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MSc Ankush Sengupta: „Dynamical structures in driven disordered systems”
S. N. Bose National Centre for Basic Sciences Kolkata, India

We study[1] the non-equilibrium steady states of a crystal whose ground state can be tuned through a square-triangular transition. Driving such a system across a quenched random background yields a complex sequence of novel dynamical states: plastic flow, anisotropic hexatics, dynamically stabilized triangle and square phases and intermediate regimes of phase coexistence with anamolously slow dynamics. Finite size scaling arguments and defect structure analysis estabish the reality of the hexatic phase, indicative of a KT type freezing transition. We investigate analogy of our continuum model with XY model in magnetic field. A detailed analysis of relevant orientation correlation functions and the drive induced anisotropy shows evidence of novel mechanisms underlining the dynamical freezing transition. The behavior of topological defects shows a possible connection to deconfinement in the two dimensional XY model. Noise studies in coexistence phase shows highly correlated motion with 1/f noise in statistical fluctuations and breakdown of the Koshelev-Vinokur proposal of shaking temperature. Dynamic nucleation under external fields, as revealed in our model, is also explored. Such states, as we btain, should be observable in transport experiments on the mixed phases of several superconductors which exhibit related structural transitions, and on adsorbed monolayer colloids with tunable interactions.

[1] Ankush Sengupta, Surajit Sengupta, Gautam I. Menon,
Driven disordered periodic media with an underlying structural phase transition.
Phys. Rev. B 75, 180201(R) (2007)