Transport phenomena in low dimensional systems in the presence of spin-orbit coupling

Abstract

Spin-orbit coupling gives rise to several interesting transport phenomena arising from the induced correlation between charge and spin degrees of free- dom. It allows one to manipulate spins without using magnetic electrodes, having as such become one of the most studied topics within the eld of spintronics. Among the many interesting e ects that arise from spin-orbit coupling, two stand out for their potential technological importance: the spin Hall e ect and the Edelstein e ect. The spin Hall e ect, whereby a charge current in the plane of the lm is partially converted into an orthogonal spin current in the same plane. The Edelstein e ect consists on a charge current which produces an in-plane, transverse spin polarization. A normal metallic lm sandwiched between two insulators may have strong spin-orbit coupling near the metal-insulator interfaces, even if spin- orbit coupling is negligible in the bulk of the lm. In this thesis we study the spin Hall and the Edelstein e ects that arise from interfacial spin-orbit coupling in metallic lms. At variance with strictly two-dimensional Rashba systems, we nd that the spin Hall conductivity has a nite value even if spin-orbit interaction with impurities is neglected and \vertex corrections" are properly taken into account. Even more remarkably, such nite value becomes \universal" in a certain con guration. This is a direct consequence of the spatial dependence of spin-orbit coupling on the third dimension, per- pendicular to the lm plane. Moving carriers in a metallic system, electrons or holes, transport both electric charge and heat. When a third quantity transported by the carriers, the spin, is connected to the previous two by spin-orbit coupling, we are able to study new transport phenomena due to this relations. An important goal of spin caloritronics is the manipulation of the spin degrees of freedom via thermal gradients. The spin Nernst e ect, i.e. the generation of a spin current transverse to a thermal gradient, stands out for its potential role in spin caloritronics. 1 In this thesis we study the connection between the spin-heat and spin- charge response in a two-dimensional disordered Fermi gas with spin-orbit coupling. We show that the ratio between the above responses can be ex- pressed as the thermopower S = 􀀀( kB)2T 0=3e times a number Rs which depends on the strength and type of the spin-orbit couplings considered. We illustrate the general results by examining di erent two-dimensional electron or hole systems with di erent and competing spin-orbit mechanisms and con- clude that a metallic system could prove much more e cient as a heat-to-spin than as a heat-to-charge converter.