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**R. G. Herb Condensed Matter Seminar**

**New Topological Excitations and Melting Transitions in Quantum Hall Effect**

**Date:**

**Thursday, October 23rd**

**Time:**10:00 am

**Place:**Chamberlin 5310

**Speaker:**Yuli Lyanda-Geller, Purdue University

**Abstract:**Topology and symmetry define states of matter and their responses to external forces. How solids melt and become fluids, or how insulators become conductors is often controlled by excitations rather than by the ground state of systems. Non-trivial topology of excitations can alter the responses. Topological excitations are notoriously difficult to predict since they cannot be obtained as a perturbation of the ground state. In this talk I will report the discovery of a new type of topological excitations that arise in two-dimensional electron systems in a magnetic field. We investigate Landau level filling factors between integer and half-integer, which exhibit the re-entrant integer quantum Hall effect (RIQHE) with vanishing longitudinal resistance and the Hall resistance quantized to a nearest integer at lowest temperatures. I will show that charge excitations in the RIQHE regime are topologically non-trivial finite size textures of electron density with charge-dependent symmetry. These topological textures explain unusual strain dependence of resistivity. At low temperatures, the textures form a crystal, whose melting leads to metal-insulator transition.

**Host:**Vavilov