Magnetic proximity effect and spin-orbital texture at the Bi2Se3/EuS interface

We investigate a magnetic proximity effect on the Dirac surface states of a topological insulator (TI) induced by a Bi2Se3/EuS interface, using density-functional theory (DFT) and a low-energy effective model, motivated by a recent experimental realization of the interface. We consider a thin ferromagnetic insulator EuS film stacked on top of Bi2Se3(111) slabs of three or five quintuple layers (QLs) with the magnetization of EuS normal to the interface (z axis), which breaks time-reversal symmetry. It is found that a charge transfer and surface relaxation makes the Dirac cones electron doped. For both three and five QLs, the top-surface Dirac cone has an energy gap of 9 meV, while the bottom-surface Dirac cone remains gapless. This feature is due to the short-ranged induced magnetic moment of the EuS film. For the five QLs, an additional Dirac cone with an energy gap of 2 meV is formed right below the bottom-surface Dirac point, while for three QLs, there is no additional Dirac cone. We also examine the spin-orbital texture of the Dirac surface states with broken time-reversal symmetry, using DFT and the effective model. We find that the p(z) orbital is coupled to the z component of the spin moment in the opposite sign to the p(x) and p(y) orbitals. The p(z) and radial p orbitals are coupled to the in-plane spin texture in the opposite handedness to the tangential p orbital. The result obtained from the effective model agrees with our DFT calculations. The calculated spin-orbital texture may be tested from spin-polarized angle-resolved photoemission spectroscopy.