Spin dynamics in anisotropic systemsin the presence of spin-orbit coupling

Abstract

In this thesis we investigated the non-trivial behaviour of spin polarization in the presence of spin-orbit coupling in anisotropic systems such as quantum wells and ultracold atomic gas. The study of spin effects has drawn large interest to create new devices with greater versatility, less heat produced and smaller size than devices based uniquely on electric current. Furthermore, spin current can be driven externally though electric current. One of the phenomena that produces the spin polarization under the influence of an external electric field is the inverse spin galvanic effect due the presence of Rashba or Dresselhaus spin-orbit coupling with surface or bulk inversion asymmetry respectively. The inverse spin-galvanic effect has been studied in many materials, mainly heterostructures. However, in contrast to the theoretical explanation, experimental results with InGaAs epilayers showed that the largest magnitude of the spin-polarization is found when the spin-orbit is the smallest. In the first part of this thesis we have extended the theory and explained theo retically the result of the experiments by adding an additional spin-orbit coupling term cubic in momentum in the Hamiltonian for quantum wells systems. By using the real-time formalism we derive the spin polarization as the Keldysh component of quasi-classical Green’s function. We describe the spin-galvanic conductivity as a fuction of frequency for the model with linear and cubic Rashba-Dresselhaus spin orbit couplings within diffusive and beyond diffusive regimes. Our results for spin polarization presented on vector diagrams qualitatively describe the experiment with InGaAs when cubic spin-orbit coupling term prevails over the linear one. The second part of the thesis is devoted to the inverse spin-galvanic effect in cold atoms, where the Rashba spin-orbit coupling is generated synthetically from the gauge potential. Differently from the 2D electron gas, in cold atoms the Fermi surface has asymmetry due to the two-fold Rashba coefficients and the magnitude of the momentum shows an angular dependence. The spin polarization was estimated numerically for different pairs of coefficients, as well as analytically starting from the Schr¨odinger equation in terms of parabolic cylinder functions, and in the adiabatic approximation they show qualitatively the same behavior. These results provide a relevant contribution for further investigations on the spin current operations, and could allow for a higher tunability of spin polarized states in quantum wells and cold atoms with Rashba and Dresselhaus spin-orbit couplings that were unlikely reachable with previously existing results.