Spin-splitting and metallic behavior in two-dimensional systems.

摘要

This thesis reports the results of low-temperature measurements related to spin-splitting, and the recently observed metallic behavior, in two-dimensional (2D) systems.; We demonstrate that the metallic behavior and apparent metal-insulator phase transition appear in a 2D electron system in an AlAs quantum well. Following this, we study the effect of spin-orbit-induced zero-magnetic-field ( B = 0) spin-splitting on the metallic behavior of 2D hole systems in GaAs quantum wells. Front and back gates are used to change the symmetry of the quantum well while keeping the density constant. This tunes the spin-splitting, which is measured using the Shubnikov-de Haas oscillations. By using both experimental results and theoretical calculations, we show that the often-used relation between the Shubnikov-de Haas oscillation frequencies and the B = 0 spin-splitting, p+/- = ehf+/-SdH , is only approximately accurate for systems with large spin-orbit coupling.; Furthermore, we find that changing the B = 0 spin-splitting significantly affects the temperature dependence of the B = 0 resistivity. Qualitatively, increasing the spin-splitting at constant density has an effect similar to decreasing the density. Our measurements are all done with the samples at densities such that they exhibit metallic behavior, although the lowest measured density is near that at which insulating behavior is expected to appear.; Finally, the 2D hole systems are studied in the presence of a magnetic field B applied in the plane of the system. We measure Shubnikov-de Haas oscillations in the presence of in-plane B and find a surprising dependence of the spin-subband densities on B. Measurements with in-plane B on lower density samples reveal a remarkable dependence of the magnetoresistance features, and in particular the value of B above which the resistivity exhibits insulating behavior, on the direction of B relative to both the crystal axes and the current direction. To explain the data, the anisotropic band structure of the holes and a re-population of the spin-subbands in the presence of B, as well as the coupling of the orbital motion to B, need to be taken into account.