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SUMMARY:Anderson localization\, topology\, and interaction - Ostrovsky\, P
  (Max Planck Institut fr Festkrperforschung)
DTSTART:20120920T105000Z
DTEND:20120920T113000Z
UID:TALK39940@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:Field-theoretical approach to Anderson localization in 2D diso
 rdered fermionic systems of chiral symmetry classes (BDI\, AIII\, CII) is 
 developed. Important representatives of these symmetry classes are random 
 hopping models on bipartite lattices at the band center. As was found by G
 ade and Wegner two decades ago within the sigma-model formalism\, quantum 
 interference effects in these classes are absent to all orders of perturba
 tion theory. We demonstrate that the quantum localization effects emerge w
 hen the theory is treated nonperturbatively. Specifically\, they are contr
 olled by topological vortexlike excitations of the sigma models by a mecha
 nism similar to the Berezinskii-Kosterlitz-Thouless transition. We derive 
 renormalization-group equations including these nonperturbative contributi
 ons. Analyzing them\, we find that the 2D disordered systems of chiral cla
 sses undergo a metal-insulator transition driven by topologically induced 
 Anderson localization. We also show that the topological terms on surfaces
  of 3D topological insulators of chiral symmetry (in classes AIII and CII)
  overpower the vortex-induced localization.\n<p></p>\nSimilar vortex excit
 ations also emerge in systems with strong spin-orbit interaction (symplect
 ic symmetry class AII). Such systems may exhibit topological insulator sta
 te both in three and two dimensions. Interplay of nontrivial topology and 
 Coulomb repulsion induces a novel critical state on the surface of a 3D to
 pological insulator. Remarkably\, this interaction-induced criticality\, c
 haracterized by a universal value of conductivity\, emerges without any ad
 justable parameters. Interaction also leads to a direct transition between
  trivial insulator and topological insulator in 2D (quantum-spin-Hall tran
 sition) via a similar critical state. The nature of this latter critical s
 tate is closely related to the effects of vortices within the Finkelstein 
 sigma model.\n
LOCATION:Seminar Room 1\, Newton Institute
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