ABSTRACTS OF TALKS - iiserkol.ac.inctcm2011/CTCM2011_files/CTCM2011Ab… · pressures ~ megabars...

ABSTRACTS OF TALKS

Active matter: spreading, sticks, stripes, sheets and cells

Sriram Ramaswamy Department of Physics,

Indian Institute of Science, Bangalore 560 012 India Abstract: I will summarize recent progress from our group on liquid drops and lamellar phases of active particles, applications to dynamics at the scale of the membrane and the nucleus of a living cell, and the physics of macroscopic, activated rodlike particles.

Investigations of Hot Dense Materials using Laser Heated Diamond Anvil Cells

N. Subramanian

Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102 India

Laser heating of materials in diamond anvil cells (LHDAC) has emerged in the recent years as a preferred route for subjecting them to extremely high temperatures, ~ thousands of kelvin, and pressures ~ megabars that can mimic planetary interiors. At such extreme conditions, the nature of the chemical bonding in elements changes and the chemical reactivity generally increase such that direct elemental reaction between even inert species becomes possible leading to the formation of novel and exotic phases. Exploration of these and establishing the high P-T phase diagrams of several substances has been made possible by coupling in situ diagnostic techniques such as synchrotron-based x-ray diffraction and Raman/IR spectroscopy to the LHDAC. This talk will focus on LHDAC-Raman spectroscopy’s unique advantages to probe P-T induced behavior of materials. Examples will include our recent work on dense hydrogen in which by monitoring the vibron changes in the LHDAC we see clear evidence of changes in bonding in the hot fluid state above a maximum in the melting curve1 and an experiment in which we demonstrate chemical bond formation by direct reaction between Ge and Sn, both belonging to Group-IV and normally non-reactive, at ~ 9 GPa and 2000 K2. -

1 N. Subramanian, Alexander F. Goncharov, Viktor V. Struzhkin, M. Somayazulu, and Russell J. Hemley, Bonding Changes in Hot Fluid Hydrogen at Megabar Pressures, Proc.Natl.Acad.Sciences, 108, 6014 (2011). 2 Y. A. Sorb , N. Subramanian, T. R. Ravindran , P. Ch. Sahu Direct Reaction between Ge and Sn in a Laser Heated Diamond Anvil Cell (2011, To be published).

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“CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011 Indian Institute of Science Education & Research-Kolkata

Probing the Enigmatic Behavior of Granular Materials: A Statistical Mechanics Approach

Bulbul Chakraborty

Department of Physics, Brandeis University, USA

Abstract: It is remarkable that we have a theoretical framework to analyze the behavior of a piece of iron or a boiling pot of water, whose constituents are not visible to the naked eye, yet we do not have the theoretical tool necessary to understand the collective response of a large number of macroscopic objects. These granular materials respond to perturbations in ways that are often at odds with our expectations. Examples are the dilation of sand under shearing, a common phenomenon at the beach, or the constant speed of sand flowing out of an hour glass. These oddities arise from the intrinsically non-equilibrium, non-thermal character of the fluctuations in granular materials, ans that is precisely what make it difficult to construct a theoretical framework. In this talk I will show that ideas of ensemble, conservation laws, and entropy go a long way in explaining the enigmatic behavior of a collection of grains

Grain Boundary Dynamics In Colloidal Crystals

Rajesh Ganapathy

International Centre for Materials Science JNCASR, Bangalore 560064, India

Colloids consist of micrometer-sized particles suspended in a fluid. The particles are Brownian and the phase behavior and long-time dynamics of the colloidal suspension as a whole is governed by the laws of statistical thermodynamics. The thermodynamic nature of colloids combined with the ease with which the particles can be imaged has allowed researchers to use colloids as model systems for studying phenomena that occur on the atomic scale but are often difficult to investigate [1-3]. In my talk, I will describe results from recent experiments where we have used fast confocal microscopy to probe the dynamics of grain boundaries (GBs) in a 3-dimensional colloidal polycrystal with single-particle resolution. Our confocal microscopy experiments show that high misorientation angle GBs (HAGBs) have structural and dynamical features that are remarkably similar to those of glass-forming liquids. Analogous to supercooled fluids, we have observed a split second peak in the radial distribution function, non-Gaussian probability distribution of particle displacements and string-like collective motion of particles at GBs [4]. Remarkably, the size of cooperatively rearranging regions related to the fragility of glasses increases with the spatial extent of the misorientation angle-dependent confinement of the GB region by adjacent crystallites. References

1. P. Schall, Itai Cohen, D. A. Weitz and F. Spapen, Nature 440, 319 (2006). 2. Rajesh Ganapathy, Mark R Buckley, Sharon Gerbode and Itai Cohen, Science 327, 445

(2010). 3. D. Kaya, N. L. Green, C. E. Maloney and M. F. Islam, Science 329, 656 (2010). 4. Hima K Nagamanasa, Shreyas Gokhale, Rajesh Ganapathy and Ajay K Sood, PNAS 108,

11323 (2011). ________________________________________________________________________________


Dynamics of Interfaces

Shankar Ghosh Tata Institute of Fundamental Research

Homi Bhabha Road, Mumbai 400 005, India

The area of interfacial physics in soft systems is, in general, relevant to problems of adhesion, wetting, lubrication and friction, and the phenomenon observed at the interface is strongly influenced by the disorder that it is subjected to. The role of this disorder in determining the state of the system has been a subject of intense research over many decades. Realizing these states in intrinsically soft systems has the added richness of new scales in length, time, energy and forces and thus affords accessibility to novel methods of experimental investigation, newer- and often more incisive - analyses, discovery of phenomena of use in whole new classes of applications and technologies. I will speak about (i) our choice of model systems (ii) experimental techniques developed to study these systems and (iii) the underlying common concepts to address questions related to the role of disorder in determining the dynamic state of a moving interface.

Stripe Order and Spin Reorientation Transition in Doped manganites

R. C. Budhani1,2, P. K. Rout2, G. N. Singh2 and Rajni Porwal2

1. National Physical Laboratory, New Delhi

2. Condensed Matter – Low Dimensional Systems Laboratory Indian Institute of Technology Kanpur

The structure of magnetic domains in epitaxial La0:625Ca0:375MnO3 films grown on (001) NdGaO3 is monitored as a function of temperature and in-plane magnetic field strength using Magnetic Force Microscopy in ultra-high vacuum. The magnetization vector M shows two distinct region of orientation order; one in-plane showing contrast-less image and the other tilted away from the film plane showing a distinct stripe pattern due to sinusoidally oscillating M . A strong fractallization of the stripe domains is seen at the boundary of two regions. The in-plane magnetic field induces a spin reorientation transition leading to disappearance of stripes from most of the film except at the boundary where it is more resilient to reorientation. The minimization of a model magnetic free energy functional and its dependence on the local strain in the film captures the physics of domain fractallization.

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The Structure and Dynamics of Mixed Lipid Membranes

Sunil K. Sinha

University of California San Diego, USA Lipid Membranes are one of the basic constituents of all living systems. Although actual biological membranes are highly complex and heterogeneous, simpler model phospholipid bilayers and multilayers are being studied as interesting materials which can still exhibit some of the flexibility and adaptability of biomembranes. Real biomembranes exhibit the phenomenon of “raft” formation, which is believed to be associated with many membrane-mediated biological functions including protein trafficking, cell surface signaling, and membrane fusion. In order to understand the physics of this type of nanophase separation at the microscopic level we have taken a simple model lipid membrane system, consisting of bilayers of a mixture of DOPC, DPPC and cholesterol. We have used X-Ray reflectivity to study the phase separation of this system in both solid-supported bilayers and multilayers to obtain evidence of this nano phase separation. I shall also present our results on cushioned bilayers where the kinetic restrictions of a solid substrate are absent, and also on the slow dynamics of the multilayers using coherent X-ray scattering. Research Supported by Division of Basic Energy Sciences, Office of Science, U.S. Dept. of Energy via Grant DE-FG02-04ER46173

.Superconductivity in Ru doped single crystals of BaFe2As2: phase diagram and critical property investigations

A. Bharathi

Indira Gandhi Centre for Atomic Research (IGCAR) Kalpakkam 603 102, Tamil Nadu, India

I will present in this talk the experimental work carried out in our laboratory on the two unusual ground states present in the 122 FeAs superconductors viz., the spin density wave (SDW) and the superconducting states. The study of the SDW state are drawn from resistivity measurements performed under magnetic field and under external pressure. The study of superconducting properties involved, in addition, magnetization measurements in external fields upto 16 Tesla. The single crystals of BaFe2-xRuxAs2 used in the study were made in our laboratory by slow cooling Ba chunks, FeAs and RuAs powders without adding any flux. Using resistivity measurements the superconducting and spin density wave transitions were identified and phase diagram was obtained that is consistent with earlier phase diagrams seen in this system in single crystal investigations. In the low concentration regime the crystals stabilize in SDW ground state. In this regime they shows linear magneto-resistance, indicating the presence of Dirac cone states. When these non-superconducting samples were subjected to pressure, a disappearance of the SDW transition is seen with the emergence of superconductivity at 35 K under an external pressure of 1.5 GPa. This is the largest TC seen in this system so far. Study of the superconducting properties show an unusual temperature dependence of the upper critical field anisotropy parameter � that peaks close to TC. The results on � are compared with our measurements on Co substituted system.

_______________________________________________________________________________________________ “CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011

Indian Institute of Science Education & Research-Kolkata

The magnetization loops on superconducting single crystals for both Ru and Co doped BaFe2As2 show the occurrence of a broad second peak, associated with a phase change in the underlying flux line lattice. The critical current density obtained from these studies are unusually large, suggestive of presence of strong flux pinning, which is corroborated by flux pinning potential values obtained from an analysis of the temperature dependent resistivity in the superconducting transition region.

Shape Transformation of Au, Ge and Carbon Nano Tube Nanostructures

P. V. Satyam Institute of Physics

Bhubaneswar – 751005, India

A detailed study on the shape, composition and arrangement of the nanostructures on the clean surfaces (ultra high vacuum, UHV condition prepared) would be very useful in terms of both fundamental and applied research. It is known that a “surface” plays a vital role in the growth (and hence their activity) of low-dimensional structures. In this talk, a detailed study of the gold and germanium nanostructures on clean surfaces of silicon (on 100, 111, 110, 557 and 5512 oriented surfaces) would be presented. Our real time in-situ electron microscopy studies on the growth of nano-rod like structures of gold silicides on Si(110) and Si(100) surfaces with and without native oxide at the interface show the important role played by the native oxide at the si surfaces. The in-situ dynamic studies using electron microscopy methods would give insight into the growth of nanoparticle to nanorod like structure growth. The results of Germanium growth on high index surfaces (Si(557) and Si(5512)) under DC current conditions would be presented. I would be showing a thickness dependent shape transformation of SixGe1-x layers on these high index surfaces. Preliminary results of the strain and the composition variation using TEM and Synchrotron Radiation would also be discussed. In the area of CNT’s, using in-situ TEM studies, we show that the Iron (Fe) nanowire axially slides inside the core of a multi-walled carbon nanotubes (MWCNT) and nanowire transformed into a faceted nanoparticles under electron-irradiation. We report the electron irradiation-induced extrusion and attogram mass transport (0.40 ags-1) inside filled MWCNT. References: (1) J K Dash et al., Journal of Physics: Condensed Matter 23 (2011) 135002 (2) A K Rath et al., Journal of Physics D: Applied Physics 44 (2011) 115301 (3) P. K. Tyagi et al., Nanotechnology: submitted

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Some consequences of disorder on strongly correlated electron systems

D. D. Sarma Solid State and Structural Chemistry Unit

Indian Institute of Science, Bangalore 560012 Strongly correlated electron systems are known to give rise to a host of interesting, and often exotic, properties, as illustrated most copiously in the electronic and magnetic properties of a large number of transition metal compounds. On the other hand, disorder in a crystalline system is also known to affect its electronic properties profoundly. It is interesting to note that simultaneous presence of strong Coulomb correlation and disorder has proven to be theoretically an intractable problem for most cases so far, though disorder in strongly correlated electronic system is inevitable in reality. We have been investigating various manifestations of disorder effects on electronic and magnetic structures of transition metal compounds for more than a decade now; we have also made limited theoretical progress in understanding specific aspects of such effects. I shall present some of these results, elucidating several nonoverlapping expressions of disorder in strongly correlated electronic systems.

New revelations about nucleation of quantized Flux Line lattice from giant vortex states in type-II superconductors

A.K. Grover* Department of Condensed Matter Physics and Materials Science

Tata Institute of Fundamental Research Homi Bhabha Road, Mumbai 400 005, India

The phenomenon of superconductivity has occupied a part of the centre stage of research in condensed matter physics ever since the discovery of abrupt transition to zero-resistance state in Hg by Kammerlingh Onnes a hundred years ago. It is therefore exciting that one can come across new revelations relating to the pristine aspects of superconductivity in contemporary times. I shall present a status report on an ongoing study at TIFR which deals with nucleation of superconductivity much above the thermodynamic upper critical field line (Hc2(T)) in elemental Nb (Tc ~ 9.3K), which is a classic example of a type-II superconductor. The so-called superheated state of superconductivity between Hc2(T) and Hc3(T) lines in the magnetic phase diagram is often discussed in terms of the notion of surface-superconductivity, first enunciated by Saint James and de-Gennes in 1963, and usually identified via a diamagnetic tail in the in-phase of susceptibility response and/or a slow approach to reach the normal state resistance value, after initiating from zero at the Hc2(T) / Tc2(H) value. Explorations in weakly pinned single crystals of Ca3Rh4Sn13 (Tc = 8.35K) and Nb have revealed in recent years [1,2] the presence of a paramagnetic response in dc magnetization measurements in the (H,T) phase-space of surface-superconductivity. ________________________________________________________________________________


These have further enabled [2,3] the unmasking of the details of the transformations that accompany the break up of the multi-quanta (L�o , L>1) giant vortex states to quantized (�o) Abrikosov vortex state from Hc3 (T) down to across the Hc2(T)-line. Such a possibility has been documented by the theorists based on Ginzburg-Landau equations about ten years ago [see, e.g., Ref. [4]). New observations [3] exemplify the existence of meta-stable giant vortex states between T2(H) and T3(H) lines, and fingerprint the transformations possible between the adjacent (L � L � 1) multi-quanta states. [1] P. Das et al., Phys. Rev. B 78, 214504 (2008). [2] P.D. Kulkarni et al., Phys. Rev. B 84, 014501 (2011). [3] U. Vaidya et al., to be published. [4] G.F. Zharkov, Phys. Rev. B 63, 214502 (2001). *[email protected] Subject Category : Non-equilibrium phenomena

Negative Refraction, Radiationless Interference and Subwavelength Focusing:

The Quest for the Superlens

R. Merlin Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA

The talk will take us from the late 1800’s, when Abbe published his ground-breaking paper on the limit of resolution of an optical instrument, to the turn of the 20th century, when the field of near-field optics experienced tremendous growth, emphasizing recent work on sub-wavelength focusing using negative-index slabs [1]. In the second half of the talk, I will introduce the concept of near-field plates [2]. These are grating-like planar structures which provide focusing well beyond the diffraction limit, at arbitrary frequencies. The subwavelength electromagnetic-field distributions of the plates closely resemble those of negative-index slabs. Practical implementations of these plates hold promise for near-field data storage, non-contact sensing, imaging, nanolithography and wireless power transfer applications. Experimental results on a microwave near-field plate will be presented, which demonstrate focusing of 1 GHz radiation at a resolution of �/20 [3,4].

[1] J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000). [2] R. Merlin, Science 317, 927 (2007); A. Grbic and R. Merlin, IEEE Trans. Antennas Propag. 56, 3159

(2008). [3] A. Grbic, L. Jiang and R. Merlin, Science 320, 511 (2008); A. Grbic, R. Merlin, E. M. Thomas and M. F.

Imani, Proc. IEEE, DOI: 10.1109/JPROC.2011.2106191 (2011).

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Fundamental instabilities in metal nanowires below 20nm and limitations on nanopatterning and nanofabrication

A.K.Raychaudhuri Unit for nanosciences

S.N.Bose National Centre for Basic Sciences JD Block , Salt Lake , Kolkata-700098

Investigation on Nanowires for interconnects as well as other nanoelectronics applications is a topic of considerable current interest. There are number of techniques for growth of nanowires which can be used for these applications that include, for example, template based growth method, use of electron-beam lithography and use of such tools as Focused ion beam (FIB) machines. All the methods have there are advantages. However, often some basic problems of very fundamental nature limits the sizes that one can go down to.

One of the fundamental instability that comes into play in metal nanowires (with large length) is the Rayleigh-Plateau instability. This generally occurs when their diameters are below 20nm This instability occurs due to predominance of surface forces over the yield strength. We will present some of our experimental results that bring out the appearance of these instabilities in nanowires of FCC metals grown by different techniques (electrochemically in templates and by FIB patterning) as well as our attempt to control the parameters that may control these instabilities.

Dynamics of Transient Vorticity Aligned Structures in Attractive Colloidal Suspensions

Ajay Singh Negi1, Michelle Bebrin2 and Chinedum Osuji3

1Department of Physics, IISER Bhopal, INDIA. 2Department of Chemical Engineering, McGill University, Canada.

3Department of Chemical and Environmental Engineering, Yale University, New Haven, CT, USA.

Shear rate jumps from high to low flow rates in an attractive colloidal suspension of carbon black particles in a non-polar solvent result in the formation of transient log-like structures aligned in the vorticity direction. Optical microscopy in situ with bulk rheology shows that the appearance of these aggregates is attended by an increase in the suspension viscosity. The viscosity shows a peak and then gradually recedes with passage of time under flow in concordance with the disappearance of the log-like structures. The time at which the viscosity reaches its maximum scales inversely with the shear rate applied to the system. This emergence of the peak in viscosity appears to be controlled by a critical strain and rescaling in these terms produces a common response across several different shear rates. Alteration of the attraction strength between particles by the addition of surfactant severely inhibits the structure formation. We present a simple model to account for these observations.

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Chaos in the Rheology of Complex Fluids

Chandan Dasgupta Centre for Condensed Matter Theory

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

The rheological behaviour of complex fluids under shear exhibits many interesting features arising from the strong coupling between mesoscopic structure and flow. Complex temporalresponse indicating the occurrence of deterministic chaos has been observed in experiments on polymer solutions, surfactant mesophases and colloidal suspensions under steady external driving. The chaotic behaviour in these systems occurs at very small values of the Reynolds number and arises from nonlinearities in the rheological constitutive equations. With a view to understanding the "rheological chaos" observed in recent experiments on a variety of orientable fluids, we study numerically the coupled nonlinear partial differential equations for the hydrodynamic velocity and order parameter fields in a sheared nematogenic fluid. In a suitable parameter range, we find irregular, dynamic shear-banding and establish by decisive numerical tests that the chaos we observe in the model is spatiotemporal in nature. We outline the dynamical phase diagram of the model and study the route to the chaotic state. We find that spatiotemporal chaos in this system sets in via a regime of spatiotemporal intermittency. In these studies, the effects of order-parameter fluctuations on the velocity field are neglected. Inclusion of these effects leads to a different steady state with coexistence of spatiotemporal chaos and well-defined shear bands.

This talk is based on work done in collaboration with B. Chakrabarti, D. Chakraborty, M. Das, S. Ramaswamy and A. K. Sood. References: [1] B. Chakrabarti, M. Das, C. Dasgupta, S. Ramaswamy and A.K. Sood, Phys. Rev. Lett. 92, 055501 (2004) and Phys. Rev. E 71, 021707 (2005). [2] D. Chakraborty, C. Dasgupta and A.K. Sood, Phys. Rev. E 82, 065301(Rapid Communication).

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Physics Issues in Graphene and Graphene analogue devices

A. K. Sood Department of Physics

Indian Institute of Science Bangalore-560 012, India

Starting from our studies on graphene based FETs ,I will present our recent in-situ Raman and transport studies of single layer MoS2 transistor. The two Raman modes E2g and A1g show different behaviour with doping. These results are understood by first principal DFT calculations. New results will also be presented on graphene. MoS2 work is done in collaborations with Biswanath Chakraborty, Achintya Bera, Dr. D.V.S. Muthu, Dr. Somnath Bhowmik, Prof. Umesh Waghmare and Prof. C.N.R. Rao.

First-principles Studies of Graphene, BCN and MoS2

Umesh V. Waghmare Theoretical Sciences Unit

J Nehru Centre for Advanced Scientific Research Jakkur PO, Bangalore 560 064 INDIA

We present first-principles theoretical investigations of structure, topological defects and vibrational properties of 2-dimensional materials such as graphene, solid solution BCN of graphene and BN, and MoS2, and identify Raman spectral signatures of various deviations from their ideal forms. Our work leads to prediction of a few surprising phenomena, that arise when these materials are subject to mechanical stress and electrical fields. We present a quasi-continuum theory to understand the mechanism of how certain defects in graphene naturally lead to its spontaneous rippling behavior.

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Metallic behaviour of polymer nanowires

Milan K Sanyal Surface Physics Division, Saha Institute of Nuclear Physics,

1/AF Bidhannagar, Kolkata 700 064, India

Physical properties of quasi-one-dimensional (1D) systems formed in chemically grown nano-wires attract much interest as they provide a unique test bed for studying the theoretically predicted novel characteristics of low-dimensional physics. Conducting polymer nanowires exhibit dramatic resistance switching that reduces resistance by several orders of magnitude above a threshold bias. Here we show that in this low resistance switched state polymer nanowires exhibit characteristics of a Drude metal. Temperature coefficient of resistance is found to be positive in the switched state and negative below the threshold bias. Positive parabolic magnetoresistance has been observed below switching, while switched state does not show any considerable magnetoresistance. We shall also discuss the appearance of a colossal photoconduction property of these polypyrrole nanowires due to the presence of correlated electrons. The photoresponse increases super-linearly and the resistance switching threshold voltage decreases with increasing illumination intensity. References:

1. Bias dependent crossover from variable range hopping to power law characteristics in the resistivity of polymer nanowires, Rahman A. and Sanyal, M.K., J. Phys.: Condens. Matter 22 175301 (2010).

2. Negative capacitance in Wigner crystal forming polymer nanowires, Rahman A. and Sanyal, M.K., Appl. Phys. Lett. 94, 242102 (2009).

3. Novel switching transition of resistance observed in conducting polymer nanowires, Rahman, A. and Sanyal, M.K., Adv. Mater. 19, 3956 (2007).

4. Observation of charge density wave characteristics in conducting polymer nanowires: possibility of Wigner crystallization, Rahman, A., Sanyal, M. K., Phys. Rev. B 76, 045110 (2007).

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The Line Widths and Lifetimes of Raman-active Phonons

Manuel Cardona Max- Planck Institute of Solid State Research, Stuttgart, Germany

I shall start with a short description of my connection with Professor A.K. Sood and our perennial mentor Sir C.V. Raman. and of Sood’s scientific production. A brief bibliometric comparison of Sood’s work and that of Raman will be presented. I shall next introduce the concept of phonons within the harmonic approximation. Anharmonic interactions result in a phonon width, which is also responsible for the phonon decay time (i.e. its lifetime). This effect can be described in terms of a complex self-energy consisting of real and an imaginary part. The imaginary part is related to the phonon width, whereas the real part corresponds to an anharmonic frequency shift. In this talk I shall discuss the self-energy induced by the anharmonic phonon interaction, isotopic disorder, and, in the case of metals, doped semiconductors, and superconductors, by electron-phonon interaction. In the latter cases, the interaction of a phonon with an overlapping background of electronic excitations leads to strikingly asymmetric phonon lineshapes, usually called Fano profiles.

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Nanophysics at Millions of Kelvin

G. Ravindra Kumar Tata Institute of Fundamental Research, Mumbai 400 005

Intense, ultrashort light pulses have revolutionized science in the last two decades. From

providing glimpses into the exotic world at high energy densities to providing cutting edge technologies, from peeking into the ultrasmall to catching the ultrafast, they have provided many breakthroughs and triggered the birth of several new areas of research.

Very fruitful opportunities present themselves when we probe the nanoworld at the high

light intensities available today. An example is the creation of MeV electrons by femtosecond infrared (1.5 eV) laser pulses at intensities above 1018 Wcm-2. It turns out that nanostructures are not just good at creating large fluxes of these electrons, but also very efficient at transporting them over large distances, quite unlike bulk media. How and why does this happen? We present examples from recent experiments performed at TIFR on carbon nanotubes and silicon nanowires and indicate interesting new directions. Illustrative references:

[1] G. Ravindra Kumar, “Intense, ultrashort light and dense, hot matter” Pramana- Journal of Physics, 73, p 113-155 (2009) [Review] [2] P.P. Rajeev et al., Phys. Rev. Lett. 90,115002 (2003); Opt. Lett. 29, 2662 (2004) [3] S. Kahaly et al. Phys.Rev.Lett. 101, 145001 (2008) [4] S. Mondal et al., Phys. Rev. B. 83, 035408 (2011) [5] K. Ohta et al. Phys.Rev.Lett. 104, 055001 (2010) [6] G. Chatterjee et al., (submitted 2011)

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Instabilities and random organization in the driven vortex matter in 2H-NbS2

Satyajit Banerjee Dept. of Physics, IIT Kanpur

The intrigue rich behaviour of vortices in type II superconductors holds similarities with diverse variety of systems like colloids, Wigner crystals, magnetic domains, etc. A common theme linking such diverse systems is the behaviour of an elastic medium driven through a random pinning environment. In type II superconductors, the elastic media is constituted by the collection of vortices whose density and intervortex interaction is easily controlled with an externally applied magnetic field. The random pinning environment is provided by atomic dislocations, vacancies, impurities etc. in the superconductor. In recent times numerous studies have revealed the presence of a diverse variety of static vortex phases, ranging from glassy solid phases to vortex liquid phases. While a lot is known about the nature of the static vortex phases and its equilibrium phase transitions, the same cannot be said about the nature of organization in the driven vortex state. Currently we are studying the phenomenon of current induced depinning of the elastic vortex medium in a type II superconductor (2H-NbS2 (Tc = 6 K)) driven through a quenched random pinning environment. In my talk I will discuss our recent results [1] on the measurement of velocity fluctuations in this driven vortex state. We observe a random organization of the flowing vortex state into an immobile, zero velocity vortex state. The dynamically generated zero-velocity state has properties distinct from the pristine pinned (undriven) vortex state. Attempts to depin this new immobile vortex state into a uniform vortex flow regime have revealed at the depinning threshold, (i) a novel moving vortex phase with a fluctuating fraction of flowing and immobile vortices producing giant velocity excursions and (ii) critical slow down in the time scale associated with the transformation from an immobile state into a flowing state or vice-versa as the critical depinning threshold is approached. The nature of the critical behaviour indicates that the dynamical transition in the driven vortex state shares features with other systems within the same universality class of driven non-equilibrium systems. The vortex-velocity distributions of driven vortex matter vividly demonstrate an underlying bimodal character, a useful basis for chaotic motion in the plastic flow regime. The new findings in vortex matter are akin to dynamical phase transition and random organization recognized in the context of driven colloidal matter. The present work is a continuation in a series of our current work on instabilities and nonlinear response of the vortex matter [1,2,3,4]. References: [1] Gorky Shaw, Pabitra Mandal, S. S. Banerjee, A. Niazi, A. K. Rastogi, A. K. Sood, S. Ramakrishnan and A. K. Grover, Jamming, un-jamming and critical behaviour in driven vortex state (submitted, 2011) . [2] Shyam; Mohan, Jaivardhan Sinha, S. S. Banerjee, Yuri Myasoedov, Phys. Rev. Lett. 98, 027003 (2007). [3] Shyam Mohan, Jaivardhan Sinha, and S. S. Banerjee, A.K. Sood, S. Ramakrishnan and A. K. Grover, Phys. Rev. Lett. 103, 167001 (2009). [4] S. S. Banerjee, Shyam Mohan, Jaivardhan Sinha, Yuri Myasoedov, S. Ramakrishnan and A. K. Grover, Nonlinear response of the static and dynamic phases of the vortex matter, in Superconductivity - Theory and Applications, ed. Assoc. Adir Moysés Luiz, Intech Open Access Publishers, 55 – 84, (2011) http://www.intechopen.com/articles/show/title/nonlinear-response- of-the-static-and-dynamic-phases-of-the-vortex-matter. ________________________________________________________________________


Thermodynamics of protein-DNA binding: a microscopic view of the conformational landscape

Amit Das(1), J. Chakrabarti(1),(2) and Mahua Ghosh(1)

(1)Department of Chemical, Biological And Macromolecular Sciences, S. N. Bose National Center for Basic Sciences, Sector III, Block JD, Salt lake City

Kolkata 700098, INDIA

(2)Advanced Materials Research Unit, S. N. Bose National Center for Basic Sciences, Sector III, Block JD, Salt lake City Kolkata 700098, INDIA

Macromolecular complexes play key role in various cellular processes. A significant contribution to the stabilization of such complexes is derived from the structural changes in the individual components over a large number of distinct binding regions. However, there is no way to know how each binding site participates in the association process. Here we calculate, based on the fundamental principles of statistical mechanics, the thermodynamics of conformational changes of protein-DNA enzyme complexes down to the level of DNA bases and protein residues. We discover that not only the thermodynamics of these enzymes are qualitatively different despite their functional similarity, but also different binding regions contribute non-uniformly to the stability of a given complex. Such analysis is essential for understanding the details of binding mechanism and specificity in macromolecular association.

Self and assisted assembly: Ordered, disordered, confined, driven an jammed states of colloidal particles.

Surajit Sengupta, Centre for Advanced Materials,

Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

In this talk I shall summarize our work on ordering of colloidal particles in a variety of situations. Starting with our earliest attempts at understanding ordering in colloids in collaboration with Prof. Ajay Sood, I shall give an overview of our theoretical and computational work on structural transitions in colloids, confined colloids, colloids in both periodic and random external fields, driven states of colloidal matter, complex and patchy colloids and colloidal assembly during controlled drying of a suspension.

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Disorder and Dynamics in Osmotically Compressed vs Uncompressed Thermo-responsive Microgel Crystals

B.V.R. Tata

Condensed Matter Physics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102, Tamil Nadu, India.

Monodisperse poly(N-isopropylacrylamide) (PNIPAM) microgel particles in aqueous medium exhibit structural ordering similar to that observed in atomic systems. Colloidal crystals of these microgel particles exhibit series of phase transitions with increase in temperature due to reduction in particle size and variation in interparticle interactions. Recently it has been shown that these particles respond not only to temperature but also to external pressure. This talk presents our new findings in regard to the response of PNIPAM particle assemblies to temperature and osmotic pressure as probed by static/dynamic light scattering, confocal microscopy and UV-Visible spectroscopy techniques. The particle dynamics across the melting of colloidal crystals of PNIPAM microgel particles which have undergone osmotic compression is found to be different. The ratio w = DL/DS (where DL is the long-time diffusion coefficient and Ds is the short-time diffusion coefficient) is found to be much smaller than 0.1 at melting/freezing in the case of compressed microgel crystals (violation of dynamical criterion of freezing) and close to 0.1 in the case of uncompressed crystals undergoing melting. The possible reasons for this discrepancy will be discussed. Another finding which is quite surprising is that osmotic compression decreases significantly the average size as well as size polydispersity of PNIPAM particles in dense suspensions resulting in the crystallization of microgel particles, which otherwise remained disordered at ambient conditions due to high size polydispersity. The effect of osmotic pressure on the tunability of Bragg diffraction of PNIPAM microgels crystals will also be presented. These results clearly reveal that PNIPAM soft sphere system not only responds to temperature but also to osmotic pressure and differ from other colloidal systems of hard and charged particles in significant way in their structure, dynamics and phase behaviour.

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Size segregation in a vibrated quasi two dimensional granular column

Ranjini Bandyopadhyay Soft Condensed Matter Group, Raman Research Institute C. V. Raman Avenue, Sadashivanagar, Bangalore 560 080

We experimentally study the dynamics of a large particle (intruder) in a 2-D vibrated column of small grains. The intruder, which is initially kept at the bottom, rises upward when the column is vibrated. We track the large particle as it moves upward using high speed video imaging and analyze the data to estimate its dynamics. We show that the rise of the intruder is driven by granular convection and depends on the peak to peak kinetic energy of shaking for a wide range of intruder densities.

Novel shear�induced phase transitions in a lyotropic mixed surfactant system

Rema Krishnaswamy Jawaharlal Nehru Centre for Advanced Scientific Research,

Jakkur Campus, Bangalore-560064

Non�equilibrium phase transitions in self�assembled amphiphilic systems occur when the internal structure of the fluid is strongly influenced by the imposed shear field. My talk will focus on the shear induced crystallization phenomena and the isotropic to lamellar transitions that we have observed recently in the lamellar and isotropic phases of a cationic�anionic mixed surfactant system [1, 2]. Shear diagrams for the isotropic and lamellar phases obtained by combining rheological measurements with polarizing microscopy, small angle light scattering and in�situ rheological X�ray scattering measurements will be presented. A remarkable characteristic of these transitions is that i) they are reversible under shear ii) they occur at surfactant volume fractions (φ) > 0.5, unlike the earlier flow induced transitions reported for sterically stabilized, dilute (φ <0.15) sponge and lamellar phases consisting of highly flexible bilayers [3]. It is likely that the shear�induced transitions presently observed arise from a redistribution of the counterions in the bilayer rather than a suppression of membrane fluctuations.

References:

1. Shear-induced reversible crystallization in a lyotropic surfactant system,Vikram Rathee, Rema Krishnaswamy, V. A. Raghunathan and A. K. Sood (preprint)

2. Reentrant phase behaviour of an anionic surfactant system with strongly binding counter-ions. S. K. Ghosh, Vikram Rathee, Rema Krishnaswamy, V. A. Raghunathan and A. K. Sood ,Langmuir 25, 8497 (2009).

3. L. Porcar, W. A. Hamilton and P.D. Butler, Langmuir 19, 10779 (2003) and references therein.

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A novel phase of amphiphile bilayers

V. A. Raghunathan Raman Research Institute,C V Raman Avenue,Sadashivanagar,

Bangalore 560 080, India

Many amphiphilic molecules self-assemble in water to form bilayers. In addition to lamellar phases, these amphiphiles also form disordered phases made up of bilayers, such as dispersions of unilamellar vesicles and the sponge phase. In this talk I shall present experimental observations on two very different amphiphile-water systems that indicate the formation of another type of disordered phase of bilayers. A few possible structures of this phase suggested by the experimental data will be discussed.

Do We Need to Revisit the Bohr Exciton Radius?

Padmashri V. Patil and Shouvik Datta Department of Physics,

Indian Institute of Science Education and Research-Pune, Pune – 411021, Maharashtra, India.

The usual definition of Bohr Exciton Radius is widely used to study optical properties of semiconductor nano-particles. Does this classic definition hold good for optical transitions much above the fundamental band gap? Is it still precise enough to be useful to describe transitions involving “hot‟ carriers? Does it get modified in presence of strong dispersions of the dielectric response? In order to investigate these issues we have studied (http://arxiv.org/abs/1105.2205v2) collisional broadening of the well established E3 excitonic absorption spectra of PbS nano-crystallites at energies much above its fundamental band gap. About this E3 exciton, reader is referred to figure 12 and subsequent discussions in Phys. Rev. B 8, 1477 (1973) and the Table 1 in the same paper for a detailed list of such higher energy excitonic transitions in lead salts. We have observed similar spectral features which clearly demonstrate the significance of extended band structure to understand the physics of exciton scattering events in semiconductor nanoparticles. Our analyses also show that quasi ballistic transport of hot excitons can actually suppress exciton-exciton scattering events required for photo-induced impact ionization in very small quantum dots. Currently, these higher energy excitonic transitions are not very well investigated in the literature even though they are very much relevant for events like multiple exciton generation or carrier multiplications in the context of next generation nano-photovoltaic devices. Most carrier multiplication papers do not even report any optical absorption spectra of the semiconductor nano-particles spanning the whole photon energy range relevant to the problem. Here we are trying to address these important issues about the very nature of higher energy excited states of 'hot' excitons in PbS nano-particles. We hope that this work will be useful to optimize the size of semiconductor nanoparticles meant to exploit such carrier multiplications events required for enhancing the power conversion efficiency of nano-photovoltaic cells. ________________________________________________________________________________


We have also introduced the concept of ‘effective Bohr exciton radius’ as a direct consequence of significant departure from the usual dielectric screening limits of coulomb interactions of „hot‟ excitons in the region of strong dispersion at energies much above the fundamental band gap. Using this ‘effective Bohr exciton radius’ one can estimate values of the all important mean free path for exciton-exciton collisions in the context of multiple exciton generation. We have observed that spectral broadening induced by exciton-exciton collisions actually decrease in nanoparticles having diameters both smaller and larger than this mean free path. Therefore, we suggest an “optimum size window” for these nanoparticles to exploit such carrier multiplications events to improve the power conversion efficiency in the next generation nano-photovoltaic cells. Further supports of the above analyses arise from studies of temperature and aging dependent spectral behaviors of the excitonic spectra.

Controlling magnetism in graphene supported on Ni(111) surface through site selective hydrogenation

Niharika Joshi and Prasenjit Ghosh

Department of Chemistry and Physics, Indian Institute of Science Education and Research (IISER),

Pune-411008, Maharashtra, India.

Graphene, a two-dimensional honeycomb lattice of sp 2 bonded C atoms, has shown lot of exceptional transport properties like very high carrier mobility, long electronic mean free path and negligible spin orbit coupling, leading to large spin relaxation time which render this material ideal for ballistic spin transport. Recently through x-ray magnetic circular dichroism studies Dedkov et al. showed that there is induced magnetism in C atoms (0.05-0.1 µB ) when graphene is supported on Ni(111) surface.[1] They also showed that the magnetic moments of the C atoms increases when Fe atoms are intercalated at the graphene Ni(111) interfaces.[2] In this work we have performed ab initio density functional theory based calculations to study the change in the induced magnetic moments in the C atoms of graphene supported on Ni(111) system at different hydrogen coverages. The two inequivalent C atoms of the clean graphene occupies the top and fcc sites with respect to the Ni atoms. Upon hydrogenating the top C atoms of graphene supported on Ni(111) with a hydrogen coverage of 0.125 ML, we find that there is significant increase in the magnetic moments of the C atoms. However, on increasing the H coverage to 0.5 ML, not only the C atoms become non-magnetic, but also the magnetism in the Ni layer which is at the graphene-Ni(111) interface is substantially quenched. However, if the hydrogen is bound to the fcc C-atoms, the total magnetic moment of the graphene sheet becomes zero. Our results suggest that if we can experimentally control the hydrogenation of the graphene sheet such that they occupy the top C atoms, then we can tune the induced magnetic moment of the C-atoms without actually using a magnetic material like Fe. References [1] M. Weser, Y. Rehder, K. Horn, M. Sicot, M. Fonin, A. B. Preobrajenski, E. N. Voloshina, E. Goering and Y. S. Dedkov, App. Phys. Lett. 96, 012504 (2010). [2] M. Weser, E. N. Voloshina, K. Horn and Y. S. Dedkov, Phys. Chem. Chem. Phys. 13, 7534 (2011)

________________________________________________________________________________________________ “CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011


ABSTRACTS OF POSTERS

Discontinuous shear thickening in confined dilute carbon nanotube suspensions

Sayantan Majumdar (a), Rema Krishnaswamy (b), and A. K. Sood (a),

(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.

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).

________________________________________________________________________________________________ “CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011


Effect of Anionic Surfactant on the Jamming Behaviour of Triblock Copolymer System.

Rajib Basak

Senior Research Fellow, Rheology and Light Scattering Lab, Soft Condensed Matter Group, Raman Research Institute,

C. V. Raman Avenue, Sadashivanagar Bangalore 560 080

Photon correlation spectroscopy and rheological measurements are performed to investigate the microscopic dynamics and mechanical responses of aqueous solutions of triblock copolymers and aqueous mixtures of triblock copolymers and anionic surfactants. Increasing the concentration of triblock copolymers in aqueous solution results in a sharp increase of the sample viscosity. This is understood in terms of the changes in the aggregation and packing behaviours of the copolymers and the constraints imposed upon their dynamics due to increased close packing. The addition of suitable quantities of an anionic surfactant to highly viscous copolymer solutions results in a decrease of the sample viscosity by several decades. We argue that the shape anisotropy and size polydispersity of the micelles comprising mixtures cause dramatic changes in the packing behaviour, resulting in sample unjamming and the observed decrease in viscosity. We construct a phase diagram in the temperature-surfactant concentration plane to summarise the jamming- unjamming behaviour of aggregates constituting triblock copolymer-anionic surfactant mixtures.

Synthesis and Characterization of valence compensated Perovskite

System Ba1-xK xSn1-xNbxO3 *Raghvendra , P. Singh, A.K. Pathaka and A.K. Raia

Department of Applied Physics, Institute of Technology, Banaras Hindu University, Varanasi-221005

a Department of Physics, Allahabad University, Allahabad. Barium stannate is a perovskite oxide having cubic unit cell. It forms a component of ceramic dielectric bodies used for thermally stable capacitors. BaSnO3 is also used as a humidity sensor. A few compositions of valence compensated system, Ba1-xKxSn1−xNbxO3 (x = 0.001, 0.005, 0.010 and 0.020) were synthesized using solid state route. All these were found to be single phase having cubic structure similar to BaSnO3 in their powder X-ray diffraction patterns. Frequency and temperature dependent conductivity were measured to understand the conductivity mechanism in these samples.Conductivity measurement revealed that the overall conductiviy process is due to the contribution of random and hopping motions of the charge particles. Impedance and dielectric behaviour of the system were also studied. Laser-induced breakdown spectroscopy (LIBS) was used for the elemental analysis of the system. The results of LIBS analysis are still awaited.



1/f noise as a probe to investigate the band structure and time reversal symmetry in 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 depend 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. At low temperature, when the phase coherence length of electrons/holes is greater than the mean free path of the system, these devices were found to show quantum interference effect. In case of graphene, both weak localization and antilocalization can be observed when the magnetic field is applied. We have measured noise in presence of magnetic field at low temperature, mainly in single layer graphene. We found that the noise is governed by the UCF (universal conductance fluctuation) and for the first time, we show that the noise magnitude goes down by exactly half the magnitude, when the time reversal symmetry is broken. 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).

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Phase Behaviour of Lipid Based Lyotropic Liquid Crystals in Presence of Colloidal Nanoparticles

Edakkal Venugopal†, Suresh K. Bhat†

Jijo J. Vallooran‡ and Raffaele Mezzenga*‡ †Complex Fluids and Polymer Engineering,

National Chemical Laboratory, Pune – 411008, India ‡Food and Soft Materials, Institute of Food, Nutrition & Health,

ETH Zurich, 8092, Switzerland We have investigated the microstructure and phase behavior of monoglyceride-based lyotropic liquid crystals in the presence of hydrophilic silica colloidal particles of size comparable to orslightly exceeding the repeat units of the different liquid crystalline phases. Using small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC), we compare the structural properties of the neat mesophases with those of the systems containing silica colloidal particles. It is found that the colloidal particles always macrophase separate in inverse bicontinuous cubic phases of gyroid (Ia3d) and double diamond (Pn3m) symmetries. SAXS data for the inverse columnar hexagonal phase (H II) and lamellar phase (Lα) suggest that a low volume fraction of thenanoparticles can be accommodated within the mesophases, but that at concentrations above a given threshold, the particles do macrophase separate also in these systems. The behavior is interpreted in terms of the enthalpic and entropic interactions of the nanoparticles with the lamellar and hexagonal phases, and we propose that, in the low concentration limit, the nanoparticles are acting as point defects within the mesophases and, upon further increase in concentration, initiate nucleation of nanoparticles clusters, leading to a macroscopic phase separation. References: (1) deGennes, P. G. The Physics of Liquid Crystals; Oxford University Press: Oxford, England, 1974. (2) Venugopal, E. et.al, Langmuir, 2011, 27, 9792-9800 (3) Mezzenga, R.; Schurtenberger, P.; Burbidge, A.; Michel, M. Understanding foods as soft materials. Nat. Mater. 2005, 4, 729–740. (4) Israelachvili, J. N., Intermolecular and Surface Forces, 2nd ed.; Academic Press Ltd: London, 1992.

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Title:TBA

Anirban Pal Research Scholar, Tata Institute of Fundamental Research,

Mumbai, India* The local electric field at the tips of an array of vertically aligned metal nanorods is very large due to the ‘lightning rod’ effect. A controlled clustering of such nanorods may greatly enhance this effect depending on the cluster size. Here we report interesting aspects of our study of the optical properties of such clustered nanorod arrays. Our study relates to the polarization dependence of surface enhanced Raman scattering from a clustered nanorod array. The polarization measurements were done with nanorods of length between 2 µm and 50 µm and having different cluster geometries. It was found that SERS intensity is directly dependent on the anisotropy of the nanorod clusters. Thus, the polarization characteristics of the SERS intensity can be used as a tool to study the structural anisotropy of the nanorod clusters.

Unconventional spin-wave excitations in composite fermions ferromagnets

Dwipesh Majumder1 1. Department of Physics,

Santipur College, Santipur, Nadia, India The fractional quantum Hall effect (FQHE) in the lowest (n=0) Landau level can be understood as integer quantum Hall effect of composite fermions (CF), electrons with an even number of flux quanta attached. CF form Landau-like levels, so called Λ-levels (ΛL) in a reduced effective mag-netic field. Magnetic order linked to spin and topological order linked to bound vortices of quasi-particles dictates the character of states in the FQHE. We find unexpected low energy excitations of fully spin polarized composite fermion ferromagnets in the FQH liquid arising from complex interplays between topological and spin order. The lowest energy modes, which involve spin reversal, are re-markable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin- flip excitonic state of CF particle-hole pairs. The striking properties of spin-flip excitons emanate from highly tun- able mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets. In the small wave vector limit (q → 0), Larmor’s theorem stipulates that the spin-wave (SW) energy is precisely equal to the bare Zeeman energy EZ = gµB Btot , where g is the Lande factor, µB is the Bohr magneton and Btot the total applied magnetic field. For a conventional fer- romagnet, such as the one at filling factor one, we expect that the SW has positive dispersion with energy that increases monotonically with wave vector reaching a large wave vector asymptotic limit of EZ , the sum of the EZ and the Coulomb energy required to create a particle-hole pair with reversed spin. Here we report that CF ferromagnets exhibit a fundamentally different behavior. Our spin-reversed study shows that there are some different mode of excitation rather than SW, what we call spin-flip excitons (SFEs), namely excitations in which a CF quasiparticle is promoted to a spin reversed level with a lowerΛ-level quantum number. _______________________________________________________________________________________________


The SFEs produce one or more deep roton minima at finite wave vectors and complexe mode mixing creates a negative dispersion of the SW at small wave vectors. This behaviour is fundamentally different from the dispersion of the conventional SW of electron ferromagnets. The appearance of roton minima below the bare Zeeman energy is a Precursor of a transfomation of the ferromagnetic state at sufficiently low Zeeman energies. References: 1. U. Wurstbauer, D. Majumder, S. S. Mandal, I. Dujovne, T. D. Rhone, B. S. Dennis, A. F. Rigosi, J. K. Jain, A. Pinczuk, K. W. West, and L. N. Pfeiffer Phys. Rev. Lett. 107, 066804 (2011) 2. T. D. Rhone, D. Majumder, B. S. Dennis, C. Hir- jibehedin, I. Dujovne, J. G. Groshaus, Y. Gallais, J. K. Jain, S. S. Mandal, A. Pinczuk, L. Pfeiffer, and K. West Phys. Rev. Lett. 106, 096803 (2011)

Optical properties of terbium doped lead-fluoroborate glass

B.P. Singha, P. Singha, S.B.Raib

aDepartment of Applied Physics, IT-BHU, Varanasi-221005 bDepartment of Physics, BHU, Varanasi-221005

Rare earth doped glasses find potential applications in laser material, optical amplifier, optoelectronic devices etc. Terbium doped lead borate glass samples were prepared by melt quench method. Emission, absorption, XRD and FT-IR spectra of the samples were studied. And enhancement in emission intensity was recorded. XRD of the prepared samples revealed the amorphous nature of glassy materials. The FT-IR analysis signified the characteristics of B-O stretching mode and confirmed the vitreous nature of Tb4+ doped borate glass. _______________________________________________________________________________________________


Structural and Magnetic Properties of Multiferroic BiFeO3 Nanoparticles Synthesized by a Novel Combustion Method

Samar Layek* and H.C. Verma

Department of Physics, Indian Institute of Technology, Kanpur 208016, India

*Corresponding AuthorEmail: [email protected]

Multiferroic materials are those which show more than one ferroic or anti-ferroic properties (like ferromagnetic, ferroelectric, ferroelastic etc,) in the single phase sample. In these material ferroic properties are coupled with each other [1]. Bismuth ferrite (BiFeO3) is one of the examples of the multiferroic material crystallizes in the distorted perovskite structure. It shows G-type antiferromagnetic ordering below Neel temperature TN = 370 0C and ferroeletricity below TC = 830 0C [2]. Pure BiFeO3 nanoparticles have been successfully synthesized by a novel combustion method without using any solvent. High purity nitrates and citric acid, which had been used as fuel, heated at 180 0C on a hot plate to produce brown colored precursor. The precursor was then subsequently annealed to prepare BiFeO3 nanoparticles. These particles have been characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), magnetization and 57Fe Mössbauer spectroscopy in order to investigated structural and magnetic properties. Rietveld refinement of the powder room temperature XRD pattern shows that prepared nanoparticles are single phase in nature and crystallize in rhombohedral distorted perovskite structure (space group-R3c). Average particle size is calculated to be about 45 nm. TEM micrograph confirms that particles are nearly spherical in nature. Room temperature ferromagnetism with coercive field of about 180 Oe has been observed in these particles by VSM measurement. Though the magnetization does not saturate upto highest applied magnetic field (1.75 T), magnetic moment in these nanoparticles increases about 2 times than that of bulk BiFeO3 sample. Increase in magnetism may arise due to suppression of spin spiral structure due to finite size effect. Single magnetic sextet with hyperfine field of about 49.8 T corresponding to Fe3+ state (isomer shift 0.35 mm/s) is resulted from the 57Fe Mössbauer spectroscopy study. Keywords: BiFeO3, Multiferroic, Combustion Method, Spin Spiral Structure, 57Fe Mössbauer spectroscopy. Reference: [1] W. Eerenstein, N. D. Mathur, and J. F. Scott, Naure 442, (2006) 759. [2] F. Lin, D. Jiang, X. Ma and W. Shi, J.Magn. Magn. Mater. 320, (2008) 691. _______________________________________________________________________________________________


Room temperature ferromagnetism in Mn-doped NiO nanoparticles

Samar Layek*and H. C. Verma Department of Physics,

Indian Institute of Technology, Kanpur-208016, India

*Corresponding author, Email: [email protected] Stoichiometric nickel oxide (NiO) crystallizes in cubic NaCl-type structure, which is well known as a strongly correlated Mott-Hubbard insulator (optical band gap is about 4eV), exhibits antiferromagnetic order below TN= 523 K [1]. It can be transformed into p-type semiconductor after the development of Ni2+ vacancies or doping with other cation.Recently room temperature ferromagnetism has been found in Fe-doped NiO samples [2,3]. We have synthesized Ni1-xMnxO (x= 0.00, 0.02, 0.04, 0.06 and 0.08) nanoparticles by a simple low temperature hydrothermal method [4]. High purity nickel and manganese acetate were taken in appropriate stoichiometric ratio and mixed together in 100ml double distilled water in a glass beaker for about 2 hours. The solution was heated at 80C on a magnetic stirrer for about 1 hour and then slowly cooled in air. The cooled solution was finally dried at 100 0C for 24 hours to form precursor. The precursors were grounded into powders and annealed using quartz crucibles in air at 400 0C for 4 hours. XRD results show that samples upto 6% of Mn-doping crystallizes in single phase fcc structure. TEM micrographs show that particles are non uniform in nature and 24-38 nm in diameter. Pure NiO nanoparticle shows weak ferromagnetism may be due to nanosize nature and the introduction of Mn within NiO lattice improves the ferromagnetism property significantly with Curie temperature above room temperature and anomalously high magnetic moment. Magnetization is highest for 2% Mn-doped samples and decreases for higher Mn-doping. References: [1] L.C.Bartel and B.Morosin, Phys.Rev.B 3,1039 (1971). [2] Y. H. Lin, J. F. Wang, J. N. Cai, M. H. Ying, R. J. Zhao, M. Li, and C.W. Nan, Phys. Rev. B 73, 193308 (2006). [3] J.H.He, S.L.Yuan, Y.S.Yin, Z.M.Tian, P.Li, Y.Q.Wang, K.L.Liu, and C.H.Wang J. App. Phys. 103, 023906 (2008). [4] C. N. R. Rao and F. L. Deepak, J. Mat. Chem., 15, 573 (2005)

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Magnetization reversal dynamics in Co nanowires with competing magnetic anisotropies

Semanti Pal, Susmita Saha, Debanjan Polley and Anjan Barman*

Department of Material Sciences, S. N. Bose National Centre for Basic Sciences,

Block JD, Sector III, Salt Lake, Kolkata, 700098, India.

Densely packed ordered arrays of magnetic nanowires with large aspect ratios (R) have tremendous potentials in technological applications including in magnetic storage [1], field sensors [2], logic devices [3], and magnonic crystals [4] due to the large volumes of the individual nanowires and large shape anisotropy along the wires axis, associated with larger areal density. If the direction of shape anisotropy and magneto-crystalline anisotropy are different then there may arise a competition between them. Here, we present a systematic study of the magnetization reversal mechanisms in Co nanowires, where the competing magnetic anisotropies are observed. To avoid the magnetostatic interactions between the nanowires, we have grown them in track etched polycarbonate membranes (PCTE, diameter: 95 nm ± 5 nm) with very low pore density using the electrodeposition technique. By varying the deposition time between 2000 s and 80 s, the lengths of the nanowires are obtained between 5.75 �m and 240 nm, which correspond to a variation of aspect ratio between about 60 and 2.5. The magnetization (M) as a function of applied magnetic field (H) for three different samples measured by VSM at T = 300 K. From static magnetometry it is found that by varying R between 2.5 and 60, we observe a cross-over of the directions of the magnetic easy and hard axis at R = 6.8 due to the competition between the two anisotropies. Micromagnetic simulations qualitatively reproduce the observed cross-over and give detailed insight to the reversal mechanisms associated with the cross-over. From the micromagnetic simulations of single nanowire, it is observed that the magnetization reversal mechanism changes from quasi-coherent rotation to formation of Bloch domains to the corkscrew [5]-like reversal mode for the long-axis loop with the decrease in the aspect ratio and the corresponding shape anisotropy. At the cross-over of the magnetic easy-axis from the long- to the short-axis of the nanowires a transformation of the reversal mechanism from quasi-coherent rotation of magnetization to Bloch-domain formation occurs. On the other hand, for aspect ratios below the cross-over region, the reversal mode in the short-axis loop transformed from nucleation and propagation of reversed domains to quasi-coherent rotation. These observations are important both from the viewpoints of fundamental science and to the applications of magnetic nanowires in various devices. Acknowledgements: We gratefully acknowledge the financial assistances from Department of Science and Technology, Govt. of India under the India-EU collaborative project "DYNAMAG" (grant number INT/EC/CMS (24/233552)) and the Nano Mission (grant number SR/NM/NS-09/2007). References

[1] B. D. Terris and T. Thomson, J. Phys. D: Appl. Phys. 38 (2005) R199. [2] E. A. Anderson, S. Isaacman, D. S. Peabody, E. Y. Wang, J. W. Canary and K. Kirshenbaum, Nano Lett. 6 (2006) 1160. [3] D. A. Allwood, G. Xiong, C. C. Faulkner, D. Atkinson, D. Petit and R. P. Cowburn, Science 309 (2005) 1688. [4] S. Neusser and D. Grundler, Adv. Mater. 21 (2009) 2927. [5] R. Hertel and J. Kirschner, Physica B 343 (2004) 206. _______________________________________________________________________________________________


Raman studies on phonon anomalies in LaCo0.5Mn0.5O3 and YFe0.6Mn 0.4O3

Venkata Srinu Bhadram†, Chandrabhas Narayana*, P. Mandal and A. Sundaresan

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

Bangalore-560064, India M. Viswanathan and P. S. Anil Kumar

Department of Physics, Indian Institute of Science, Bangalore-560012, India LaCo0.5Mn0.5O3 is a ferromagnetic perovskite with monoclinic structure and can exists in two phases with distinct curie temperatures (Tc) 138K and 243K. Such intriguing characteristics are assigned to lattice effects. From the temperature dependent Raman studies we found that the stretching mode frequency deviates from its regular anharmonic dependence ωanh(T) = ωo-C(1+(2/e(hωo/2Πk

BT)-1)), for T< Tc in both the phases. This anomalous behavior is attributed to the

spin-phonon coupling near Tc. The full width at half maximum (FWHM) of the Raman modes which are low in high Tc phase indicates the higher B-site ordering in this phase compared to low Tc phase. Similar studies were done on Orthorhombic YFe0.6Mn0.4O3 which is an antiferromagnet below TN

(370K). In the antiferromagnetic phase a spin reorientation transition and ferroelectric transitions are observed at TSR (320K) and TC (120K) respectively. Magnetodielctric coupling is seen at TN, TSR and magnetoelectric coupling is observed at TC. Temperature dependent Raman stretching modes show anomalies at all these transition temperatures indicating the spin-phonon coupling. Our results suggest that the spin-phonon coupling is involved in magnetodielectric effect and ferroelectricity in YFe0.6Mn0.4O3 although it is not clear whether these two effects have a common origin.



Time-resolved Excitation and Detection of GHz Frequency Magnonic Spectra in Nanoscale Cobalt Antidot Lattices

R. Mandal, S. Saha, D. Kumar, S. Pal, B. Rana, K. Das,

A. K. Raychaudhuri and A. Barman Department of Materials Science, S. N. Bose National Centre for Basic Sciences, Block JD,

Sector III, Salt Lake, Kolkata 700 098 Y. Fukuma2 and Y. Otani 2,3

2RIKEN ASI, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 3Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba

277-8581, Japan

Ordered arrays of magnetic nanostructures inspire a number of technological developments including magnetic storage, memory, sensors, biomedicine and diagnostics. Emerging areas such as spin torque nano-oscillators and magnonic crystals show exciting promises in the development of on-chip microwave sources and data communications and processing. Magnonic crystals are based upon the manipulation of magnonic band structures in spatially modulated magnetic structures and magnetic antidot lattices are one of the prime candidates for magnonic crystals [1-2]. Among the various issues in the successful development of magnonic crystals are the fabrication of high quality ordered nanoscale structures and control of magnonic spectra in such structures. Here we present the fabrication and time-resolved excitation and detection of magnonic spectra in Cobalt antidot lattices with 100 nm antidot diameter and with variable lattice constants of 200nm, 300nm, 400nm and 500nm. The antidotes were prepared by focused ion beam lithography on 20 nm Co films deposited by electron beam evaporation. The magnetic parameters of the unpatterned film were determined by vibrating sample magnetometery and the scanning electron micrographs confirm the physical structures of the antidot lattices. The magnonic spectra was excited and detected by a home built all-optical time-resolved magneto-optical Kerr effect microscope using a femtosecond laser with pulse width < 80 fs. The magnonic spectra was measured as a function of the lattice constant, and strength and relative orientation of the bias magnetic field with the lattices. Two distinct bands of magnons are observed in the spectra and significant tunability of both band positions and hence the bandgaps are observed with the variation of lattice constants and bias magnetic field. The observations are interpreted by performing finite difference method based micromagnetic simulations, which shows that the formation of demagnetized regions within the anitidot lattices modify the microscopic magnetic potentials to the propagating magnons varying the magnonic band structures. Acknowledgement: We gratefully acknowledge the financial supports from Department of Science and Technology, Government of India under the grant numbers SR/NM/NS-09/2007 and INT/JP/JST/P-23/09 and Japan Science and Technology Agency Strategic International Cooperative Program under the grant numbers 09158876. . [1] Chumak, A. V.; Tiberkevich, V. S.; Karenowska, A. D.; Serga, A. A.; Gregg, J. F.; Slavin, A. N.; Hillebrands, B. Nat. Commun. 2010, 1, 141. [2] Neusser, S.; Grundler, D. Adv. Mater. 2009, 21, 2927. _______________________________________________________________________________________________


Domain wall motion in notched nanowire by spin-transfer torque

A. Ganguly, A. Barman and S. Barman S. N. Bose National Centre for Basic Sciences

JD Block, Saltlake, Sector III, Kolkata 700098, India

We have investigated domain wall motion in notched permalloy nanowires driven by spin polarized current. We solve modified Landau-Lifshitz Gilbert equation including the effect of spin-transfer torque by Micromagnetic Simulation. The ferromagnetic ground states for the studied geometries shows vortex domain walls located at different positions depending upon the position, shape, dimension and symmetry of the notch. We have investigated the effect of the spin torque on domain-wall displacement and domain-wall velocity. We find that the domain wall velocity is higher for single notched nanowire as compared to multiple notches. We also find that the critical spin current density, above which the domain wall motion starts, decreases with increasing temperature. Acknowledgement: We gratefully acknowledge the financial supports from Department of Science and Technology, Government of India under the grant number SR/NM/NS-09/2007. Perovskite Oxides at High Pressures: Raman and X-Ray Diffraction Study

Abhisek Basu, Sanhita Paul, Satyabrata Raj and Goutam Dev Mukherjee

Department of Physical Sciences, Indian Institute of Science Education and Research Kolkata,

Mohanpur Campus, Nadia 741252, India Understanding of perovskite oxides with general formula ABO3 that consists of corner linked array of BO6 octahedra with A atoms at the centre of the unit cell hold great importance to both fundamental and application related research. These oxide systems exhibit many interesting phase transition behaviour induced by structural distortions, which can occur due to two main reasons: a rotation (tilt) of the BO6 octahedra and/or cationic displacement. Application of high pressure can tune the lattice parameters precisely and hence will help to evaluate the effects of volume contraction on their structures and properties due to the above structural distortions.

In this work we report high pressure investigations on two different types of perovskite oxide

systems: electrochromic Na0.025WO3 and ferroelectric Pb1-xCaxTiO3. High pressure was generated using a diamond anvil cell. High pressure Raman spectra were measured using Horiba Jobin-Yvon LabRAMHR 800 micro-Raman spectrometer with 1800 g/mm grating (resolution ~ 2 cm-1) by exciting the sample with 488 nm line of Ar+ ion laser. High pressure x-ray diffraction (XRD) measurements were carried out at the Elettra synchrotron light source, Italy using a monochromatic x-ray radiation (λ=0.6888 Å).

High pressure investigations on Na0.025WO3 reveal a phase transition at about 2 GPa by a change in

space group symmetry in the monoclinic lattice followed by a second structural transition to a triclinic lattice at about 18 GPa [1].

___________________________________________________________________________


There are large volume changes associated with these transitions, which are driven by rotation and significant distortions of WO6 octahedra. Investigations on Pb1-xCaxTiO3 show a ferroelectric to paraelectric transition induced by a tetragonal to cubic structural transition with pressure [2]. However persistence of certain Raman bands in the paraelectric phase of these systems indicate to the local polar clusters in these systems, even they show an overall cubic structure from x-ray diffraction measurements. On further increase in pressure these systems show a re-entrant ferroelectric behaviour with a transition to polar tetragonal phase. However these transition pressures can be tuned with Ca content. [1] A. Basu, S. Paul, M. Polentarutti, G. Bais, S. Oishi, S. Raj and G.D. Mukherjee, J. Phs.:Condens. Matter 23, 365401 (2011). [2] A. Chandra, A. K. Tyagi, V. Vijaykumar, G. D. Mukherjee and R. Boehler, J Electroceram. 26, 191 (2011). [3] A. Basu, A. Chandra, A. K. Tyagi, and G. D. Mukherjee, Communicated.

Ultrafast Precessional Dynamics of Magnetization in Square Arrays

of Thin Magnetic Nanoelements

B. Rana, D. Kumar, S. Barman, R. Mandal, S. Pal and A. Barman Department of Materials Science,

S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700 098

Y. Fukuma2 and Y. Otani 2,3

2RIKEN ASI, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 3Institute for Solid State Physics,

University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan

Nanomagnet arrays are technologically important because of their potential applications in magnetic storage [1], memory, logic [2], sensors and magnonic crystals [3]. The precessional dynamics occurring in the picosecond timescale can potentially increase the operation time for all these devices. It is also very important to study the ultrafast magnetization dynamics at various length scales. Here we present all optical

excitation and detection of magnetization dynamics of square arrays of square permalloy (Ni81Fe19) nanoelements with dimensions varies as 50 nm, 75 nm, 100 nm, 150 nm and 200 nm and also edge to edge separation varying from 50nm to 200 nm in the similar way keeping width to separation ratio constant (one). The scanning electron micrographs of the dot arrays show that the samples are well fabricated except a small rounded edge. The magnetization of the nanoelements were excited and detected by a home built all-optical time-resolved magneto-optical Kerr effect microscope using a Ti-sapphire mode locked femtosecond laser with pulse width < 80 fs and repeatation rate 80 MHz. Experimental results show that for the dot arrays with width (W) = 50 nm and separation (S) = 50 nm, two closely spaced modes are observed in the frequency domain. Lower mode corresponds to backward volume mode and higher mode corresponds to uniform collective mode. Whereas for W = 75 nm and S = 75 nm, same kind of trend is observed with an additional higher frequency mode. The lowest frequency mode is a kind of edge mode for the whole array. But the mode at the middle comes from the edges of the individual elements and higher mode comes from the uniform excitation of the whole array. For W = 100 nm, S = 100 nm, only two modes are observed, which indicates the transition from a single domain to a multi-domain region. The lower frequency mode is the edge mode of the individual elements and higher frequency mode is the uniform mode. For W =200 nm and S = 200 nm another transition is observed. In this case three well separated modes are observed correspond to long wavelength collective mode, edge mode and centre mode. ________________________________________________________________________________


Acknowledgement: We gratefully acknowledge the financial supports from Department of Science and Technology, Government of India under the grant numbers SR/NM/NS-09/2007 and INT/JP/JST/P-23/09 and Japan Science and Technology Agency Strategic International Cooperative Program under the grant numbers 09158876. .[1] T. Thomson, G. Hu, and B. D. Terris, Phys. Rev. Lett. 96, 257204 (2006). [2] A. Imre, G. Csaba, L. Ji, A. Orlov, G. H. Bernstein, and W. Porod, Science 311, 205 (2006). [3] S. Neusser, D. Grundler, Adv. Mater. 21, 2927 (2009).

Effect of laser modulation on network formers

Chandana Mondal Centre for Advanced Materials,

Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India Using molecular dynamics simulations we study the effect of an external periodic field on a simple model system of particles in two dimensions. In addition to translational degrees of freedom, each particle has two internal states and interacts with a modified Lennard-Jones potential which depends on relative positions as well as the internal states. We find that, despite its simplicity, the model has a rich phase diagram showing many features of common network-forming liquids such as water and silica, including polymorphism and thermodynamic anomalies viz. density maxima(pressure minima), increase in the isothermal compressibility etc. When an external laser modulation with period of the triangular lattice is applied along y-direction, the honeycomb-triangular-liquid triple point shifts towards higher temperature. The higher the amplitude(V 0) of the modulating field, the higher the triple temperature as well as the temperature of minimum pressure. With the increase in V 0 the range of temperature, where the anomalous, network-forming liquid is stable, decreases and beyond a certain value of V 0 the liquid shows no anomalous behaviour.

Magnetization Reversal in Magnetic Nanoparticles and Microparticles of Different Shapes and Sizes

B. K. Mahato, A. Ganguly, B. Rana and A. Barman

Department of Materials Science, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake, Kolkata 700

098

Magnetic nanoparticles have a number of potential applications in magnetic data storage, biomedicine and biotechnology, catalysis, magnetic fluids and magnonic crystals. For most of the applications ordered arrays of nanoparticles are required. It is also important to know the magnetization reversal of magnetic nanoparticles for a number of applications. For a single domain magnetic nanoparticle the reversal occurs through coherent rotation of magnetization and rectangular hysteresis loop is observed [1]. In this case strong short range exchange interaction controls the reversal. Whereas for multidomain nanoparticle, reversal occurs through the formation of different kinds of well defined domain states, like C-state, S-state, flower state, vortex state etc. depending upon the size and shape of the nanoparticles and microparticles. Here the reversal mechanism is mainly controlled by long range dipolar interaction. For a microparticle with well defined shape, the reversal mechanism also depends on the direction of the applied field. Here we present the reversal mechanism of different kinds of cluster, chains of single domain Ni nanoparticles.

___________________________________________________________________________ “CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011


We have also shown the reversal mechanism of multidomain Co-microparticles with spherical and hexagonal and octahedral shape and made a comparison among themselves. The nanoparticles and microparticles were synthesized chemically using reduction method [2] and solvothermal method and experimentally the hysteresis loops were measured by vibration sample magnetometer (VSM) at room temperature. We have explained the reversal mechanism with the aid of micromagnetic simulation using finite different method and finite element method. Acknowledgement: We gratefully acknowledge the financial assistances from Department of Science and Technology, Govt. of India under the India-EU collaborative project "DYNAMAG" (grant number INT/EC/CMS (24/233552)) and the Nano Mission (grant number SR/NM/NS-09/2007). . [1] B. Rana et al., J. Appl. Phys. 107, 09B513 (2010). [2] Szu-han et al, J. Colloid and Interface Sci. 259, 282 (2003).

Caging Dynamics in Colloidal Glasses of Laponite Debasish Saha Research Scholar,

Rheology and Light Scattering Laboratory, Soft Condensed Matter Group, Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore- 560 080

The glassy state refers to an out-of-equilibrium, disordered state of matter which originates due to the jamming or freezing of the constituents comprising the system. In this jammed or frozen state, the motion of the constituents appears to be arrested and the dynamics of the system continues to slow down with time in a phenomenon known as "aging". Colloidal glasses are soft glasses (i.e. they have a very low shear modulus when compared to hard glasses) and exhibit all the properties typically expected in hard glassy materials, for example, jamming, disorder and non-equilibrium aging behavior. In this poster, we study glass formation in the context of aging colloidal suspensions prepared by dissolving a glass former (Laponite- a synthetic hectorite clay) in deionized water. Incolloidal glasses, the constituent particles or clusters are trapped in cages formed by their neighbors, which restricts their motion and slows down their dynamics. Such caging dynamics, whose time scales are typically expected to be ~100 microsecond, are studied in Dynamic Light Scattering (DLS) experiments. The intensity autocorrelation function (ACF) exhibited by a glassy Laponite suspension has a stretched exponential form due to the presence of competing relaxation rates in the system. The origin of this non-exponential decay in aging colloidal glasses is investigated by performing rheology, mobility and conductivity experiments. A simple toy model is developed to understand the results.

___________________________________________________________________________ “CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011


Raman signatures of pressure induced electronic topological and structural transitions in Bi2Te3.

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-1and 135 cm-1. The mode Grüneisen parameters are determined in both α and β-phases.

Magneto Optic investigation of coexistence of superconductivity and magnetism in Iron-Arsenide superconductor

Pabitra Mandal1, Gorky Shaw1, and S. S. Banerjee1,1, Neeraj Kumar2, S .K. Dhar2, A.Thamizhavel2

1Department of Physics, Indian Institute of Technology, Kanpur-208016, India

2Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai- 400005, India

Studies on recently discovered pnictide superconductors reveal that superconductivity and magnetism appear in close proximity of each other. The parent pnictide compound exhibits long range antiferromagnetic orderi,ii which is suppressed by doping and superconductivity appears. In some compounds of this class show an abrupt suppression of antiferromagnetic order at the onset of superconductivity while in others superconductivity apparently coexists with short range magnetic order,iii along with microscopic phase separation into purely magnetic and purely superconducting fractions37. We investigate how do coexisting magnetic correlations affect the magnetization response of the superconducting state? By imaging of the local magnetic field distribution at low fields using high sensitivity Magneto-optical imaging (MOI) technique in less investigated CaFe1.94Co0.06As2 single crystal. Our findings show the presence of anomalous remnant magnetization within Meissner like regions of the superconductor when the magnetic field is cycled to zero from a value below the lower critical field. While increasing the field, the pristine Meissner state of the superconductor shows a spatially uniform enhancement in magnetization without prior appearance of local field gradients. Magnetization response within regions of the sample progressively transforms from diamagnetic negative to positive value with increasing temperature in isofield measurement. With further increasing temperature, the positive magnetization dies down

__________________________________________________________________________


gradually to zero. The strength of positive magnetization increases with increasing field indicating coexisting magnetic order. The area of these regions exhibits a critical divergence like behavior with temperature which suggests, along with the observation of remnant magnetization in Meissner like state, coexistence of ferromagnetism and superconducting order in this compoundiv. 1 [email protected]

Preparation, structural and magnetic studies on BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) multiferroic nanoparticles

Samar Layek, Santanu Saha* and H. C. Verma Department of Physics,

Indian Institute of Technology, Kanpur, 208016, India. Multiferroic materials are those which show more than one ferroic or anti-ferroic properties (like ferromagnetic, ferroelectric, ferroelastic etc,) in the single phase sample. In thesematerial ferroic properties are coupled with each other [1]. Bismuth ferrite (BiFeO3) is one of the rare examples of the multiferroic compounds which shows simultaneously ferroelectric (TC = 830 0C) and G-type antiferromagnetic (TN = 370 0C) properties above room temperature in its rhombohedrally distorted perovskite structure [2]. BiFe1-xCrxO3 (x=0.0, 0.05 and 0.1) nanoparticles have been prepared by a novel simple combustion method without using any solvent. Pure and upto 10% Cr-doped particles are single phase in nature and crystallize in distorted perovskite structure (space group 3Rc) is known from XRD result. Average crystallite size is of the order of 50 nm for all the samples investigated. The morphology of the BiFeO3 nanoparticles is investigated by scanning electron microscopy (SEM). The TEM studies showed that the particles are nearly spherical shape and the average crystallite size is about 50±5 nm which is in accordance with that calculated from XRD studies. The enhancement of room temperature magnetization in pure BiFeO3 nanoparticles has been observed by the VSM measurement than that of bulk counterpart. Saturation magnetization and coercive field increase with increasing Cr-doping. Local magnetic behaviors have been investigated by 57Fe Mössbauer spectroscopy. Mössbauer data of BiFeO3 nanoparticles can be fitted with single sextet of isomer shift of about 0.35 mm/s and hyperfine field of about 49.5 T corresponding to single magnetic state in which Fe atoms are in 3+ oxidation states in these systems.Keywords: BiFeO3, Nanoparticles, Multiferroic, Combustion Method, Cr-doping, 57Fe Mössbauer spectroscopy. Reference: [1] W. Eerenstein, N. D. Mathur, and J. F. Scott, Naure 442, (2006) 759. [2] F. Lin, D. Jiang, X. Ma and W. Shi, J.Magn. Magn. Mater. 320, (2008) 691. __________________________________________________________________________


Correlation between hardness and basicity (pKa) of substituted anilines Abhishek Das (1), Someswar Chatterjee (2)*

(1) Dept. of Chemistry, GERF (Accredited Institution of University of Mysore)

Kalyani, Nadia, West Bengal, (INDIA) (2) Dept. of Chemistry, K.N.College (Affiliated to The University of Burdwan), Murarai,

Birbhum,West Bengal, PIN-731219, (INDIA) In this work the correlation between hardness and basicity (pKa) of substituted anilines and N- Alkylated anilines has been studied. The hardness values of PhNH2, o-, m-, and p- substituted, NH2-PhNH2, Me-PhNH2, Cl-PhNH2, NO2-PhNH2 and N-alkylated anilines (PhNHMe, PhNHEt, PhNHPh) have been calculated using Koopmans’ relation. The calculation of the energies of HOMO & LUMO of the above compounds has been done semi-empirical quantum mechanically (using AM1 & PM3 semi-empirical Hamiltonian) with the help of ArgusLab4.0 software. It is found that hardness value of aniline decreases along with substitution. The high correlation between pKa values and hardness of substituted anilines and N-Alkylated anilines indicate the hardness bears direct relationship to the basicity of substituted anilines.

OPTICAL TRAPPING AND MANIPULATION OF MICROMETER SIZED ‘PEAPODS’

Vishvas Srivastava, Basudev Roya, Bibudha Parasarb, Atharva Sahasrabudheb,

Ayan Banerjee*, Prashant Panigrahi*, Soumyajit Roy** Indian Institute of Science Education and Research, Kolkata, Mohanpur - 741252, Nadia,

W.B., India

Biological phenomena of cellular movement using flagella and the dynamic nature of actin filaments in human body are a few examples of molecular motors in action in nature. The modeling of such motion has become an exciting area of research. Use of simple chemical building blocks and physical forces to emulate such phenomena is a promising step in this direction. We contribute to this rapidly growing field by presenting our findings on „Optical Trapping and Manipulation of Micrometer Sized Softoxometalate based particles‟. These particles, based on Ammonium Phosphomolybdate Keggin, resemble closely to a peapod in their structure and hence are termed as inorganic “peapods”. Softoxometalates are large single molecular clusters of polyoxometalates that show superstructure formation and properties of soft condensed matter. Their dielectric nature and inherent anisotropy allow them to act as good starting materials to mimic complex biological motions. Our findings are based on optical trapping and rotation of the 2-micron sized peapods. These particles are trapped using only a TEM00 Gaussian beam and standard optical tweezers configuration. Rotation can be induced in the trapped particles by using a cover slip of specific thickness that enhances spherical aberration inside the trapping chamber. The frequency of rotation can be controlled by varying the power of the trap. Till now only anisotropic transparent materials such as borosilicate glass and polystyrene beads have been successfully trapped and manipulated in optical traps. Higher order beams such as Laugerre-Guassian and Bessel beam are generally used for this purpose. However, we are successful in optically trapping and manipulating light absorbing “peapods” only with a spherically aberatted Gaussian beam. __________________________________________________________________________


The peapods are spontaneous assemblies of ammonium salt of the phosphomolybdate Keggin .Their structure is composed of [P2MoO11]6- spheres, encased by a “skin” proposed to be made up of a protonated MoO3 sheet. This shows that the surface of these particles is charged and they have an inherent polarization. Therefore, due to their intrinsic charge distribution and anisotropic light scattering property they can be oriented and manipulated in optical traps. Thus we have developed a novel system based on simple chemical building blocks to construct complex biological machines with the aid of optical forces.

Title: TBA

D. Sai krishna Kakatiya University, AP, India

Graphene, a one-atom layer of graphite, possesses a unique two-dimensional structure and excellent mechanical, thermal, and electrical properties. Thus, it has been regarded as an important component for making various functional composite materials. Graphene can be prepared through physical, chemical and electrochemical approaches. Among them, chemical methods were tested to be effective for producing chemically converted graphene (CCG) from various precursors (such as graphite, carbon nanotubes) in large scale and at low costs. Therefore, CCG is more suitable for synthesizing high-performance graphene based composites. In this work the efficient synthesis of graphene using pyrrole as a reducing agent was explored. The obtained graphene sheets were dispersed in many polar organic solvents. Prepared graphene sheets were used as a catalyst support for Pt and Pd for oxygen reduction reaction, methanol oxidation and formic acid oxidation.. All prepared samples were characterized using TEM (Transmission electron microscope), XRD (X-ray diffraction), Raman, IR (Infrared spectroscopy) UV-Visible spectra and Cyclic Voltammetry.

_______________________________________________________________________________________________


A Novel Magnetic Nanoparticle Based Drug Delivery System

Satyam Singhal, Sulal S, Soumyajit Roy* Indian Institute of Science Education and Research, Kolkata, Mohanpur - 741252, Nadia,

W.B., India

Today, technologies based on magnetic nanoparticles (MNPs) are routinely applied to biological systems with diagnostic or therapeutic purposes. Cancer nanotherapeutics are rapidly progressing and are being implemented to solve several limitations of conventional drug delivery systems like oral chemotherapy and surgery. Although chemotherapy and surgery can cure cancer to a certain extent, the side effects of these therapeutic methods are inevitable. In such cases drug delivery using nanoparticles is an effective tool. Here we propose to design Silica Coated Nanoparticles by sol gel Technique. They are able to carry loaded active drugs to cancer cells by selectively using the unique pathophysiology of tumors, such as their enhanced permeability and over expressed cell protein signaling. Iron oxide (hematite) magnetic nanoparticles coated with silica increases the stability of Nanoparticle. These stabilized nanoparticles are then coated with carbohydrates, complementary to the recognition sites on the cancer tissues. The core/shell structure enhances the thermal and chemical stability of the nanoparticles, improves solubility, makes them less cytotoxic and allows conjugation of other molecules to these particles. Aggregation of nanoparticles can be prevented by coating the particles with other materials. The magnetic property of these particles can be used for their controlled delivery using an external magnetic field. This will help in effective transfer of these drug coated nanoparticles to targeted organs thereby increasing the efficiency and safety of drug delivery.

Structure-Property Correlation Studies of Pr1-xCaxMnO 3 (x = 0.3, 0.5) Nanoparticles

Anustup Sadhu and Sayan Bhattacharyya*

Advanced Functional Materials Laboratory,Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur - 741252, Nadia,

W.B., India In a typical doped perovskite manganite system such as Pr1-xCaxMnO3, the charge ordered state is characterized by a long range ordering of the Mn3+ and Mn4+ ions, due to competition between Coulomb interactions between the charges, exchange interactions between the Mn moments, and the electron–lattice coupling through Jahn–Teller distortions of the oxygen octahedron surrounding the Mn3+ ions. Although mostly studied as thin films, single crystals or in the bulk, when the physical dimensions of this material are brought down to the nanoscale, the charge ordering transition is altered bringing in novel properties. In this study, 40-50 nm Pr1-xCaxMnO3 (x = 0.3, 0.5) particles were synthesized by microwave irradiation to form polycrystalline samples containing high percentage of defects and subsequently annealed at above 1000oC to yield perfectly crystalline materials. __________________________________________________________________________


Optical spectroscopy at 295 K showed that the intensity of the intraband polaron transition Mn3d eg1 � Mn3d eg2 at 1.4 eV was comparable to the O2p � Mn3d band at 3.2 eV for x = 0.3 sample, whereas for x = 0.5, the former was suppressed, indicating a better charge ordering for x = 0.3 sample. Raman spectroscopy showed signatures of disorder and the typical Raman modes for this particular system. The electrical resistivity data of the annealed samples agreed with the model of thermally activated conduction of small polarons. Magnetic measurements revealed the presence of a glassy state at low temperatures and the coexistence of ferromagnetic and anti-ferromagnetic charge-order phases.

Light scattering: A probe to self-similarity in normal and cancerous tissue

Jalpa Soni (a)*, Harsh Purwar (a), Gregor P. Jose (b), Asima Pradhan (c), Tapas K. Sengupta (b), Prasanta K. Panigrahi (a) and Nirmalya Ghosh (a)

(a) DPS, IISER-Kolkata, Mohanpur 741 252, West Bengal, India. (b) DBS, IISER-Kolkata, Mohanpur 741 252, West Bengal, India.

(c) Dept. of Physics, IIT-Kanpur, Kanpur 208 017, India. Elastically scattered light from a complex random medium like biological tissue contains rich morphological and functional information of potential biomedical importance. Both the angular and wavelength dependence of the scattered light from tissue can be analyzed to extract and quantify subtle morphological changes taking place during progression of a disease like cancer, and thus may be exploited as a sensitive tool for early diagnosis of cancer. In order to explore this possibility, we have applied a combined Fourier based and discrete wavelet based analysis for extraction and quantification of the nature of the fluctuations hidden in the elastic scattering spectra recorded from normal and pre-cancerous human cervical tissues. The light scattering spectra (wavelength �, 400 nm – 800 nm) recorded (from the tissue sections) at varying scattering angles (�, 10o – 150o, angular interval �� chosen to cover the entire range of the scattering vector spanned) were subjected to the combined Fourier and Wavelet Based Multi-Fractal De-trended Fluctuation Analysis (WB-MFDFA). The analysis revealed otherwise hidden signatures of self-similar (fractal) behavior in the spectral fluctuations of both normal and pre-cancerous tissues, with interesting differences in the nature of self-similarity. Further, the spectral fluctuations in both these tissues showed multi-fractality (non-stationarity in fluctuations). Interesting differences were observed in the extracted values for the fractal scaling exponent � (and Hurst parameter - H) and the strength of multi-fractality (quantified through width of the singularity spectrum) between normal and pre-cancerous tissues. While the value for the Hurst parameter for the pre-cancerous tissues were higher at forward scattering angles (10o � � � 70o), the reverse was the trend in the backscattering angles (120o � � � 150o). Moreover, the strength of multifractality was observed to be marginally higher for the pre-cancerous tissues as compared to their normal counter parts. The observed multifractality in the light scattering spectral fluctuations has been identified to originate from the multi-scale self-similar nature of the local refractive index fluctuations in the tissues, which was confirmed further by analyzing the phase contrast images of the same tissues. The details of these results will be presented and their implications for early diagnosis of cancer will be discussed. __________________________________________________________________________

“CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011


References: 1. S. Ghosh, J. Soni, H. Purwar, J. Jagtap, A. Pradhan, N. Ghosh and P. K. Panigrahi, “Differing self-similarity in light scattering spectra: a potential tool for pre-cancer detection”, Optics Express, 19, issue 20, 19717-19730 (2011). 2. J. Soni, G. Jose, S. Ghosh, A. Pradhan, T. K. Sengupta, P. K. Panigrahi and N. Ghosh, “Probing Tissue Multifractality Using Wavelet based Multifractal Detrended Fluctuation Analysis: Applications in Precancer Detection”, To appear in the the 4th International Conference on BioMedical Engineering and Informatics, to be held on to be held on 15- 17 October 2011, Shanghai, China. i C. de la Cruz et al., Nature (London) 453, 899 (2008). ii J. Zhao et al., Nature Mater. 7, 953 (2008). iii J. T. Park et al., PRL 102, 117006 (2009). iv Pabitra Mandal et al, submitted. __________________________________________________________________________


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