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Ilya Eremin and Flavio S. Nogueira
A thin film of ferromagnetically ordered material proximate to the surface of a three-dimensional topological insulator explicitly breaks the time-reversal symmetry of the surface states. For an out-of-plane ferromagnetic order parameter on the surface, the parity is also broken, since the Dirac fermions become massive. This leads in turn to the generation of a topological Chern-Simons term by quantum fluctuations. On the other hand, for an in-plane magnetization the surface states remain gapless for the non-interacting Dirac fermions. In my talk I study the possibility of spontaneous breaking of parity due to a dynamical gap generation on the surface in the presence of a local, Hubbard-like, interaction of strength g between the Dirac fermions. A gap and a Chern- Simons term are generated for g larger than some critical value, gc, provided the number of Dirac fermions, N, is odd. For an even number of Dirac fermions the masses are generated in pairs having opposite signs, and no Chern-Simons term is generated. I discuss our results in the context of recent experiments in EuS/Bi2Se3 heterostructures. |
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