Spin-Orbit-Induced Electronic Phenomena in Superconductors and Antiferromagnets
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- Institutt for fysikk 
In this thesis, we present three scientific papers that are all related to electronic phenomena induced by spin-orbit coupling. Papers 1 and 2 concern spin-orbit induced effects in superconductors and Paper 3 treats similar effects in antiferromagnets. Paper 1 focuses on transport effects induced by spin-orbit coupling in diffusive systems. In diffusive systems, spin-orbit coupling is mediated by impurities. We use a non-equilibrium Green's function approach to derive coupled transport equations for a diffusive superconductor and include effects of a spin-orbit potential. These equations describe charge, spin and energy transport in superconductors. The spin-orbit potential provides extra contributions to the transport equations that are responsible for additional spin-transport mechanisms. These extra transport mechanisms are spin Hall effect, spin swapping and spin relaxation. In Paper 1, we present a microscopic derivation of these transport equations. We demonstrate that both the spin Hall effect, spin swapping and spin relaxation become energy dependent for the superconducting state. By applying the transport equations to non-local geometries, we calculate the voltage responses for these geometries in the normal and superconducting state. We find that the value of the spin-Hall-voltage signal in the superconducting state is the same, or smaller, than that in the normal state. Conversely, the spin-swapping-voltage signal is enhanced in the superconducting state. In Paper 2, we investigate the Meissner response in a superconductor-normal metal heterostructure. We include the effects of Rashba spin-orbit coupling due to inversion-symmetry breaking along an axis, n, and an external magnetic field and calculate the Meissner response in the normal metal. We find that the Meissner response is anisotropic with regard to the angle between the external-magnetic-field direction and n. Because of this anisotropy, the Meissner response can go from diamagnetic to paramagnetic when the external magnetic field is rotated. In addition, we find that at temperatures that are very low compared to the superconducting critical temperature, the susceptibility in the normal metal can decrease with increasing temperature. In Paper 3 we present a numerical study of the spin Hall effect in a metallic antiferromagnet. The conduction electrons experience a staggered potential because of the underlying magnetic order. Although there is no net magnetization, we find that the spin Hall conductance is significantly larger in this system, as compared to normal metals and ferromagnets.