Spin-Dependent Quantum Phenomena: Spin Hall Effect and Spin Pumping

• Main Authors: Son-Hsien Chen, 陳松賢
• Other Authors: Ching-Ray Chang
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/42277272346347454369

博士 === 國立臺灣大學 === 物理研究所 === 97 === Spin-dependent quantum phenomena, the spin-Hall effect and spin-pumping, are investigated with employing the Landauer-Keldysh formalism, the Keldysh-nonequilibrium-Green-function technique applied to the Landauer setup. As one advantage from this method, physical quantities such as, charge and spin occupations, charge and spin currents, and conductance can all be obtained in a unified way. Analysis will be given mainly on these quantities. The spin-Hall effect in the two-dimensional sample made of semiconductor heterostructure with Rashba and Dresselhaus spin-orbit interaction (coupling) is studied. The spin-precession, which originates from these two SO couplings and plays an important role in the spin Hall effect, can be elucidated by the spin propagator constructed via the non-Abelian (non-commutable) spin-orbit gauge. Applications based on the spin-precession are proposed by considering a square ring etched from the two-dimensional system mentioned above. The spin-Hall effect in the presence of magnetic field, non-magnetic defect, and magnetic impurity (-ies) are also discussed. In particular, the exchange between two magnetic impurities is non-collinear, reflecting the existence of the spin precession of mediating electrons. Furthermore, the quantum spin Hall effect in graphene is also examined. We point out that the size of the sample is relevant to the quantization of the spin Hall conductance; the size of graphene has to be large enough to get the quantized conductance. On the issue of spin-pumping, we consider one-, two-, and three-dimensional systems. In the one-dimensional tight-binding model, the analytical form of the pumped spin currents yield fundamental understanding of the pumping; a plain and insightful physical picture is established to explain the pumping mechanism. In the two-dimensional topological-insulator graphene, a setup based on the interplay of the quantum spin Hall effect and spin pumping is proposed. This setup offers an experimental proof via electric means for the existence of the topological-insulator phase. Distinguishable from most of the present theoretical results, in the three-dimensional case, our calculations yield the same order of magnitude of the converted charge voltage measured in a magnetic tunneling junction with spin-pumping.