Abstract
Proximity-induced magnetic effects on the surface Dirac spectra of topological insulators are investigated by scanning tunneling spectroscopic studies of bilayer structures consisting of undoped Bi2Se3 thin films on top of Cr-doped Bi2Se3 layers. For thickness of the top Bi2Se3 layer equal to or smaller than 3 quintuple layers, a spatially inhomogeneous surface spectral gap Δ opens up below a characteristic temperature which is much higher than the bulk Curie temperature determined from the anomalous Hall resistance. The mean value and spatial homogeneity of the gap Δ generally increase with increasing c-axis magnetic field (H) and increasing Cr doping level (x), suggesting that the physical origin of this surface gap is associated with proximity-induced c-axis ferromagnetism. On the other hand, the temperature (T) dependence of Δ is non-monotonic, showing initial increase below which is followed by a ‘dip’ and then rises again, reaching maximum at T ≪ These phenomena may be attributed to proximity magnetism induced by two types of contributions with different temperature dependences: a three-dimensional contribution from the bulk magnetism that dominates at low T, and a two-dimensional contribution associated with the RKKY interactions mediated by surface Dirac fermions, which dominates at ≪ T < In addition to the observed proximity magnetism, spatially localized sharp resonant spectra are found along the boundaries of gapped and gapless regions. These spectral resonances are long-lived at H = 0, with their occurrences being most prominent near and becoming suppressed under strong c-axis magnetic fields. We attribute these phenomena to magnetic impurity-induced topological defects in the spin texture of surface Dirac fermions, with the magnetic impurities being isolated Cr impurities distributed near the interface of the bilayer system. The long-term stability of these topologically protected two-level states may find potential applications to quantum information technology.
Highlights
The research of topological matter is an exciting frontier where the classification of quantum states of matter beyond the principle of symmetry breaking has stimulated many conceptual advances and experimental discoveries [1,2,3]
We have demonstrated STS evidences for magnetism-induced massive Dirac spectra and topological defects in the surface state of molecular beam epitaxy (MBE)-grown 3D-topological insulators (TIs) that consisted of bilayers of undoped Bi2Se3 on top of Cr-doped Bi2Se3
For an undoped layer thinner than or equal to 3-quintuple layers (QLs), proximity-induced ferromagnetism was manifested by the appearance of a surface gap Δ below a transition temperature Tc 2D > 200 K, and the value of Tc 2D was found to be Cr-doping (x) dependent and much higher than the bulk ferromagnetic transition temperature Tc 3D ( 30 K)
Summary
The research of topological matter is an exciting frontier where the classification of quantum states of matter beyond the principle of symmetry breaking has stimulated many conceptual advances and experimental discoveries [1,2,3]. Topological insulators (TIs) [4,5,6,7,8] are bulk insulators in two or three dimensions with strong spin–orbit coupling and gapless surface states protected by the timereversal invariance. Gapping the Dirac cones of TIs by introducing superconductivity [9,10,11] or magnetism [8, 9, 12, 13] via either doping or proximity effects. The underlying physics for a magnetism-induced surface gap in three-dimensional (3D) TIs is based on the assumption that the long-range ferromagnetism has a net magnetization M perpendicular to the surface of the 3D-TI, and that the in-plane component of the Hamiltonian 0 for the surface Dirac fermions remains intact after the introduction of c-axis magnetization.
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