Equilibrium magnetization measurements of two-dimensional electron systems

The thermodynamic equilibrium magnetization of two-dimensional electron systems, embedded in GaAs-(Al,Ga)As heterojunctions, has been measured in the temperature range 50 mK to 1 K in experiments using a highly sensitive torque magnetometer. Clear de-Haas-van-Alphen oscillations are observed for Landau-level filling factors between 3 and 52. Theoretical fits to the data show that the Landau levels are close to the idealized delta-function density of states, with a residual broadening of similar to 0.3 meV or less. A field- and energy-independent background density of states is invoked as a possible explanation of the shape of the oscillations. In one of the samples studied there is a similar to 60% increase in Landau-level broadening, despite an increase in transport mobility of about the same percentage, on illumination. Analysis of the de-Haas-van-Alphen oscillations at small odd-integer filling factors reveals a g-factor exchange enhanced by up to 14.5 times