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...

We study magnetoresistivity oscillations in a high-mobility two-dimensional electron system subject to both microwave and dc electric fields. First, we observe that the oscillation amplitude is a periodic function of the inverse magnetic field and is strongly suppressed at microwave frequencies near half-integers of the cyclotron frequency. Second, we obtain a complete set of conditions for the differential resistivity extrema and saddle points. These findings indicate the importance of scattering without microwave absorption and a special role played by microwave-induced scattering events antiparallel to the electric field. Over the past few years, it was realized that dc resistivity of a high-mobility two-dimensional electron system (2DES) in very high Landau levels (LLs) exhibits a variety of unexpected features when subject to external electric fields and sufficiently low temperatures. Relevant phenomena include microwave (ac)-induced resistance oscillations (MIRO) 1,2 and Hall field (dc)-induced resistance oscillations (HIRO). 3 Microwave-illuminated 2DES became a subject of intense experimental 4 and theoretical 5 interest, following the discovery of zero-resistance states (ZRS). 6,7,8 More recently it was demonstrated that the effects of dc electric field 9,10 (or its combination with microwaves 11 ) on electron transport can also be quite dramatic leading to zero-differential resistance states.MIRO and HIRO are periodic in inverse magnetic field 1/B and originate from inter-LL transitions owing to microwave absorption and elastic scattering off short-range disorder, respectively. MIROs are governed by a parameter ǫ ac ≡ ω/ω C , where ω = 2π f is the microwave frequency and ω C = eB/m * is the cyclotron frequency. 1 HIROs in turn are controlled by ǫ dc ≡ ω H /ω C , where ω H ≃ 2eER C (E is the Hall field and R C is the Larmour radius) is the Hall voltage across the cyclotron orbit. 3 Experimentally, MIRO maxima 12,13 are found at ǫ ac ≃ n − φ ac (n, m ∈ Z + , throughout this paper; φ ac 1/4) and HIRO maxima 14,15,16 (measured in differential resistance r = dV/dI) occur at ǫ dc ≃ n.Recently, experimental studies of 2DES were extended into the regimes where MIRO and HIRO coexsist. 11 Magnetic field sweeps under microwaves illumination at fixed I suggested that the peaks in r occur atEquation (1) represents an important result since it indicates a dominant role of combined inter-LL transitions that consist of an energy jump due to microwave absorption and a space jump in the direction parallel to the electric field due to scattering off impurities. 4,11 However, there are several issues that remained unresolved. First, while Eq. (1) correctly reproduces HIRO in the limit of vanishing ǫ ac , it fails to describe MIRO. Second, according to Eq. (1) the resistivity peaks should continuously move to higher B (lower ǫ ac ) with increasing dc I (higher ǫ dc ). Instead, it is observed that the peaks first evolve into the minima without changing their positions and then abruptly jump to catch up with Eq. (1). Finally...

We report on a state characterized by a zero differential resistance observed in very high Landau levels of a high-mobility two-dimensional electron system. Emerging from a minimum of Hall field-induced resistance oscillations at low temperatures, this state exists over a continuous range of magnetic fields extending well below the onset of the Shubnikov-de Haas effect. The minimum current required to support this state is largely independent on the magnetic field while the maximum current increases with the magnetic field tracing the onset of inter-Landau level scattering.Over the past decade it was realized that high-mobility two-dimensional electron systems ͑2DESs͒ exhibit an array of fascinating phenomena occurring in very high Landau levels where the Shubnikov-de Haas oscillations ͑SdHOs͒ are not yet resolved. Among these are three classes of magnetooscillations, namely, microwave-, 1-6 phonon-, 7-10 and Hall field-11-14 induced resistance oscillations ͑HIROs͒. Remarkably, the minima of microwave-induced oscillations can evolve into states with zero resistance. [15][16][17][18][19] These exotic states are currently understood in terms of the absolute negative resistance which leads to an instability with respect to formation of current domains. 20-22 Unfortunately, direct experimental confirmation of the domain structure has proven difficult in irradiated 2DES and awaits future studies. It is therefore of great interest to explore if other classes of oscillations give rise to phenomenologically similar states. Recently, experiments revealed states with zero differential resistance which emerged from the maxima of microwaveinduced resistance oscillations 23,24 and from the maxima of the SdHOs. 25,26 Such states are analogous to the radiationinduced zero-resistance states in a sense that they can also be explained by the domain model. 25 In this Rapid Communication we report on another state characterized by a zero differential resistance which requires neither microwave irradiation nor the Shubnikov-de Haas effect. This state emerges from a minimum of HIROs in a high-mobility 2DES at low temperatures. Appearing in very high Landau levels, this state is observed over a continuous magnetic field range extending well below the onset of the SdHOs. The minimum current required to support such a state is largely independent on the magnetic field while the maximum current increases roughly linearly with the magnetic field tracing the onset of inter-Landau level scattering. According to the domain model, 25 these currents should be associated with currents inside the domains.The data presented in this Rapid Communication were obtained on a Hall bar ͑width w = 100 m͒ etched from a symmetrically doped GaAs/AlGaAs quantum well. After a brief low-temperature illumination with visible light, density and mobility were n e Ӎ 3.8ϫ 10 11 cm −2 and Ӎ 1.0 ϫ 10 7 cm 2 / V s, respectively. Differential resistivity, r xx ϵ dV xx / dI, was measured using a quasi-dc ͑a few hertz͒ lock-in technique at temperatures ranging from T = 1....

We report on magneto-oscillations in differential resistivity of a two-dimensional electron system subject to intense microwave radiation. The period of these oscillations is determined not only by microwave frequency but also by its intensity. A theoretical model based on quantum kinetics at high microwave power captures all important characteristics of this phenomenon which is strongly nonlinear in microwave intensity. Our results demonstrate a crucial role of the multiphoton processes near the cyclotron resonance and its harmonics in the presence of strong dc electric field and offer a unique way to reliably determine the intensity of microwaves acting on electrons.

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.