Session W40: Magnetotransport in 2DEGs

Microwave mode of a two dimensional electron system in a spatially varying magnetic field

We find a resonance in the microwave absorption spectrum of a high mobility two dimensional electron system (2DES) in a spatially varying magnetic field produced by a long ferromagnetic cylinder of Dy placed, with its axis perpendicular to the 2DES, on the surface of the sample. An external field B$_{0}$ perpendicular to the 2DES is also applied. The resonance is present for B$_{0}$ less than about 0.5 T, and depends hysteretically on B$_{0}$, apparently due to the Dy magnetization. The resonance peak frequency, f$_{pk}$, decreases with B$_{0}$, and is about 1.4 GHz for a 1 mm diameter cylinder and B$_{0}$=0.2 T. For sufficiently large B$_{0}$, f$_{pk}\sim $1/ B$_{0}$, reminiscent of an edge magnetoplasmon [1]. This work is supported by MARTECH. [1] See for example, V. A. Volkov and S. A. Mikhailov, Sov. Phys.-JETP \textbf{67}, 1639(1988).   

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

We report on non-equilibrium transport measurements in a high- mobility 2D electron system subject to weak magnetic and strong DC electric fields. Detailed study of DC-induced magneto-oscillations, first observed by Yang {\em et al}, reveals a resonant condition which is qualitatively different from that reported earlier. In addition, we explore new experimental regime of separated Landau levels and observe dramatic reduction of resistance induced by a relatively weak DC field. These results demonstrate similarity of transport phenomena in DC-driven and microwave-driven systems and have important implications for experiments on quenching of microwave-induced zero-resistance states by a DC current.   

Magnetoresistance oscillations in two-dimensional electron systems induced by both AC and DC fields

We report on magnetotransport measurements in a high-mobility two-dimentional 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, with some new minima appearing as zero-resistance states. Most of our observations are explained in terms of indirect electron transitions using a new, ``combined'' resonant condition. Strong coupling and interplay of AC- and DC-induced effects call for a theory treating both excitation types within a single framework.   

Non-linear Resistivity of a Two-Dimensional Electron Gas in a Magnetic Field

We develop a theory of nonlinear response to an electric field of a two-dimensional electron gas (2DEG) placed in a classically strong magnetic field. The latter leads to a non-linear current-voltage characteristic at a relatively weak electric field. The origin of the non-linearity is two-fold: the formation of a non-equilibrium electron distribution function, and the geometrical resonance in the inter-Landau-levels transitions rates. We find the dependence of the current-voltage characteristics on the electron relaxation rates in the 2DEG. Our results can be applied for analysis of measurements at low [1] and high [2,3] current densities. [1] J. Zhang, S. Vitkalov, A. A. Bykov, A. K. Kalagin and A. K. Bakarov, cond-mat/0607741. [2] C. L. Yang, J. Zhang, R. R. Du, J. A. Simmons and J. L. Reno, Phys. Rev. Lett. 89, 076801 (2002). [3] W. Zhang, H. -S. Chiang, M. A. Zudov, L. N. Pfeiffer and K. W. West, cond-mat/0608727.   

B-periodic oscillations in microwave irradiated high-mobility 2D electron gas

Recently a new type of B-periodic magneto-oscillations was observed [1] in the Hall bar samples of a 2D electron gas under the irradiation of microwaves (MW, frequency $\omega )$. The period $\Delta B$ is determined by $\omega $, the electron density$ n_{s}$, and the distance between potential probes $L$, $\Delta B\propto n_s /\omega L$. The phenomenon is explained by coherent excitation of edge magnetoplasmons in the region near the contacts. Using very high-mobility (8- 20 $\times $ 10$^{6}$ cm$^{2}$/Vs) GaAs/Al$_{x}$Ga$_{1-x}$As heterostructures, we were able to observe both the MW-induced resistance oscillations, which is 1/B -periodic, and the B-periodic oscillations in the same sample, in the frequency range 27 to 130 GHz. Experimental data as well as a brief discussion will be presented. [1] Kukushkin et al, Phys. Rev. Lett. 92, 236803 (2004).   

DC-current induced magneto-oscillations in very high-mobility 2D electron gas

We report on a systematic experimental study of DC-current induced magneto-oscillations [1] using Hall bar samples of very high-mobility (8-20 $\times $ 10$^{6}$ cm$^{2}$/Vs) GaAs/Al$_{x}$Ga$_{1-x}$As heterostructures. Previously we show that remarkable nonlinear resistance and $1/B$ oscillations can arise when a high bias current ($I_{x})$ is passed through a Hall bar (width $w)$, and the effect can be explained by a Zener tunneling model in the presence of a tilting Hall field [1]. Data of resistance $R_{xx} \equiv V_x /I_x $, differential resistance $r_{xx} \equiv \partial V_x /\partial I_x $, and $r_{xx} '\equiv \partial r_{xx} /\partial I_x $ in higher mobility samples, which show higher order oscillations, have confirmed the validity of this model. Our temperature dependent date show that this effect can persist to $k_B T>\hbar \omega _c $, where $\hbar \omega _c $ is the cyclotron energy. [1] Yang et al, Phys. Rev. Lett.\textbf{ 89}, 076801 (2002).   

Magnetoresistance and microwave photoresistance of a periodically modulated high-mobility 2D electron gas

We have measured the magnetoresistance ($R_{xx})$ and the microwave (MW) photoresistance on a high-mobility 2D electron gas patterned with a large period (1200 and 1500 nm) triangular antidot lattice [1]. Our experiments were performed in a MW frequency range from 26 to 150 GHz and at temperatures (T) from 0.3 to 10 K, samples were Hall bars having low T mobility as high as 2.5 x 10$^{6}$ cm$^{2}$/Vs after the patterning. We observed remarkably sharp (up to seventh order) geometrical resonance (GR) peaks in $R_{xx}$. Moreover, under irradiation, MW-induced resistance oscillations (MIRO) and magnetoplasmon resonance (PR) modes were observed. Analysis shows that MIRO, MP, and GR are decoupled from each other in these large-period modulated 2D electron gas samples. [1] Yuan \textit{et al}, Phys. Rev. B \textbf{74}, 075313(2006).   

Resistivity of a high mobility two-dimensional hole gas on (100) GaAs in the vicinity of the metal-to-insulator transition

We report on the density and temperature dependence of the resistivity of an extremely high mobility, carbon-doped, two-dimensional hole system (2DHS) in the vicinity of the putative metal-to-insulator (MIT) transition. The high mobility of our structures allows us to probe the conduction properties at very low 2D densities, $\sim $10$^{9}$cm$^{-2}$, a regime in which interactions are expected to play an important role. Using a back-gated structure, a mobility of 2.2x10$^{6}$cm$^{2}$/Vs is achieved at a density of 2.9x10$^{10}$cm$^{-2}$ at T=50mK. Backgating allows us to monitor the evolution of the resistivity as the density is continuously tuned from 2.9x10$^{10}$cm$^{-2}$ to 2.9x10$^{9}$cm$^{-2}$. From analysis of the temperature dependence of the resistivity, the sample becomes insulating at 3.5x10$^{9}$cm$^{-2}$. We compare our data to existing models of the MIT in high mobility, low density, structures.   

Electronic Transport Studies in Thin GaAs/AlGaAs Quantum Wells

The results of experimental transport studies involving a series of thin GaAs/AlGaAs quantum wells with varying well widths will be reported. The mobility, $\mu$, of thin GaAs/AlGaAs quantum wells is typically limited by electron scattering from the interfacial roughness of the quantum well. The total scattering rate due to all scattering mechanisms is determined from the mobility through $\tau^{-1}=e/\mu m^{*}$ where $m^*$ is the effective electron mass. Our series of samples consists of well widths of $L=7.9, 9.9, 12.9, 16.0$ and 33.0 nm. For constant electron density ($n_e\sim5.5\times 10^{10}$ cm$^{-2}$) we find that interfacial roughness is the dominant scattering mechanism for $L\leq 16.0$ nm and successfully fit the data using a finite quantum well model$^1$ with adjustable interfacial roughness parameters. We will also present the magnetotransport properties of this series of samples. $^1$J.M. Li et al. Semicond. Sci. and Tech. 20, 1207 (2005).   

Effect of DC electric field on longitudinal resistance of two dimensional electrons in a magnetic field

The effect of a DC electric field on the longitudinal resistance of highly mobile two dimensional electrons in heavily doped GaAs quantum wells is studied at different magnetic fields and temperatures. Strong suppression of the resistance by the electric field is observed in magnetic fields at which the Landau quantization of electron motion occurs. The phenomenon survives at high temperature where Shubnikov de Haas oscillations are absent. The scale of the electric fields essential for the effect is found to be proportional to temperature in the low temperature limit. We suggest that the strong reduction of the longitudinal resistance is the result of a nontrivial change in the distribution function of 2D electrons induced by the DC electric field. Comparison of the data with recent theory yields the inelastic electron-electon scattering time $\tau_{in}$ and the quantum scattering time $\tau_q$ of 2D electrons at high temperatures, a regime where previous methods were not successful.   

Experimental analysis of tunneling from a two-dimensional electron gas into the bulk in the presence of strong scattering.

An asymmetric double-barrier heterostructure of InGaAs/InAlAs was grown lattice-matched to InP, and electrical contact was made to the InGaAs quantum well layer as well as both bulk InGaAs regions.~ The tunnel current out of the quantum well across one barrier was monitored while varying the electric field across the other (thicker) barrier.~ This measurement yielded the dependence of the tunnel current on the carrier concentration in the quantum well.~ Samples with ErAs dot scattering centers within the quantum well were measured and compared with device simulations to confirm the impact of scattering on tunneling out of a quantum well.   

Undoped Electron-Hole Bilayers in a GaAs/AlGaAs Double Quantum Well

There is intense interest in exciton condensation effects that can occur in bilayer systems. While exciton condensation effects have been studied in quantum hole bilayers, transport experiment in the exciton condensation regime of electron-hole bilayers have proved to be extremely difficult. We present the fabrication details and device measurements of completely undoped electron-hole bilayer devices in a GaAs/AlGaAs double quantum well heterostructure. The quantum wells are separated by a 90{\%} AlGaAs barrier with thicknesses of 20 nm or 30 nm depending upon the device. These devices have independently tunable densities of the two-dimensional electron gas and two-dimensional hole gas. We report four-terminal transport measurements of the independently contacted electron and hole layers with balanced densities from 1.2 x 10$^{11 }$cm$^{-2}$ down to 4 x 10$^{10 }$cm$^{-2}$ at T = 300 mK. Coulomb drag results from these devices will be presented. The mobilities can exceed 1 x 10$^{6}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for electrons and 4 x 10$^{5}$ cm$^{2}$ V$^{-1}$ s$^{-1}$ for holes. This work has been supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract No. DE-AC04-94AL85000.