J. H. Smet

Demonstration of a 1/4-cycle phase shift in the radiation-induced oscillatory magnetoresistance in GaAs/AlGaAs devices.

We examine the phase and the period of the radiation-induced oscillatory magnetoresistance in GaAs/AlGaAs devices utilizing in situ magnetic field calibration by electron spin resonance of diphenyl-picryl-hydrazal. The results confirm a f-independent 1/4-cycle phase shift with respect to the hf=j variant Plancks over 2pi omega(c) condition for j>/=1, and they also suggest a small ( approximately 2%) reduction in the effective mass ratio, m(*)/m, with respect to the standard value for GaAs/AlGaAs devices.

Radiation-induced oscillatory Hall effect in high-mobility GaAs/Al x Ga 1-x As devices

We examine the radiation induced modification of the Hall effect in high-mobility GaAs/Al x Ga 1 - x As devices that exhibit vanishing resistance under microwave excitation. The modification in the Hall effect upon irradiation is characterized by (a) a small reduction in the slope of the Hall resistance curve with respect to the dark value, (b) a periodic reduction in the magnitude of the Hall resistance R x y that correlates with an increase in the diagonal resistance R x x , and (c) a Hall resistance correction that disappears as the diagonal resistance vanishes.

Current-induced nuclear spin depolarization at Landau level filling factor nu=1/2

In this work, we demonstrate that significant changes in electron temperature and nuclear spin polarization can be created by applying an electric current in a two-dimensional electron system at Landau level filling factor nu = 1/2. The current induced effects on nuclear spins can be attributed to electron heating and the efficient coupling between the nuclear and electron spin systems at nu = 1/2. The electron temperature, elevated by the current, can be measured with a thermometer based on the measurement of the nuclear spin relaxation rate. The electron temperature is found to be proportional to the square root of the current density at nu = 1/2. Electron spin transitions at nu = 2/3 and 1/2 are utilized for the measurement of the current induced changes in nuclear spin polarization. Consistent results are obtained from these two different methods of nuclear magnetometry. The finite thickness of the electron wave function is found to be important for evaluation of the nuclear spin polarization even in a narrow quantum well. The nuclear spin polarization follows a Curie law dependence on the electron temperature. This work also allows us to evaluate the electron g factor in high magnetic fields as well as the polarization mass of composite fermions.