Abstracts of Talks

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 curve 1 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 K 2. - 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). ________________________________________________________________________________ “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). ________________________________________________________________________________ “CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011 Indian Institute of Science Education & Research-Kolkata 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. Budhani 1,2 , P. K. Rout 2, G. N. Singh 2 and Rajni Porwal 2 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 La 0:625 Ca 0:375 MnO 3 films grown on (001) NdGaO 3 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. ________________________________________________________________________________ “CURRENT TOPICS IN CONDENSED MATTER” OCTOBER 7-9, 2011 Indian Institute of Science Education & Research-Kolkata 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 BaFe 2As 2: phase diagram and critical property investigations A. Bharathi Indira Gandhi Centre for Atomic Research (IGCAR) Kalpakkam 603 102, Tamil Nadu, India I will

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