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Magnetic proximity effect at Bi$_2$Se$_3$/EuS interface with broken inversion symmetry

We investigate a magnetic proximity effect on Dirac surface states of topological insulator (TI) induced by a Bi$_2$Se$_3$/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 Bi$_2$Se$_3$(111) slabs of three or five quintuple layers (QLs) with the magnetization of EuS normal to the interface ($z$ axis), which breaks both time-reversal and inversion symmetry. It is found that a charge transfer and surface relaxation makes the Dirac cones electron-doped. For both 3 and 5 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 5 QLs, an additional Dirac cone with an energy gap of 2 meV is formed right below the bottom-surface Dirac point, while for 3 QLs, there is no additional Dirac cone. We also examine the spin-orbital texture of the Dirac surface states with broken inversion 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.