We report on nonequilibrium transport measurements in a high-mobility two-dimensional electron system subject to weak magnetic field and dc excitation. Detailed study of dc-induced magneto-oscillations, first observed by Yang et al., reveals a resonant condition that is qualitatively different from that reported earlier. In addition, we observe dramatic reduction of resistance induced by a weak dc field in the regime of separated Landau levels. These results demonstrate similarity of transport phenomena in dc-driven and microwave-driven systems and have important implications for ongoing experimental search for predicted quenching of microwave-induced zero-resistance states by a dc current.Nonequilibrium magnetotransport in very high Landau levels (LLs) of two-dimensional electron systems (2DESs) is of intense current interest. Major efforts, both theoretical and experimental, have been directed toward microwave photoresistance phenomena, such as microwave-induced resistance oscillations 1,2 (MIRO) and zero-resistance states 3,4 (ZRS). Conversely, other novel effects observed in 2DESs, not irradiated by microwaves, have not received due attention. These include magneto-oscillations from interface acoustic phonon scattering 5 and those from Zener tunneling between tilted LLs, 6 both relying on large-angle scattering required by momentum and/or energy conservation.Experimentally, MIRO appear in photoresistance,where ω = 2π f is the microwave frequency, ω C = eB/m * is the cyclotron frequency of an electron, and B is the magnetic field. MIRO were initially explained in terms of impurity scattering, 7,8,9 but it is currently believed that the electron distribution function effects play a dominant role. 10 The fact that MIRO minima evolve into ZRS is linked to microscopic negative resistivity and its instability, which results in formation of current domains. 11 While the concept of negative resistivity has recently found some support in bichromatic experiments, 12 the conjecture that ZRS only exist below some critical current density 11 has not been experimentally verified. To systematically approach this problem, it is important first to better understand the effects of the dc current on 2DESs without microwaves, as it itself can strongly modify magnetotransport properties. 6 In this paper we report on magnetotransport measurements in a high-mobility 2DES under dc current excitation driving the system into a nonequilibrium state. Our sample was cleaved from a symmetrically doped GaAs/Al 0.24 Ga 0.76 As 300-Å-wide quantum well grown by molecular beam epitaxy. A Hall bar mesa of a width w = 100 µm was fabricated using photolithography. Ohmic contacts were made by evaporating Au/Ge/Ni and thermal annealing in forming gas ambient. The experiment was performed in a 3 He cryostat, equipped with a superconducting solenoid, at a constant coolant temperature T ≃ 1.5 K. After illumination with visible light, electron mobility µ and density n e were ≃ 1.2 × 10 7 cm 2 /Vs and 3.7 × 10 11 cm −2 , respectively. The differential r...

Using dc excitation to spatially tilt Landau levels, we study resonant acoustic phonon scattering in two-dimensional electron systems. We observe that dc electric field strongly modifies phonon resonances, transforming resistance maxima into minima and back into maxima. Further, phonon resonances are enhanced dramatically in the nonlinear dc response and can be detected even at low temperatures. Most of our observations can be explained in terms of dc-induced (de)tuning of the resonant acoustic phonon scattering and its interplay with inter-Landau level impurity scattering. Finally, we observe a resistance maximum when the electron drift velocity approaches the speed of sound and a dc-induced zero-differential resistance state.

We report on magnetotransport measurements in a high-mobility two-dimensional electron system subject simultaneously to ac (microwave) and dc (Hall) fields. We find that dc excitation affects microwave photoresistance in a nontrivial way. Photoresistance maxima (minima) evolve into minima (maxima) and back, reflecting strong coupling and interplay of ac- and dc-induced effects. Most of our observations can be explained in terms of indirect electron transitions using a new, combined resonant condition. Observed quenching of microwave-induced zero resistance by a dc field cannot be unambiguously linked to a domain model, at least before a systematic theory treating both excitation types within a single framework is developed.

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We study magnetoresistivity oscillations induced by microwave radiation or acoustic phonons in high-mobility two-dimensional electron systems subject to dc electric field. In microwave-irradiated samples the response is governed by combined electron transitions, composed of microwave absorption and scattering off impurities. In non-irradiated samples, acoustic phonon resonances are tuned by dc electric field. Here, we show that in both experiments scattering off impurities without microwave or phonon absorption plays an important role and might even dominate the response.