Gapless surface states that are protected by time-reversal symmetry are among the manifestations of 3D topological insulators.

We studied how electron-electron interaction may lead to spontaneous breaking of time-reversal symmetry on the surface of such insulators.

Our work (arXiv:1110.4037) explores the conditions under which the surface of a strong topological insulator may lower its energy by a spontaneous breaking of time reversal symmetry through the formation of a uniform spin polarization.

In our analysis, we find that there is a critical interaction strength above which the spin polarized ground state is favorable in energy compared to the non-interacting ground state. We find this to be the case for every value of the chemical potential, for all temperatures much smaller than the bulk energy gap, and for both contact interaction and Coulomb interaction.