電子與複合費米子在砷化鎵二維系統之傳輸

Abstract

This thesis focus on electron and composite fermion transport properties in GaAs two-dimensional electron systems. The experiments fall into two sets.

1. Transport and Quantum Lifetime Dependence on Electron Density in Gated GaAs/AlGaAs Heterostructures We present a study of the transport and quantum lifetime dependence on electron density in two completely di erent kinds of two-dimensional electron gas systems. We observed that both the two scattering times increase with increasing the electron density. But for the GaAs heterostructure with InAs self-assembled quantum dots, the two lifetimes and the ratio of them are much smaller than those of the conventional GaAs heterostructures. We concluded that the short-range scattering dominates the two-dimensional electron gas as a consequence of the InAs quantum dots. Furthermore, for another GaAs heterostructure with ultra high mobility, we showed that the dominated scattering mechanism is the long-range scattering associated with the ionized donors. Besides, a substantial increase of the quantum lifetime with carrier density, which do not conform to the conventional theory, were observed and we speculate that the screening e ects need to be considered in order to explain our experimental results. The theoretical calculations were also made to investigate our experimental results.

2. Evolution of the Second Lowest Extended State as a Function of Effective Magnetic Field in the Fractional Quantum Hall Regime It has been shown that at a Landau level filling factor is equal to 1/2, a two-dimensional electron system can be mathematically transformed into a composite fermion system interacting with a Chern-Simons gange field. Besides, since it is believed that all state should be localized in a 2D system at B = 0, a profound question concerning the fate of the extended states as B approach to 0 arose soon after the discovery of the quantum Hall effect. Integrating these two ideas, we present the first study of the evolution of the second lowest extended state of the composite fermions in a vanishing effective magnetic field in the fractional quantum Hall regime.