Quantum magneto-transport in two-dimensional GaAs electron gases and SiGe hole gases

We have measured the low-temperature transport properties of two-dimensiona l (2D) GaAs electron gases and 2D SiGe hole gases. Our experimental results fall into three categories. (i) Collapse of spin-splitting and an enhanced Lande g-factor at Landau level filling factors both nu = 3 and nu = I in a 2D GaAs electron gas are observed. Our experimental results show direct ev idence that the effective disorder is stronger at nu = I than that at nu = 3 over approximately the same perpendicular magnetic field range. (ii) We p resent evidence for spin-polarisation of a dilute 2D GaAs electron gas. The Lande g-factor of the system is estimated to be 1.66. This enhanced g valu e is ascribed to electron-electron interactions at ultra low carrier densit y limit. (iii) In a high-quality SiGe hole gas, there is a temperature-inde pendent point in the magnetoresistivity rho (xx) and rho (xy) which is ascr ibed to experimental evidence for a quantum phase transition between nu = 3 and nu = 5. We also present a study on the temperature(T)-driven flow line s in our system. (C) 2001 Elsevier Science Ltd. All rights reserved.