ABSTRACT
Various materials and nanostructures with large Rashba coefficient are a subject of study in spintronic material science as they present templates for voltage-controlled spintronic applications. Some of these material systems were mentioned in Chapter 3. There is a specific class of semiconductors called topological insulators (TI) that presents new opportunities to spintronics. TI surface electron states experience a strong spin-orbit interaction and can be manipulated by an external electric field. A large linear-k Rashba coefficient has been reported in Bi2Te3-based TI [1].A topological insulator is a semiconductor with a specific band structure that makes the bulk electrically insulating and the surfaces-conducting. More specifically, the electron spectrum has an energy gap in the bulk (semiconductor) and it is gapless at the surfaces (metal). If the surface electron spectrum comprises odd number of minima in the Brillouin zone, the minima are gapless (Dirac cones), they are protected by time reversal symmetry, and lie in the bulk energy gap. Perturbations which do not violate the symmetry cannot open the gap at the surface and destroy the conduction state [2-4]. The position of the TI surface branches of the spectrum relative to the bulk bandgap is illustrated in Fig. 7.1.The linear dispersion of gapless surface excitations, shown in Fig. 7.1, forms a Dirac cone in variables E, kx, ky. Due to the spinorbit interaction, the electron momentum is locked to the spin Wide Bandgap Semiconductor Spintronics Vladimir Litvinov Copyright © 2016 Pan Stanford Publishing Pte. Ltd. ISBN 978-981-4669-70-2 (Hardcover), 978-981-4669-71-9 (eBook) www.panstanford.com
orientation so that electrons with opposite spins move in opposite directions. This makes non-magnetic scattering on the surface ineffective as the change of momentum is associated with the spin rotation and thus the scattering is forbidden unless the localized impurity spin takes part in the scattering process preserving the total (electron plus impurity) spin conservation. Non-dissipative edge currents are illustrated in Fig. 7.2, where a two-dimensional TI is shown as an example.
