-12:30Selective Single Scan (S3) NMR Spectroscopy Techniques

T13 12:30-12:45

T14 12:45-1:00Spinning Black Holes

1:00-2:00 Lunch

Department of Physics, IISc Bangalore

Inhouse Symposium 2013

Programme

Chair: Subroto Mukerjee

Anindya DasHigh-efficiency Cooper pair splitting demonstrated by two-particle conductance resonance and positive noise cross-correlation

R.V. Sudheer KumarA Modified DAPT Pulse Sequence for Measuring Large Dipolar Couplings in Solid State NMRSarika C K

Cluster Formation of Gold Nanoparticles in Two Dimension

Nitin Kumar

Dona Cherian Decoupling of magneto-structural transition in Fe

1+y Te and the onset

of superconductivity in FeSexTe

1−x

Saquib ShamimSpontaneously broken time reversal symmetry in strongly interacting two dimensional electron layers in Si and Ge

Chair: R. C. Mall ik

K. D. Rathod

Ashoka Bali

Thermoelectric properties of bulk PbTe with encapsulated bismuth and indium phases

Kingshuk SarkarFluctuation diamagnetism from a Ginzburg-Landau-like free energy functional for high-Tc superconductors

Vidya Kochat

Inducing spin-triplet superconducting correlations in graphene

Ch. Raju

Thermoelectric properties of doped quaternary chalcogenide materials

Kowsalya Devi Pavuluri

Debarghya BanerjeeMultiscaling in Hall-Magnetohydrodynamic turbulence: insight from a shell model study

Banibrata Mukhopadhyay

Session III 2:00-4:30 Poster Session

Session IV 4:30-5:30

T15 4:30-4:45

T16 4:45-5:00

T17 5:00-5:15

T18 5:15-5:30

5:30-6:00 High Tea

Session V 6:00-7:15

T19 6:00-6:15

T20 6:15-6:30

T21 6:30-6:45

T22 6:45-7:00

T23 7:00-7:15Turning Shear Thickening “On” and “Off ”

7:15-7:30 Best Poster Award

7:30-8:30 Dinner

Chair: S. Ramakumar

K. RajanFunctional Near Infrared Optical Tomography (fNIROT): Issues and Challenges

Pranab Jyoti BhuyanShort-time Dynamics of Glass-forming Liquids in Metabasins of the Energy LandscapeShreyas GokhaleGrowing Dynamical Facilitation on Approaching the Colloidal Glass Transition

Ashok GaraiForce induced biomolecular transition through an intermediate: Theory, analysis and interpretation

Chair: R. Ganesan

Manish JainGap renormalization of molecular crystals from density functional theory

Pranjal Mahanta The role of N-terminal Valine Residue in the structural stability of a Bacillus xylanase

V. VadakkumbattTrapping and manipulating multielectron bubbles in liquid helium to investigate 2D electron systems at unexplored densities

Himanshu Joshi

Structure Stability and Elasticity of DNA Nanotubes

Vikram Rathee

Vishwamitra Memorial Prize

LIST OF POSTERS

No. Presenter Title

P01 M. A. Aamir

P02 Aabhaas V Mallik t-J Model Revisited: Relevance for Cuprate Problem

P03 Raghav Nallani

P04 Semonti Bhattacharyya Dephasing in 3D Topological Insulators at Low Temperatures

P05 Gopal Hazra

P06 Physics in Synthetic Dimensions

P07 Adhip Agarwala Routes to Calculation of DC conductivity of Disordered Materials

P08 Kimberly Hsieh Sui Mee

P09

P10 Demonstration of room temperature spin injection in Fe3O4/MgO/GaAs

P11

P12 Anindita Sahoo

P13 Subhamoy Ghatak

P14 Sandip Mondal Spin Coated Dielectrics: Organic Vs. Inorganic

P15 R. Koushik Shot noise in disordered ultra-thin films of NbN

P16 PBS Mahapatra Thermal Properties of Ultrathin Molybdenum Disulfide layers

P17 Tanweer Ahmed

P18 Rakesh M Optical tomography in scattering gel dosimeters

P19 Ranjan Modak

P20 Kazi Rafsanjani Amin Noisy Graphene Gas Sensor

P21 Magneto transport and 1/f noise in 2DEG at LaAlO3/SrTiO3 interface

P22 Lora Rita Goveas Magnetic and EPR studies of nanoparticles and bulk Sm0.65Ca0.35MnO3

P23 Siddharth Madhav Khare

P24 Debabrata Pramanik

P25 Vijay B. Shenoy Feshbach Resonance in a Synthetic Non-Abelian Gauge Field

Giant Suppression of Universal Conductance Fluctuation in Bilayer Graphene - Boron Nitride Heterostructure

Molecular mechanism of permeation of water through helium leak-tight graphene oxide membrane

Is a deep one-cell meridional circulation essential for the flux transport solar dynamo?

Sudeep Kumar Ghosh

Systematic study of the effect of various organic solvents on PMMA-495 and Scotch Tape residues in exfoliated graphene on Si-SiO2 wafers

Aditya N. Roy ChoudhuryMagnetic Field dependence of the Chemical Potential in Semiconductors Probed at Room Temperature

Shwetha G. Bhat

Gyan PrakashUltrafast Temperature Dependent Electron and Lattice Dynamics of Sb2Te3

n avalanche-based detection of the insulator-metal transition in epitaxial samarium nickelate thin films through higher order statistics of conductivity noise

The impact of crystalline inhomogeneity on electrical transport and 1/f noise in MoS2 field effect transistor

Absolute charge retentivity and photon counter like characteristics from interface engineered photoresponsive Graphene – MoS2 hererostructure.

Finite size scaling in crossover among different random matrix ensembles in microscopic lattice models

Gopi Nath Daptary

Measurement of force pattern of C. elegans moving on agarose pad using colored micropillars

Confinement Induced Stochastic Sensing of Charged Coronene in α-Hemolysin Nanochannel: A Molecular Dynamics Study

P26 Amit Kumar Majhi

P27 Srabani Kar Probing Carrier Dynamics in Graphene by Terahertz Spectroscopy

P28 K.S. Vasu

P29 Jagtap Amardeep Manikrao

P30 Kallol Roy Optoelectronic properties of graphene-MoS2 hybrids

P31 Manjari Gupta

P32 Paritosh Karnatak

P33 Vishal MV

P34 Pradeep Kumar

P35 T. Phanindra Sai Contact limited resistance noise in high mobility single layer graphene

P36 Suresh Kumar

P37 Bhagyashree K. S

P38 Suman Saurabh ATOMISTIC SIMULATION OF STACKED NUCLEOSOME CORE PARTICLES

P39 Vineeth Mohanan P

P40 Vijay K Ravi

P41 Achintya Bera New Raman modes in graphene layers using 2 eV excitation

Polymer devices with optically transparent horizontal electrodes for rapid optimization of cell electroporation

Yielding and Thixotropy in an anisotropic nanoparticle dispersion formed by Few layer Graphene oxide platelets

Temperature dependence of photoluminescence properties of CdHgTe nanocrystals

Superfluid flow in a 2D annular ring with a barrier : A strong coupling perturbation theory approach

Observation of Coulomb Interaction Effects via Defect Spectroscopy in Graphene on Boron Nitride

Revision of Neutron Drip Density of Magnetised Matter: Exciting Prospect of Massive Neutron Stars

Orbital-Ordering and Ca(Fe0.97Co0.03)2As2 Phonon Anomalies in iron pnictide

NANOHYBRIDS OF SINGLE LAYER GRAPHENE OXIDE-MnFe2O4 NANOPARTICLES FOR EFFICIENT REMOVAL OF HEAVY METALS IN WATER

Electron-Hole Asymmetry in Bulk and Nanoparticles of Nd1-xCaxMnO3 (x = 0.6, 0.4)

Chirality Dependent Pinning and Depinning of Magnetic Domain Walls at Nano-constriction

DNA and siRNA functionalized Dendrimer and Single Wall Carbon Nanotubes: An atomic force microscopy based investigation

TALK ABSTRACTS

High-efficiency Cooper pair splitting demonstrated by two-particle conductance resonance and positive noise cross-correlation

Abstract: Entanglement, being at the heart of the Einstein-Podolsky-Rosen (EPR) paradox, is a necessary ingredient in processing quantum information. Being non-local, its demonstration for two, separated, entangled particles must involve simultaneous, non-local, measurements. While fully entangled photons are readily provided by low efficiency parametric down conversion of higher energy photons, such a feat is not readily available for electrons. On the other hand, starting with Cooper pairs in superconductors - being composites of two fully entangled electrons - and splitting them adiabatically, is a promising means to create entangled electrons. In this talk I will show how we fabricate such an electron splitter by contacting a superconductor strip at the center of a suspended InAs nanowire; terminated at both ends with two normal metallic drains. Intercepting each half of the nanowire by a gate-induced resonant tunneling quantum dot strongly impedes the flow of Cooper pairs due to large charging (addition) energy, while still permitting passage of single electrons. I will provide conclusive evidence of extremely high efficiency Cooper pairs splitting by the following observations: (i) Simultaneous split electrons conductance resonance in both sides of the nanowire; (ii) Time coincidence of split electrons via observing positive cross-correlation of currents fluctuations in the two drains; and (iii) The actual charge of the injected quasiparticles’ via shot noise measurements.

Anindya Das

A Modified DAPT Pulse Sequence for Measuring Large

Dipolar Couplings in Solid State NMR

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

Department of Physics$, NMR research centre#

Indian Institute of Science, Bangalore, India

Solid state NMR has developed into one of the most informative and direct experimental approaches for structural characterization. Dipolar couplings provide valuable information on order and dynamics of liquid crystals. Experiments to measure heteronuclear dipolar couplings are very powerful and widely important in solid state NMR, since it provides site specific dipolar couplings in aligned samples. Interpretation of these heteronuclear dipolar couplings are hampered by the chemical shift anisotropy (CSA) and dipolar interactions among abundant spins (1H-1H couplings). Additionally, multiple 13C-1H dipolar couplings found in complicated systems make the spectra more difficult to use. Hence we need to design techniques which give heteronuclear dipolar coupling information uniquely.

For static samples the complications are resolved partially by separated local field (SLF) spectroscopy where the local field due to heteronuclear dipolar coupling and chemical shift interactions are separated into two frequency domains in a 2D experiment. A method of separated local field experiment is presented for measuring heteronuclear dipolar couplings in oriented systems. This method is based on Dipolar Assisted Polarization Transfer (DAPT). Compared to rotating frame techniques based on Hartmann-Hahn match, this approach is easy to implement and is independent of any matching condition. DAPT pulse sequence is replaced with BLEW-48 to increase the efficiency of homonuclear decoupling and the polarization transfer from proton to carbon. DAPT is implemented as Proton Encoded Local Field (PELF) technique by incrementing BLEW-48 blocks in indirect dimension to get the long range dipolar coupling information. The long duration of BLEW-48 cycle gives small spectral width in indirect dimension which results the folding of strong range dipolar coupling peaks. This problem is eliminated by decreasing rf amplitude of one of the BLEW-48 blocks in indirect dimension. This method is implemented successfully on two liquid crystal systems and the results are explored.

References:

1. S Jayanthi et.al Chemical Physics Letters 439 (2007) pp.407 2. DP Burum et.al. Journal of Magnetic Resonance (1969) 44 (1981) 173-88. 3. S.V.Dvinskikh et.al. Journal of Magnetic Resonance 223 (2012) 73-79

Cluster Formation of Gold Nanoparticles in Two Dimension

Sarika C K and Jaydeep K Basu

We present a simple and efficient method to assemble gold nanoparticles into two dimensional clusters.

Organized structures of gold nanoparticles are known for its tunable optical, electronic and magnetic

characteristics. The existing method to prepare 2D ensemble of gold nanoparticles is using pre-

fabricated structured templates1. But we find that the inherent properties of grafted polymer chains can

be employed to control the interaction which leads to self assembly. Our system is Polystyrene grafted

Gold Nanoparticles (PGNPs) and it is found that when a solution of PGNP in chloroform is spread at air-

water interface in Langmuir-Blodgett trough, clusters are formed with a height confirming them to be a

monolayer. Morphology is studied using Atomic Force Microscopy and the distribution of gold

nanoparticles inside is probed by Transmission Electron Microscope, (images are shown below). The

interparticle distance inside the cluster is controlled by the length of grafted polystyrene chains. We could

also tune the size of clusters by varying the concentration of the initial solution, temperature of the water

surface, different solvent and diffusivity of PGNP.

AFM and TEM images of clusters transferred to glass substrate

As the thickness of the film decreases further, spinodal dewetting comes into action. The wetting of the

film depends upon the local concentration of PGNPs because of its hydrophobic nature. Since the film is

unstable, it tries to lower the energy by modulating its concentration over the surface2. As a result, film

decomposes and PGNP enriched regions in the form of quasi-spheres are formed. Once the evaporation

of chloroform is complete, the grafted chains coil down and compact clusters float on the water surface.

In the absence of chloroform, particles cannot diffuse further and therefore the clusters are freezed on

the water surface. If the evaporation time of the solvent is large, particles diffuse, undergo coarsening

process and form bigger clusters. A detailed study over the mainstream parameter space i.e.,

concentration, temperature, solvent diffusivity etc shows results which are consistent with this model.

[1] Roy Shenhar; T B Norsten; et al. Adv.Mater 2005, 17, 657.

[2] Uwe Thiele; D V Todorova; et al. Phys.Rev.Lett 2013, 111, 117801.

With an insight from the literature, a model for

mechanism of the self assembly is derived from

the concept of dewetting of grafted polystyrene

chains on water surface. The solution in

chloroform when spread onto water forms a

uniform film. Evaporation of chloroform starts

and the film undergo nucleation dewetting from

the defects making random distribution of voids

on the water surface.

Collective order-disorder transition in active granular matter

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

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

Flocking, the collective motion of vast numbers of living creatures, ranging from microscopic to macroscopic, relies on organisms sensing each other's presence, orientation and direction of movement. Asymmetric granular rods confined in a 2D geometry, under constant vertical shaking, display a persistent motion and serve a perfect laboratory imitation of flocking through direct physical contact which is possible only at high concentrations. Here we show, in striking contrast, a flocking transition where rods communicate over several rod lengths through a medium of bead particles. Communication is the result of local 'packets' of long range correlations in the medium, powered by the motion of the rods, which later grows into a single vortex spanning the whole surface. We provide a phase diagram in the plane of rod and bead concentrations and show a power-law spatial correlations as we approach the phase boundary. The experimental findings are well supported by numerical simulations.

_____________

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

Decoupling of magneto-structural transition in Fe1+yTeand the onset of superconductivity in FeSexTe1−x.

Dona Cherian and Suja Elizabeth

November 12, 2013

Abstract for Talk

The binary members of Fe superconductors, Fe1+yTe and its doped compositions have at-tracted much attention because of their interesting structural, magnetic and superconductingproperties. Under atmospheric pressure, highest superconducting transition is observed with50% doping by Se at the Te site. The simpler layered structure and the unique electronicstructure make them interesting materials for investigation despite the low superconduct-ing transition temperature. The parent compound, Fe1+yTe is a bicollinear antiferromag-net(AFM) at low temperature and demonstrates distinguishable properties in accordancewith the concentration of excess Fe (y) [1, 2]. When doped with Se, superconductivity setsin and reaches a maximum TC = 15 K [4]. In this study, we have grown single crystals ofFe1+yTe and Fe1+yTe1−xSex by modified horizontal bridgman method.

Fe1+yTe demonstrates an AFM transition at 67 K (TN)which is identified as a magne-tostructural transition. By varying the concentration of excess Fe, we have tuned TN overa range of temperature from 67 K to 57 K. For a critical composition of excess Fe, at 57 K,the transition decouples to a magnetic transition TN= 57 K and a structural transition, TS

= 46 K [3]. Below TS, the system goes to a lower symmetry. In an attempt to study theeffect of the split transitions on superconductivity, the double peak system is systematicallydoped with Se (x=2% to 25%). Magnetization and specific heat measurements clearly showthe merging of split peak as concentration of Se increases to 5%. For higher doping, theAFM transition shift to lower temperature and vanishes with the onset of superconductiv-ity. A formulated phase diagram of Fe1+yTe1−xSex for x= 0 to 0.5 clearly demonstrates theevolution of superconductivity from the AFM parent compound.

References

[1] W. Bao and Y. Qiu et al. Phys. Rev. Lett., 102:247001, 2009.

[2] S. Li and C. de la Cruz et al. Phys. Rev. B., 79:054053, 2009.

[3] S. Roßler and Dona Cherian et al. Phys. Rev. B, 84:174506, 2011.

[4] K. W. Yeh and T. W. Huang et al. Eurphys. Lett., 84:37002, 2008.

1

Spontaneously broken time reversal symmetry in strongly interacting two

dimensional electron layers in Si and Ge

Saquib Shamim,1 Suddhasatta Mahapatra,2 Giordano Scappucci,2 W.M.Klesse,2 Michelle Y.

Simmons,2 and Arindam Ghosh1

1Department of Physics, Indian Institute of Science, Bangalore 560012, India 2Centre of Excellence for Quantum Computation and Communication Technology, University of New South

Wales, Sydney NSW 2052, Australia

Time reversal symmetry is a fundamental symmetry in nature which protects many exotic

states like the surface states of a topological insulator [1] or the quantum spin hall state in

HgTe quantum well [2]. Weak localization which is a ubiquitous phenomenon at low

temperatures for disordered conductors is a consequence of time reversal symmetry.

Violation of time reversal symmetry modify these states and results in phenomena ranging

from delocalization of electrons, quantum Hall liquid [3], the quantum anomalous Hall effect

in topological insulators [4] and appearance of nontrivial states like chiral superconductivity

predicted in graphene [5]. Here we show experimental evidence for spontaneous time

reversal symmetry breaking in two dimensional systems formed by atomically confined

doping of phosphorus (P) atoms inside bulk crystalline silicon (Si) and germanium (Ge).

Weak localization corrections to the conductivity and the universal conductance fluctuations

were both found to decrease with decreasing doping in the Si:P and Ge:P δ-layers, suggesting

delocalization driven by Coulomb interactions. In-plane magnetotransport measurements

indicate the presence of intrinsic local spin fluctuations at low doping, resulting in

spontaneous lifting of the time reversal symmetry. We extract the spin-scattering time and its

dependence on density and temperature and compare the result with theoretical predictions

[6-8]. Our experiments suggest the existence of a new many-body quantum state in two

dimensions in the regime intermediate between the Anderson and Mott insulating phases,

when two dimensional electrons are confined to narrow half-filled impurity bands.

[1] Hsieh, D. et al. Nature 460, 1101–1105 (2009)

[2] Knig, M. et al. Science 318, 766–770 (2007) [3] Klitzing, K. v., Dorda, G. & Pepper. Phys. Rev. Lett. 45, 494 (1980)

[4] Chang, C.Z. et al. Science 340, 167–170 (2013)

[5] Nandkishore, R., Levitov, L. S. & Chubukov, A. V. Nat. Phys. 8, 158–163 (2012).

[6] Paalanen, M. A., Graebner, J. E., Bhatt, R. N. & Sachdev, S. Phys. Rev. Lett. 61, 597–600 (1988)

[7] Sachdev, S. Phys. Rev. B 39, 5297–5310 (1989).

[8] Shamim, S. et al. (To be published)

Continuous beam of laser-cooled Yb atoms

K. D. Rathod and V. Natarajan

Abstract

We demonstrate the launching of laser-cooled Yb atoms in a continuousatomic beam. The continuous cold beam has significant advantages overthe more-common pulsed fountain, which was also demonstrated by usrecently. The cold beam is formed in the following steps: i) atoms froma thermal beam are first Zeeman-slowed to a small final velocity; ii) theslowed atoms are captured in a two-dimensional magneto-optic trap (2D-MOT); and iii) atoms are launched continuously in the vertical directionusing two sets of moving-molasses beams, inclined at ±15◦ to the vertical.The cooling transition used is the strongly allowed 1S0 →1 P1 transition at399 nm. We capture about 7×106 atoms in the 2D-MOT, and then launchthem with a vertical velocity of 13 m/s at a longitudinal temperature of125(6) mK.

1

Thermoelectric properties of bulk PbTe with encapsulated bismuth

and indium phases

Ashoka Bali and Ramesh Chandra Mallik

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

Abstract: Lead telluride (PbTe) is a well-established thermoelectric material in the

temperature range 350K-850K. Recently, the thermoelectric figure of merit zT(=S2σT/κ,

where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity,

and T is the temperature) of 1.5-1.7 has been achieved in doped PbTe. Since doping primarily

involves change of the band structure of the material, one approach of improving the zT

without affecting the band structure is to introduce interfaces in the bulk of the material,

which are expected to reduce thermal conductivity by additional scattering of phonons. In this

work, PbTe with bismuth (i.e. PbTe-Bi) and indium (PbTe-In) secondary phases has been

synthesized by matrix encapsulation method. In the PbTe-Bi system, bismuth did not

substitute at either Pb or Te site, as was found by Rietveld, Raman and X-Ray Photoelectron

Spectroscopy analyses. Scanning Electron Microscopy (SEM) results showed that Bi phase

was distributed throughout the bulk of PbTe, while Transmission Electron microscopy (TEM)

results showed that Bi did not exist in nanometer size. Measurement of temperature

dependent Seebeck coefficient, electrical resistivity and thermal conductivity showed an

increased scattering of electrons from PbTe-Bi and their possible electron acceptor role. For

the PbTe-In system, similar results were obtained where indium too did not enter the PbTe

sublattice and micrometer sized In phase was found throughout the PbTe matrix. Transport

properties showed decreased temperature dependence throughout the measurement range. It

was found that electrons play a more important role at both the PbTe-Bi and PbTe-In

interfaces than the phonons as opposed to the initially expected phonon scattering. A

reasonable zT of 0.8 at 725 K was achieved for undoped PbTe, but no improvement was

found for bismuth or indium added samples with micrometer inclusions due to reduction in

power factor.[1,2]

References

1. A. Bali, E. Royanian, E. Bauer, P. Rogl, and R. C. Mallik, J. Appl. Phys. 113 123707 (2013)

2. A. Bali, Il-Ho Kim, P. Rogl and R. C. Mallik, J. Electron. Mater. (2013) doi: 10.1007/s11664-013-

2819-1

Fluctuation diamagnetism from a Ginzburg-Landau-like free

energy functional for high-Tc superconductors

Kingshuk Sarkar and Subroto Mukerjee

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

Sumilan Banerjee

Department of Physics, The Ohio State University, Columbus, Ohio, 43210, USA

T. V. Ramakrishnan

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

Department of Physics, Banaras Hindu University, Varanasi 221005, India

Using a recently proposed Ginzburg-Landau-like energy functional due to Baner-

jee et. al. Phys. Rev. B 83, 024510 (2011), we calculate the fluctuation diamagnetism

of high-Tc superconductors as a function of doping concentration x in addition to

the magnetic field H and temperature T by employing classical Monte-Carlo simu-

lations. We show that our results are in good qualitative agreement and reasonable

quantitative agreement with recent experimental data. Our calculations show that a

model of classical superconducting fluctuations can produce features of the observed

magnetization in the pseudogap region. In particular we show that the magnetiza-

tion tracks the superconducting dome and also comment on the determination and

doping dependence of Hc2(T = 0).

Inducing spin-triplet superconducting correlations in graphene

Vidya Kochat and Arindam Ghosh

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

Graphene has emerged as a very promising material in terms of its rich and unique physics as well as its

technological applications, all owing to its 2D hexagonal lattice structure and linear dispersion at low energies.

Although, ferromagnetism and superconductivity have been predicted in intrinsic graphene due to defects and at

high doping, their experimental signatures still remain elusive. But it is has been shown that superconductivity and

ferromagnetism can be induced in graphene via contacts with superconducting and ferromagnetic materials. Here,

we look into the possibility of creating hybrid structures of superconducting/ferromagnetic contacts on graphene for

the realization of spin-triplet superconducting correlations in graphene. We investigate the interplay of

superconductivity and magnetism in graphene by interfacing it with Palladium, a material known to have an

exchange-enhanced spin susceptibility and Aluminium, which becomes an s-wave superconductor below 1.2K. We

study the symmetry properties of graphene in the presence of Andreev reflection from the superconducting contacts

and observe a gate-tunable breaking of time reversal symmetry at high densities suggestive of spin-triplet

correlations. We observe an anomalous magnetoresistance in graphene and gold films contacted by Pd/Al, where the

critical field required to completely destroy superconductivity is increased by more than an order of magnitude in

comparison with the critical field of Al (10mT). We also present evidence of tuning the spin-singlet and triplet

correlations by changing the thickness of the Pd layer. Our work could form an ideal platform for merging

ferromagnet - superconductor heterostructures with graphene which can lead to many new applications like a spin-

tunable superconducting graphene switch.

Thermoelectric properties of doped quaternary chalcogenide materials Ch. Raju and Ramesh Chandra Mallik

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

Abstarct: Quaternary - Cu2+xZnSn1-xSe4 (0≤x≤0.15) and quinary - Cu2.1Zn0.9Sn1-xInxSe4 (0≤x≤0.1) compounds were prepared by melting (1170K) followed by annealing (773 K) for 172 h. Powder X-ray diffraction (XRD) data accompanied by electron probe microanalysis (EPMA) and Raman spectra of all the samples confirmed the formation of a tetragonal kesterite structure with Cu2FeSnS4-type as a main phase. In addition to the main phase, small amounts of secondary phases like ZnSe, SnSe and CuSe were observed. The thermoelectric properties of all the samples were measured as a function of temperature in the range of 300–780K. In quaternary compound Cu2+xZnSn1-xSe4 (0≤x≤0.15), electrical resistivity of all the doped samples exhibits metal like whereas undoped sample showed semiconducting like behavior. In quinary compounds Cu2.1Zn0.9Sn1-xInxSe4 (0≤x≤0.1), electrical resistivity of all samples increased with temperature showed metal like behavior. The positive values of the Seebeck coefficient and the Hall coefficient reveal that holes are the majority charge carriers. Thermal conductivity of all the samples in both the series of compounds decrease with increase in temperature. In quaternary compound Cu2+xZnSn1-xSe4 (0≤x≤0.15), thermal conductivity of doped samples was high compared to Cu2ZnSnSe4 and this may be due to the larger electronic contribution and the presence of the ZnSe phase in the doped samples. In quinary compounds Cu2.1Zn0.9Sn1-xInxSe4

(0≤x≤0.1), lattice thermal conductivity is not significantly modified as the doping content may infer negligible mass fluctuation scattering for copper/zinc and indium/tin substitution. The maximum zT = 0.3 at 720 K occurs for Cu2.05ZnSn0.95Se4 for which a high-pressure torsion treatment resulted in an enhancement of zT by 30% at 625 K. The maximum zT=0.45 at 773K was obtained for Cu2.1Zn0.9Sn0.925In0.075Se4 due to its smaller value of thermal conductivity [1-2]. References 1. Ch. Raju, M. Falmbigl, P. Rogl, X. Yan, E. Bauer, J. Horky, M. Zehetbauer, R.C. Mallik, AIP Adv., 3 (2013) 032106-032112. 2. R. Chetty, M. Falmbigl, P. Rogl, P. Heinrich, E. Royanian, E. Bauer, S. Suwas, R.C. Mallik, Physica Status Solidi A, (2013). DOI: 10.1002/pssa.201329264

Selective Single Scan (S3) NMR Spectroscopy Techniques

KowsalyaDevi Pavuluri$# and K. V. Ramanathan#

Department of Physics$, NMR Research Centre#

Indian Institute of Science, Bangalore

Multidimensional acquisitions play a central role in the progress and applications of Nuclear

Magnetic Resonance (NMR) spectroscopy. Such 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 ultrafast manner. In ultrafast NMR the usual 𝑡1 encoding

is replaced by spatial encoding followed by conventional mixing period and by a detection

block based on echo planar imaging (EPI) [1]. Ultrafast NMR has been successfully used for

many biological and chemical applications like real time kinetic studies of protein, RNA

folding elucidation and mechanistic details of organic reaction monitoring. A limitation

affecting ultrafast experiments rests in their ability to cover large spectral ranges while

preserving an acceptable resolution, due to limitations in gradient amplitudes and filter

bandwidths [2]. A new approach based on selective pulses has been proposed to simplify the

ultrafast NMR spectra of complex molecules. Selective pulses excite only single or a range of

frequencies thus simplifying complex spectra. The present talk introduces main principles and

recent developments of selective pulse experiments in subsecond approach of NMR

spectroscopy. Some of the results obtained recently by us will also be presented.

References:

1. L. Frydman et.al J.Am.Chem.Soc. 125, 9204(2003)

2. Patrick Giraudeau et.al J.Magn.Reson. 205, 171(2010)

Multiscaling in Hall-Magnetohydrodynamic Turbulence: Insights from a Shell Model

Debarghya Banerjee, Samriddhi Sankar Ray, Ganapati Sahoo, and Rahul Pandit

We show that a shell-model version of the three-dimensional Hall-magnetohydrodynamic (3D Hall-MHD) equations provides a natural theoretical model for investigating the multiscaling behaviors of velocity and magnetic structure functions. We carry out extensive numerical studies of this shell model, obtain the scaling exponents for its structure functions, in both the low-k and high-k power-law ranges of three-dimensional Hall-magnetohydrodynamic, and find that the extended-self-similarity procedure is helpful in extracting the multiscaling nature of structure functions in the high-k regime, which otherwise appears to display simple scaling. Our results shed light on intriguing solar-wind measurements.

Spinning Black Holes

Banibrata Mukhopadhyay

Department of Physics, Indian Institute of Science, Bangalore 560012bm@physics.iisc.ernet.in

November 13, 2013

Abstract

I will give the basic overview of the issues related to the spinof black holes and how to resolve them out.

1

Functional Near Infrared Optical Tomography (fNIROT): Issues and Challenges K. Rajan

Functional magnetic resonance imaging (fMRI) has been in regular use in medical field tosegregate, identify and relate the spatial regions in the brain for various activities such as memoryassociation, thinking process, motor functions, audio, visual and language processing. MRI is animaging modality that is considered to be clean; does not have any side effects on the patient.However, the patient suffers from high noise level due to switching and is exposed to very highmagnetic field (1.5 to 3 Tesla) in an enclosed dome for over an hour. Another entrant to this field isthe functional positron emission tomography (fPET). This provides molecular and cellular levelimages for various stimulations. fPET is used for modelling drug delivery and drug design andsimilar studies. Staging of the Alzheimer’s disease is a typical application. However, the approach isbased on a radio-isotope that is injected in to the blood stream. Fluoro Deoxy Glucose (FDG) is afrequently used probe. The isotope accumulates at the activity locations depending on themetabolism and then decomposes, emitting positrons, which combine with electrons to generate apair of gamma rays which run in opposite directions, and are detected by the gamma ray detectors.Compared to the above probes, light is a very good probe for tissue studies. It is not ionizing andhas many advantages which include portability, and cost effectiveness. This is very helpful forfrequent exposure required for staging the cancer growth and subsequent identification ofmalignancy. However, the tissue has constituents such as fat, lipids, oxy- and deoxy hemoglobinthat scatter light while light traverses through a tissue. Light transport through a tissue is highlyscattering, and this makes the problem highly ill-posed and ill-conditioned. Unlike an X-ray probewhich runs through the tissue in a straight line, thereby making reconstruction quite easy throughthe use of filtered backprojection (FBP), we can not use simple algorithm for light basedreconstructions. This calls for solving the problem as an inverse problem and is tackled throughiterative methods. A cost factor is attached to the problem, and optimization is used for arriving atthe best solution. The forward light transport is modelled as radiative transfer equation (RTE) orcan be modelled using Monte Carlo approaches. The onset of cancer causes marked changes in thetissue optical parameters such as absorption and scattering coefficients. It is found that tissueabsorption coefficient is orders of magnitude smaller compared to scattering coefficient. Under thiscondition, the RTE can be simplified to a simple diffusion equation. The discretization of the objectdomain is carried out using finite element method (FEM). In this study, we analyze the issues andchallenges to be sorted out for making the functional near infrared optical tomography (fNIROT) areality.

Short-time Dynamics of Glass-forming Liquids inMetabasins of the Energy Landscape

Pranab Jyoti Bhuyan, Prof Chandan Dasgupta

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

Abstract:

A detailed study of the short-time dynamics of glass-forming liquids in metabasins of the

energy landscape will be presented. A metabasin is identified as the collection of the

basins of attraction of a series of correlated inherent structures in the potential energy

landscape. The spectrum of harmonic excitations is obtained from the eigenvalues of the

Hessian matrices of the inherent structures. An excess density of states in comparison

to the Debye squared-frequency law is observed in the low-frequency range, manifesting

as the boson peak. An analysis of the magnitudes of the components of the polarization

vectors shows that the vibrational modes constituting the boson peak and modes with

frequencies lower than that of the boson peak are substantially different in nature from

the modes in other parts of the spectrum. We find that dynamical quantities computed in

the harmonic approximation deviate from those obtained from exact molecular dynamics

simulations at a time scale much shorter than the time scale of β relaxation. When the

effects of anharmonicity are incorporated in an approximate way and dynamical quantities

are calculated using the eigenvalues and eigenvectors of a modified Hessian matrix with

the added contribution of anharmonic terms, the results show a better agreement with

those of molecular dynamics and metabasin dynamics up to a time scale that extends

beyond the β-regime at lower temperatures.

Growing Dynamical Facilitation on Approaching the Colloidal Glass Transition

Shreyas Gokhale, K. Hima Nagamanasa, Rajesh Ganapathy and A.K. Sood

Abstract

The dramatic slowing down of dynamics with no apparent change in structure is perhaps the

best-known and least understood feature of the glass transition in molecular, polymeric and

colloidal liquids. Despite extensive research, the microscopic origin of this slowdown

remains mysterious. In particular, it is unclear whether structural relaxation is governed by a

growing length scale, as advocated by thermodynamic theories, or by the concerted motion

of mobile defects termed excitations, a phenomenon known as dynamical facilitation. Here,

by performing video microscopy on a binary colloidal glass-former, we show that dynamical

facilitation unambiguously grows on approaching the glass transition. Interestingly, the

spatial frequency of occurrence of excitations is anti-correlated with the distribution of local

hexatic order, suggesting intriguing connections between structural evolution and relaxation

dynamics on approaching the glass transition. We discuss our findings in the context of

growing static and dynamic length scales observed in experiments and simulations on glass-

forming liquids.

Title: Force induced biomolecular transition through an intermediate: Theory, analysis and interpretation Speaker: Ashok Garai

Abstract Single molecule force spectroscopy studies are widely used to probe biomolecular conformational transitions, such as protein unfolding and ligand-receptor dissociation, in real time and with spectacular resolution. Such transitions under applied force are highly influenced by the structure of the underlying free energy landscape. In this talk, I will show the presence of an intermediate in the unfolding/unbinding pathway leads to non-trivial features in the key experimental observables, the mean unfolding rate, the distribution of unfolding forces and the most probable forces. I will present a general analytical theory for such force induced conformational transitions through an intermediate. The applications of the theory will be illustrated with data obtained from computer simulations that mimic optical tweezers and atomic force spectroscopy experiments.

Gap renormalization of molecular crystals from density functional theory

Manish Jain

The role of N-terminal Valine Residue in the structural stability of a Bacillus xylanase

Mahanta P1, Bhardwaj A

2, Reddy VS

2, Ramakumar S

2

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

2International Centre

for Genetic Engineering and Biotechnology, New Delhi-110067

Abstract

Thermally stable proteins have variety of applications in both industrial and biotechnological

research. Despite significant progress that has been made in understanding the structural basis

of proteins thermostability; the stabilization strategies used by proteins are still enigmatic.

Mutational analysis has been extensively used to pinpoint specific interactions that contribute

to the stability of proteins. Such studies have shown that mutations can stabilize, destabilize

or may not have any deleterious effect on proteins. In a previous study, we observed that

different amino acid substitutions at first position of a GH10 xylanase (BSX), with a TIM-

barrel structure, have different impact on thermostability. In the present work, we have solved

the 3D crystal structures of different N-terminal mutants to investigate the role of first

hydrophobic residue, valine (Val1) on the structural stability of BSX. Studies of N-terminal

mutants showed no major changes in 3-D structures of these mutants with respect to the wild

type protein. However, there are subtle changes in both local and distal non-covalent

interactions as result of mutation. Changes in non-covalent interactions such as hydrophobic

interaction patterns, Cβ contact order and Cβ packing density are used to represent the impact

of mutations that are transmitted near and far from mutation site. Our analysis of different N-

terminal mutant structures revealed that increasing the strength of N-C terminal direct as well

as indirect interactions may result in an increase in the overall stability of BSX. These

observations may be useful in the rational design of enzymes for biotechnological

applications, especially for those enzymes in which N and C-termini are in close contact.

Trapping and manipulating multielectron bubbles in liquid helium

to investigate 2D electron systems at unexplored densities

V. Vadakkumbatta, E. M. Josephb, A. Pala, and A. Ghoshb

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

bCentre for Nano Science and Engineering, Indian Institute of Science, Bangalore, India

Multielectron bubbles (MEB) are micron sized cavities in liquid helium that contain electrons confined within a nanometer thick layer on the inner surface of a bubble. These objects present a rich platform to study the behavior of a two dimensional electron gas (2DES) on a curved surface. Most crucially, the surface electron densities in MEBs can vary over a wide range, making it a suitable candidate for studying classical Wigner crystallization and quantum melting in a single system. So far, there has been only limited experimental study of MEBs, with most of the previous investigation transient in nature. As we discuss in our presentation, we have developed a new technique of generating MEBs, and trapping them in a Paul trap for more than hundreds of milliseconds. This allows the MEBs to be further manipulated with buoyant and electric forces, such as to obtain reliable measurements of their physical properties. As we observe experimentally, the surface charge density of a single MEB can vary by orders of magnitude during the course of one measurement, thereby covering a previously unexplored section of the 2DES phase diagram.

“Structure Stability and Elasticity of DNA

Nanotubes”

Himanshu Joshi

1 , Anjan Dwaraknath

2, and Prabal K. Maiti

1

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

Bangalore 560 012, India

2Indian Institute of Technology Madras 600 036, India

Abstract:

Recently DNA has emerged as unique material for the bottom up synthesis of nano devices and

has led to a variety of motifs which are topologically similar to cubes, octahedrons, other

polyhedral and nanotubes. DNA Nanotubes are tubular structures created by self-assembling

DNA strands whose sequences are designed so that the resulting molecule takes the shape of a

nanotube. Here, we study the molecular mechanics involve in the design and characterization of

these DNA nanostructure using state of the art all atom molecular dynamics simulation. We

report the algorithm to construct variety of DNA nanotube topology like six helix, eight helix

bundles as well as triangular nanotubes. We study their thermodynamic stability as a function of

sequence and number of DNA helices forming the nanotube. The RMSD and radius analysis

averaged over several ns long simulation demonstrate the stability of these tubes. Steered

molecular dynamics simulations have been performed to study the elastic response of these

nanostructures. The six-helix and eight-helix structures have stretch moduli of the order of 4000

pN which is 4 times larger than the ds-DNA and have persistence length of the order of 6-7 µm.

Key Words: DNA Nanotubes, Molecular dynamics simulation, RMSD, Stretch modulus.

Turning Shear Thickening “On” and “Off ”

Vikram Rathee1, Srishti Arora

2, Rajesh Ganapathy

2, Ajay Sood

1

1Deartment of Physics, Indian Institute of Science, Bangalore-560012 2International Centre for Material Science, Jawaharlal Neharu Centre for Advanced Research, Bangalore - 560064

Shear-thickening the anomalous increase in viscosity above a critical shear rate and

is feature of dense granular and colloidal suspensions. The numerous studies on shear

thickening have predicted the formation of hydroclusters which results in enhanced dissipation

and increased viscosity. Thus by controlling the formation of hydroclusters one can in principle

control shear thickening. Here, we demonstrate for the first time that by coating colloids with

temperature tunable polymer brush we can turn ‘On’ and ‘Off’ shear thickening. Further we

show using confocal microcopy coupled with rheology that hydroclusters which forms due to

strong hydrodynamic lubrication forces percolate in 3d and results in discontinuous shear

thickening even for the dilute suspension of anisotropic particles.

POSTER ABSTRACTS

Giant Suppression of Universal Conductance Fluctuation in

Bilayer Graphene - Boron Nitride Heterostructure

M. A. Aamir, Kallol Roy and Arindam Ghosh

Department of Physics, Indian Institute of Science, Bangalore

Integrating graphene systems with boron nitride has opened a plethora of avenues to explore,

ranging from many-body physics of Dirac electrons to uncanny mesoscopic devices of chiral carriers.

In this work, we study bilayer graphene which is in direct contact with boron nitride acting as a

dielectric to the top gate. We find that universal conductance fluctuations (UCF) is suppressed by

3 orders of magnitude at the charge neutrality point. This indicates formation of an insulating state

which is unexpected for a 2D system with parabolic dispersion. We suggest this is because of

nanometer-scale periodic potential imposed by boron nitride on the upper layer of bilayer graphene

which intrinsically breaks the inversion symmetry of the system and thereby opens a gap in the bulk.

In this picture, UCF is suppressed because the current carriers are forced to move along the edges

where it encounters minimal scattering. This is an exciting way of inducing a topological insulator

phase in a graphene system.

t-J Model Revisited: Relevance for Cuprate Problem

Aabhaas V Mallik, Umesh K Yadav,H R Krishnamurthy, Vijay B Shenoy, T V Ramakrishnan

Nov 2013

Abstract

We have analyzed the unprojected fermionic t-J model using functional integral formalism abouta d-wave superfluid saddle point. We have also explored, what happens when projection is imple-mented at a mean field level. In the latter part of the analysis, we have discovered some featureswhich seem to have relevance for the high Tc cuprate superconductor problem. In my presentationI will discuss some of these findings.

1

Molecular mechanism of permeation of water through helium leak-tight graphene oxide membraneRaghav Nallani and Prabal K MaitiDepartment of Physics, Indian Institute of Science, Bangalore 560012

Geim et al's experimental observation1 showed that the layers of graphene oxide (GO) are good for permeation of water but not for helium. They argued that the GO layers are dynamic in the formation of permeation route depending on the environment they are in (i.e, water or helium). To probe this hypothesis we calculate the potential of mean force (PMF) of GO sheets, with inter-planar distance as reaction coordinate in helium and water. Our PMF calculation shows that equilibrium interlayer distance between GO sheets in helium is 4.5 Å leaving no space for helium permeation. In contrast PMF in water shows two minima one at 4.5 Å and another at 7.0 Å corresponding to no water and water filled region thus giving rise to permeation path. We also present the entropy calculations of water trapped between graphene sheets at different inter-sheet spacing.

1 Unimpeded permeation of water through helium leak tight Graphene based membranes-R. R. Nair, H. A. Wu, P. N. Jayaram, I. V. Grigorieva, A. K. Geim : Science Vol 335

Dephasing in 3D Topological Insulators at Low Temperatures

Semonti Bhattacharyya, Mitali Banerjee, Saurav Islam , Hariharan N

Suja Elizabeth and Arindam Ghosh

Department of Physics, Indian Institute of Science, Bangalore 560012

ABSTRACT

Topological insulator (TI), a new quantum state of matter, exhibits exotic metallic surface states (SS) in the bulk insulating band gap. These SS are protected from back scattering by high spin-orbit coupling and time reversal symmetry. When combined with Superconductors and Ferromagnets TI SS can give rise to novel physics phenomena like Majorana Fermions or Magnetic Monopoles. Although extensive ARPES (Angle Resolve Photoemission Spectroscopy) studies have been done, electrical transport is poorly understood in this field. Low temperature Quantum transport in these materials requires more investigation both for scientific understanding and technological applications.

In this work we have studied the electrical transport properties of thick (~100 nm) and thin (~10 nm) flakes of Bi1.6Sb0.4Te1.7Se1.3, a material which has shown relatively less bulk doping (1) compared to Bi2Se3 and Bi2Te3.The resistance (R) of thick flakes, which has contribution from both surface and bulk bands, show an activated kind of behavior in the temperature (T) range 70 K to 300 K before showing a saturation at 70 K. Thin flakes, which has contribution from only SS show metal-like behavior in R~T in the same T range. However, both thin and thick flakes show an upturn beyond 25K which finally ends in a saturation at T< 2K.Although this upturn has been mostly attributed to e--e- interaction in earlier studies (2), this kind of saturation has not been reported yet.

To probe further, we have done Magnetoresistance (MR) measurements. The phase coherence

length (lϕ) extracted from MR shows T-0.5 dependence above ~2K , which is commonly observed

in 2D systems because of Nyquist dephasing. At T< 2K, lϕ also exhibits a saturation. This indicates that there is an emergence of a different dephasing mechanism at T ~ 2K , which may result from spin-spin scattering.

1. A. A. Taskin, Zhi Ren, Satoshi Sasaki, Kouji Segawa, and Yoichi Ando, PRL 107, 016801

2. Jian Wang, Ashley M. DaSilva, Cui-Zu Chang, Ke He,2 J. K. Jain, Nitin Samarth, Xu-Cun Ma,

Qi-Kun Xue, and Moses H. W. Chan, PRB83, 245438 (2011)

PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com

Is a deep one-cell meridional circulation essential

for the flux transport solar dynamo?

Gopal Hazra, Bidya Binay Karak, and Arnab Rai Choudhuri

November 11, 2013

Abstract

The solar activity cycle is successfully modeled by the flux transportdynamo, in which the meridional circulation of the Sun plays an impor-tant role. Most of the kinematic dynamo simulations assume a one-cellstructure of the meridional circulation within the convection zone, withthe equatorward return flow at its bottom. In view of the recent claimsthat the return flow occurs at a much shallower depth, we explore whethera meridional circulation with such a shallow return flow can still retainthe attractive features of the flux transport dynamo (such as proper but-terfly diagram, proper phase relation between the toroidal and poloidalfields). We consider additional cells of the meridional circulation belowthe shallow return flow—both the case of multiple cells radially stackedabove one another and the case of more complicated cell patterns. Aslong as there is an equatorward flow in low latitudes at the bottom ofthe convection zone, we find that the solar behavior is approximately re-produced. However, if there is either no flow or a poleward flow at thebottom of the convection zone, then we cannot reproduce solar behavior.On making the turbulent diffusivity low, we still find periodic behavior,although the period of the cycle becomes unrealistically large. Also, witha low diffusivity, we do not get the observed correlation between the polarfield at the sunspot minimum and the strength of the next cycle, whichis reproduced when diffusivity is high. On introducing radially downwardpumping, we get a more reasonable period and more solar-like behavioreven with low diffusivity.

1

Synthetic Dimensions

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

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

Applying a standing wave optical potential on a system of atoms having hyperfine states createsan one dimensional optical lattice having a synthetic direction provided by the hyperfine statesof the atoms. The hyperfine states are coupled using Raman transitions and due to the positiondependence of the Rabi coupling the problem effectively becomes a finite rung ladder problemhaving a finite amount of flux per plaquette. When the depth of the optical lattice is very large(compared to the recoil energy), we can analyze the problem using a nearest neighbor tight bindingmodel. But for shallow optical potential higher order hoppings will also become important andwe need an exact solution. We study the problem using the tight binding model and exact Blochdiagonalization techniques for different lattice depths and different fluxes per plaquette with twokinds of boundaries open and periodic in the synthetic direction in different parameter regimes. Wefind that for experimentally relevant lattice depths the tight binding bandwidths are more than 20 %larger than that of the exact bands. Our results will help the experimentalists to design experimentswith suitable parameters and help the theorists to prepare appropriate models to analyze the physicsof the problem. We also study the two body problem with SU(m) (m is the number of hyperfinestates) invariant interaction. The formation of the bound states with the variation of the strengthof the interaction is studied.

Routes to Calculation of DC conductivity of Disordered Materials

Adhip Agarwala∗ and Vijay B. Shenoy†

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

The calculation of conductivity for both diagonal and off-diagonal disorder in tight-binding sys-tems, has been attempted using various ways including Kubo Formalism, Transfer Matrix etc. Inthis work, we propose to calculate conductivity using current current commutator and contrast itwith the method of calculation of conductivity using the current-current correlation. We observethat this method is well controlled in low temperatures and features of drude conductivity areobtained from the Anderson Model.

∗ adhip@physics.iisc.ernet.in † shenoy@physics.iisc.ernet.in

Title of abstract: Systematic study of the effect of various organic solvents on PMMA-495 and Scotch

Tape residues in exfoliated graphene on Si-SiO2 wafers.

Authors: Kimberly Hsieh Sui Mee (1), Tathagata Paul (1), Arindam Ghosh (1)

Affiliation: (1) Department of Physics, Indian Institute of Science, Bangalore, 560012.

Abstract: Obtaining a clean exfoliation, free from residues and other forms of contamination is one of

the major challenges and the first step in making any nanostructured devices. A cleaner sample leads

to better devices and hence, better results. There is a need to systemize this process of cleaning whilst

finding new and better solvents which remove the residues without affecting the working material (in

this case exfoliated graphene). In this work we have enlisted a few organic solvents (from the

numerous present) which have these exact properties, are easily procurable and are non-toxic. The

effectiveness of these solvents in removing PMMA-495 coating and Scotch tape residues from the

surface of Si-SiO2 wafers containing exfoliated graphene has been checked in a systematic manner.

The characterization techniques used were optical microscopy for a rough idea of the effect of the

solvents, SEM & AFM for their higher resolving power which gives a better understanding of the

cleaning power of these solvents and Raman spectroscopy for determining any adverse chemical

effect of the solvent on graphene.

Magnetic Field dependence of the Chemical Potential in Semiconductors Probed at Room Temperature

Aditya N. Roy Choudhury and V. Venkataraman

ABSTRACT

In a magnetic field the Density of States break up into Landau Levels and are spin split. Consequently the chemical potential or the Fermi Level changes in a magnetic field. In the non-degenerate limit, this change, for a given magnetic field, is independent of the carrier concentration. For n-GaAs, a typical estimate is 200 µeV for 6.5 Tesla at room temperature. For semiconductors with higher electron effective mass such as Si, or for metals, this value is much lower.

Magnetic susceptibility measurements, by standard methods, yield a total contribution from the itinerant (Landau + Pauli) as well as localized (Larmor + Curie) magnetic moments. Our experiment opens a new window towards separating the free carrier contribution. Our method works best in the non-degenerate limit where all other methods cease to work owing to very low carrier concentration.

We make a Metal Oxide Semiconductor (MOS) capacitor and measure the voltage across it in a pulsed magnetic field of ~ 7 Tesla. To reduce the inductive pick-up generated in the sample leads from the time varying field, a small chip sits just next to the sample, inside the bore of the magnet, and amplifies the signal which is otherwise of the order of ~ 10 – 100 µV. A high capacitance of the sample and a high leakage resistance are essential to obtain any good data.

The idea of the experiment was proposed way back in 1957-1963 and has been tried out ever since in various metals, semiconductors and superconductors. Till date, no record exists of a successful attempt (except for 2DEGs or thin films at LHe temperatures). One major reason is a spurious Hall Voltage that arises out of the changing magnetic field and the eddy current induced by it. Unless properly handled, this spurious effect completely overshadows the signal.

We report a successful observation of the Fermi Level’s magnetic field dependence in n-GaAs (ND ~ 1×1017/cc) at room temperature. Comparatively, nothing was observed in n-Si.

REFERENCES

1. I.O. Kulik and G.A. Gogadze, Sov. Phys. JETP 17, 361-364 (1963). 2. M. Peter, D.L. Randles and D. Schoenberg, Phys. Lett. 33A, 357-358 (1970). 3. V.I. Nizhankovskii and S.G. Zybtsev, Phys. Rev. B 50, 1111-1118 (1994).

Demonstration of room temperature spin injection in Fe3O4/MgO/GaAs

Shwetha G. Bhat and P. S. Anil Kumar

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

Spin injection, manipulation and detection are the important aspects of semiconductor spintronics. In order to achieve these aspects, it is necessary to choose the right combination of materials which yield better results for the existing device technology. The quest for an ideal spin injector with high spin injection efficiency for the semiconductors like GaAs, Si etc. is still on for more than a decade now. Conventional metallic ferromagnets were the first choice of materials. But for the first time using an oxide spin injector Fe3O4 we demonstrate the spin injection into GaAs at room temperature with the help of a tunnel barrier MgO using 3-terminal Hanle technique. The advantage of the half metallicity nature of Fe3O4 helps to have a better spin injection efficiency in combination with the spin filtering effect of the tunnel barrier MgO. Spin relaxation time of about 320 ps is obtained for 1 x 1019/cm3 Zn-doped GaAs at 300 K.

Ultrafast Temperature Dependent Electron and Lattice Dynamics of Sb2Te3

Gyan Prakash1, Srabani Kar1 and A. K. Sood1

Center for Ultrafast Laser Applications (CULA) and Department of Physics, Indian Institute of Science, Bangalore - 560 012, India

Abstract

Sb2Te3 , Bi2Se3 and Bi2Te3 belong to a class of materials known as 3D-topological Insulator. These new class of materials have electronic band gap in the bulk but due to spin orbit coupling and time-reversal symmetry have gapless surface states. A recent study of Sb2Te3has shown that the temperature dependence of the linear thermal expansion along the hexagonal ‘c’ axis has an anomalous behavior in the temperature range of 204-236K , without any specific heat anomaly suggesting that an electronic or structural phase transition may not be responsible for the anomaly in the thermal expansion.

We will report the temperature evolution of coherent phonons and carrier relaxation in Sb2Te3 topological insulator by the femto-second pump probe differential reflectivity over a wide range of temperatures (3-300 K). Anomalous behavior of electron and phonon dynamics around 225K is observed. The rise time of the signal associated with the thermalization and energy relaxation of hot carriers excited increases linearly with temperature up to 200K followed by an abrupt rise and fall around 225K. Carrier recombination time of around 0.9 to 1.3ns is observed in the temperature region 3-300K with a linear increase with temperature. Strain pulse model is used to study the acoustic phonon and temperature dependence of optical constants extracted.

Title of Poster: An avalanche-based detection of the insulator-metal transition in epitaxial samarium

nickelate thin films through higher order statistics of conductivity noise

Authors: Anindita Sahoo1, Sieu D. Ha2, Shriram Ramanathan2, and Arindam Ghosh1

Affiliation: 1Department of Physics, Indian Institute of Science, Bangalore 560 012, India 2School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA

Abstract:

We have experimentally studied the avalanche dynamics of SmNiO3 thin film (perovskite nickelate)

during the electronic as well as structural metal-insulator phase transition using time-varying resistance

fluctuation or noise. The second spectrum of resistivity noise shows a temperature dependent strong

non-Gaussian component arising due to the long-range correlation. Our noise study indicates an

avalanche-mediated athermal first order insulator-metal phase transition during which parent and

product phases co-exist. We also analyzed the response of non-Gaussian component to the temperature

ramp rate during the phase transition. Besides, the non-Gaussian component of noise measures the

insulator-metal transition temperature very accurately which may provide a significant way of

measuring accurate transition temperatures in other correlated oxide systems.

The impact of crystalline inhomogeneity on electrical transport and

1/f noise in MoS2 field effect transistor

Subhamoy Ghatak*, Sumanta Mukherjee, D. D. Sarma and Arindam Ghosh

We show that both conductivity and low frequency 1/f noise are strongly influenced by the presence of localized

trap states in ultra-thin MoS2 field effect transistor. The origin of this localized trap states are addressed by high

source-drain I-V measurement which indicate trap-assisted space charge limited conductivity and hence

presence of an exponential distribution of trap states. The trap states not only create Coulomb scattering of

charge carriers but also slowly exchange carrier with channel. The trap density is quantitatively calculated

which turns out two orders of magnitude higher than the typical SiO2 surface trap density. This suggests a

structural origin of the trap states in MoS2 films. The result was also supported by similar noise measurement on

MoS2 devices on trap free hexagonal boron nitride substrate. The origin of these states is also investigated by

spectroscopic studies, which indicate a possible presence of metallic 1T-patches inside the major

semiconducting 2H phase.

[1] Ghatak, S. et al. ACS Nano 2011, 5, 7707.

[2] Ghatak, S. et al. Manuscript under preparation.

Spin Coated Dielectrics: Organic Vs. Inorganic Sandip Mondal & V Venkataraman

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

The dielectric layer is an important part of modern opto-electronic devices. Its great impact on today’s technology

has encouraged technologists and scientists to search for better materials in terms of cost effectiveness, low dimensionality

and large area applicability[1-6]. Several organic dielectric materials such as PDDA , PMMA and semiconducting materials

such as P3HT, MEH-PPV have been investigated[7,8]. But the issues with such materials are contact making, high leakage, low

reliability and less repeatibility. There are also several inorganic, carbon free, spin-coated dielectric materials reported in the

literature viz TiO2, SiO2, ALPO(Aluminium Oxide Phosphate) which are found to have better impact on devices applications[1-

3, ,9]. Here, we explored and compared the optical and electrical properties of few organic and inorganic dielectric materials

through their device applications.

MOS (Metal-Oxide-Semiconductor) capacitor devices have been fabricated on p-Si substrate with water soluble

organic material PDDA, non-aqueous PMMA and inorganic ALPO as dielectric materials for comparison. The PDDA and PMMA

solution have been prepared with 3.3 wt% in water and Toluene respectively. Then the precursor solution was stirred for 24

h at moderate temperature of 50 - 60 ⁰C to achieve the uniformly concentrated solution. ALPO precursor solution was

prepared by dissolving Al(OH)3 (Alfa) in 2 mole equivalents of HCl (aq) (Sigma Aldrich , ACS 37%) mixing with appropriate

amount of H3PO4 (aq) (Fisher, ACS 85%). The dissolution was accomplished by stirring under moderate heat (80-90 °C) in a

water bath for 24 h. Dielectric films were deposited by spin coating at a speed of 3000 rpm for 30 seconds on pre-cleaned

highly doped p-Si wafer followed by exposing of oxygen plasma at pressure 0.005 mbar for 20 mins. Typical film thickness for

one deposition cycle for ALPO with a 0.5 M Al solution was 40 nm following an annealing at 1000 °C for 30 min for ALPO-

MOS. Polymer MOS device with dielectric thickness ~200nm was annealed at 150 ⁰C for 6 hours. The top gate metal of 200

nm aluminium was deposited at 1x10-5

mbar pressure on the dielectric film with shadow masks of diameter 230, 414 and

600µm respectively. The thickness of the film has been measured with SEM and verified with ellipsometry by fitting with

Cauchy’s model.

The preliminary investigation of all the devices under 10X optical microscope showed that the PDDA device were

damaged by aluminium metal gate diffusion through the polymer film and therefore not suitable for further study. Next,

ALPO and PMMA films were characterized by Electron Diffraction Spectroscopy (EDS), X-ray Photoelectron Spectroscopy

(XPS) and Atomic Force Microscope (AFM). The AFM image shows that for a scanning area of 75x75 microns, the PMMA film

is rougher with respect to ALPO film. The ALPO-MOS device shows excellent C-V characteristics with dielectric constant ~4.8.

The PMMA device also shows good CV behaviour but not as good as like ALPO-MOS. The PMMA-MOS device shows the

dielectric constant ~2.6. The leakage current is extremely low for 120 nm thick 600 µm diameter top electrode ALPO-MOS

devices. Leakage current density (J) at 4.1MV/cm (I/A) = 2.33 nA/ cm2

and J at 8.3 MV/cm (I/A) = 53 nA/cm2. There is no

breakdown found up to +/- 100 volt for this device. But the leakage current for PMMA-MOS device is extremely high at 1

MV/cm (I/A)= 63 A/Sq cm. Hence inorganic APLO is very much superior to organic PMMA because of its good interface

properties, large-area uniformity, very low leakage and excellent capacitive characteristics.

The above devices can be useful for various optoelectronic applications such as random access memory (RAM), photo-

transistors, and solar cells. The authors thank Centre for Excellence in Nanoelectronics (CEN) for the facilities.

o References:

1) NATURE ,VOL 428 , 29 April 2004 2) NATURE, SCIENTIFIC REPORTS , 3 : 2275, 24 July 2013 3) Chem. Rev. 2010, 110, 3–24 4) Appl. Phys. Lett. 96, 141116 (2010) 5) Appl. Phys. Lett. 96, 082116 (2010) 6) Nature Nanotechnology 2, 378 - 384 (2007) 7) J. Phys. Chem. C 2010, 114, 20609–20613 8) Optics Letter, Vol. 33, No. 21, nov1, 2008 9) Chem. Mater. 2007, 19, 4023-4029

e-mail: sandipmondal@physics.iisc.ernet.in Contact Number: 080-2293-3344

Shot noise in disordered ultra-thin films

of NbN

R. Koushik, T. Phanindra Sai, Avradip Pradhan & Arindam Ghosh

Department of Physics, IISc, Bangalore

Shot noise is the temporal fluctuations in current originating from the partial

transmission of quantized charge in a mesoscopic conductor. It gives information about the

charge as well as the statistics of the quasi-particles in current carrying conductors. In this

work, we report measurements of shot noise in disordered ultra-thin films of niobium

nitride (NbN). NbN is a type-2 superconductor and shows a transition from 3D to 2D

superconductivity as the film thickness is reduced below the coherence length (~6 nm).

These films are also characterized by superconducting fluctuations above the critical

temperature (Tc). Thus shot noise can serve as a direct tool to establish the presence of

cooper pairs above Tc. Our technique involves measurements of current fluctuations near 2

MHz using a low noise cryogenic amplifier operating at 4 K whose output is further amplified

by an amplifier operating at room temperature and finally measured using a digitizer card.

The gain of the amplifiers have been measured using Johnson noise thermometry with

amplifier noise level < 1 nV/√Hz. The measurements on patterned ultra-thin films of NbN

yield an excess noise much larger than the expected Poissonian value for cooper pairs and

the sample resistance shows an insulating behaviour. We attribute the results to hopping

phenomenon for a disordered system though we do not rule out the possibility of multiple

Andreev reflections.

References

1. DiCarlo et al., Rev. Sci. Instr. 77, 073906 (2006)

2. Hoffman et al., Phys. Rev. B 70, 180503(R) (2004)

Abstract for Poster: Thermal Properties of Ultrathin Molybdenum Disulfide

layers

Authors: PBS Mahapatra, Priyamvada Bhaskar, Subroto Mukerjee, Arindam Ghosh

The recent increasing importance of the thermal properties of materials is explained both by practical needs and fundamental science. Heat removal has become a crucial issue for continuing progress in the electronic industry owing to increased levels of dissipated power. In recent years, the thermal conduction in low-dimensional structures has revealed truly intriguing features. Scaling down to lower dimension (2D), the Umklapp limited intrinsic thermal conductivity no longer remains finite because of the phase-space reduction of higher order scattering process of phonons. Even if the impurity and boundary scattering removes this divergence, thermal conductivity still remains much higher than usual good thermal conductors like normal metals. Reported values of Single layer suspended Graphene can go as high as 5300 Wm−1K−1, which is orders of magnitude higher than the normal metals. The electrical transport properties of this 2D layers on supported structures are thoroughly studied over last few decades but the thermal properties of such structures are vastly unknown owing to unique difficulties both theoretically and experimentally. Also, inconsistent results from various experimental techniques for measuring thermal conductivity of ultrathin materials further worsen the scenario. Search for adequate theory for heat transport in Ultrathin Molybdenum Disulfide (MoS2) layers and its measurement with classical 2ω and 3ω techniques strongly motivates this work. In this work, I present the experimental model to study the thermal conductivity of supported ultrathin MoS2 layers.

Title and Abstract (poster)

Absolute charge retentivity and photon counter like characteristics from interface engineeredphotoresponsive Graphene – MoS2 hererostructure.

Tanweer Ahmed, Kallol Roy, Arindam Ghosh

Molybdenum disulphide (MoS2) has opened up a large possibility in potential optoelectronicapplications due to its optical range band-gap1. Binary hererostructure consisting of Graphene – MoS2,achieving very high photoresponsivity and optoelectronic-memory functionality has already beenreported2. Here we fabricate a tertiary heretostructure where the interfaces of Graphene and MoS2 areseparated by an insulating multilayer boron nitride (BN), to prevent any electrical charge transferbetween the two. Though the inclusion of the dielectric layer between MoS2 and Graphenecompromises with the device's photoresponsivity which is found to be 5E8 A/W at 165K, more than anorder of magnitude smaller compared to the highest reported value2, but this MoS2 – BN – Graphenetertiary heterostructure shows immediately achievable persistent state which is absolutely constant withtime. This absolute charge retentivity can be exploited to make photon counter and in optoelectronicmemory applications as well.

References:1. Mak, K. F. et al. Atomically Thin MoS2: A New Direct-Gap Semiconductor.DOI: 10.1103/PhysRevLett.105.136805

2. Roy, K. et al. Graphene–MoS2 hybrid structures for multifunctional photoresponsive memorydevices. DOI: 10.1038/NNANO.2013.206

Optical tomography in scattering gel dosimetersRakesh M∗, Sharath M S and Rajan K∗∗

Dept. of Physics. Indian Institute of Science, Bangalore, IndiaEmail:∗rakesh@physics.iisc.ernet.in, ∗∗rajan@physics.iisc.ernet.in

Abstract—Optical tomography setup using laser light to esti-mate dose information in gels is described. A model based onMonte Carlo light propagation is proposed for dose reconstruc-tion. Limits on the detector dynamic range and system design isdiscussed. Some preliminary results by using polarized light areshown.

Keywords [Polarized light scattering, Gel dosimetry, MonteCarlo simulation]

I. INTRODUCTION

Gel Dosimetry uses the principle of change in attenuationcoefficient due to radiation exposure and is used in radiother-apy [1]. The change is due to ionization and polymerizationof monomers in the gel matrix. The particle size distributionincreases with higher dose further increasing the turbidity.Diode laser is used to scan plane by plane through thedosimeter and the transmitted light is collected using a photo-detector. Filtered back-projection (FBP) method has been usedto reconstruct the optical attenuation map inside the dosimeter.A resolution of 1mm and a good dose accuracy is essential indosimetry [2].

II. LIGHT PROPAGATION MODELS

A. Ballistic propagation model

For light that propagates along a straight line without anyscattering, the Beer Lamberts law

I = I0e−(µtx) (1)

is the governing equation that gives the value of intensityof ballistic light that exits out, where µt is the attenuationcoefficient and x is the ballistic path length traversed by light[3]. Total attenuation is due to absorption and scattering. Theattenuation and path length effectively determine the dynamicrange required for the scanning system.

B. Monte Carlo light propagation model

In gel dosimeter, scattering coefficient change can be abruptdependening on the irradiated dose profile [4]. In order tobetter model the light matter interaction, Monte-Carlo lightpropagation is considered [5]. Attenuation due to scattering isdependent on concentration and geometry (size distribution,shape) of the scatterer. Polarized light scattering is modeledby using mie-theory as the mean size of particles is inmicrons. In each of the 1mm voxel, a scattering coefficientvalue is obtained by considering size and concentration of thepolymerized scatterer as a input parameter, and it is updatediteratively.

III. DEPOLARIZATION BY SCATTERING

Polarized light as it traverses a scattering medium getsdepolarized. The amount of depolarization depends on thesize parameter and the kind of polarization i.e linear orcircular [6]. A plot of measured degree of polarization oflinearly polarized light (DOPL) of the transmitted light withdifferent concentration of TiO2 suspension and correspondingsimulation using Monte Carlo is shown in the figure below.

Fig. 1. Monte Carlo simulation and experimental plot of depolarization oflinearly polarized light for various concentration

IV. CONCLUSION

A study of depolarization by scattering and its applicabilityto dosimetry is presented. The results with Polarized lightMonte Carlo simulations match with experimental measure-ments. Further studies establishing correlation between depo-larization and dose need to be carried out for dose irradiatedgels.

REFERENCES

[1] M. Oldham, “Optical-CT scanning of polymer gels” J. Phys.: Conf. Ser., vol. 3, 122-135, 2004.

[2] C. Baldock, “Polymer gel dosimetry ” Phys. Med. Biol. , vol. 55, no.R1, 2010.

[3] Lihong. Wang, “ Biomedical Optics : Principles and Imaging” WileyInterscience, 2007.

[4] C. S. Wuu and Y. Xu, “Three-dimensional dose verification for intensitymodulated radiation therapy using optical CT based polymer gel dosime-try ” Med. Phys. , vol. 3, no. 5, pp 1412-1419, 2006.

[5] J. C. Ramella-Roman and S. A. Prahl and S. L. Jacques, “ Three MonteCarlo programs of polarized light transport into scattering media :Part I”Opt. Exp., vol. 13. pp 10392–10405, 2005.

[6] D. Bicout, “Depolarization of multiply scattered waves by sphericaldiffusers: Influence of the size parameter” Phy. Rev. E, vol. 49. no. 2,pp. 1767–1770, 1994.

Finite size scaling in crossover among different random matrix ensembles inmicroscopic lattice models

Ranjan Modak1 and Subroto Mukerjee1,21 Department of Physics, Indian Institute of Science, Bangalore 560 012, India and

2 Centre for Quantum Information and Quantum Computing,Indian Institute of Science, Bangalore 560 012, India

Using numerical diagonalization we study the crossover among different random matrix ensem-bles [Poissonian, Gaussian Orthogonal Ensemble (GOE), Gaussian Unitary Ensemble (GUE) andGaussian Symplectic Ensemble (GSE)] realized in two different microscopic models. The specificdiagnostic tool used to study the crossovers is the level spacing distribution. The first model is a onedimensional lattice model of interacting hard core bosons (or equivalently spin 1/2 objects) and theother a higher dimensional model of non-interacting particles with disorder and spin orbit coupling.We find that the perturbation causing the crossover among the different ensembles scales to zerowith system size as a power law with and exponent that depends on the ensembles between whichthe crossover takes place. This exponent is independent of microscopic details of the perturbation.We also find that the crossover from the Poissonian ensemble to the other three is dominated bythe Poissonian to GOE crossover which introduces level repulsion while the crossover from GOE toGUE or GOE to GSE associated with symmetry breaking introduces a subdominant contribution.We also conjecture that the exponent is dependent on whether the system contains interactionsamong the elementary degrees of freedom or not and is independent of the dimensionality of thesystem.

Noisy Graphene Gas Sensor

Kazi Rafsanjani Amin and Aveek BidDept of Physics, Indian Institute of Science, Bangalore, India

The demonstration of the existence of a perfectly two dimensional crystal, graphene was one of the most celebratedinventions in present day condensed matter science 1. Since then, graphene has been a focus of research, both frombasic science as well applied point of view. The conductance of graphene extremely sensitive to the ambient and thepresence of a single molecule adsorbed on the surface of graphene can significantly modify its electrical characteristics.Change in electrical conductance of graphene arising from adsorption of gas molecules on its surface, which acts ascharge carrier donors or acceptors, makes graphene a promising candidate for solid-state gas sensor.2. Where graphenebased gas sensors are excellent in low cost fabrication, room temperature operation, high sensitivity and simplicity,disadvantage of graphene sensors working on the principle of conductance change up on exposure to gas moleculeslies in large reset time, and lack of specificity. Graphene gas sensors working on the principle of measurement of lowfrequency conductance fluctuation rather than measuring average conductance opens up a new direction includingpromise of detection with specificity. 3

Low frequency (1Hz to 1KHz) conductance fluctuation (noise) of graphene field effect transistor device, fabricatedinvolving conventional electron beam lithography technique, was measured continuously in time segments of 1 minute,while the device was exposed to known volume of different chemical vapours. As the resistance of the device increasedup on exposure, the 1/f noise of the device also increased, and the increase in noise was much more profound. Similarto change in resistance, the relative variance (RV ) of the resistance fluctuation also reaches a steady state value after acertain amount of time. The maximum value of RV scales with amount of chemical applied to sensor device. When thevapour molecules were pumped out of the experimental chamber, in contrary to very slow response in resistance, theRV switches back to its original value very fast. The sensor response in RV was found to be very much reproducibleas the device was exposed to same volume of methanol vapour for consecutively three times. As the device wereexposed to methanol, a hump with a particular frequency was observed in the 1/f noise spectrum, which allows oneto detect presence of methanol in a mixture of two different chemical vapours.

The work can be concluded by stating that measurement of low frequency 1/f noise in resistance is a efficientmethod in application of graphene as gas sensors.

1 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science306, 666 (2004).

2 F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, and K. S. Novoselov, Nat Mater 6, 652(2007).

3 S. Rumyantsev, G. Liu, M. S. Shur, R. A. Potyrailo, and A. A. Balandin, Nano Letters 12, 2294 (2012).

Magneto transport and 1/f noise in 2DEG at LaAlO3/SrTiO3 interface

Gopi Nath Daptary, Shelender Kumar and Aveek Bid

Indian Institute of science, Bangalore-560012

We have studied through magneto transport the two dimensional electron gas (2DEG) at the interface of two insulators LaAlO3 and SrTiO3, formed due to electronic reconstruction. At low temperatures and in high carrier density range this 2DEG has a superconducting ground state, whose critical temperature can be controlled by the electric field induced by a gate voltage perpendicular to the direction of carrier confinement. At lower carrier densities the system goes ferromagnetic as seen from magnetization studies. The nature of the charge carriers at the interface and their origin are still not fully understood. There have been various theoretical predictions regarding the zero magnetic field ground state of the system – ranging from ferromagnetic to spiral.

We have studied in detail the magnetoresistance of a 10 unit cell device at various values of back gate voltage Vg. We observe that the magnetoresistance at 245 mK has a field sweep rate dependent hysteresis in the field range 0T to 2T. The hysteresis is largest at large negative Vg (low carrier densities) and decreases as Vg is made increasingly positive. At the highest positive Vg (high carrier density) the hysteresis in magnetoresistance gives way to a transient superconductivity that decays to the normal state over a time period of a few seconds. All these intriguing properties motivated us to probe the hysteretic nature of this material through 1/f noise measurement. We find that the relative variance of resistance fluctuations or noise tracks the hysteretic behaviour of MR curve. Noise at 0T magnetic field shows a peak at the Vg where the hysteresis is the largest while the noise at 8 Tesla is almost independent of the gate voltage. Resistance fluctuation measurements and hysteresis in magnetoresistance indicate that there probably is a co-existence of two different electronic states in the system.

[1] D.A.Dikin et al, PRL 107, 056802(2011)

[2] 3 June 2007; doi: 10.1038/nmat1931

[3] 4 December 2008|doi:10.1038/nature07576

Magnetic and EPR studies of nanoparticles and bulk Sm0.65Ca0.35MnO3

Lora Rita Goveas1, K. S. Bhagyashree

2, Anuradha K.N.

1, S.V.Bhat

2

1Department of Physics, Dr. Ambedkar Institute of Technology, Bangalore-560056, India

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

When the size of particles is reduced to nanometer scale, some of their basic magnetic properties differ

significantly from their bulk counterpart due to the quantum confinement effect and extremely high

surface/volume ratio. Size reduction of the charge ordered manganites has resulted in the disappearance of

charge order, weakening of antiferromagnetism and emergence of ferromagnetism1,2

. Here we present a

comparative study of the temperature dependent magnetic properties and EPR parameters of Sm0.65Ca0.35MnO3

manganite of nanometer size (SCMON) prepared by sol-gel method with micrometer size prepared by two

different techniques: (i) sintering the nanoparticles at higher temperature (SCMOBN) (ii) solid state reaction

method (SCMOBS). The samples were characterized by XRD, SEM and TEM. The magnetic measurement

carried out by SQUID magnetometer showed that SCMOBN underwent charge order transition (Tco) at 240K

and ferromagnetic transition (TC) at 120K, where as the charge order peak was found to be absent in SCMON

and the TC was reduced to 70K. The field cooled (FC) curve shows that the magnetization increases with the

size reduction. The Electron Magnetic Resonance (EMR) studies carried out using a commercial EPR

spectrometer between the temperature 4K and 300K confirm the existence of ferromagnetism in the bulk sample

as well as in the nanosample but with enhanced strength. SCMOBN and SCMOBS showed similar temperature

dependence of EPR parameters viz intensity, line width and resonance field. We conclude that mixed magnetic

state exists in the bulk sample whereas it is suppressed in the nano sample and strong ferromagnetism is induced

instead. We ascribe the magnetic behaviour of the nanoparticles to a coreshell scenario3

.

1. S. S. Rao, K. N. Anuradha, S. Sarangi, and S. V. Bhat, Appl. Phys. Lett. 2005, 87, 182503.

2. SSRao, S Tripathi, D Pandey and SV Bhat, Phys. Rev. B, 2006 , 74,144416.

3. Lopez-Quintela M.A., Hueso L.E., Rivas J., and Rivadulla F., Nanotechnology, 2003, 14(2), 212.

Measurement of force pattern of C. elegans moving on agarose pad using colored micropillars

Siddharth Madhav Khare*, Anjali Awasthi**, V. Venkataraman*, Sandhya P. Koushika***

* Department of Physics, IISC, Bangalore, INDIA

** NCBS-TIFR, Bangalore, INDIA & BITS-Pilani, INDIA

***DBS-TIFR, Mumbai, INDIA

Abstract:

Locomotive behavior of an organism is largely influenced by the environmental cues. These include

chemical, thermal, electrical, mechanical etc. We plan to study the effect of mechanical obstacles on the

force pattern produced by the nematode Caenorhabditis elegans (C. elegans)[5]. As already reported [1],

we use Poly Dimethyl Siloxane (PDMS) micro-pillars to produce structured environment[3,5] as well as

to measure forces exerted by the worm C. elegans. Soft-lithography on SU-8 has been used for PDMS

replica moulding. Micro-pillars are deflected by the worm and the deflection is proportional to the force

exerted.

To the best of our knowledge, quantitative analysis of the forces exerted by the C. elegans strains with

mechano-sensory defect has not been done using this technique. A custom made software has been

developed in matlab to help analyse large amount of data. Cross-correlation method was used to detect

the pillar positions in the image. We expect the colored pillar technique to be useful for real time image

processing and also in other applications where image clarity is an issue. Measurement and comparison of

force patterns of different strains of C. elegans may be useful to correlate the sensory-motor responses of

the worm[1].

We calibrated the micro-pillars using optical microscope (Olympus IX51) and MEMS based force sensor

from FemtoTools. Mean length and diameter of the pillars was measured to be 153µm and 49µm

respectively. Force constant for a 125µm long pillar was approximately 0.13µN/µm. Finite element

model of the micro-pillar was implemented in Comsol to estimate the Young’s modulus of the colored

PDMS (approx. 350kPa). Time lapse images were recorded at 15fps for C. elegans strains: N2, mec12-

e1605, mec10-e1515, mec10-tm1552, mec7-e1443, mec4-e1339 and jsls609. We see that there is an

inherent difference between mean body length and body diameter of one day adult animals of different

strains. The force pattern shows a correlation between body length of the animal and the average force,

but the body diameter does not seem to affect the force as much. We are trying to understand this

correlation and represent the differences in the force patterns of different strains quantitatively.

References:

1. “On-chip analysis of C. elegans muscular forces and locomotion patterns in microstructured

environments”, Shazlina Johari et. al., Labchip, 10.1039/c3lc41403e (2013)

2. “A micropillar-based on-chip system for continuous force measurement of C. elegans”, Ali Ghanbari

et. al., J. Micromech. Microeng. 22, 095009, (2012)

3. “Artificial Dirt: Microfluidic Substrates for Nematode Neurobiology and Behavior”, S. R. Lockery et.

al., J. Neurophysiol 99: 3136-3143, (2008)

4. “SU-8 force sensing pillar arrays for biological measurements”, Joseph C. Doll et. al., Lab Chip, 9,

1449-1454, (2009)

5. “Enhanced Caenorhabditis elegans Locomotion in a Structured Microfluidic Environment”, S. Park

et. al., PLoS ONE 3(6): e2550. doi:10.1371/journal.pone.0002550

Confinement Induced Stochastic Sensing of Charged

Coronene in α-Hemolysin Nanochannel: A Molecular

Dynamics Study

Debabrata Pramanik1, R. Shivanna

2, K. S. Narayan

2 and Prabal K Maiti

1

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

Bangalore - 560012, India

2Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific

Research, Bangalore, India

Abstract

Biological nanopores provide optimum dimensions and environment to study early aggregation

kinetics of charged polyaromatic molecules in the nano-confined regime. It is expected that

probing early stages of nucleation will enable design strategy for supramolecular assembly and

biocrystallization processes. Specifically, we have studied translocation dynamics of coronene

and perylene based salts, through the α-hemolysin (α-HL) protein nanopore using all atom MD

simulation. The blocking events which arises in the ionic currents provides interesting features

about the aggregation kinetics and size. Two types of blockades have been observed and arise

due to different aggregation process.

Reference: 1. “Confinement Induced Stochastic Sensing of Charged Coronene and Perylene

Aggregates in alpha-Hemolysin Nanochannel”, R. Shivanna, Debabrata Pramanik, H.

Kumar, K. Venkata Rao, Subi J. George, Prabal K. Maiti and K. S. Narayan; Soft Matter,

9, 10196-10202 (2013).

Feshbach Resonance in a Synthetic Non-Abelian Gauge Field

Vijay B. Shenoy∗

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

(Dated: November 12, 2013)

We study the Feshbach resonance of spin-1/2 particles in a uniform synthetic non-Abelian gaugefield that produces spin orbit coupling and constant spin potentials. We develop a renormalizablequantum field theory including the closed channel boson which engenders the resonance, in thegauge field. We show that the gauge field shifts the Feshbach field where the low energy scatteringlength diverges. In addition the Feshbach field is shown to depend on the centre of mass momentumof the particles. For high symmetry gauge fields which produce a Rashba spin coupling, we showthat the system supports two bound states over a regime of magnetic fields when the backgroundscattering length is negative and resonance width is comparable to the energy scale of the spin-orbitcoupling. We discuss interesting consequences useful for future theoretical and experiment studies,even while our predictions are in agreement with recent experiments.

Polymer devices with optically transparent horizontal electrodes for rapid optimization

of cell electroporation

Amit Kumar Majhi, V. Venkataraman

Department of Physics IISC, Bangalore 560012.

Some of the outstanding issues in the area of electroporation include the in situ observation of

cells during electric field treatment and ability to achieve higher efficacy at lower voltage. In

order to address these issues, the efficacy of a new device design concept is investigated in

the present work. Polymer devices have been fabricated from polydimethylsiloxane(PDMS)

with transparent indium tin oxide (ITO) parallel plate electrodes in horizontal geometry. This

device geometry permits longer observation of cells in both brightfield transmission and

fluorescence reflection microscopy. Using propidium iodide (PI) as a marker dye, the number

of electroporated cells was measured in-situ as a function of applied voltage from 10V to 90V

in a series of ~2ms pulses across 0.5mm electrode spacing. The electric field at the interface

and device current was calculated using a model, which takes into account bulk screening of

the transient pulse. The measured device current is in agreement with the calculations without

any fitting parameters. The voltage dependence of the number of electroporated cells

could be explained using a stochastic model for the electroporation kinetics and the free

energy for pore formation was found to be 45.6 kT at room temperature. Another advantage

of the newly designed device is the ability to achieve a maximum of 40% electroporation

efficiency at applied device voltage of 50V across 0.5 mm spaced electrodes. With this

device, the optimum electroporation conditions can be quickly determined by monitoring the

uptake of propidium iodide (PI) marker dye in situ in a typical culture volume of 10 µL to

100 µL under the application of millisecond voltage pulses. The electroporation efficiency is

quantified using ex-situ fluorescence assisted cell sorter (FACS) as well as morphological

studies of cultured cells. Importantly, the efficacy of the developed device was tested

independently using two cell lines(C2C12 mouse myoblast cells and Yeast cells) as well as in

three different electroporation buffers (Phosphate buffer saline (PBS); electroporation buffer

(EPBH) and10% glycerol).

This work was carried out in collaboration with Greeshma Nair and Prof. Bikramjit Basu

from Materials Research Centre, IISc.

"Abstract for Poster"

Probing Carrier Dynamics in Graphene by Terahertz

Spectroscopy

Srabani Kar1, Gyan Prakash

1, Dipti Mohapatra

1, Eric Freysz

2, A. K. Sood

1

1Department of physics, Center for Ultrafast Laser Application, Indian Institute of Science,

Bangalore 560012, India. 2UniVersity of Bordeaux 1, CPMOH, UMR CNRS 5798, 351,

Cours de la liberation, 33405 Talence cedex, France

Linear band dispersion of monolayer grapheme offers many interesting possibilities such as

tuning the carrier density (electrons and holes) by arbitrary small electric field and flat

transmission spectrum covering a large spectral window. The response of the carriers to

external perturbations plays an important role in opto-electronic applications of grapheme.

The carriers photo-excited by an ultrafast pump pulse can relax through intra-band and inter-

band scattering processes which can be probed by pump -probe experiments. The photo-

induced change in conductivity can be positive or negative depending on the dominance of

intraband vis interband scattering at the probe frequency. We report photo-induced changes

of conductivity (∆σre) by optical pump and terahertz probe experiment in the terahertz range

of 0.5-2.5 THz. In this frequency range intra-band contribution to ∆σre is positive and the

inter-band contribution can be either positive or negative depending on the Fermi energy.

However, the inter-band contribution (∆σreinter

) is about 1000 time less than the intra-band

part (∆σreintra

). We will present our recent experiments on CVD grown single layer grapheme,

and nitrogen doped grapheme .The results will be explained in the frame work of hot carrier

scattering and super collisions.

Yielding and Thixotropy in an anisotropic nanoparticle dispersion formed by Few layer Graphene oxide platelets

K.S. Vasua, Rema Krishnaswamyb, S. Sampathc and A.K. Sooda*

a Departmetn of Physics, Indian Institute of Science, Bangalore – 560012. b Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India c Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012,India. Thixotropy (Aging), where the suspension viscosity decreases (increases) with time under steady shear is not well understood for anisotropic particles of large aspect ratios (>100) that form viscoelastic gels at very low volume fractions. The most relevant examples are carbon nanotubes, graphene or graphene oxide (GO) used as nanofillers to enhance the mechanical properties of the host material. The study of yielding behavior in bare suspensions of these systems is an important step towards understanding the mechanical processing of nanocomposites. Recently, we have observed a thixotropic yield stress behavior with shear localization below a critical shear rate in a bare, non-aggregating aqueous suspension of giant Graphene Oxide (GO) platelets (aspect ratio ~5000) that undergo a rigidity percolation transition above a critical volume fraction of 0.000375 with a percolation exponent of 2.4± 0.1 [1]. We observe a shear-induced jamming and rejuvenation in creep measurements that possibly arises from the formation and break up of contacts on shearing the 3D percolated network of disks. Our recent results which examine the fluidization behaviour at applied stresses close to the yield stress in GO suspensions will also be presented [2]. References:

1. K. S. Vasu, Rema Krishnaswamy, S. Sampath, and A. K. Sood , Soft Matter, 2013 9, 5874 . 2. K. S. Vasu, Rema Krishnaswamy and A. K. Sood, (unpublished).

Abstract for Poster

Temperature dependence of photoluminescence properties of CdHgTe nanocrystals

Jagtap Amardeep Manikrao*, Jaykrishna Khatei, K.S.R.K. Rao Indian Institute of Science, Bangalore*E-mail:amar@physics.iisc.ernet.in

Abstract— We report the temperature dependent (15-300K) photoluminescence spectrum and relaxationmechanism in CdHgTe nanocrystals grown byhydrothermal method. Photoluminescence quenchingwith increasing temperature is observed.

Index Terms- Semiconductor nanocrystals, photoluminescence

I. INTRODUCTION

Semiconductor quantum dots (QDs) with a size-tunablebandgap have been widely applied in light-emittingdiodes(LEDs), photovoltaic cells. A detailed knowledgeof the radiative and nonradiative relaxation processes ofQDs is therefore fundamental for future applications ofcolloidal QDs in optoelectronic devices.

In this work, we have studied the temperaturedependent photoluminescence of CdHgTe NCs.

II. EXPERIMENTAL DETAILS

The cadmium chloride (CdCl2 ) ,Mercury chloride(HgCl2) and Sodium telluride (Na2TeO3) were used forCd, Hg and Te source respectively. Sodium borohydride(NaBH4) and 3-mercaptopropionic acids (MPA) wereused as reducing agent and surface capping ligandsrespectively. The synthesis was carried out in 50mlTeflon lined stainless steel autoclave kept at 180 ºC for15 to 50 minutes. Molar ratio of Hg/Cd was kept 05%

III. RESULTS & DISCUSSIONS

Figure (a) shows UV-Vis absorption spectrum of theCdHg(05%)Te QDs for different synthesis time . Itshows that the first excitonic peak for CdHg(05%)Te 15was at 567 nm and redshifts from 567 – 663 nm assynthesis time varies 15 to 50min.

400 450 500 550 600 650 700 750 800 850 900

0.0

0.5

1.0

1.5

CdHg(05%)Te 15

CdHg(05%)Te 25

CdHg(05%)Te 35

CdHg(05%)Te 40

CdHg(05%)Te 45

CdHg(05%)Te 50

Ab

sorb

an

ce(a

.u.)

Wavelength(nm)

567nm

663nm

611nm647nm

(a)

Figure (b) shows temperature dependent PLspectrum of 35 minute synthesized NCs. The detailPL quenching mechanism with temperature will bepresented in poster .

(b)

550 600 650 700 750 800 850

0.000

0.002

0.004

0.006

0.008

0.010

0.012

25.7k 31k 40k 50k 60k 70k 82k 91k 100k 110k 120k 130K 140k 150K 160K 170k 170k 180K 190K 200k 210k 220k 230k 240k 250K 260k 282.5k 292.4k 302.4k

Inte

gra

ted

PL

Inte

nsi

ty(a

.u.)

Wavelength(nm)

Optoelectronic properties of graphene-MoS2 hybrids

Kallol Roya, Medini Padmanabhan

a, Srijit Goswami

a, T. Phanindra Sai

1, Gopalakrishnan

Ramalingamb, Srinivasan Raghavan

b, and Arindam Ghosh

a

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

bCentre for Nano Science and Engineering and Materials Research Center, Indian Institute of

Science, Bangalore 560012, India

Graphene is an interesting layered material because of its high quality electronic characteristics

which can be tuned by application of a gate voltage [1-3]. However optical absorption property

of single layer graphene is poor, and hence most of the light (> 97%) gets transmitted [4].

Molybdenum di-sulphide (MoS2) is another layered material which is optically active and its

bandgap changes with number of layers [5]. In our work we have shown that the atomically thin

heterostructures of graphene and MoS2 can be made to integrate their electronic and optical

characteristics respectively and show gate tunable multifunctional optoelectronic response [6,7].

Optical responsivity of such hybrid devices are measured as high as ~1010 A W

-1 at 130 K, and is

greater than 108 A W

-1 at room temperature. These devices can also act as re-writable

optoelectronic memory devices and can be controlled by application of suitable light and gate

pulses.

References:

1. Novoselov, K. S. et al. A roadmap for graphene. Nature 490, 192–200 (2012).

2. Dean, C. R. et al. Boron nitride substrates for high-quality graphene electronics. Nat. Nanotechnol. 5, 722–6 (2010).

3. Bolotin, K. I. et al. Ultrahigh electron mobility in suspended graphene. Solid State Commun. 146, 351–355 (2008).

4. Nair, R. R. et al. Fine Structure Constant Defines Visual Transparency of Graphene. Science. 320,

(2008).

5. Mak, K. F., Lee, C., Hone, J., Shan, J. & Heinz, T. F. Atomically Thin MoS2: A New Direct-Gap

Semiconductor. Phys. Rev. Lett. 105, 136805 (2010).

6. Roy, K. et al. Graphene – MoS2 hybrid structures for multifunctional photoresponsive memory

devices. Nature Nanotechnology 8, 826–830 (2013)

7. Roy, K. et al. Optically active heterostructures of graphene and ultrathin MoS2. Solid State Commun.

1, 1–8 (2013) (http://dx.doi.org/10.1016/j.ssc.2013.09.021).

Superfluid flow in a 2D annular ring with a barrier : A strong coupling

perturbation theory approach

Manjari Gupta and H. R. Krishnamurthy

Center For Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560012, India

J. K. Freericks

Department of Physics,Georgetown University, Washington, D.C. 20057, USA

(Dated: November 12, 2013)

There have been recent experimental studies on Josephson junctions using cold atomic

gases in a 2D annular ring with a potential barrier across the annulus serving as a weak-link

[1]. These provide an opportunity to better understand the properties of Josephson junc-

tions in an atomic superfluid. We have developed a strong-coupling (t � U ) perturbation

technique for Bose Hubbard model (BHM) [2] which can be used to simulate the physics

of these systems for low atomic densities (〈n〉j,max ≤ 0.5; j being the site index). This

method also allows one to study different complex atomic circuits, that can be formed with

cold atoms in an optical lattice with an overall annular trap, and barriers as weak links, for

higher atomic densities (〈n〉j,(max) > 1.0).

[1] K. C. Wright, R. B. Blakestad, C. J. Lobb, W. D. Phillips, and G. K. Campbell, Phys. Rev. Lett. 110,

025302 (2013).

[2] M. Gupta, H. R. Krishnamurthy, and J. K. Freericks, arXiv:1308.4112v2 (????).

Observation of Coulomb Interaction Effects via Defect Spectroscopy in Graphene on

Boron Nitride

Paritosh Karnatak*, Srijit Goswami*, Arindam Ghosh

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

Discrete switching events are observed in electrical transport across a high mobility

Graphene - Boron Nitride heterostucture. This switching behaviour arises from the random

capture and emission of charge carriers by a defect, in proximity to a metal contact. When the

Fermi level in the Graphene lines up with the defect state, the conductance steps attain values

close to ~e2/h, while away from this resonance condition, the step size decreases. In addition,

the emission rates are also maximum close to resonance, indicative of Coulomb interaction

effects, consistent with the finite temperature Fermi Edge Singularity picture. We also show

that in this defect-Graphene system, noise serves as a sensitive probe to emerging gaps in the

density of states of Graphene at finite magnetic fields.

Revision of Neutron Drip Density of MagnetisedMatter: Exciting Prospect of Massive Neutron Stars

Vishal MV, Banibrata Mukhopadhyay

Department of Physics, Indian Institute of Science, Bangalore 560012, Indiavishalmv555@gmail.com, bm@physics.iisc.ernet.in

November 12, 2013

Abstract

We consider a system of relativistic, degenerate gas consisting of electrons andheavy ions under the influence of a strong magnetic field, which describes magnetizedmatter of compact objects. We show that strong magnetic fields can significantlymodify the density of onset of neutron drip due to the Landau quantization ofthe electrons. We obtain the drip density-field relations by minimising the energyof the system thus finding the most stable configuration. The drip densities areseen to increase in the presence of high magnetic fields and this can lead to veryinteresting cases such as incrementing the masses of highly magnetised neutronstars, e.g. magnetars.

1

Orbital-Ordering and Phonon Anomalies in iron pnictide

Ca(Fe0.97Co0.03)2As2

Pradeep Kumar1, D. V. S. Muthu

1, L. Harnagea

2, S. Wurmehl

2, B. Büchner

2 and A. K.

Sood1

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

2Institute for Solid State Research, IFW Dresden, D-01171 Dresden, Germany

We will present our inelastic light scattering studies on Ca(Fe0.97Co0.03)2As2 in a wide

spectral range 120-5200 cm-1 from 5K to 300K, covering the tetragonal to orthorhombic

structural transition as well as magnetic transition at Tsm ~ 160K. The mode frequencies of

two first-order Raman modes B1g and Eg, both involving displacement of Fe atoms, show

sharp increase below Tsm. Concomitantly, the linewidths of all the first-order Raman modes

show anomalous broadening below Tsm, attributed to strong spin-phonon coupling. The high

frequency modes observed between 400-1200 cm-1 are attributed to the electronic Raman

scattering involving the crystal field levels of d-orbitals of Fe2+. The splitting between xz and

yz d-orbital levels is shown to be ~ 25 meV which increases as temperature decreases below

Tsm. A broad Raman band observed at ~ 3200 cm-1 shows anomalous temperature dependence

and is assigned to coupled orbital and magnetic excitations.

References

1. P. Kumar et al., (To be submitted).

Contact limited resistance noise in high mobility single layer graphene

T. Phanindra Sai1, Sanjeev Koushal2 and Arindam Ghosh1

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

2Tokyo Electron Ltd., Akasaka Biz Tower, 3-1 Akasaka 5-Chome, Minato-ku,

Tokyo 107-6325, Japan.

In this work, we report an experimental study of 1/f noise in dual gated high mobility

single layer graphene based device. The fabricated device consists of single layer graphene

etched in Hall probe geometry with gold contacts, with highly doped Si and air bridged Cr-

Au acting as back and top gates respectively. The device was systematically studied in two

probe and four probe geometry for 1/f noise as function of back gate, top gate voltages and

temperature. The normalized noise spectral density showed M-shaped gate bias

dependence and this behavior was observed to be identical for both back and top gates.

Even though previous independent studies on 1/f noise mechanism in graphene indicated

contributions from charge carrier fluctuations, mobility fluctuations, contacts, water and

residue contamination, the source of noise is not conclusively established.

Our results from the two and four probe measurements, clearly point to the fact that

the contact resistance between gold contact and graphene dominates and limits the noise

spectral density, showing similar dependence on gate voltage as that of the contact

resistance. We also compare our contact noise dependence on contact resistance with

theoretical model describing complicated and complication free contact of metal and

semiconductor.

References

Fengnian Xia et.al, Nature Nanotechnology, 6, 179 (2011)

Balandin A, Nature Nanotechnology, 8, 549 (2013)

Atindra Nath Pal et.al, ACS Nano, 5, 2075 (2011)

Vandamme L K J, IEEE Transactions on Electron Devices, 41, 2176 (1994)

NANOHYBRIDS OF SINGLE LAYER GRAPHENE OXIDE-MnFe2O4

NANOPARTICLES FOR EFFICIENT REMOVAL OF HEAVY METALS IN WATER

Suresh Kumar1, Rahul Nair2, Prem Kumar1, Satyendra Nath Gupta1, M.A.R. Iyengar1 and A.

K. Sood1.

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

2School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK.

The increasing concentration of heavy metals contaminants like arsenic and lead in groundwater has become a challenging issue worldwide due to their serious effects on human health. The role of nanoscale sorbents in water purification is promising. Herein we show that nanohybrids of a few layer graphene oxide- MnFe2O4 nanoparticles prepared by co-precipitation technique are novel adsorbents for the co-removal of Pb (II), As (III) and As (V) from contaminated water. Batch experiments were carried out to investigate the adsorption kinetics and adsorption capacities of the nanohybrids. The adsorption data from these experiments fit the Langmuir model, yielding a high value of the maximum absorption capacity. The nanohybrid showed better heavy metal adsorption capacity than bare nanoparticles and are the best reported so far in the literature. The high removal efficiency, easy magnetic separability and fast kinetics make these hybrid nanostructures potentially attractive candidates for low cost adsorbent for the effective co- removal of heavy metals from contaminated water.

Electron-Hole Asymmetry in Bulk and Nanoparticles of Nd1-xCaxMnO3 (x = 0.6, 0.4)

*Bhagyashree K. S. and S. V. Bhat

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

*bhagyashree@physics.iisc.ernet.in

The phase diagrams of doped rare earth manganites RE1-xAxMnO3, where RE is a trivalent rare earth ion and A is a

divalent alkaline earth ion, exhibit a striking asymmetry across half-doping, i.e. x=0.5. This asymmetry is termed

electron-hole asymmetry since for 0 < x < 0.5, more and more holes are introduced in REMnO3 by the substitution of

Mn3+

by Mn4+

and for 0.5 < x < 1, Mn4+

in AMnO3 are replaced by Mn3+

, effectively doping with more and more

electrons. An early study on Pr1-xCaxMnO3 (PCMO), showed1 that electron paramagnetic resonance (EPR) can be an

effective tool to characterize this electron-hole asymmetry. Recently it was observed2 that the electron hole asymmetry

as reflected in EPR ‘g’ factor disappeared in nanoparticles of Pr1-xCaxMnO3. Here we study and compare magnetic and

EPR behaviours of bulk and nanoparticles of hole doped (x = 0.4) and electron doped (x = 0.6) Nd1-xCaxMnO3

(NCMO).

The nanosamples of these compounds were prepared by the sol-gel method by sintering the precursor at 6000C for 12

hrs. The bulk samples were prepared by sintering the nanosamples at 14000C. In order to confirm the phase formation

we have done XRD on all the four samples. The Reitveld refinement shows that all the four samples crystallize in

orthorhombic structure with space group Pnma. We have also done EDAX on all the samples which confirms the

stoichiometric compositions. TEM shows that the average size of the nanoparticles is ~ 30 nm. According to the earlier

reports, the hole doped compound undergoes a charge ordering (CO) transition at Tco ~ 250 K, AFM transition at ~170

K and a transition to a canted AFM phase at ~ 80 K. The electron doped compound has a Tco of ~ 280 K. There are no

reports of magnetization measurements done on the nanoparticles of these compositions to the best of our knowledge.

Our magnetization measurements done on the bulk Nd0.6Ca0.4MnO3 shows that it undergoes CO transition at ~ 250 K

and AFM transition at ~ 160 K and canted AFM transition at ~ 70 K. Hysteresis is observed only below 70 K, which

confirms the canted AFM phase below 70 K. The nanoparticles of the same composition show a very weak peak

slightly below 240 K which might be because of the residual CO. The hysteresis is seen below 70 K but the EPR

measurements confirm that it is due to FM phase unlike in the bulk. Magnetization measurement of the Nd0.4Ca0.6MnO3

bulk shows that it undergoes a CO transition at ~ 280 K and the hysteresis is not observed at any temperature. The

magnetization measurement of the nanosample shows a broad hump at ~ 240 K. But hysteresis starts to appear only

below 50 K.

EPR measurements have been carried out on all the samples from room temperature down to 4 K using a commercial

X- band EPR spectrometer. All the signals obtained at various temperatures fit well with broad Lorentzian line shapes

except those from the nanosample of Nd0.6Ca0.4MnO3, where the signals are asymmetric below 70 K. This is due to the

FM phase existing below this temperature. We have plotted the g vs temperature in order to study the electron-hole

asymmetry. It is seen that the asymmetry is very small even in the bulk NCMO compounds unlike in the case of

PCMO1,2

. In the nanosamples it becomes even less. We understand these results in the light of destabilization of charge

order in nano dimensions.

References

1. J. P. Joshi et al., J. Phys. Cond. Mat. 16(2004)2869.

2. Padmalekha K. G. and S. V. Bhat, to be published.

ATOMISTIC SIMULATION OF STACKED NUCLEOSOME CORE PARTICLES

Suman Saurabh, Prabal K. Maiti

DNA, for its storage inside a cell, needs to be severely compacted, owing to its long length and the small

space available to it inside a cell nucleus. In the first stage of this essential compaction, the DNA winds

1.6 turns around a histone protein core. The system thus formed is called a Nucleosome Core Particle

(NCP). Histone protein has a structured core region and an unstructured tail region. Tails come under a

special category of proteins called Intrinsically Disordered Proteins (IDPs). This kind of proteins need a

binding partner to fold to a definite structure, and are otherwise unfolded. A lot of work has been done

in studying the properties of a single NCP and the role of tails in its structural stability through

experiments and molecular simulations. The NCPs, stacked over each other, form the chromatin fiber.

As a first step to understand the properties of the chromatin fiber, this work involves molecular

dynamics simulation of two NCPs stacked over each other in which a special emphasis has been given on

studying the role of histone tails in mediating the interaction between the two NCPs, trying to find

molecular level explanations of various experimental findings. Also, the effect of salt concentration on

the stacking properties are studied.

Chirality Dependent Pinning and Depinning of Magnetic Domain

Walls at Nano-constriction

Vineeth Mohanan P

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

The implementation of magnetic domain wall (DW) based memory and logic devices

critically depend on the control over DW assisted magnetization reversal processes. Here we

investigate the magnetization reversal by domain wall (DW) injection, pinning and depinning at

a geometrical constriction in Permalloy (Py) nanowire (NW) driven by external in-plane

magnetic field, using localized electrical probes. We could controllably inject vortex DWs of

different chiralities into the NW and estimate chirality dependent pinning probability at a nano-

constriction in the wire. The interaction of DWs with a notch is investigated through experiments

and micromagnetic simulations. The pinning probability is found to be higher for DWs of

clockwise chirality since the potential barrier seen by it at the notch is higher compared to DWs

of counter clockwise chirality. This implies that a careful discretion is required for the selection

of the type of domain wall which might be used in DW based devices. This study also reveals

that we need to further improve our control over the DW injection and the pinning probability

substantially to realize the desired memory and logic devices. The stochastic nature of the DW

based reversal driven by magnetic field is evaluated. The results demonstrate the difficulties in

achieving deterministic switching behavior that are vital for technological applications.

DNA and siRNA functionalized Dendrimer and Single Wall Carbon Nanotubes: An atomic force microscopy based investigation

Vijay K Ravi1, K. S. Vasu1, Abirami. L2, Ranjitha.T3, S. Das3, N. Jayaraman2 ,Prabal. K . Maiti1 and A. K. Sood1

1 Dept. of Physics, IISc, 2 Dept. of Organic Chemistry, IISc, 3 Dept. of Microbiology and

Cell biology, IISc Bangalore, Bangalore-560012. Email: asood1951@gmail.com

Abstract

With progress of nanotechnology and bio-nanotechnology, dendrimer and the water

soluble single walled carbon nanotubes have emerged as gene and drug delivery vector

and as well as ‘building blocks’ in nano/microelectronic devices. In this context, the

dendrimer, single walled carbon nanotubes have been reported as delivery vector for

gene and drug. Here we have studied efficient complexation of DNA and siRNA non-

covalently functionalized PETIM dendrimer and SWNT. Atomic force microscopy of

PETIM dendrimer and its complexes were done after depositing on the mica surface.

The PETIM dendrimer-pEGFP DNA complex showed an increase in the average z-

height and dendrimers decorating on the DNA strand, without causing a distortion of the

DNA structure. Further, the physicochemical properties, its toxicity and transfection

efficiency in different cell lines were studied and it was established as efficient gene

delivery vector (1). In the next studies, we have shown the complexation of siRNA with

PETIM dendrimer for application in gene silencing for liver cancerous cell lines. In

addition we have studied complexation of pDNA-SWNT upon functionalization with

PETIM which has shown significant complexation.

Reference:

(1) Lakshminarayanan, A., Ravi, V. K., Tatineni, R., Rajesh, Y. B. R. D., Maingi,V.,

Vasu, K. S., Nandhitha, M., Maiti, P. K., Sood, A. K., Das, S. and Jayaraman, N. (2013)

Efficient dendrimer-DNA Complexation and Gene Delivery Vector Properties of

Nitrogen-Core Poly(Propyl Ether Imine) Dendrimer in Mammalian Cells. Bioconjugate

Chemistry 24, 1612−1623.

New Raman modes in graphene layers using 2 eV excitationAchintya Bera, Biswanath Chakraborty, Dipti Ranjan Mahapatra, Harini Barath and A. K. Sood

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

We show selective excitation energy (EL) dependent Raman spectral features in PMMAtreated single, bilayer and trilayer graphene samples. New Raman modes at 1529 cm−1 and1445 cm−1 are observed only for excitation energy of 1.96 eV. The modes are not observed inpristine samples. The intensity of these modes are found to scale with disorder as measuredby the intensity of the D-band. Significantly the D band is shown to split into two sub-bandswhen red excitation (1.96 eV) is used. Raman images using EL = 1.96 eV show that the Dmode is present at the edges as well as in the central portion of the samples which otherwisedoes not exhibit D band with EL = 2.41 eV. When annealed at 400 0C, the modes disappear.We propose that graphene-PMMA clusters are formed all over the samples resulting inhydrogen terminated edges. The new modes originated from the H-terminated armchair (1529cm−1 ) and zigzag (1445 cm−1 ) edges of the clusters. The resonant behaviour has beenexplained by the resemblance of the edge structure with trans and cis-polyacetylene whichtoo has optical band gap in 2 eV range. The results are of particular importance in regard todevice fabrication where treatment with PMMA is unavoidable.