Theoretical Physics Seminar:
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Nov 28, 2008 11:00 AM, Theoretical Physics Seminar Room, LC.3.17
Spin current and spin Hall effects
Speaker: Li, You-Quan
Abstract: In terms of SU(2) Yang-Mills field formalism, we give a natural definition of spin current for the systems with Rashba or Dresselhaus spin-orbit couplings etc., and we obtain a covariant form of continuity equations for the corresponding spin current. The traditional Kubo formula is generalized to describe the linear response with respect to non-Abelian fields. We find that the covariant form we proposed plays an essential role in guaranteeing the consistency of SU(2) Kubo formula. We also derive the classical counterpart of quantum mechanical covariant “continuity-like” equation for the spin current, and present an intuitive picture for elucidating the non-conservation of the spin current. We discuss the spin-relaxation time for two-dimensional systems with a hierarchy of spin-orbit couplings, and found that the spin-relaxation time can be infinite if the coupling strengths certain condition which correspond to the vanishing Yang-Mills “magnetic” field. The intrinsic spin Hall conductivity in a two dimensional electron gas with Rashba spin-orbit coupling is investigated by taking account of impurities. Our evaluation of the vertex corrections for the anisotropic magnetic impurities gives a nonvanishing spin Hall conductivity which depends on the momentum relaxation time, spin-orbit splitting and the anisotropic coefficient of interaction between itinerant electrons and magnetic impurities. The influence of inelastic relaxation time is also studied. We reveal that, due to the existence of inelastic scattering which may arise from electron-electron interaction, the spin Hall conductivity does not vanish when the impurity concentration diminishes to zero no matter if it is non-magnetically or magnetically disordered. Spin transport properties of a coupled bilayer electron gas with Rashba spin-orbit coupling are studied. Our investigation on the impurity effect manifests that an arbitrarily small concentration of nonmagnetic impurities does not suppress the spin Hall conductivity to zero in the bilayer system. Based on these features, an experimental scheme is suggested to detect a magnification of the spin Hall effects.
