Spontaneous ferromagnetism and finite surface energy gap in the topological insulator Bi2Se3 from surface Bi-se antisite defects

We perform ab initio calculations on Bi-sc antisite defects in the surface of Bi2Se3, finding strong low-energy defect resonances with a spontaneous ferromagnetism, fixed to an out-of-plane orientation due to an exceptionally large magnetic anisotropy energy. For antisite defects in the surface layer, we find semi-itinerant ferromagnetism and strong hybridization with the Dirac surface state, generating a finite energy gap. For deeper lying defects, such hybridization is largely absent, the magnetic moments become more localized, and no energy gap is present