Research: Quantum Condensed Matter

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Properties of material at very low temperature has been fascinating physicists for almost a century after the discovery of superconductivity by Kammerlingh Onnes in 1911. The enigmatic quantum world reveals itself here on a macroscopic scale. The discovery of Quantum Hall Effect(1980, 1982) and the observation of Bose-Einstein condensation of cold atoms(1995) have particularly provided us two such interesting opportunities, where we can enrich ourselves with the knowledge of quantum phases that atomic and subatomic constituents of matter can form over an extended range of external paramaters. Experiments and theory have been going hand in hand in both these branches and possible technological applications in future. Over the past few years I have been learning about these quantum phases of ultra-cold material with my collaborators.

· BOSE-EINSTEIN CONDENSATION: The above figures reveal some of the interesting aspects we have learnt in this process of studying cold atoms (~nK). The first figure from the left, for example, depicts the interplay between the dipole moments associated with certain type of cold atoms and the consequent modulation of their density profile (PRL, 98,260403(2007)) with (M. Takahashi, T. Mizushima and K. Machida). The second figure tells about the temporal evolution of the momentum distribution of such cold atoms restricted in a narrow one dimensional channel as it passes through a disorder potential created by superimposing two or more laser beams( cond-mat/0610579). The third picture depicts, what will happen the time evolution of such cold atoms in a disorder potential if they form a lump like object called soliton (With Eric Akkermans and Ziad Musslimani). The last two pictures (with Assa Auerbach and Dan Arovas) are the schematic diagram of a vortex-antivortex pair created out of such cold atoms a sphere ( a geometry which does not have any edge) and their excitation spectrum (Phys. Rev. B, 74, 064511). We have used this data to evaluate how such vortex ( such quantum matter can rotate only by creating vortex like whirlpool) tunnels from one point to another. To summarize with my collaborators I have investigated how various types of interactions and external potential modifies the quantum phases of these cold atoms and how those various phases manifest themselves in certain properties, for example transport.

· QUANTUM HALL EFFECT: Here I have mostly studied the effect of spin and spin like degrees (with R. Rajaraman) of freedom (on the quantum mechanical ground states of two dimensional electron system in a transverse magnetic field at very low temperature ( ~mK) . Currently I am looking at similar properties of graphene, where a relativistic two-dimensional electron system can be formed . The above picture depicts an interesting topological object, dubbed as CP₃ soliton (see the above figure), where spin and layer degrees of freedom of two dimensional get intertwined to provide a exotically textured object. Some experimental signatures of this object was also found at later stage.