Time reversal symmetric topological exciton condensate in bilayer HgTe quantum wells.
We investigate a bilayer system of critical HgTe quantum wells, each featuring a spin-degenerate pair of massless Dirac fermions. In the presence of an electrostatic interlayer Coulomb coupling, we determine the exciton condensate order parameter of the system self-consistently. Calculating the bulk topological Z2 invariant of the resulting mean-field Hamiltonian, we discover a novel time reversal symmetric topological exciton condensate state, coined the helical topological exciton condensate. We argue that this phase can exist for experimentally relevant parameters. Interestingly, due to its multiband nature, the present bilayer model exhibits a nontrivial interplay between spontaneous symmetry breaking and topology: Depending on which symmetry the condensate order parameter spontaneously picks in combined orbital and spin space, stable minima in the free energy corresponding to both trivial and nontrivial gapped states can be found.
Department of Physics, Stockholm University, Se-106 91 Stockholm, Sweden and Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, 6020 Innsbruck, Austria and Institute for Theoretical Physics, University of Innsbruck, 6020 Innsbruck, Austria.
Jan Carl
JC
Budich
Institute for Theoretical Physics and Astrophysics, University of Würzburg, 97074 Würzburg, Germany.
Björn
B
Trauzettel
Institute for Theoretical Physics and Astrophysics, University of Würzburg, 97074 Würzburg, Germany and Institute of Theoretical Physics, Technische Universität Dresden, 01062 Dresden, Germany.
Paolo
P
Michetti