In an ultraclean 2D electron system (2DES) subjected to crossed millimeterwave (30-150 GHz) and weak (B<2 kG) magnetic fields, a series of apparently dissipationless states emerges as the system is detuned from cyclotron resonances. Such states are characterized by an exponentially vanishing low-temperature diagonal resistance and a classical Hall resistance. The activation energies associated with such states exceed the Landau level spacing by an order of magnitude. Our findings are likely indicative of a collective ground state previously unknown for 2DES.

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Developments in the physics of 2D electron systems during the last decade revealed a new class of nonequilibrium phenomena in the presence of a moderately strong magnetic field. The hallmark of these phenomena is magnetoresistance oscillations generated by the external forces that drive the electron system out of equilibrium. The rich set of dramatic phenomena of this kind, discovered in high mobility semiconductor nanostructures, includes, in particular, microwave radiation-induced resistance oscillations and zero-resistance states, as well as Hall field-induced resistance oscillations and associated zero-differential resistance states. The experimental manifestations of these phenomena and the unified theoretical framework for describing them in terms of a quantum kinetic equation are reviewed. This survey also contains a thorough discussion of the magnetotransport properties of 2D electrons in the linear-response regime, as well as an outlook on future directions, including related nonequilibrium phenomena in other 2D electron systems.

We report on the temperature dependence of microwave-induced resistance oscillations in high-mobility two-dimensional electron systems. We find that the oscillation amplitude decays exponentially with increasing temperature, as exp(-alphaT;{2}), where alpha scales with the inverse magnetic field. This observation indicates that the temperature dependence originates primarily from the modification of the single particle lifetime, which we attribute to electron-electron interaction effects.

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...