DEPARTMENT OF PHYSICSIN-HOUSE SYMPOSIUM 2011

INDIAN INSTITUTE OF SCIENCEBANGALORE 560012

(NOVEMBER 26, 2011)

FOREWORD

As a periodic review of its activities, the Department of Physics has been organizing In-house Symposium on annual basis during recent years. This one-day symposium usually consists of oral presentations by faculty members, post-docs and students, and poster presentations by all those who would like to present their recent results. This is the first Inhouse Symposium in the New Physical sciences building. This year we have a total of 19 talks and 51 posters. I hope this package would be a reasonable representation of the ongoing research activities in the department. This event is also particularly useful to freshers to familiarize themselves with the current research activity in our Department in various branches of Physics.

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

Prof. H. R. KrishnamurthyChairmanNovember 26, 2011

Department of Physics, IISc Bangalore

In-house Symposium 2011

November 26, 2011

Auditorium, New Physical Sciences Building

Programme

Session I 9:00-10:30 Chair: H. R. Krishnamurthy

T01 9:00-9:30 Sriram Ramaswamy

Motile, shaken, stuck and relaxed

T02 9:30-9:45 Aveek Bid

T03 9:45-10:00

T04 10:00-10:15

T05 10:15-10:30

10:30-11:00 Tea

Session II 11:00-01:00PM

T06 11:00-11:15

T07 11:15-11:30

T08 11:30-11:45

T09 11:45-12:00

Neutral modes in the fractional quantum hall regime

M. Haridas

Optical properties of semiconducting QD- metal nanoparticle hybrid arrays

Mattaparthi Venkata Satish Kumar

Structure of DNA-functionalized dendrimer nanoparticles

Anurag Misra

Crystal structure of a peptide inhibitor of human islet amyloid polypeptide (hIAPP) fibrillization: Implications for the treatment of Type II diabetes

Chair: Tarun Deep Saini

K. P. Ramesh

Ultra high field and temperature dependence of NMR T

1 and charge transport studies in organic

conductor (PF6- doped p3MT)

Amal Medhi

Synchronous and asynchronous Mott transitions in topological insulator ribbons

Vinod E. M.

A composition dependence study on (GeTe)1-xSex

chalcogenide alloys

V. S. Manu

Use of genetic algorithm for quantum information processing by NMR

T10 12:00-12:15

T11 12:15-12:30 Sayantan Majumder

T12 12:30-12:45

T13 12:45-1:00

1:00-2:00 Lunch

Session III 2:00-3:15 Poster Session

Session IV 3:15-4:00

T14 3:15-3:30

Using light and sound to study electrons in Helium

T15 3:30-3:45

T16 3:45-4:00

4:00-4:30 High Tea

Session V 4:30-5:30

T17 4:30-4:45

Gas physics of galaxy clusters

T18 4:45-5:00

T19 5:00-5:15

5:15-5:30

Bidya Binay Karak

Modelling Maunder minimum using a dynamo model

Discontinuous shear thickening in confined dilute carbon nanotube suspensions

Sakshath S

A new magnetic memory concept based on ferromagnetic nanorings

Biswanath Chakraborty

Phonon softening in top gated single layer MoS2 field effect transistor: in-situ Raman scattering and density functional theory

Chair: Prabal K. Maiti

Ambarish Ghosh

Saroj Kumar Nandi

Jamming and large deviations in micellar gels: a model

Nitin P. Lobo

Adiabatic cross-polarization applied to solid state NMR experiments

Chair: Vijay B. Shenoy

Prateek Sharma

Atindra Nath Pal

1/f noise as a probe to investigate the band structure of graphene

Mogurampelly Santosh

Nucleic acid interaction with carbon nanotube and graphene

Concluding Remarks, Best Poster Award and Vote of Thanks

List of Posters

No. Presenter Title

P01

P02

P03

P04

P05

P06

P07

P08

P09

P10 Yogeshwar P. Saraswat

P11

P12

P13

P14

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

R. V. Sudheer KumarSolid state NMR Methodological Development for Structural Characterization

S. M. Kamil Validity of Dynamical Density Functional Theory

Nitin KumarSymmetry properties of Large-Deviation Functions of the Velocity of a Self-Propelled Granular Rod

Amit Kumar Majhi Electroporation of cells with very short electric pulses

Medini PadmanabhanOpto-electronic properties of graphene-semiconductor hybrids

Chinkhanlun Guite

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

Bidisha Nandy DNA compaction by dendrimer

S. M. Mohanasundaram

Over 100-fold increase in strain sensitivity of a metal based piezoresistive MEMS transducer through nanoscale inhomogenization

Fermionic Superfluid State in an Optical Lattice via Band-Insulator to Superconductor Transition

Aditya N. Roy Choudhury A 10 Tesla Table-top Controlled Waveform Magnet

Saquib ShamimUnconventional noise in two dimensional doped Silicon

Manjari Gupta

Real Space Distribution of Ultra-cold Bosons in an Optical Lattice at a Finite Temperature Using Strong Coupling Expansion

M. A. AamirTuning between an antidot lattice and quantum dot lattice in a double-gated GaAs/AlGaAs heterostructure

P15 Jayantha P. Vyasanakere

P16

P17

P18 Sudeep K. Ghosh

P19

P20

P21

P22

P23

P24

P25

P26

P27

P28

P29

P30

P31

P32 Membrane coupled to Active Fluid

P33

Ultracold Fermions with Artificial Rashba Spin-Orbit Coupling

Y. Jayasubba ReddyHeteronuclear Correlation between Carbon and Double Quantum Proton Chemical shifts in Solids

Semonti Bhattacharyya Topological Insulator: Basic concepts and preliminary results

Trapped fermions in a synthetic non-Abelian gauge field

Siddharth KhareMicrofluidic Devices For Measuring and Exerting Micro-Newton Forces For Biological Applications

Subhamoy Ghatak Nature of Electronic States in Ultrathin MoS2

Field Effect Transistor

Gurucharan V. Karnad Design of a polymer based Infra-red sensor

Vidya Kochat1/f noise as a probe to investigate the band structure and quantum interference in graphene

Tapan Chandra Adhyapak Active smectics

Achintya BeraRaman signatures of pressure induced electronic topological and structural transitions in Bi2Te3

Debabrata PramanikDNA and Dendrimer assisted dispersion of nanotubes

Sandip Mondal Optical and Electrical Investigation of CdTe Qds/PDDA Bistable Devices

T Phanindra SaiFabrication of tunable potential barrier in Bilayer graphene

Vishal Maingi DBT: A versatile Dendrimer Building ToolKit

KowsalyaDevi PavuluriDevelopment of Methodologies in Ex-Situ NMR Spectroscopy

Marsha M. Parmar Strongly correlated transport in ultrathin gold nanowires

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

Ananyo Maitra

K. S. BhagyashreeTemperature Dependent Sign Reversal of Magnetocrystalline Anisotropy in Nanoparticles of La0.875Sr0.125MnO3

P34

P35

P36 Amal Medhi

P37

P38

P39 Multi Electron Bubbles in Liquid Helium

P40 Naveen Jingade

P41

P42

P43

P44

P45

P46 Sapam Ranjita Chanu

P47

P48

P49

P50

P51 Vijay B. Shenoy

Ashoka Bali Thermoelectric properties of PbTe with Bi precipitates

M. PrashanthaEvidence for shift of rigidity percolation to higher coordination numbers

Topological Insulator on the Edge: Boundary Conditions Revisited

Dona CherianStructural and magnetic transition in Fe1+yTe single crystals

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

Vaisakh V

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

Himanshu Joshi Structure of DNA Nanotubes

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

Shibu SawConfinement Induced Density Modulation and Spatially Resolved Dynamics of Confined Liquids

Ruchika Yadav Structural and magnetic properties of Nd1-xYxMnO3 (0.1 x≤ 0.≤ 6)

Swarna M Patra Structure of Cytolysin A (ClyA) pore in Lipid Bilayer

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

Debarghya BanerjeeReal-space manifestations of Energy-spectra Bottlenecks: Insights from Hyperviscous Hydrodynamical Equations

Sakshath SIn-plane magnetic anisotropy in epitaxial ultrathin Fe films

Rupamanjari Majumder

Scroll-wave dynamics in human cardiac tissue: lessons from a mathematical model with inhomogeneities and fiber architecture

Pramod Kumar Verma Density functional studies of electrons and phonons in pyrochlores

“Upper Branch Fermi Gas and Tan's ”theorem

TALK ABSTRACTS

Motile, shaken, stuck and relaxed

Sriram Ramaswamy

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

Neutral modes in the fractional quantum hall regime

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

Current propagates in the fractional quantum Hall effect (FQHE) regime along the edges of the two-

dimensional-electron gas (2DEG) via chiral edge modes with a chirality dictated by the applied magnetic field.

For some fractional states so called, holes-conjugate states), such as ½<vb<1 of the lowest Landau level, and in

particular vb=2/3, early predictions suggested the presence of counter propagating edge modes, a downstream

mode with the expected chirality, and an upstream mode with an opposite chirality. Since experiments did not

find upstream propagating edge modes, Kane & Fisher suggested that in the presence of disorder and

interactions edge reconstruction may take place with resultant charge modes accompanied by neutral modes -

with the latter carrying only energy. In holes-conjugate fractional states the upstream modes are expected to be

neutral, thus explaining why they were not detected thus far. Moreover, a neutral upstream Majorana mode is

also expected for selected wavefunctions proposed for the even denominator state v=5/2. Here we report on the

direct observation of upstream neutral modes in vb=2/3, vb=3/5, vb=5/3 & vb=5/2. This was done by allowing

these modes to impinge on a quantum point contact (QPC) constriction, with the observed following effects: (a)

A resultant shot noise being proportional to the applied voltage on the injecting contact; (b) With simultaneously

partitioning also a charge mode, the presence of the neutral mode was found to affect significantly the Fano

factor and the temperature of the partitioned charged mode. In particular, for the vb=5/2 fractional state, this

observation may single out the non-abelian wavefunctions for the quasiparticles provided one assumes that the

plausible abelian states lack upstream neutral modes.

Optical properties of semiconducting QD- metal nanoparticle hybrid arrays. M. Haridas and J. K. Basu

Soft nano Materials Physics Laboratory

Department of Physics.

Controlling the emission properties of materials has been a challenging task for the material

physicist over the past several decades. Recent interests in this field have been mainly

focused on the study of interaction between different types of nano particles. A system

consisting of metal nanoparticle (MNP) and semiconducting quantum dot(QD) is an example

for such system. The interaction between surface plasmons of metal nanoparticles and

excitons in semiconductors generates unique optical properties which is not achievable with a

single type of nano structure. Optical properties can be tuned by various parameters like

interparticle distance, density and spectral positions of QDs with MNPs. However the main

challenge is to prepare the ordered arrays with well-defined periodicities. Self-assembly

process emerges as a simple and versatile method because of their simplicity and reliability

over other methods for the preparation of such arrays. We have prepared QD arrays and

hybrid arrays using self assembly technique with di block copolymer as template. Chemically

synthesised cadmium selenide (CdSe) QDs were dispersed inside the particular block by

tuning the chemical property of the capping legends. Hybrid arrays were also prepared using

the same template with polymer capped gold nanoparticles (Au NPs) dispersed inside the

other block. Density and dispersion of Au NPs were varied by controlling the grafting density

of the capping polymer. This method provides better control over the density and interparticle

distance for the arrays. Emission properties of the ordered arrays were studied spatially

resolved and time resolved spectroscopy techniques.

Photoluminescence (PL) spectra collected from the hybrid arrays shows

enhancement/quenching with respect to the density and dispersion of Au NPs in the PS block

compared to the corresponding QD films. Hybrid films with lower grafting density of Au

NPs shows systematic enhancement in the PL emission, while the PL spectra from the

samples with higher polymer grafting density Au NPs is quenched with respect to increase in

Au NP density. Exciton lifetime for the hybrid arrays were estimated from the

photoluminescence decay measurements, by fitting a double exponential to the decay profile.

The longer time corresponds to radiative electron hole recombination in isolated CdSe QD,

while the shorter time seems to originate from collective interaction amongst QDs. The

lifetime data shows systematic reduction in t1 and t2 with increasing volume fraction of Au

NPs. Similarly, the amplitude (a1, a2) of the two relaxations switch over with increase in

volume fraction of Au NP. It is clear that the shorter time t1, becomes the dominant relaxation

mode with increasing φAu, and is affected much more than t2, due to incorporation of Au NP.

However samples with higher polymer grafting density of Au NPs shows slower decay

compared to the samples with same volume fraction of Au NPs with lower polymer grafting

density and same volume fraction of CdSe QDs. Variation of lifetime with respect to the

volume fraction of Au NPs is less compared to the films with Au NPs of lower grafting

density. The observed variation in the optical properties has been explained in terms of the

energy transfer between CdSe QDs and Au NPs.

References

M. Haridas and J. K. Basu, Nanotechnology, 21, 415202 (2010).

M. Haridas, J. K. Basu, D. J. Gosztola and G. P. Wiederrecht, Appl. Phys. Lett, 97,

083307 (2010).

Structure of DNA-Functionalized Dendrimer Nanoparticles

Mattaparthi Venkata Satish Kumar and Prabal K Maiti

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

Abstract

Atomistic molecular dynamics simulations have been carried out to reveal the

characteristic features of ethylenediamine (EDA) cored protonated (corresponding to neutral pH)

poly amido amide (PAMAM) dendrimers of generation 3 (G3) and 4 (G4) that are functionalized

with single stranded DNAs (ssDNAs). The four ssDNA strands that are attached via alkythiolate

[-S (CH2)6-] linker molecule to the free amine groups on the surface of the PAMAM dendrimers

observed to undergo a rapid conformational change during the 25 ns long simulation period.

From the RMSD values of ssDNAs, we find relative stability in the case of purine rich (having

more adenine and guanine) ssDNA strands than pyrimidine rich (thymine and cytosine) ssDNA

strands. The degree of wrapping of ssDNA strands on the dendrimer molecule was found to be

influenced by the charge ratio of DNA and the dendrimer. As G4 dendrimer contains relatively

more positive charge than G3 dendrimer, we observe extensive wrapping of ssDNAs on the G4

dendrimer than G3 dendrimer. This might indicate that DNA functionalized G3 dendrimer is

more suitable to construct higher order nanostructure. The ssDNA strands along with the linkers

are seen to penetrate the surface of the dendrimer molecule and approach closer to the center of

the dendrimer indicating the soft sphere nature of the dendrimer molecule. The effective radius

of DNA-functionalized dendrimer nanoparticle was found to be independent of base composition

of ssDNAs and was observed to be around 19.5 Å and 22.4 Å when we used G3 and G4

PAMAM dendrimer as the core of the nanoparticle respectively. The observed effective radius of

DNA-functionalized dendrimer molecule apparently indicates the significant shrinkage in the

structure that has taken place in dendrimer, linker and DNA strands. As a whole our results

describe the characteristic features of DNA-functionalized dendrimer nanoparticle and can be

used as strong inputs to design effectively the DNA-dendrimer nanoparticle self-assembly for

their active biological applications.

Crystal structure of a peptide inhibitor of human islet amyloid polypeptide (hIAPP) fibrillization: Implications for the treatment of Type II diabetes

Anurag Misraa, Aseem Mishrab, T. Vaishnavi Murthyb, Madhavi Guptab, Virander Singh Chauhanb

Suryanarayanarao Ramakumara, aDepartment of Physics, Indian Institute of Science, Bangalore-560012, India.

bInternational Centre for Genetic Engineering and Biotechnology, New Delhi-110067, India. E-mail: anurag@physics.iisc.ernet.in

Type-2 diabetes mellitus (T2DM), an endocrine metabolic disorder affects more than 100

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

Ultra high field and temperature dependence NMR T1 and charge

transport studies in Organic Conductor ( −6PF doped p3MT).*

K. Jugeshwar Singh1, W.G. Clark2 G. Gaidos2, W.G. Moulton3, A.P. Reyes3, P. Kuhns3, J. D.

Thompson4, R. Menon1 and K.P. Ramesh1

1 Department of Physics, Indian Institute of Science, Bangalore-560012, India. 2Department of Physics and Astronomy, UCLA, Los Angeles, California 90095-1547, USA 3National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA 4 Los Alamos National Laboratory, Los Alamos, NM 87545, USA Email: jshwar@physics.iisc.ernet.in; kpramesh@physics.iisc.ernet.in

Ultra high field and ultra low temperature charge transport phenomenon is studied in doped

organic conductor, poly 3 methylthiophene (p3MT) using NMR relaxation rates (1/T1), magnetic

susceptibility and electrical conductivity techniques. The magnetic susceptibility data show

contributions from both Pauli and Curie spins, with the size of the Pauli term depending strongly

on the doping level. Proton NMR relaxation (1H-T1) measurements are carried out over a wide

temperature (T) range 3-300 K at magnetic fields, (B): 0.9, 9.0, 16.4 and 23.4 T, and fluorine

NMR relaxation (19F-T1) measurements were done at B = 9 T over the range 3 < T < 300 K. The

doping level has been varied to enable to investigate the role of carrier density and electron-

electron interactions (EEI) on the relaxation mechanism. Three dominant mechanisms, (i) due to

conduction electron, (ii) via spin diffusion to the paramagnetic centers (SDPC) and (iii) due to

reorientation motion of symmetric subgroups, are observed and each mechanism is found to

dominate differently at different temperature regions. Frequency dependence of T1 shows that

system behaves as a quasi 1-Dimensional (q1-D) at high temperatures and 3-D at low

temperatures. 19F-T1 shows Intra chain diffusion of charge carriers other than

reorientation. The cross relaxation among the

−6PF

1H and 19F nuclei give the more insight into the

role of both inter-chain and intra-chain conduction mechanisms. Further, a good correlation

between electrical conductivity, magnetic susceptibility and NMR T1 data has been observed.

* This work is funded by the DST and NSF.

1

Synchronous and Asynchronous Mott Transitions in Topological Insulator Ribbons

Amal Medhi,∗ Vijay B. Shenoy,† and H. R. Krishnamurthy‡

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

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

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

∗ amedhi@physics.iisc.ernet.in

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

A composition dependence study on (GeTe)1-xSex chalcogenide alloys

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

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

Abstract:

In the age of technology revolution human modern living standard requires to efficient

and cheaper memory materials for the devices. Chalcogenide based memory materials

can full fill this requirement. Therefore, this study demonstrate the synthesis and

characterizations of bulk (GeTe)1-xSex (X = 0.02, 0.10, 0.20 and 0.50) chalcogenide

glasses. Obtained outcomes from X-ray Diffraction (XRD), Differential Scanning

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

Scanning Electron Microscopy (SEM) measurements reveal that alloys structures

changes with additive element (Se) concentration. Immiscibility of Ge-Te and Ge-Se

phases have been verified at lower concentration of Se (X = 0.02, 0.10, 0.20) while a

miscible single phase GeTeSe homogeneous amorphous structure is obtained for

optimum composition (X = 0.50) alloy. Structural changes in described materials could

be explained with help of bond theory of solids.

Use of Genetic Algorithm for Quantum Information Processing by NMR

V. S. Manu and Anil Kumar

Abstract

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

Modelling Maunder minimum using a dynamo model

Bidya Binay Karak* and Arnab Rai Choudhuri

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

______________________

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

Discontinuous shear thickening in confined dilute carbon nanotube suspensions Sayantan Majumdara, Rema Krishnaswamyb, and A. K. Sooda, b

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

Abstract A monotonic decrease in viscosity with increasing shear stress is a known rheological response to shear flow in complex fluids in general and for flocculated suspensions in particular. Here we demonstrate a discontinuous shear-thickening transition on varying shear stress where the viscosity jumps sharply by four to six orders of magnitude in flocculated suspensions of multiwalled carbon nanotubes (MWNT) at very low weight fractions (approximately 0.5%). Rheo-optical observations reveal the shear-thickened state as a percolated structure of MWNT flocs spanning the system size. We present a dynamic phase diagram of the non-Brownian MWNT dispersions revealing a starting jammed state followed by shear-thinning and shear-thickened states. The present study further suggests that the shear-thickened state obtained as a function of shear stress is likely to be a generic feature of fractal clusters under flow, albeit under confinement. An understanding of the shear-thickening phenomena in confined geometries is pertinent for flow-controlled fabrication techniques in enhancing the mechanical strength and transport properties of thin films and wires of nanostructured composites as well as in lubrication issues. References 1. Sayantan Majumdar, Rema Krishnaswamy, and A.K. Sood, PNAS, 108, 8996-9001 (2011).

A new magnetic memory concept based on Ferromagnetic nanorings

Sakshath S and P S Anil KumarDepartment of Physics

Indian Institute of ScienceBangalore 560012; India

Ferromagnetic rings and wires, on a mesoscopic scale show different stable domain

configurations when subjected to external magnetic fields. These different configurations can be

used to implement digital memory states. In addition, magnetic effects induced by the spin torque of

a spin polarised electric current can be used to switch between different magnetic configurations.

Resistance of a nanoscale ferromanetic wire or ring depends on the presence or absence of a domain

wall. In materials where the AMR effect is higher, the presence of a domain wall decreases the

resistance whereas in low AMR materials, the effect of domain wall is opposite. We have made

Cobalt and NiFe nano-rings with ferromagnetic contacts and demonstrated a new memory device

with well defined resistance states that can be electrically controlled. We also propose that three

dimensional integration is possible.

Phonon softening in top gated single layer MoS2 field effect

transistor: in-situ Raman scattering and density functional theory

Biswanath Chakraborty1∗, Achintya Bera1, D. V. S. Muthu1, Somnath

Bhowmick2, U. V. Waghmare2, A. K. Sood1, and C. N. R. Rao2

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

2 Chemistry and Physics of Materials Unit,

Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore-560064, India

Abstract

Phonon dynamics, particularly electron-phonon coupling plays an important role in transport

properties of semiconductors where the mobility is largely affected by scattering from optical

phonons. Raman scattering has already been established to probe electron-phonon interaction in

doped materials ranging from bulk semiconductors to one dimensional carbon nanotube through

two dimensional graphene. Here we report an in-situ Raman scattering study of an electrochemi-

cally doped single layer of MoS2. The zone center A1g phonon shows a softening with simultaneous

broadening as the doping increases while the other Raman active zone center E12g mode is almost

inert to increasing carrier concentration. We use first-principles density functional theory based

calculations to develop understanding of why the A1g mode specifically exhibits a strong electron

phonon coupling, and hence the sensitivity to electron doping. Electrical characterization of a

similar top gated single layer device shows an high on-off ratio ∼ 105.

∗ electronic mail:biswanath@physics.iisc.ernet.in

1

Using light and sound to study electrons in Helium

Ambarish Ghosh

Jamming and large deviations in micellar gels: a model

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

1Centre for Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560012, India

2Martin Fisher School of Physics, Brandeis University,

Mail Stop 057, Waltham, Massachusetts 02454-9110, USA

3Department of Physics, Indian Institute of Science, Bangalore 560012, India

Abstract

We present a simple model for the rheological behavior observed in some recent experiments

performed on micellar gels. The model combines attachment-detachment kinetics with stretching

due to shear, and shows well-defined jammed and flowing states. The large deviation function

(LDF) for the coarse-grained velocity becomes increasingly non-Gaussian with larger applied forces.

The power fluctuations are found to obey a steady-state fluctuation relation. We argue that a wider

class of physical problems with attachment-detachment kinetics can be understood in terms of the

present theory.

∗Electronic address: snandi@physics.iisc.ernet.in†Electronic address: bulbul@brandeis.edu‡Electronic address: sriram@physics.iisc.ernet.in; Also at: JNCASR, Bangalore 560 064 India§Electronic address: asood@physics.iisc.ernet.in

1

Adiabatic Cross-Polarization Applied to Solid State NMR Experiments

Nitin P. Lobo1 and K. V. Ramanathan2

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

Abstract

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

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

(8) S. Caldarelli, M. Hong, L. Emsley and A. Pines, J. Phys. Chem. 100, 18696 (1996). 

Gas Physics of Galaxy Clusters

Prateek Sharma

I will describe the main area of my current research, the baryonic physicsof galaxy clusters. Structures in the universe form because of gravitationalinstability in an expanding universe. Gravitationally bound structures, dom-inated by dark matter, form in a bottom up fashion with the smallest struc-tures forming the earliest. Larger and larger structures form due to mergersof smaller halos and filaments. Galaxy clusters are the most massive grav-itationally bound objects in the universe, reaching up to 1015 solar masses.The gravitational force (and mass) in galaxy clusters is dominated by darkmatter, and baryons are only 15% of the total mass. In addition, most of thebaryons are in the form of diffuse, hot plasma called the intracluster medium(ICM); stars in constituent galaxies have less than 10% of baryons.

Galaxy clusters serve as independent probes of cosmological parameterssuch as dark energy (accelerated expansion of the universe). However, theobserved propertied such as X-ray luminosity, temperature, etc. have to becalibrated with the cluster mass dominated by dark matter. In addition tobeing cosmological probes, galaxy clusters are also home to most massivegalaxies and black holes. The hot gas in cluster cores has a short coolingtime and is expected to cool and form stars at a tremendous rate. However,this is not observed; the star formation and cooling rates are 10-100 timessmaller than expected. It is now believed that massive black holes in clustercenters heat the cooling cluster cores in a feedback loop. More the cooling,more is the amount of cooling gas that can feed the black hole, and more isthe core heating.

I will describe an idealized model of this interplay of heating and coolingin cluster cores and show that a lot of observations can be explained ina physical framework. I will also describe some of the research projectsthat I am involved in and intend to carry out in near future. Most of mywork involves numerical simulations of physical processes, such as turbulence,cooling, heating, winds, magnetic fields, etc. in the context of astrophysics.Studying such processes in idealized setups has a far-reaching impact onunderstanding wide range of astrophysical phenomena.

1/f noise as a probe to investigate the band structure of graphene

Atindra Nath Pal* and Arindam Ghosh

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

The flicker noise or low frequency resistance fluctuations in graphene depends explicitly on its

ability to screen external potential fluctuations and more sensitive compared to the conventional

time average transport. Here we show that the flicker noise is a powerful probe to the band

structure of graphene that vary differently with the carrier density for the linear and parabolic

bands. We have used different types of graphene field effect devices in our experiments which

include exfoliated single and multilayer graphene on oxide substrate, freely suspended single

layer graphene, and chemical vapor deposition (CVD)-grown graphene on SiO2. We find this

difference to be robust against disorder or existence of a substrate. Also, an analytical model has

been developed to understand the mechanism of graphene field effect transistors. Our results

reveal the microscopic mechanism of noise in Graphene Field Effect Transistors (GraFET), and

outline a simple portable method to separate the single from multi layered graphene devices.

References

1. Atindra Nath Pal and Arindam Ghosh, Physical Review Letters 102, 126805 (2009). 2. Atindra Nath Pal and Arindam Ghosh, Appl. Phys. Lett., 95, 082105 (2009). 3. Atindra Nath Pal, Ageeth A. Bol, and Arindam Ghosh, Appl. Phys. Lett. 97, 133504

(2010). 4. Atindra Nath Pal et al., ACS Nano 5, 2075 - 2081 (2011).

Nucleic Acid Interaction with Carbon Nanotube and Graphene

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

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

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

Abstract

(Dated: November 17, 2011)

In an effort to design efficient platform for siRNA delivery, we combine all atom classical and

quantum simulations to study the binding of small interfering RNA (siRNA) by pristine single wall

carbon nanotube (SWCNT) and graphene. Our results show that siRNA strongly binds to SWCNT and

graphene via unzipping its base-pairs and the propensity of unzipping increases with the increase in the

diameter of the SWCNTs and maximum for graphene. The unzipping and subsequent wrapping events

are initiated and driven by van der Waals interactions between the aromatic rings of siRNA

nucleobases and the SWCNT/graphene surface. However, MD simulations of double strand DNA

(dsDNA) of the same sequence show that the dsDNA undergoes much less unzipping and wrapping on

the SWCNT/graphene in the simulation time scale of 70 ns. This interesting difference is due to smaller

interaction energy of thymidine of dsDNA with the SWCNT/graphene compared to that of uridine of

siRNA, as calculated by dispersion corrected DFT methods. After the optimal binding of siRNA to

SWCNT/graphene, the complex is very stable which serves as one of the major mechanisms of siRNA

delivery for biomedical applications.

POSTER ABSTRACTS

Mechanical properties of ZnS nanowires and thin films:

Microscopic origin of the dependence on size and growth direction

Taraknath Mandal, Prabal K Maiti and Chandan Dasgupta

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

Bangalore 560012

Abstract

Mechanical properties of ZnS nanowires and thin films are studied as a function of size and

growth direction using all-atom molecular dynamics simulations. Using the stress-strain

relationship we extract the Young's moduli of nanowires and thin films at room temperature. Our

results show that the Young's modulus of [0001] nanowires has a strong size dependence. On the

other hand, 0110][ nanowires do not exhibit a strong size dependence of the Young's modulus in

the size range we have investigated. We provide a microscopic understanding of this behaviour

on the basis of bond stretching and expansion due to the rearrangement of atoms in the surface

layers. The ultimate tensile strengths (UTS) of the nanowires do not show much size

dependence. To investigate the mechanical behaviour of ZnS in two dimensions, we calculate the

Young's modulus of thin films under tensile strain along the [0001] direction. The Young’s

modulus of thin films converges to the bulk value more rapidly than that of the [0001] nanowire.

Solid state NMR Methodological Development for Structural

Characterization

R.V.Sudheer Kumar

NMR Research Centre, Department of Physics

Indian Institute of Science, Bangalore-560012, India.

Solid state NMR has developed into one of the most informative and direct

experimental approaches for structural characterization, particularly of peptides

and proteins. There are several techniques such as REDOR, DRAWS, 13C MQ

NMR, fp-RFDR-CT etc are available for carrying out such studies. Based on these

techniques complete high resolution three dimensional structures can be

determined which forms the way for further biomedical applications. With this

goal, we have started producing simulated powder patterns in solids using Matlab

by considering chemical shift anisotropy, dipolar coupling and dipolar dephasing

curve of REDOR pulse sequence etc. Experimental skills required for carrying out

the above studies are being acquired. Such experiments are proposed to be carried

out on systems such as peptides and later on amyloid fibrils and discussion with

various groups interested in the area is in progress.

The second part of the study is to obtain proton chemical shifts in solid state

indirectly. For this we propose to use Deuterium NMR. However deuterium is a

spin 1 nucleus and gives rise to quadrupolar broadening. The use of magic angle

spinning gives rise to centre band and side bands. If the spinning speed is fast

enough, then side bands occur far away and do not interfere with chemical shift

range of deuterium. Studies on the methods for polarization transfer between

heteronuclei for static oriented systems are also started. Then we can establish

correlation between deuterium and its attached carbon in a deuterated liquid

crystal. Experimental results and further plan of work are under progress.

Validity of Dynamical Density Functional Theory

S. M. Kamil and Chandan Dasgupta

Abstract

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

1

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

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

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

Abstract

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

Reference :

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

_____________ 

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

Electroporation of cells with very short electric pulses

Amit Kumar Majhi and V. Venkataraman

The application of short electric field pulses makes it possible to render cell

membranes temporarily permeable to substances that otherwise would not be

able to effectively enter the cell interior. This technique known as Electroporation

is widely used in genetic engineering. Typically, millisecond pulses of electric field

strengths are applied such that a transmembrane potential exceeding the natural

cellular transmembrane potential difference of approximately 1V is generated

which results in permeabilization (poration) of cell membranes [1].

A simple electrical model for living cells predicts [2, 3] an increasing probability for

electric field interactions with intercellular substances of both prokaryotic cells

and eukaryotic cells when pulse duration is reduced into sub-microsecond range.

To investigate this, we have carried out Electroporation on yeast cells using

propidium iodide (PI) as a fluorescent marker. Preliminary experiments have been

carried out with pulse widths ranging from 3 milliseconds to 100 microseconds.

The effects on yeast cells due to the pulses are examined with inverted

fluorescence microscope. It is known that PI moves into the cell during

electroporation and emits fluorescence when it binds with nucleic acid. We have

observed the rapid intake of PI immediately after the pulse is applied. In both

cases e.g. millisecond and microsecond pulse, intake of PI happens immediately.

Further experiments are in progress to quantify the effect and also to reduce the

pulse widths to sub-microsecond regime.

References:

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

2. “Nanosecond Pulse Electroporation of Biological Cells: The Effect of Membrane

Dielectric Relaxation”, Elham Salimi et. al.

3. “The Effects of Intense Submicrosecond Electrical Pulses on Cells”, Biophys J. 2003

April; 84(4): 2709–2714.

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

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

Frequency Modulated Spin Polarization

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

We propose and demonstrate a technique for electrical detection of polarized spins in semiconductors in

zero applied magnetic fields. Spin polarization is generated by optical injection using circularly polarized light

which is modulated rapidly using an electro-optic cell. The modulated spin polarization generates a weak time-

varying magnetic field which is detected by a sensitive radio-frequency coil. Using a calibrated pick-up coil

and amplification electronics, clear signals were obtained for bulk GaAs and Ge samples from which an optical

spin orientation efficiency of 4.8% could be determined for Ge with 1342nm excitation wavelength at 127K. In

the presence of a small external magnetic field, the signal decay according to the Hanle-effect, from which a

spin lifetime of 4.6±1.0 ns for electrons in bulk Ge was extracted.

Abstract for Poster Inhouse Symposium 2011

DNA compaction by dendrimer

Bidisha Nandy , Prabal K Maiti

At physiological pH, a PAMAM dendrimer is positively charged and can effectively bind negatively

charged DNA. Currently there has been great interest in understanding this complexation reaction both

for fundamental (as a model for complex biological reactions) as well as for practical (as a gene

delivery material and probe for sensing DNA sequence) reasons. We have studied the complexation

between double stranded DNA (dsDNA) and various generations of PAMAM dendrimers (G3-G5)

through atomistic molecular dynamics simulations in presence of water and ions. We report the

compaction of DNA on nanosecond time scale. This is remarkable given the fact that such a short DNA

duplex of length close to 13 nm is otherwise thought to be a rigid rod. Using several nanoseconds long

MD simulations we have observed various binding modes of dsDNA and dendrimer for various

generations PAMAM dendrimers at varying charge ratio and confirms some of the binding modes

proposed earlier. The binding is driven by the electrostatic interaction and larger the dendrimer charge

stronger the binding affinity. As DNA wraps/binds to the dendrimer, counter ions originally condensed

onto DNA (Na+) and dendrimer (Cl-) get released. We calculate the entropy of counter ions and show

that there is gain in entropy due to counter ion release during the complexation. MD simulations

demonstrate that when the charge ratio is greater than 1 (as in the case of G5 dendrimer) the optimal

wrapping of DNA is observed. Calculated binding energies of the complexation follow the trend

G5>G4>G3 in accordance with the experimental data. For lower generation dendrimer like G3 and to

some extent for G4 also we see considerable deformation in the dendrimer structure due to their

flexible nature. We have also calculated the various helicoidal parameters of DNA to study the effect of

dendrimer binding on the structure of DNA. B form of the DNA is well preserved in the complex as is

evident from various helical parameters justifying the use of PAMAM dendrimer as a suitable delivery

vehicle.

Over 100-fold increase in strain sensitivity of a metal based piezoresistive MEMS transducer through nanoscale

inhomogenization

S. M. Mohanasundaram1, Rudra Pratap

2 and Arindam Ghosh

1

1Department of Physics, Indian Institute of Science

2Center for Nano Science and Engineering, Indian Institute of Science

Abstract:

Metal-based electromechanical sensing devices could find a much wider

applicability if their sensitivity to mechanical strain could be substantially improved. In

spite of direct integrability to suboptical scale structures, convenient interfacing with high

frequency readout, and, above all, simplicity in device fabrication process, metals are

unpopular in micro and nano-electromechanical systems (MEMS and NEMS) because

their strain sensitivity is one to two orders of magnitude lower in comparison to common

semiconductor-based piezoresistors. Here, we demonstrate a simple method to enhance

the strain sensitivity of metal films integrated to specially designed micro-cantilevers by

over two orders of magnitude. By locally inhomogenizing thin gold films using

controlled electromigration, we have achieved a logarithmic divergence in the strain

sensitivity with progressive microstructural modification. The enhancement in strain

sensitivity could be attributed to a non-universal tunneling-percolation transport, which

creates a robust platform to engineer low resistance, high gauge factor metallic

piezoresistors that may have profound impact on nanoscale self-sensing technology.

Fermionic Superfluid State in an Optical Lattice via

Band-Insulator to Superconductor Transition

Yogeshwar P. Saraswat, Vijay B. Shenoy

Abstract

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

the central problems in the area of cold quantum gases due to cooling problem. The idea hinges on

using a characteristically low entropy system, a band-insulator and tuning the interaction in such

a system to realize a superfluid state. By performing a detailed mean field analysis of the model,

we show that the superfluid state is indeed stable and other competing phases like charge density

wave, supersolid, will not come into picture.

1

A 10 Tesla Table-top Controlled Waveform Magnet

Aditya N. Roy Choudhury and V. Venkataraman

Department of Physics, Indian Institute of Science, Bangalore 560012

ABSTRACT

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

REFERENCES

1. D. B. Montgomery, “Pulse Magnets”, in Solenoid Magnet Design, Wiley-Interscience, 1969. 2. L.J. Campbell and J.B. Schillig, “Controlled Waveform Magnets”, in High Magnetic Fields Science

and Technology Vol.1, edited by Fritz Herlach and Noboru Miura, World Scientific Publishing, 2003. 3. R. Grossinger et al, Physica B 294-295, 555-561 (2001). 4. L.J. Campbell et al, Physica B 216, 218-220 (1996). 5. G.S. Boebinger et al, Physica B 294-295, 512-518 (2001). 6. J.J.M. Franse et al, Physica B 201, 217-226 (1994). 7. http://www.irf.com/technical-info/appnotes/an-990.pdf

Unconventional noise in two dimensional doped Silicon

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

Arindam Ghosh1

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

As classical information processing technology approaches the sub-20nm node, it is

becoming increasingly important to control the exact number and position of dopants in

electronic devices. Recent progress in using scanning tunnelling microscopy (STM) as a

lithographic tool allows positioning of dopants with atomic scale precision. Combined with

molecular beam epitaxy, this technology has been employed to realize heavily δ-doped planar

nanostructures, such as tunnel gaps, nanowires and quantum dots. The same approach can

also be used to fabricate vertically-stacked, multiple electrically-active layers. The time

averaged transport properties of Si:P δ-doped layers have now been studied in detail, but very

little is known about its long term charge stability. The importance of this issue is paramount

to the overall development of devices with controlled dopant positioning at the nano-scale

and in particular for single dopant spin based qubits.

We perform low frequency noise measurements in degenerately doped Si:P δ-layers and find

that the noise magnitude in these systems is suppressed by several orders of magnitude as

compared to bulk doped metallic silicon and is one of the lowest values reported for doped

semiconductors. Though magneto-conductivity data at low temperatures indicate weak

localisation, the noise magnitude is nearly constant with magnetic field at low fields. The

noise magnitude reduces by a factor of two at high fields (Zeeman field) in both parallel and

perpendicular magnetic field. This might indicate that the low temperature Hamiltonian of

these heavily doped systems has very different symmetry properties than conventional

disordered conductors.

Real Space Distribution of Ultra-cold Bosons in an Optical Lattice at a

Finite Temperature Using Strong Coupling Expansion

Manjari Gupta∗ and H. R. Krishnamurthy†

Center For Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560012, India

J. K. Freericks‡

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

(Dated: October 28, 2011)

Ultra-cold bosonic atoms trapped in deep optical lattices are well described by the Bose-

Hubbard model, in which atoms can hop to nearest neighbour sites with amplitudet and have

onsite repulsionU . In addition there is an overall harmonic trap present. Thiscauses an effective

chemical potential variation throughout the lattice, which results in coexistence of consecutive

annular regions of Mott and superfluid phases.

Recently, real space density profiles of atoms in optical lattices has been measured experimen-

tally for finite temperature systems. To compare with the experimental data a perturbative strong

coupling (t/U) expansion at finite temperature and finiteU is carried out upto second order about

the Mott phase including superfluidity at mean field level by using mean field decomposition of the

kinetic energy term of the Bose Hubbard model. We have also calculated the entropy per particle,

which is a relevent experimental quantity, upto second order and the dominant contribution to it

from different parts of the overall trap at very low temperatures.

∗Electronic address:manjari@physics.iisc.ernet.in†Electronic address:hrkrish@physics.iisc.ernet.in‡Electronic address:freericks@physics.georgetown.edu

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

Ultracold Fermions with Artificial Rashba

Spin-Orbit Coupling

Jayantha P. Vyasanakere and Vijay B. Shenoy

Cold atomic gases enjoy the prospect of shedding light on outstanding andlongstanding puzzles of quantum condensed matter physics. The recent gener-ation of synthetic Rashba spin-orbit coupling (SOC), which is equivalent to anon-Abelian gauge potential, in 87Rb atoms by the NIST group is a landmarkin that respect. Motivated by these developments, we theoretically investigateinteracting fermions with spin-orbit coupling.

The clue that fermions with SOC admits novel physics stemmed up fromthe study of the two-fermion problem in three spatial dimensions. In absenceof SOC the two-body problem permits a bound state only if the attractionbetween them exceeds a threshold value and the binding energy is independentof their center of mass momentum. Remarkably, SOC induces a two-body boundstate, in the zero center of mass momentum sector, for any attraction, howeversmall. In contrast to this, for center of mass momenta much larger than theSOC, any attraction, however large fails to produce the two-body bound state.The two-body bound state wave function has a triplet content and associatedspin-nematic structure similar to those found in liquid 3He.

In absence of SOC, it is known that the ground state of the weakly attractingmany-body problem is a BCS superfluid with large overlapping pairs. As theattraction is increased, the system undergoes a crossover to a BEC of tightlybound fermion-pairs. Our study reveals that at a fixed attraction, however small,increasing SOC invokes a crossover from the just discussed BCS superfluid stateto a new type of BEC state. The BEC state that emerges is a condensate ofbosons which are tightly bound pairs of fermions. Remarkably, at large SOC,the nature of these bosons is determined solely by the Rashba SOC and is neitherinfluenced by the attraction nor by the density of particles – hence these bosonsare referred to as rashbons. For a general SOC, rashbons have exotic propertieslike anisotropic dispersion.

Our study estimates the transition temperature of the BEC of rashbons andsuggests a route to enhance the exponentially small transition temperature ofthe system with a fixed weak attraction to the order of the Fermi temperatureby tuning the strength of SOC.

References : arXiv: 1101.0411, 1104.5633, 1108.4872.

1

Heteronuclear Correlation between Carbon and Double Quantum Proton

Chemical shifts in Solids

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

a NMR Research Centre, b Department of Physics,

Indian Institute of Science, Bangalore-560012, India

c Corresponding Author : kvr@nrc.iisc.ernet.in

In recent times, Solid State NMR (ssNMR) spectroscopy has dramatically overcome the severe

limitations in resolution and sensitivity that had limited its application to biomolecules and has

established itself as an important tool for studying large and poorly soluble systems such as

membrane proteins and protein fibrils. NMR studies in the solid state have been hampered mainly

by the presence of very large 1H-1H dipolar couplings. Currently available structures in solids

mainly rely on measurements made for rare spins, though 1H chemical shifts and 1H-1H proximities

are being increasingly used for the above purpose. However, direct detection of protons does not

provide enough resolution even with the use of windowed sequences, which require additional time

and effort for optimization. We are, therefore, exploring a method for obtaining correlation between

double quantum proton resonances and single quantum carbon resonances in which one of the

proton is coupled to a carbon. Correlations based on both scalar and dipolar couplings are being

explored. Such experiments have the following advantages, viz., (a) provide more definite

assignments; (b) uncertainties in proton chemical shifts can be reduced by making use of the

redundant information available due to possibilities of several DQ cross peaks present for the same

carbon; (c) provide chemical shifts of protons, bonded to some other atoms such as the amide

protons; (d) enable 13C-13C correlation. The application of this technique to several amino acids and

possible applications to small peptides with definite structures will be presented.

Abstract for poster

Topological Insulator: Basic concepts and preliminary results

Mitali Banerjee1 ,Semonti Bhattacharyya1,Nethra C2,

N. Ravishankar2, Arindam Ghosh1

1Dept. of Physics,2Material Research Centre,

IISc,Bangalore - 560012

Topological insulators, a new quantum state of matter, have conducting surface states apart from insulating bulk states and those surface states are quite robust to disorder. Such surface states have been predicted to host various exotic quantum phenomenon and quasi particle excitations. Angle Resolved Photo Emission Spectroscopy (ARPES) has been an efficient tool to find such surface states in quite a few materials but probing of such states by electrical transport studies has been pretty challenging so far. Preliminary electrical transport measurements are performed on Bi2Se3, BiSbTe3 nanocrystals. These nanocrystals are prepared in microwave-stimulated solvo-thermal method and been characterized by X-ray diffraction(XRD), tunneling electron microscopy (TEM), atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS) etc.

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Trapped fermions in a synthetic non-Abelian gauge field

Sudeep Kumar Ghosh,∗ Jayantha P. Vyasanakere,† and Vijay B. Shenoy‡Centre for Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560 012, India

On increasing the coupling strength (λ) of a non-Abelian gauge field that induces a generalizedRashba spin-orbit interaction, the topology of the Fermi surface of a homogeneous gas of noninter-acting fermions of density ρ ∼ k3

F undergoes a change at a critical value, λT ≈ kF [Phys. Rev. B 84,014512 (2011)]. In this paper we analyze how this phenomenon affects the size and shape of a cloudof spin- 1

2fermions trapped in a harmonic potential such as those used in cold atom experiments. We

develop an adiabatic formulation, including the concomitant Pancharatnam-Berry phase effects, forthe one particle states in the presence of a trapping potential and the gauge field, obtaining approx-imate analytical formulae for the energy levels for some high symmetry gauge field configurationsof interest. An analysis based on the local density approximation reveals that, for a given numberof particles, the cloud shrinks in a characteristic fashion with increasing λ. We explain the physicalorigins of this effect by a study of the stress tensor of the system. For an isotropic harmonic trap,the local density approximation predicts a spherical cloud even for anisotropic gauge field configura-tions. We show, via a calculation of the cloud shape using exact eigenstates, that for certain gaugefield configurations there is systematic and observable anisotropy in the cloud shape that increaseswith increasing gauge coupling λ. The reasons for this anisotropy are explained using the analyt-ical energy levels obtained via the adiabatic approximation. These results should be useful in thedesign of cold atom experiments with fermions in non-Abelian gauge fields. An important spin-offof our adiabatic formulation is that it reveals exciting possibilities for the cold-atom realization ofinteresting condensed matter Hamiltonians by using a non-Abelian gauge field in conjunction withanother potential. In particular, we show that the use of a spherical non-Abelian gauge field witha harmonic trapping potential produces a spherical geometry quantum hall like Hamiltonian in themomentum representation.

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

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

Title: Microfluidic Devices For Measuring and Exerting Micro-Newton Forces For Biological Applications

Author: Siddharth Khare

Research Advisor: Prof. V. Venkataraman

Abstract:

Forces right from pico-newton to mega newton are observed in nature. Usually we do not feel

the forces below a few tens of milli newton since they are too small compared to most of the forces to

which we are subjected. But the microorganisms deal with much smaller forces and their activities are

largely influenced by such forces. Therefore measuring and exerting these small forces is of interest to

biologists.

The forces exerted by microorganisms of sizes around a few 100's of microns are typically in 10's

of micronewton range. We are trying to develop sensors and actuators for measuring and exerting

forces in this range.

We use photo-lithography on a photo-sensitive polymer SU-8 (from Micro-chem). The structures

made in SU-8 are used for replica molding of PDMS, which is a transparent and bio-compatible polymer.

The molded PDMS forms the final device. For actuation, we use Iron-PDMS mixture and external

magnetic field. These devices are flexible pillars of dimensions 50µm X 50µm X 250µm.

For biology part, we are working in collaboration with Prof. Sandhya Kaushika (NCBS). We use

the micro-force sensors and actuators to probe a nematode called Caenorhabditis Elegans in order to

study the neuro-biological problems.

References:

1. Joseph C. Doll et. al., Lab Chip, 2009, 9, 1449–1454

2. F M Sasoglu et al., J. Micromech. Microeng. 17 (2007) 623–632

3. Jimmy le Digabel et. al., Lab Chip, 2011, 11, 2630

4. S. N. Khaderi et. al., Lab Chip, 2011, 11, 2002

Nature of Electronic States in Ultrathin MoS2 Field Effect Transistor

Subhamoy Ghatak, Atindra Nath Pal and Arindam Ghosh

Molybdenum disulphide (MoS2) is a transition metal dichalcogenide. It is a layered material

with a Mo layer sandwiched between two S layers (S-Mo-S), which forms the basic unit of its

trigonal prismatic structure. Though the intra-layer bonding in a unit is quiet strong, each S-

Mo-S unit is attached to other S-Mo-S units only with weak Van der Waals force. This

enables to make an atomically thin single layer of MoS2. It is an indirect bandgap (1.2 eV)

semiconductor in bulk, and becomes a direct bandgap (1.9 eV) semiconductor in single layer

form. The presence of bandgap has made it an interesting material in thin film transistors as

channel material. It has been reported [1] recently that very high on/off ratio (~108) can be

obtained in single layer MoS2 transistor due to the presence of this bandgap. But though the

on/off ration is very high, mobility in these transistors is considerably law compared to those

commercial Si transistors. Here we have investigated the origin of such low mobility. From

our temperature dependent study we find that atomically thin MoS2 layer becomes highly

disordered in the presence of the substrate and electron got localised in the traps created by

the charge impurities at substrate-MoS2 interface. We propose that high mobility can be

obtained in these transistors by removing the charge impurity background.

[1] Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Nature

Nanotechnology 2011, 6, 147–150.

Email: ghatak@physics.iisc.ernet.in

Design of a polymer based Infra-red sensor

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

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

Cell (1). It is based on the detection of the deflection of a membrane caused due to the thermal expansion of the

gas trapped in a cavity. The temperature in the cavity is elevated due to the radiations absorbed by the absorber

material, which in turn heats the gas, thus causing it to expand. While this principle has been well demonstrated

in the form of a single element Golay cell, there has not been research demonstrating an array of Golay cells

due to the inherent thermal cross talk produced due to high thermal conductivity of silicon. The low thermal

crosstalk in PDMS based arrayed devices has been demonstrated (2) in research involving design of polymer

based micro-mirror devices.

We therefore propose to build an array of Golay Cells using Poly(dimethylsiloxane) (PDMS) to reduce

the thermal crosstalk. We have fabricated a PDMS based sensor by replica molding of the SU-8 structures,

which were used as masters. Carbon Nanotubes (2) have been used to act as the infra-red absorber material,

while a 60µm PDMS membrane has been used as the detection membrane. Preliminary results, involving the

deflection of the detection membrane with variation in external temperature have been observed and measured

using a Coherence Correlation Interferometer.

References1. Pneumatic Heat Detector . Golay, Harold A. Zahl AND Marcel J. E. 11, s.l. : The Review of ScientificInstruments , November,1946, Vol. 17.2. Thermo-pneumatically actuated, membrane-based micro-mirror devices. Zappe, Armin Werber and Hans.s.l. : J. Micromech. Microeng, 2008, Vol. 16.3. Multiwall carbon nanotube absorber on a thin-film lithium niobate pyroelectric detector. John H. Lehman,Katherine E. Hurst, Antonije M. Radojevic, Anne C. Dillon, Richard M. Osgood, Jr. 7, s.l. : OpticsLetters, April, 2007, Vol. 32.

1/f noise as a probe to investigate the band structure and quantum interference in graphene

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

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

Abstract

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

Active smectics

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

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

2Department of Physics, Indian Institute of Technology Madras, Chennai 600 036, India

3Department of Physics and Institute of Theoretical Science,University of Oregon, Eugene, OR 97403, USA

(Dated: October 28, 2011)

We show that smectic liquid-crystalline order in systems of active particles is long-ranged indimensions d = 3 and quasi-long-range in d = 2.This is in contrast to the situation for systems atthermal equilibrium with the same spatial symmetry. We find that strong enough active stressescan disrupt smectic order, through instabilities whose character is oscillatory if the stresses areextensile. Our results apply to any active system that spontaneously develops one-dimensionalspatial periodicity, including driven monolayers of rods and the Rayleigh-Benard instability.

Ramansignaturesofpressureinducedelectronic

topologicalandstructuraltransitionsinBi2Te3.

Gopal K Pradhan, Achintya Bera, Pradeep Kumar, D V S Muthu

and A K Sood.

Department of Physics, Indian Institute of Science, Bangalore - 560012, India In recent years, high-pressure studies of technological important Bi2Te3 have

revealed giant improvement of thermoelectric power factor, superconductivity

and reconstruction of Fermi surface topology giving rise to an electronic

topological transition (ETT). We report Raman signatures of electronic

topological transition (ETT) at 3.6 GPa and rhombohedral (α-Bi2Te3) to

monoclinic (β-Bi2Te3) structural transition at ~ 8 GPa. At the onset of ETT, a

new Raman mode appears near 107 cm-1

which is dispersionless with pressure.

The structural transition at ~ 8 GPa is marked by a change in pressure derivative

of A1g and Eg mode frequencies as well as by appearance of new modes near

115 cm-1

and 135 cm-1

. The mode Grüneisen parameters are determined in both

α and β-phases.

DNA and Dendrimer assisted dispersion of nanotubes

Debabrata Pramanik and Prabal K. Maiti.

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

560012, India.

DNA and dendrimers are emerging out to be strong dispersive agents for nanotubes from the bundle

geometry. To understand the efficiency of DNA and dendrimer in reducing the binding strength of

bare nanotube we have calculated potential of mean force (PMF) between two single walled

nanotubes which are wrapped either by single stranded DNA or PAMAM dendrimer. The calculated

PMF between dendrimer nanotube is ~ 8 kcal/mol in contrast to the effective interaction of ~30

kcal/mol between two bare nanotube. We find that poly A DNA wrapped nanotube has almost no

binding affinity as revealed by the PMF calculation. These results demonstrate that DNA is most

effective in nanotube dispersion. Dendrimer can also be use as dispersing materials as it lowers the

effective interaction between nanotubes significantly.

Optical and Electrical Investigation of CdTe QDs/PDDA

Bistable Devices

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

Electrical bistability is a phenomenon in which a device exhibits two states of different conductivities at the same appliedvoltage. This behaviour is ideal for switch-able memory applications and has been widely studied recently. Quantum dotsare promising candidates for future memory devices due to their attractive features like miniaturized dimensions and thepossibility for nano-scale design through chemical synthesis. In recent years, A. J. Pal and his group have observed electricalbistable phenomena in various types of semiconductor and metallic nano-particle films such as CdS, PbS, CdSe and Au. Incase of CdSe nano-particles, they have shown that higher the quantum confinement more is the electrical bistability. Onthe basis of that, we have investigated the electrical bistable phenomena in CdTe QDs devices. We are trying to model thereason of bistability by transport phenomena in different device geometry. CdTe QDs are studied as a subclass of CdSe QDs,but in the former case one can achieve strong quantum confinement more easily due to their large Bohr exciton radius.

CdTe QDs were synthesized via colloidal hydrothermal route. This is a one pot synthesis method carried out in a Teflonlined stainless steel autoclave. In a typical procedure, devices for electrical bistable measurements were fabricated by spincoating a cationic polymer (PDDA) and MPA-capped CdTe QDs (anionic nature) on ITO coated glass substrate via layerby layer (LbL) deposition using the principle of electrostatic attraction.

We have performed photoluminescence and optical absorption spectroscopic measurements for the devices prepared witha different number of CdTe/PDDA bilayers in order to investigate the influence of the film thickness on the electronic andoptical properties. The excitation wavelength was 400 nm for 15 different devices of (CdTe-PDDA)*n with n=10,20, 30,40 and 50, respectively. An increase in photoluminescence and optical absorption intensity was observed for the 50-bilayerfilm compared to the 30-bilayer devices. However, the intensity was much lower for the sample of 10 bilayers. So an clearincrement of the intensity has been found with increasing number of bilayers. From the first excitonic peak of the absorptionspectra the size of the QDs were calculated.

We recorded the current-voltage (I-V) characteristics of the CdTe QDs films using Pt/Ir tips of STM. A typical set ofI-V plot for a voltage has been found. For the devices, I-V characteristic shows an electrical bistability with low conductingstates when the voltage is swept from 0 to +2.5 V and high conducting states with voltage sweep from +2.5 to 0 V inforward bias direction and vice-versa in reverse bias direction. The switching is reversible and cyclic and the dependence ofI-V characteristics with sweep direction results in two values of current at one voltage and is generally quantified as on/offratio. An interesting phenomenon has been observation is that the bistability increases as the size of CdTe QDs decreases,similar to what has been reported for CdSe QDs.

Acknowledgements:

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

Reference:

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

1

Fabrication of tunable potential barrier in Bilayer graphene

T Phanindra Sai, Arindam Ghosh

Department of Physics, Indian Institute of Science, Bangalore.

Bilayer graphene which shows a bandgap opening on application of dual electronic gating

has emerged as a prospective candidate for device applications as well as for validating

many theoretical predictions. It has been recently predicted that using quantum point

contact (QPC) structure on a bilayer graphene, in the first conductance plateau the QPC

produces a strong polarization of valleys, provided the constriction has zig-zag edges along

the direction of current flow. Their findings signify that two valleys can be individually

addressed as independent internal degrees of freedom of conduction electrons by

controlling the gate voltage. There is also a theoretical prediction that the edge band

dispersion of graphene can be continuously changed by tuning the onsite energies on the

boundary of the system. And under certain values of boundary potential, the edge band can

completely merge with the bulk band or become valley dependent gapless chiral modes.

These chiral modes are interesting because they are analogues to edge states in quantum

spin hall effect (QSHE), where the valley index playing the role of spin.

As first step to realise device structures with which these theoretical predictions can be

validated, we fabricated bilayer graphene devices using state of the art lithographic

technique, with top split gates as QPC structures over the bilayer as well as across the edge

using cross-linked PMMA as dielectric between the top gate and bilayer graphene.

Transport studies were performed on such devices at low temperatures which showed that

the bandgap was tunable using both top and bottom gates, but the presence of cross-linked

PMMA lead to low mobility in the devices. To overcome the low mobility issue we had

fabricated elevated split gates over the bilayer graphene with air gap as dielectric.

Preliminary transport measurements indicated considerable improvement in mobility.

Experiments are underway to measure the transport properties at low temperatures of the

elevated split gated devices.

DBT: A versatile Dendrimer Building ToolKit

Vishal Maingi1, Vaibhav Jain2, Mattaparthi V. Satish Kumar1, Prasad V. Bharatam2 and Prabal K. Maiti1*

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

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

We have developed a graphical user interface (GUI) based Dendrimer Builder Toolkit (DBT)

which can be used to generate the dendrimer configuration of desired generation for various

chemical functionality. The validation of structures generated by this tool was carried out by

studying the structural properties of polyamidoamine (PAMAM, G3-G6 generations) and

poly(propyl) imine (PPI, G5 generation) dendrimers. We also studied the interaction and release

pattern behaviour of ligands (two soluble drugs namely Salicylic acid; Sal, L-Alanine; Ala and

two insoluble drugs namely Phenylbutazone; Pbz, Primidone; Prim) with G5 PAMAM

dendrimer using atomistic MD simulations. We have computed the Potential of Mean Force

(PMF) variation with G5-PAMAM dendrimer complexed with drug molecules. The order of ease

of release pattern was found to be: Ala > Sal > Prim > Pbz. We find that PMF energy barrier to

be less for the drug when it is bound to non protonated dendrimer compared to the case of

protonated dendrimer. Our results suggest that encapsulation of drug molecule into the host

PAMAM dendrimer should be carried out at higher pH values (near pH 10).

Development of Methodologies in Ex-Situ NMR Spectroscopy

KowsalyaDevi Pavuluri and K.V.Ramanathan

NMR Research Centre, Department of Physics

Indian Institute of Science, Bangalore-560012, India.

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

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

Abstract for poster

Strongly correlated transport in ultrathin gold nanowires

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

N. Ravishankar# and Arindam Ghosh*

*Department of Physics and #Materials Research Center,

Indian Institute of Science, Bangalore 560012, INDIA.

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

References:

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

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

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

(2011).

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

Strongly magnetized cold electron degenerate gas:

Mass-radius relation of the collapsed star

Upasana Das, Banibrata Mukhopadhyay

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

E-mail: upasana@physics.iisc.ernet.in, bm@physics.iisc.ernet.in

Abstract.

We consider a relativistic, degenerate electron gas at zero-temperature under the

influence of a strong, uniform, static magnetic field, neglecting any form of interactions.

Since the density of states for the electrons changes due to the presence of the magnetic

field (which gives rise to Landau quantization), the corresponding equation of state also

gets modified. In order to investigate the effect of very strong magnetic field we focus

only on systems in which a maximum of either one, two or three Landau level(s)

is/are occupied. This is important since, if a very large number of Landau levels are

filled, it implies a very low magnetic field strength which yields back Chandrasekhar’s

celebrated non-magnetic results. The maximum number of Landau levels occupied

is fixed by the correct choice of two parameters, namely the magnetic field strength

and the maximum Fermi energy of the system. We study the equations of state of

these one-level, two-level and three-level systems and compare them by taking three

different maximum Fermi energies. We also find the effect of the strong magnetic field

on the mass-radius relation of the underlying star composed of the gas stated above.

We obtain an interesting theoretical result that, it is possible to have an electron

degenerate static star with a mass significantly greater than the Chandrasekhar limit,

provided it has an appropriate magnetic field strength and central density.

Keywords : degenerate Fermi gases, stellar magnetic fields, Landau levels, equations

of state of gases, white dwarfs

PACS : 67.85.Lm, 97.10.Ld, 71.70.Di, 51.30.+i, 97.20.Rp

Membrane coupled to Active Fluid

Ananyo Maitra, Sriram Ramaswamy, Jean­Francois Joanny

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

Temperature Dependent Sign Reversal of Magnetocrystalline Anisotropy in Nanoparticles of La0.875Sr0.125MnO3

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

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

Many properties of materials drastically change when they are prepared in nanoscale. For example, it has

been recently shown that the charge ordered, antiferromagnetic manganite Nd0.5Ca0.5MnO3 loses its charge

order and becomes ferromagnetic when the size of the particles is reduced to ~ 20-30 nm. In this report we

present a ferromagnetic resonance study of magnetocrystalline anisotropy in nanoparticles (dia ~ 20 nm) of

La0.875Sr0.125MnO3(LSMO) as a function of temperature in the range 4 K – 300 K. We find that the nano

LSMO shows negative uniaxial anisotropy at low temperatures which switches over to positive anisotropy

at ~ 250 K. While composition dependent sign reversal of anisotropy is known to occur, to the best of our

knowledge this is the first report of temperature dependent sign reversal of magnetocrystalline anisotropy.

The nanoparticles of LSMO were prepared using the sol-gel technique. The composition was checked with

inductively coupled plasma emission spectroscopy. XRD and Rietweld fitting showed that the structure of

the nanoparticles is rhombohedral analyzed using hexagonal space group R-3cH. TEM was used to find

the particle size which was ~ 20 nm. SQUID magnetometry showed that the sample underwent a

ferromagnetic transition at 300 K. The ferromagnetic resonance signals were recorded using a commercial

EPR spectrometer between the temperatures 4 K - 300 K. FMR signal lineshape is a definitive indicator of

the nature and sign of magnetocrystalline anisotropy of the sample. The signal shape of the LSMO

nanoparticles at the lowest temperature used (i.e.4 K) is indicative of negative anisotropy. As the

temperature is increased the lineshape gradually changes and around 250 K a signal characteristic of

positive anisotropy is observed. We believe that the co-operative Jahn Teller transition occurring around

room temperature in LSMO drives the sign change in the magnetocrystalline anisotropy.

Thermoelectric properties of PbTe with Bi precipitates

Ashoka Bali and Ramesh Chandra Mallik

Thermoelectric Materials and Devices Laboratory,

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

Abstract

Lead telluride is an established thermoelectric material in the temperature range 350K - 800K. The

maximum figure of merit (zT) obtained in doped bulk PbTe is 1.5 at 773K [1], which is too less for

commercial applications. Embedding precipitates in the bulk of PbTe is one method of increasing this zT.

By doing this, a larger number of interfaces are introduced in the bulk, which is expected to reduce

thermal conductivity by increased phonon scattering. In addition, embedding metallic precipitates has

been theoretically predicted to enhance the power factor. Bismuth, being a semi-metal, and an n-type

dopant in PbTe, comes across as a good choice for the precipitate element. Here, PbTe with precipitates

of Bi embedded in the bulk of the material has been prepared by matrix encapsulation technique, where

powdered PbTe and Bi were heated above the melting point of PbTe, and rapidly quenched in water.

XRD result showed crystalline PbTe, even though Bi was not detected. Scanning electron micrographs

confirmed the presence of a secondary phase in the bulk. Seebeck coefficient, electrical conductivity and

thermal conductivity were measured from room temperature to 750K. Degenerate semiconductor

behavior was observed from electrical conductivity data, which also showed a decrease with increasing Bi

concentration. Negative values of Seebeck coefficient indicated n-type nature of the samples. Seebeck

coefficient also showed a decrease with increase in Bi concentration, leading to an overall decrease in the

power factor. Thus, no improvement in zT as compared to undoped PbTe was observed, for which the

maximum value calculated was 0.8 at 725K.

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

Reference: J. P Heremans et al, Science 321, 554-557 (2008)

Evidence for shift of rigidity percolation to higher coordination numbers

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

Chalcogenide glasses based on tellurium are difficult glass formers as they require higher cooling rates. They cannot be prepared over a wide composition range like Se based chalcogenide glasses by the melt quenching method. For example, in Ge-As-Te system, some compositions are difficult to form glasses as the cooling rate (102 K/s) achieved in normal melt quenching is not sufficient. Hence, there are two glass forming regions in Ge-As-Te system separated by these compositions. To achieve higher cooling rates and form glasses over a wide composition range without any discontinuity we rapidly quenched the melts with the help of a home built twin roller melt spinning apparatus. In covalent network glasses, the bond bending and bond stretching constraints are balanced against the number of degrees of freedom available. At an average coordination number (Zav) = 2.40, the constraints and the degrees of freedom are balanced. Various properties as a function of average coordination number exhibit distinct changes at this point and it is called rigidity percolation threshold (RPT). Another threshold usually occurs at higher coordination number which is purely chemical in origin is called chemical threshold (CT). RPT occurs at Zav = 2.40 and CT occurs around 2.67. For example, in Ge-As-Se glasses (which is considered to be a model system for Se based glasses) exhibit RPT at Zav = 2.4 and CT at Zav = 2.67. In the same way Ge-As-Te can be considered as a model system for Te based glasses. But the difficulty with Te systems is the preparation of glasses over a wide range covering both the thresholds. We could overcome this difficulty by quenching the melt of the glasses rapidly by melt spinning method. Thin flakes of Ge-As-Te glasses have been prepared covering average coordination numbers (Zav) from 2.20 to 2.85 in a single composition tie line for the first time. The glass transition (Tg) vs. average coordination number plots do not show any signatures of RPT at the critical coordination number Zav = 2.40, instead a maximum in Tg is observed at Zav = 2.70. This shift has been understood based on the modified constraint theory taking into the account of the metallic nature of tellurium. The metallic nature of Te affects the degree of covalent nature of the Ge-As-Te network, which in turn affects the balance between the constraints and the number of degrees of freedom available to the atoms. The metallic nature of Te, the decrease in the degree of covalency and the imbalance between constraints and the degrees of freedom shifts the RPT to higher coordination number <r> = 2.70.

Topological Insulator on the Edge: Boundary Conditions Revisited

Amal Medhi∗ and Vijay B. Shenoy†

Center for Condensed Matter Theory, Indian Institute of Science, Bangalore 560012, India(Dated: November 16, 2011)

We present an analytic formulation for derivation of the boundary conditions and edge statesin topological insulators with finite boundaries. By considering model Hamiltonians of topologicalinsulators in the continuum limit, we show that there are two types of boundary conditions possible-the usual fixed boundary condition and a free boundary condition. By comparing with resultsfrom the corresponding tight binding model calculations, we argue that the appropriate boundarycondition for topological insulators is the latter. We also elucidate the nature of edge states of aparticular model (BHZ model) of topological insulators in detail.

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

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

Structural and magnetic transition in Fe1+yTe single crystals.

Dona Cherian 1, S. Röβler2 , U. Schwarz 2 , S. Wirth2 , H. L. Bhat1 and Suja Elizabeth1 .

1Department of Physics, Indian Institute of Science, Bangalore , India. 2 Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straβe, Dresden, Germany.

Email : donacherian@physics.iisc.ernet.in

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

layer resulting in a complex magnetic order[2]. We have investigated structural and magnetic properties of Fe1+yTe single crystals.

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

References

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

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

Geetanjali Singh and S V Bhat

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

Electron-hole asymmetry1, which refers to the asymmetry in the phase diagram across x = 0.5 in doped rare earth manganites such as Re1-xAxMnO3 is shown2 to vanish on reducing the size of the particles to a few nanometers. Bismuth based manganites provide an interesting model system for comparison: e.g. though the doped rare earth manganites show disappearance of charge order (CO) on size reduction to nanoscale3, CO in Bi manganites is found to be more robust. Here we study the effect of size reduction on EPR parameters of electron (x = 0. 6, BCMOE) and hole (x = 0.4, BCMOH) doped Bi1-

xCaxMnO3. Nearly spherical nanoparticles (d ~ 18 nm) of BCMO were prepared by sol-gel synthesis and the bulk samples using the solid state reaction route. X-band EPR was carried out between 5 and 300 K. Lineshape fitting was carried out using double Lorentzian function accounting for clockwise and counterclockwise rotating components of the microwave field. The extracted EPR parameters, namely, the linewidth, intensity and the resonance field for the bulk and the nano samples indicate that the differences observed in the EPR parameters for the electron and hole doped bulk samples persist in the nanosamples as well in contrast with the results on Pr1-xCaxMnO3

2. We understand this in terms of the presence of the highly polarizable 6s2 lone pairs on bismuth which is understood to cause many interesting departures from the behavior of rare earth manganites. References: 1. Janhavi P. Joshi, A.K. Sood, S.V. Bhat, Sarathy, Vijay, C.N.R. Rao, J.Phys. Cond. Matter. 16, 2869 (2004) 2. K.G. Padmalekha and S.V. Bhat, arxiv-cond-mat/1010.3556. 3. S. S. Rao, S. Tripathi, D. Pandey and S. V. Bhat, Phys. Rev, B 74, 144416 (2006)

Multi Electron Bubbles in Liquid Helium

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

Abstract

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

We present our experimental setup and few preliminary experimental results.

Numerical studies of dynamo action in a linear

shear flow with turbulence

Nishant K. Singh1,2, Naveen Jingade2 & S. Sridhar11Raman Research Institute, Bangalore, India2Indian Institute of Science, Bangalore, India

Abstract

Large–scale cosmic magnetic fields are believed to originate fromdynamo action in the electrically conducting fluids. Conversion ofkinetic energy present in the fluid into the magnetic energy is re-ferred to as the dynamo action. The framework of mean–field electro-hydrodynamics which has been developed/evolved by using Maxwell’sequations together with Ohm’s law during the past few decades aimsto address some of the outstanding puzzles related to magnetic fieldsin the universe. One particular challenge is to explain the generationof magnetic fields over length scales exceeding the length scale of tur-bulence. It is known that the helical turbulent flows on its own canamplify seed magnetic fields through the α–effect but there has beenno evidence yet which has shown the dynamo action to produce large–scale mean magnetic field due to non–helical homogeneous isotropicturbulence alone. Thus it is legitimate to think what large–scale prop-erties such systems should possess in order to generate large–scale meanmagnetic field if the turbulence is non–helical? The role of mean veloc-ity shear as the large–scale feature has received some attention. Directnumerical simulations now provide a strong support for such a sheardynamo. While the shear dynamo has been conclusively demonstratedto function, it is not yet clear what makes it to work.

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

1

Structure of DNA Nanotubes

Anjan Dwarknath1, Himanshu Joshi2, Prabal K. Maiti2

1 Indian Institute of Technology, Chennai

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

Bangalore.

DNA Nanotubes are tubular structures created by self assembling DNA strands whose sequences

are designed so that the resulting molecule takes the shape of a nanotube. Here we report for

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

helix bundles as well as triangular nanotubes. We use state of the art all atom molecular

dynamics (MD) simulations, to study microscopic picture of the DNA nanotubes. We comment

on their thermodynamic stability as a function of sequence and number of DNA helix forming the

nanotube. We also calculate the stretch modulus of these structures by stretching them in

constant velocity simulation. The stretch moduli of the DNA nanotubes are of the order of 4000

pN in contrast to 1000 pN for a single DNA double helix.

Exploration of the statistical properties of

Gross-Pitaevskii turbulence in two dimensions.

Vishwanath Shukla and Rahul Pandit

Centre for Condensed Matter Theory, Department of Physics,

Indian Institute of Science, Bangalore 560012, India

November 18, 2011

Abstract

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

i∂Ψ(r, t)

∂t=

(−∇2 + g|Ψ|2

)Ψ(r, t) (1)

where Ψ(r, t) is the complex classical field describing a weakly interactingBose gas, and g is the effective interaction strength, that is used to describezero-temperature superfluids and optical turbulence. In particular, weexplore the time evolution of initial conditions in which

Ψ(k, 0) =1√

π1/2σexp

(− (k − k0)

2

2σ2

)exp iΘ, (2)

where Ψ(k, 0) is the Fourier transform of Ψ(r, 0), k =√

k2x + k2

y, andΘ = Θ(kx, ky) are uniformly distributed random numbers in the interval[0, 2π

]. We study the time evolution of such an initial condition as a

function of k0 = n0∆k and σ = β∆k where n0 is an integer, β is a realnumber and ∆k = 2π/L . We use a pseudospectral direct numericalsimulation for our study.

In any practical calculation we must truncate the number of Fouriermodes that we retain in our pseudospectral study. Thus, we have a trun-cated Gross-Pitaevskii system, which is a finite-dimensional Hamiltoniansystem that thermalizes at long times. We obtain the time evolution of(a) spectra for compressible, incompressible, interaction, and quantumpressure parts of the energy, (b) occupation-number spectra, (c) the pop-ulation in the zero-wave-number mode, (d) probability distribution func-tions (PDFs) for the velocity, and (e) pseudocolour plots of the vorticity.Some universal features emerge as the system approaches thermalizationbut not before that.

1

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

Authors: Shibu Saw and Chandan Dasgupta

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

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

Structural and magnetic properties of Nd1-xYxMnO3 (0.1 ≤ 0.6 ≥ ݔ)

Ruchika Yadav1, Harikrishnan S2, Shilpa Adiga2, H.L.Bhat1 and Suja Elizabeth1 1Indian Institute of Science, Bangalore-560012, India

2 Peter Grünberg Institute-4/Jülich Centre for Neutron Sciences-2, Forschungszentrum Jülich, 52425 Jülich, Germany.

E-mail: liz@physics.iisc.ernet.in

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

N decreases with increasing ݔ. Data also show a similar trend in AC susceptibility measurements. For the sample with 0.3 = ݔ, results of magnetic measurement indicate ferrimagnet like behavior. The insulating nature of the samples is evident from the studies in temperature evolution of electrical resistivity. Optimization of single crystal growth using Float Zone mirror furnace is underway, for direction dependent studies on oriented single crystals. References: [1] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, Y. Tokura, Nature 426 (2003) 55–58 [2] S. Issing, A. Pimenov, V. Y. Ivanov, A. A. Mukhin, J. Geurts, Phys. Rev. B 81(2010) 24304. [3] J. Hemberger, F. Schrettle, A. Pimenov, P. Lunkenheimer, V. Ivanov, A. Mukhin, A. Balbashov, A. Loidl, Physical Review B 75 (2007) 35118.

Structure of Cytolysin A (ClyA) pore in Lipid Bilayer

Swarna M Patra*, K Ganapathy Ayappa** and Prabal K Maiti* *Centre for Condensed matter theory

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

**Department of Chemical Engineering Indian Institute of Science, Bangalore 560 012, India

Abstract:

Membrane proteins are subject to same consideration as soluble proteins, but structural study of

membrane proteins appear to be harder. Simulations of membrane proteins are a booming field

with great application. Cytolysin A (ClyA) is a cytolytic protein expressed by Escherichia coli

and other enterobacteria. The crystal structure of ClyA, 400kDa dodecameric trans membrane

pore in DDM detergent micelles is reported. The soluble monomer structure and the tertiary

structure of ClyA protomers are significantly different. In this study through molecular dynamics

we have studied the structural aspects of ClyA (2WCD pdb) pore forming toxin in homogeneous

lipid bilayer. In the present study we have taken two systems with different salt concentration.

The work is under progress; our preliminary result shows the structural stability of the pore in

lipid bilayer and the selectivity of pore towards Na ions. This observation is further supported

with the Adaptive Poisson-Boltzmann Solver (APBS) calculation, indicating inside of the pore

has strong negative potential. Our present work gives a better understanding of the structural

aspect at molecular level. This has a potential application for the pharmaceutical industry in new

drug discovery.

Conversion between electromagneticallyinduced transparency and absorption in a

degenerate lambda system

Sapam Ranjita Chanu and Vasant NatarajanDepartmant of Physics,Indian Institute of Science

Bangalore-560012, INDIA

November 21, 2011

Abstract

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

1

Real-space manifestations of Energy-spectra

Bottlenecks: Insights from Hyperviscous

Hydrodynamical Equations

Uriel Frisch, Samriddhi Sankar Ray, Ganapati Sahoo,

Debarghya Banerjee and Rahul Pandit

November 17, 2011

Abstract

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

1

In-plane magnetic anisotropy in epitaxial ultrathin Fe films

Sakshath S, Padmalekha K G, S V Bhat and P S Anil KumarDepartment of Physics

Indian Institute of ScienceBangalore 560012; India

Epitaxially grown Fe films show a plethora of magnetic anisotropies depending on the

environment. The control of magnetic anisotropy in thin ferromagnetic films is important for

several device applications such as Spin torque oscillators, Magnetic memories etc as well as from

the point of view of understanding fundamental physics. Factors such as the material of the capping

layer, underlayer used in growth, deposition technique, geometry of deposition, substrate material

etc are observed to strongly influence the growth of Fe films. An understanding of the causes of

anisotropy, particularly the uniaxial anisotropy in these films has not been concrete. We try to shed

some light into this aspect with studies of anisotropy in Fe films.

Fe thin films were grown at room temperature in an Ultrahigh Vacuum chamber with base

pressure less than 2x 10-10 Torr with the following confiigurations:

1. MgO/Fe/GaAs

2. Au/Fe/GaAs

3. MgO/Fe/MgO/GaAs

Structural characterisation was done using Low Energy Electron diffraction . Magnetic

characterisation has been done using Magnetooptic Kerr effect in the longitudinal configuration and

Ferromagnetic Resonance. The initial growth of Fe during the first few layers as well as the the

electronic structure of the interfaces seem to be the most important reasons behind the uniaxial

Anisotropy. For a smooth well oriented substrate, the electronic band structure of the substrate gives

rise to the unexpected Uniaxial anisotropy in Fe films.

Scroll­Wave   Dynamics   in   Human   Cardiac   Tissue:   Lessons   from   a Mathematical Model with Inhomogeneities and Fiber Architecture

Rupamanjari Majumder1, Alok Ranjan Nayak1, Rahul Pandit1,2

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

1 Department of Physics, Centre for Condensed Matter Theory, Indian Institute of Science, Bangalore, India, 2 Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India

Density Functional Theory Studies of electronsand phonons in pyrochlores

Pramod Kumar VermaDepartment of Physics

Indian Institute of Science , BangaloreResearch Supervisor : H R Krishnamurthy

November 23, 2011

Abstract

Density functional theory calculations have been performed to ob-tain the lattice constant and electronic properties of the ideal py-rochlores with the composition Y2B2O7 (where B = Ti, Zr and Ir).From the band structure and the density of states we infer that Y2Ti2O7

and Y2Zr2O7 are insulators while Y2Ir2O7 is a metal. The dynami-cal atomic charges (also called Born effective charges) are obtained toquantify the degree of covalency or ionicity. A large anomalous con-tribution to the dynamical charge is observed for both the Zr and Ti(specially for Ti). It is attributed to the hybridization between theoccupied 2p states of the oxygen and unoccupied d states of the Bcation. Density functional perturbation theory (DFPT) calculationshave been performed at the Gamma point to obtain the phonon prop-erties of the system.While for Y2Zr2O7 and Y2Ir2O7 all the phononfrequencies are positive as one would expect. Y2Ti2O7 shows insta-bilities with respect to some optical phonon distortions is that 6 ofthe frequencies are imaginary. This is likely to underlie the anoma-lous temperature dependent of the phonons that have been seen inother titanate pyrochlores. A pressure of the order of 3.5GPa or asmall distortion to the atomic positions in the unit cell stabilizes thesystem.

1

“Upper Branch” Fermi Gas and Tan’s Theorem

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

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