Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface

Abstract

Topological insulators are new states of quantum matter in which surface states residing in the bulk insulating gap of such systems are protected by time-reversal symmetry. The study of such states was originally inspired by the robustness to scattering of conducting edge states in quantum Hall systems. Recently, such analogies have resulted in the discovery of topologically protected states in two-dimensional and three-dimensional band insulators with large spin-orbit coupling. So far, the only known three-dimensional topological insulator is Bi R ecently, the subject of time-reversal-invariant topological insulators has attracted great attention in condensed-matter physics [1][2][3][4][5][6][7][8][9][10][11][12] . Topological insulators in two or three dimensions have insulating energy gaps in the bulk, and gapless edge or surface states on the sample boundary that are protected by time-reversal symmetry. The surface states of a three-dimensional (3D) topological insulator consist of an odd number of massless Dirac cones, with a single Dirac cone being the simplest case. The existence of an odd number of massless Dirac cones on the surface is ensured by the Z 2 topological invariant 7-9 of the bulk. Furthermore, owing to the Kramers theorem, no time-reversalinvariant perturbation can open up an insulating gap at the Dirac point on the surface. However, a topological insulator can become fully insulating both in the bulk and on the surface if a timereversal-breaking perturbation is introduced on the surface. In this case, the electromagnetic response of three-dimensional (3D) topological insulators is described by the topological θ term of the form S θ = (θ /2π)(α/2π) d 3 x dt E · B, where E and B are the conventional electromagnetic fields and α is the fine-structure constant 10 . θ = 0 describes a conventional insulator, whereas θ = π describes topological insulators. Such a physically measurable and topologically non-trivial response originates from the odd number of Dirac fermions on the surface of a topological insulator.Soon after the theoretical prediction 5 , the 2D topological insulator exhibiting the quantum spin Hall effect was experimentally observed in HgTe quantum wells 6 . The electronic states of the 2D HgTe quantum wells are well described by a 2 + 1-dimensional Dirac equation where the mass term is continuously tunable by the thickness of the quantum well. Beyond a critical thickness, the Dirac mass term of the 2D quantum well changes sign from being positive to negative, and a pair of gapless helical edge states appears inside the bulk energy gap. This microscopic mechanism for obtaining topological insulators by inverting the bulk Dirac gap spectrum can also be generalized to other 2D and 3D systems. The guiding principle is to search for insulators where the

Ambipolar Surface State Thermoelectric Power of Topological Insulator Bi2Se3

“…At higher temperatures T ≥ 150 K thermal activation is significant enough that the sign change cannot be observed within the range of accessible V g . The electron-hole asymmetry observed in both σ and S is consistent with the asymmetric surface band structure of Bi 2 Se 3 [3,27]. The peak value of S reaches ~170…”

“…Thermopower of Au leads (~2 µV/K at 300 K ) at a given temperature is expected to be negligible compared to that of Bi 2 Se 3 and was not subtracted from the measured S. Carrier density was tuned through 300 nm thick SiO 2 dielectric back gate. To stay in the linear response regime, heater current was adjusted (typically 3 to 8 mA) so that the condition ΔT << T at given temperature is satisfied 9 .…”

Proximity-induced high-temperature superconductivity in the topological insulators Bi2Se3 and Bi2Te3

“…This proximity effect has been proposed as a powerful resource for practical applications in optoelectronics 3 and has been predicted to produce the elusive Majorana fermion 4 by combining a superconductor with a topological insulator (TI) [5][6][7][8][9][10] . These goals have been pursued actively, including the recent demonstrations of novel semiconductor light sources 11 and induced superconductivity in TIs [12][13][14][15][16] such as Bi 2 Se 3 and Bi 2 Te 3 , which have been shown to have topologically protected surface states [17][18][19] . Nonetheless, all such experiments performed to date have employed low critical-temperature (low-T c ) materials that require extreme cooling, and the important ratio of the superconducting gap to the Fermi energy in such systems is very small.…”

Topological Insulator Nanowires and Nanoribbons

“…[3][4][5][6][7] In particular, the surface states of Bi2Se3 forms a single Dirac cone inside a large bulk band gap of 0.3 eV, thus being suggested as the reference material for the 3D TIs. 2,4,8 The unique properties of the Bi2Se3 TI may pave the way for dissipationless quantum electronics and room temperature spintronics applications.1 To date, the surface properties of the 3D TIs have been mainly investigated by ARPES measurements on the cleaved surface of bulk crystals. [3][4][5][6][7] Single crystalline nanostructure, on the other hand, offers an attractive alternative system to study the surface states, for example, by transport measurements or surface scanning techniques.…”

“…1 Recently, research on Bi2Se3 and related compounds (Bi2Te3 and Sb2Te3) has attracted much interest because they are predicted to be three-dimensional (3D) topological insulators (TIs), a new class of quantum matter possessing conducting surface states with nondegenerate spins. 2 In TIs, the strong spin-orbit coupling dictates robust, nontrivial surface states, which are topologically protected against back scattering from time-reversal invariant defects and impurities. Angleresolved photoemission spectroscopy (ARPES) measurements on bulk single crystals of BixSb1-x, Bi2Se3, and Bi2Te3 have verified the existence of the 3D TI phase.…”