Spin‐helical transport in normal and superconducting topological insulators

Abstract

31 85141In a topological insulator (TI) the character of electron transport varies from insulating in the interior of the material to metallic near its surface. Unlike, however, ordinary metals, conducting surface states in TIs are topologically protected and characterized by spin helicity whereby the direction of the electron spin is locked to the momentum direction. In this paper we review selected topics regarding recent theoretical and experimental work on electron transport and related phenomena in two-dimensional (2D) and three-dimensional (3D) TIs.The review provides a focused introductory discussion of the quantum spin Hall effect in HgTe quantum wells as well as transport properties of 3DTIs such as surface weak antilocalization, the half-integer quantum Hall effect, s þ p-wave induced superconductivity, superconducting Klein tunneling, topological Andreev bound states and related Majorana midgap states. These properties of TIs are of practical interest, guiding the search for the routes towards topological spin electronics. 1 Introduction The situation when a material behaves as a metal in terms of its electric conductivity and, at the same time, as an insulator in terms of its band structure is extremely unconventional from the viewpoint of the standard classification of solids. That is why the recent discovery of a class of such materials -topological insulators (TIs) -has generated much interest (see e.g., reviews [1,2]). The dual properties of the TIs are especially well pronounced in two-dimensional (2D) systems which are also known as quantum spin-Hall insulators (QSHIs) [3][4][5][6][7]. In QSHIs, the metallic electric conduction is associated with propagating states that occur only near the sample edges, while the conduction in the interior is suppressed by a band gap like in ordinary band insulators. These edge states originate from intrinsic spin-orbit (SO) coupling and are profoundly different from those appearing in quantum Hall systems in a strong perpendicular magnetic field [8,9]. The key distinction lies in the role of the time reversal symmetry. In the QSHIs the SO coupling preserves the time-reversal symmetry, resulting in a pair of counter-propagating channels on the same edge as opposed to one-way directed (chiral) edge states in quantum Hall systems. Remarkably, the spin and momentum directions of the two QSHI edge channels are locked in the opposite ways so that these states are characterized by opposite spin helicities and orthogonal

Growth, characterization, and transport properties of ternary (Bi1−xSbx)2Te3topological insulator layers

“…[48][49][50][51] Weak antilocalization is observed in topological insulators, since spin-momentum locking leads to a suppression of time-reversal coherent backscattering. As Hall measurements also shown in Fig.…”

Nonsinusoidal Current-Phase Relationship in Josephson Junctions from the 3D Topological Insulator HgTe

“…The CPR in an S/TI/S junction is a key diagnostic [8,[25][26][27][28][29][30][31][32]. Weak disorder in the TI far from the superconducting contacts theoretically does not affect the induced superconducting state [33,34]; therefore, Andreev bound states should form in hightransmittance surface channels [8,26,27,29,31].…”

Nonlocal edge state transport in topological insulators

“…This distribution will define the degree of the response nonlocality. For example, in the case of N = 4, 14 if the current flows from terminal 1 to terminal 4, we may be interested in the resistance between these terminals and the nonlocal response, expressed in terms of the resistance between terminals 1 and 2, 2 and 3, and 1 and 3. It is evident that with the increase in the number of terminals N , the resistance between terminals 1 and N , through which the current flows, will tend to unity.…”