Topological Exciton Condensate

Babak Seradjeh (IU)

Recent advances in the study of band insulators have revealed the

existence of new topological invariants that characterize these

materials. Among the three-dimensional time-reversal invariant

insulators a "strong" topological insulator (STI) was predicted to

exist, shortly followed by experimental confirmations in several

Bi-related materials with strong spin-orbit interaction. The STI is

physically distinguished by surface states with an odd number of Fermi

level crossing pairs, which remain metallic in the presence of weak

disorder. These states exhibit linear dispersion and behave as

massless Dirac fermions familiar from the physics of graphene. Having

an odd number of Dirac fermions leads to some exotic properties

associated with surfaces of a STI, such as a fractional quantum Hall

conductivity.



We have recently predicted that a "topological exciton condensate" is

spontaneously formed by the Coulomb interaction in a thin-film STI,

which intriguingly supports vortices with a precisely fractional value

of charge, e/2. This is a distinct correlated phase of matter enabled

by the special properties of topological insulators. I shall review

these developments and present our theory of the topological exciton

condensate. I will also discuss recent results on the effects of

particle-hole imbalance which show a spatially modulated condensate

can form in this case akin to the elusive

Fulde-Ferrel-Larkin-Ovchinikov state in an s-wave superconductor. I

will conclude by suggestions for the experimental observation of this

novel condensate.

 

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This week's CM/CAM seminar will be given by Babak Seradjeh, a new professor at Indiana University-Bloomington. His will speak about his work onthin films of topological insulators, but there are are also interesting connections with graphene.

Dont miss it! Refreshments will be provided before.