Interaction and Mixing Effects in Two and One Dimensional Hole Systems

Abstract

This thesis describes electrical measurements performed on low dimensional p- type devices, fabricated from GaAs/AlGaAs heterostructures. The Coulomb interaction between holes is similar to that between electrons. However, the kinetic energy is suppressed, which makes interaction effects particularly important. Holes may also be used to study band structure effects which arise from spin-orbit coupling in the valence band. The effects of Coulomb interactions in low dimensional electron systems are currently being studied extensively. Experiments presented in this thesis indicate the possible importance of Coulomb exchange interactions in both one and two dimensional hole systems (1DHSs,2DHSs). Tilted magnetic field studies of 2DHSs in the quantum Hall regime indicate that Landau levels at even filling factors will not cross. For high filling factor, this is attributed to a spin-orbit mixing effect which arises from the low symmetry of the system. At lower filling factor, activation-energy measurements verify that the energy gaps decrease and then increase as the field is tilted. However, the energy gap versus field dependences do not exhibit the curvature that might be expected from a perturbative anticrossing. It is speculated that the origin of this effect is a phase transition driven by the exchange interaction. Balanced arguments contrasting the relative strengths of the mixing and interactions theories are provided. The second part of this thesis describes a new method for the fabrication of ballistic 1DHSs, which exhibit clear conductance quantization. The quantization changes from even to odd multiples of e2/h as a function of the magnetic field in the plane of the heterostructure, as "spin splitting" causes the 1D subbands to cross. Measurements of the 1D subband energy spacings are used together with the magnetic fields at which the crossings occur to calculate the in-plane g factors of the 1D subbands. These are found to increase as the number of occupied 1D subbands decreases. This enhancement of the g factor is attributed to exchange interactions; possible mixing explanations are also discussed. At higher magnetic fields, the pattern of quantization features shows that the subbands have crossed many times, and that the 1DHS can be strongly magnetized.EPSR