Current-induced gap opening in interacting topological insulator surfaces

Understand

Two-dimensional topological insulators (TIs) host gapless helical edge states that are predicted to support a quantized two-terminal conductance.

• Quantization is protected by time-reversal symmetry, which forbids elastic backscattering.
• Paradoxically, the current-carrying state itself breaks the time-reversal symmetry that protects it.
• Here we show that the combination of electron-electron interactions and momentum-dependent spin polarization in helical edge states gives rise to feedback through which an applied current opens a gap in the edge state dispersion, thereby breaking the protection against elastic backscattering.