Linear ac ͑888 Hz͒ resistance of highly mobile two-dimensional electrons in GaAs heavily doped quantum wells is studied at different magnitudes of dc and ac ͑10 KHz to 20 GHz͒ excitations. In the dc excitation regime the differential resistance oscillates with the dc current and external magnetic field similar to that observed earlier in AlGaAs/ GaAs heterostructures ͓C. L. Yang et al., Phys. Rev. Lett. 89, 076801 ͑2002͔͒. At external ac excitations the resistance is also found to be oscillating with the magnetic field. However the form of the oscillations is considerably different from the dc case. We show that at frequency below 100 KHz the difference is the result of a specific average of the dc differential resistance during the period of the external ac excitations.

The longitudinal resistivity of two dimensional (2D) electrons placed in strong magnetic field is significantly reduced by applied electric field, an effect which is studied in a broad range of magnetic fields B and temperatures T in GaAs quantum wells with high electron density. The data are found to be in good agreement with theory, considering the strong nonlinearity of the resistivity as result of non-uniform spectral diffusion of the 2D electrons. Inelastic processes limit the diffusion. Comparison with the theory yields the inelastic scattering time τin of the two dimensional electrons. In the temperature range T = 2 − 10K for overlapping Landau levels, the inelastic scattering rate 1/τin is found to be proportional to T 2 , indicating a dominant contribution of the electronelectron scattering to the inelastic electron relaxation. In a strong magnetic field, the nonlinear resistivity demonstrates scaling behavior, indicating a specific regime of electron heating of wellseparated Landau levels. In this regime the inelastic scattering rate is found to be proportional to T 3 , suggesting the electron-phonon scattering as the dominant mechanism of the inelastic relaxation. At low temperatures and separated Landau levels an additional regime of the inelastic electron relaxation is observed: τin ∼ T −1.26 .

No abstract