Photoemission Spectroscopy Studies of New Topological Insulator Materials
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As the size of a solid shrinks, the ratio of surface area to bulk volume grows and surface effects become more important. In a world where technologies advance with the shrinking size of electronic devices, one phase of matter has emerged which is fit for the near future of surface-dominated performance. Moreover, it has brought a new set of ideas to solid-state physics and chemistry, especially the understanding that the discipline of topology can be applied to classify the electron band structures. The topological insulator phase yields an exotic metal surface state in which the orientation of the electron’s spin is locked perpendicular to its momentum. This property suppresses backscattering (making it possible to pass spin-polarized currents through the material without loss), offers a crucial ingredient for innovative approaches to quantum computation, and provides the basis for observing unique magnetoelectric effects. However, the surface states of materials in the topological insulator phase can wildly differ, so it is of interest to systematically characterize new materials to understand how the structure in position-space is related to the spin-resolved structure of electrons in energy- and momentum-space. We will discuss this relationship as it is probed through spin- and angle-resolved photoemission spectroscopy experiments on three topological (Bi₂)m(Bi₂Se₃)n superlattices: (a) Bi₂Se₃ (m = 0, n = 1), (b) Bi₄Se₃ (m = 1, n = 1), and (c) BiSe (m = 1, n = 2). Our studies have not only proven the topological nature of these materials, but also demonstrate how bulk band structure and polar chemical bonding control the surface metal’s concentration, dispersion, and spin-orbital character. Case (a) is considered to provide an ideal model of the topological surface metal. Case (b) provides the three important findings: (1) the chemical identity of the surface-termination controls the orbital composition and energy distribution of the surface states, (2) there are two topological states in sequential bulk band gaps, (3) of these, one of topological state undergoes a hybridization effect that yields a momentum-dependent gap in the band structure as large as 85 meV. Case (c) has a practical significance in that the surface metal has a potentially record-breaking carrier density of ~10¹³cm⁻² (estimated from the Fermi surface area), more than an order of magnitude higher than in Bi₂Se₃. This occurs as a result of charge transfer from the Bi₂ layers to the Bi₂Se₃ layers.
Table of Contents
Introduction -- Electron spectroscopies applied to bismuth cahlcogenides -- Topological semimetal composed of bismuth-bilayers and bismuth selenide layers stacked in a 1:1 ratio -- Topological insulator composed of bismuth-bilayers and bismuth selenide layers stacked in a 1:2 ration -- Summary and outlook