Magnetotransport in a two-dimensional electron system in dc electric fields

Abstract

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...

Zero-Differential Resistance State of Two-Dimensional Electron Systems in Strong Magnetic Fields

“…[14]. Another notable nonlinear effect: strong reduction of the longitudinal resistance by the dc electric field has been observed at considerably smaller biases [15,16,17]. In the paper [16] the strong reduction of the resistance has been attributed to substantial changes of the electron distribution function induced by the DC electric field E dc .…”

“…It was found that the microwave radiation makes the response to the dc electric field to be more linear. Strongly nonlinear I-V curves were observed in this experiment, however, the transition to the zero differential resistance state has not been detected (see also recent experiments [15,16,17]). We suggest that the lower electron mobility in our samples can be the possible reason of the stronger nonlinearity.…”

“…[13] Another interesting nonlinear phenomena have been observed in response to DC electric field. [14,15,16,17] Oscillations of the longitudinal resistance, periodic in inverse magnetic field, have been observed at relatively high DC biases satisfying the condition n ×hω c = 2R c E H , where R c is the cyclotron radius of electrons at the Fermi level and E H is the Hall electric field induced by the DC bias in the magnetic field. The effect has been attributed to "horizontal" Landau-Zener tunneling between Landau levels, tilted by the Hall electric field E H .…”

“…The weaker nonlinearity requires stronger dc biases for the same nonlinear change of the resistance. The higher dc bias makes the inter Landau level scattering to be more intensive [14,15,16,17,18], increasing the longitudinal resistance.…”

Oscillatory nonlinear differential magnetoresistance of highly mobile 2D electrons in high Landau levels

“…Recently, the oscillatory behavior in the nonlinear magnetotransport has attracted much attention: in a 2D system even without irradiation, a relatively weak current can induce drastic suppression and strong oscillations of the differential magnetoresistance, and may result in a state of zero-differential resistance. 22,23,24,25,26 Several theoretical models have been proposed in an attempt to explain this interesting nonlinear phenomenon. 27,28,29,30 Numerical examinations based on the current-control transport scheme were shown in good agreement with the experimental observation for differential magnetoresistivity as a function of the ratio of the current density to the magnetic field in both the magnetic-field sweeping and the current-sweeping configurations covering both separated and overlapping Landau level regimes.…”