Electron-heating induced by a tunable, supplementary dc-current (Idc) helps to vary the observed magnetoresistance in the high mobility GaAs/AlGaAs 2D electron system. The magnetoresistance at B = 0.3 T is shown to progressively change from positive to negative with increasing Idc, yielding negative giant-magnetoresistance at the lowest temperature and highest Idc. A two-term Drude model successfully fits the data at all Idc and T. The results indicate that carrier heating modifies a conductivity correction σ1, which undergoes sign reversal from positive to negative with increasing Idc, and this is responsible for the observed crossover from positive- to negative- magnetoresistance, respectively, at the highest B.

Radiation-induced magnetoresistance oscillations are examined in the GaAs/AlGaAs 2D system in the regime where an observed concurrent giant magnetoresistance is systematically varied with a supplementary dc-current, I dc. The I dc tuned giant magnetoresistance is subsequently separated from the photo-excited oscillatory resistance using a multi-conduction model in order to examine the interplay between the two effects. The results show that the invoked multiconduction model describes the observed giant magnetoresistance effect even in the presence of radiation-induced magnetoresistance oscillations, the magnetoresistance oscillations do not modify the giant magnetoresistance, and the magnetoresistance oscillatory extrema, i.e., maxima and minima, disappear rather asymmetrically with increasing I dc. The results suggest the interpretation that the I dc serves to suppress scattering between states near the Fermi level in a strong magnetic field limit.

The reflected microwave power from the photo-excited high mobility GaAs/AlGaAs 2D device has been measured over the wide frequency band spanning from 30 to 330 GHz simultaneously along with diagonal magnetoresistance as a function of the magnetic field. Easily distinguishable resonances in the reflected power signal are observed at the same magnetic fields as a reduced amplitude in the Shubnikov-de Haas (SdH) oscillations of the diagonal magnetoresistance. The reflection resonances with concurrent amplitude reduction in SdH oscillations are correlated with cyclotron resonance induced by microwave, mm-wave, and terahertz photoexcitation. The magnetoplasma effect was also investigated. The results suggest a finite frequency zero-magnetic-field intercept, providing an estimate for the plasma frequency. The experimentally measured plasma frequency appears to be somewhat lower than the estimated plasma frequency for these Hall bars. The results, in sum, are consistent with an effective mass ratio of m*/m = 0.067, the standard value, even in these high mobility GaAs/AlGaAs devices, at very large filling factors. Preliminary findings from this article have been published as conference proceedings, see Kriisa, A., et al., J. of Phys. Conf. Ser. 864, 012057 (2017).

A small and narrow negative-magnetoresistance (MR) effect that appears about null magnetic field over the interval −0.025 ≤ B ≤ 0.025 T in magnetotransport studies of the GaAs/AlGaAs 2D system with μ ≈ 107cm2/Vs is experimentally examined as a function of the sample temperature, T. The temperature dependent magnetoresistance data were fit using the Hikami et al. theory, without including the spin-orbit correction, to extract the inelastic length, li, which decreases rapidly with increasing temperature. It turns out that li < le, where le is the elastic length, for all T. Thus, we measured the single particle lifetime, τs, and the single particle mean free path ls = vFτs. A comparison between li and ls indicates that li > ls. The results suggest that the observed small and narrow magnetoresistance effect about null magnetic field could be a manifestation of coherent backscattering due to small angle scattering from remote ionized donors in the high mobility GaAs/AlGaAs 2DES.

No abstract

We examined the influence of microwave radiation on both the amplitude of Shubnikov-de Haas (SdH) oscillations and the null field longitudinal magnetoresistance at liquid helium temperatures, in GaAs/AlGaAs Hall bar devices. Microwave radiation over the frequency range 25 ≤ f ≤ 50 GHz with source power 0 ≤ P ≤ 4 mW served to photo-excite the high mobility (10 7 cm 2 /V s) 2D electron system (2DES) as magnetoresistance traces were obtained as a function of the microwave power P and temperature T. Lineshape study of SdH oscillations has been carried out over the span 2.3 < ωc/ω ≤ 5.2, where ωc = eB/m * , ω = 2πf , B is the magnetic field, m * is the effective mass and f is the microwave frequency. Here, fits of the SdH lineshape served to determine the electron temperature (Te) as a function of P and T. Theory has proposed that, in the ωc/ω ≥ 1 regime, both the electron temperature and radiation energy absorption rate (Sp) exhibit relatively small response, while in the ωc/ω ≤ 1 regime, both Te and Sp are enhanced and exhibit oscillatory behavior. We compare the experimental results with these theoretical predictions, and comment upon relative role of electron heating in the microwave photo-excited high mobility 2DES.

We examine the microwave frequency(f)-variation of the angular-phase-shift, θ 0 , observed in the polarization-angle-dependence of the microwave-induced magnetoresistance oscillations in the high mobility GaAs/AlGaAs two-dimensional electron system. By fitting the diagonal resistance R xx vs. θ plots to an empirical cosine square law, we extract the θ 0 and trace its quasi-continuous variation with f. The results suggest that the overall average of θ 0 extracted from Hall bar device sections

Millimeter wave radiation-induced magneto-resistance oscillations are examined in the GaAs/AlGaAs 2D electron system under bichromatic excitation in order to study the evolution of the oscillatory diagonal magnetoresistance, Rxx as the millimeter wave intensity is changed systematically for various frequency combinations. The results indicate that at low magnetic fields, the lower frequency millimeter wave excitation sets the observed Rxx response, as the higher frequency millimeter wave component determines the Rxx response at higher magnetic fields. The observations are qualitatively explained in terms of the order of the involved transitions. The results are also modeled using the radiation-driven electron orbit theory.