Abstract
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Topological insulators are quantum materials that have an insulating bulk state
and a topologically protected metallic surface state with spin and momentum
helical locking and a Dirac-like band structure [1–3]. Two-dimensional (2D)
topological insulators are associated with gapless edge states, and three-dimensional
(3D) topological insulators with gapless surface states [4]. A variety of
compounds have been identified as 2D or 3D topological insulators, including
HgTe/CdTe, Bi2Se3, Bi2Te3, Sb2Te3, BiTeCl, Bi1.5Sb0.5Te1.8Se1.2, SmB6 and so on
[5–8]. The topological surface (edge) states in these materials have been mainly
investigated by first-principle theoretical calculation, electronic transport,
angle-resolved photoemission spectroscopy (ARPES), and scanning tunneling
microscopy (STM) [6]. Unique and fascinating electronic properties, such as
the quantum spin Hall effect, quantum anomalous Hall effect, topological magnetoelectric
effect, magnetic monopole image, and Majorana fermions, have
been observed in the topological insulator materials [9, 10]. With these unique
properties, topological insulator materials have great potential applications in
spintronics and quantum information processing, as well as magnetoelectric
devices with higher efficiency and lower energy consumption [11, 12].