The surface states of a topological insulator are described by an emergent relativistic massless Dirac equation in 2 + 1 dimensions. In contrast with graphene, there is an odd number of Dirac points, and the electron spin is directly coupled to the momentum. We show that a magnetic impurity opens up a local gap and suppresses the local density of states. Furthermore, the Dirac electronic states mediate an RKKY interaction among the magnetic impurities which is always ferromagnetic, whenever the chemical potential lies near the Dirac point. Through this exchange mechanism, magnetic atoms uniformly deposited on the surface of a topological insulator could naturally form a ferromagnetically ordered film. These effects can be directly measured in STM experiments. We also study the case of quenched disorder through a renormalization group analysis.
Following the recent theoretical proposal and experiment on quantum spin Hall
effect in HgTe/CdTe quantum wells, we consider a single magnetic impurity
localized in the bulk of the system, which we treat as a classical spin. It is
shown that there are always localized excited states in the bulk energy gap for
arbitrarily strong impurity strength in inverted region, while the localized
excited states vanish for very strong impurity strength in normal region.
Similar conclusion also applies to three-dimensional topological insulators.
This distinct difference serves as another novel criterion for the conventional
and topological insulating phases when the time-reversal symmetry is broken,
and can be easily experimentally observed through the STM and/or ARPES
experiments.Comment: 5 pages, 4 figures
Topological Weyl semimetals (TWSs) with pairs of Weyl points and topologically protected Fermi arc states have broadened the classification of topological phases and provide superior platform for study of topological superconductivity. Here we report the nontrivial superconductivity and topological features of sulfur-doped -phase MoTe with enhanced T compared with type-II TWS MoTe It is found that -phase S-doped MoTe (MoTe S , ∼ 0.2) is a two-band -wave bulk superconductor (∼0.13 meV and 0.26 meV), where the superconducting behavior can be explained by the pairing model. Further, measurements of the quasi-particle interference (QPI) patterns and a comparison with band-structure calculations reveal the existence of Fermi arcs in MoTe S More interestingly, a relatively large superconducting gap (∼1.7 meV) is detected by scanning tunneling spectroscopy on the sample surface, showing a hint of topological nontrivial superconductivity based on the pairing of Fermi arc surface states. Our work demonstrates that the -phase MoTe S is not only a promising topological superconductor candidate but also a unique material for study of superconductivity.
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