Current-induced Gap Opening in Interacting Topological Insulators*

Gapping the conducting surface states of topological insulators (TIs) by time reversal symmetry breaking perturbations, such as coupling to a ferromagnet or circularly polarized light, is important for realizing a variety of intriguing new phenomena in these exotic materials. Here we propose a new mechanism for gapping TI surface states, using applied DC currents. The current-carrying non-equilibrium state breaks time-reversal symmetry, and furthermore hosts a nontrivial spin polarization due to strong spin-orbit coupling. Due to electron-electron interactions, this spin polarization produces an internal exchange field that breaks the degeneracy between Kramers partner states. We illustrate this gap opening mechanism for one-dimensional quantum spin Hall edge states and two-dimensional Dirac surface states, and discuss the parameters that control the size of the induced gaps as well as experimental signatures.