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Johannes Wanner
Acoustically induced spin relaxation in two-dimensional electronic systems
Supervisor: Prof. Dr. Ulrich Eckern [Theoretical physics II]
Date of oral examination: 11/25/2015
145 pages, english
Spintronics represents a promising candidate for the succession of conventional electronics systems. As implied by its name, spintronics is the concept of manipulating the spin polarisation of charge carriers additionally to their motion. Due to spin-orbit interaction the spins are, however, not independent from charge dynamics. Hence, scattering processes of electrons are an important source of spin relaxation, known as Dyakonov-Perel spin-dephasing mechanism. For new spintronic devices a reliable storage of information is mandatory and the question of how to prolong the spin lifetime arises. In this work, we investigate the influence of a surface acoustic wave (SAW) on the two-dimensional electron gas of a quantum well as a promising approach for coherent transport of spin-polarized wave packets. In principle, there are two ways a SAW affects the spin relaxation rate. On the one hand, the piezoelectric field evoked by the SAW reduces the motional degree of freedom, and enhances thus the spin lifetime. On the other hand, the induced deformation of the material leads to additional spin-orbit interactions which in turn are responsible for an increase of the relaxation rate. To study these competing processes, we calculate with the help of symmetry arguments effective Hamiltonians which are capable to describe single charge carriers in the crystal. Subsequently, this single particle approach is generalized by the concepts of many-body physics where, in particularly, we rely on the methods of quasiclassical transport theory. Eventually, the resulting Boltzmann-like equation is utilized to determine the influence of a SAW on spin relaxation rates. Due to the spatial anisotropy of the underlying material, we found a strong dependence of the spin lifetime on the experimental boundary conditions such as growth direction of the quantum well, propagation direction and acoustic power of the SAW or initial conditions of the spin polarization.