Millimeter wave transmission spectroscopy of gated two-dimensional hole systems

We developed a differential transmission to study cyclotron resonance of GaAs/AlxGa1 xAs two-dimensional hole samples. The technique utilizes a modulated AuPd gate isolated by a Si3N4 dielectric from the sample, which is irradiated opposite the gate by millimeter waves ranging from 2 to 40GHz. This technique effectively removes the background signal and yields a hole effective mass of 0:41me with a cyclotron scattering time of 20 ps, consistent with the previous results using different techniques

Quantum Lifetime of Two-Dimensional Holes

The quantum lifetime of two-dimensional holes in a GaAs/AlGaAs double quantum well is determined via tunneling spectroscopy. At low temperatures the lifetime is limited by impurity scattering but at higher temperatures hole-hole Coulomb scattering dominates. Our results are consistent with Fermi liquid theory, at least up to r_s = 11. At the highest temperatures the measured width of the hole spectral function becomes comparable to the Fermi energy. A new, tunneling-spectroscopic, method for determining the in-plane effective mass of the holes is also demonstrated.Comment: 5 pages, 4 figures. Published versio

Strongly Anisotropic Transport in Higher Two-Dimensional Landau Levels

Low-temperature, electronic transport in Landau levels N>1 of a two-dimensional electron system is strongly anisotropic. At half-filling of either spin level of each such Landau level the magnetoresistance either collapses to form a deep minimum or is peaked in a sharp maximum, depending on the in-plane current direction. Such anisotropies are absent in the N=0 and N=1 Landau level, which are dominated by the states of the fractional quantum Hall effect. The transport anisotropies may be indicative of a new many particle state, which forms exclusively in higher Landau levels.Comment: 12 pages, 3 Postscript figure

Resistively Detected NMR in Quantum Hall States: Investigation of the anomalous lineshape near ν=1\nu=1

A study of the resistively detected nuclear magnetic resonance (RDNMR) lineshape in the vicinity of $\nu=1$ was performed on a high-mobility 2D electron gas formed in GaAs/AlGaAs. In higher Landau levels, application of an RF field at the nuclear magnetic resonance frequency coincides with an observed minimum in the longitudinal resistance, as predicted by the simple hyperfine interaction picture. Near $\nu=1$ however, an anomalous dispersive lineshape is observed where a resistance peak follows the usual minimum. In an effort to understand the origin of this anomalous peak we have studied the resonance under various RF and sample conditions. Interestingly, we show that the lineshape can be completely inverted by simply applying a DC current. We interpret this as evidence that the minima and maxima in the lineshape originate from two distinct mechanisms.Comment: 5 pages, 3 figures, EP2DS 17, to be published in Physica

Shifted Landau ladders and low field magneto-oscillations in high-mobility GaAs 2D hole systems

We present well-developed low-field magneto-resistance oscillations originating from zero-field spin splitting (ZFSS) of heavy holes in high mobility GaAs/AlGaAs quantum wells. This low field oscillation is 1/B-periodic and emerges before the onset of Shubnikov–de Haas oscillations. The effect can be explained by resonant scattering between two Landau ladders shifted by the ZFSS gap, which in turn can be measured by comparing with the hole cyclotron energy. A front gate is fabricated to tune the ZFSS and hence the oscillation period

Evidence for 2k_F Electron-Electron Scattering Processes in Coulomb Drag

Measurements and calculations of Coulomb drag between two low density, closely spaced, two-dimensional electron systems are reported. The experimentally measured drag exceeds that calculated in the random phase approximation by a significant, and density dependent, factor. Studies of the dependence of the measured drag on the difference in density between the two layers clearly demonstrate that previously ignored q=2k_F scattering processes can be very important to the drag at low densities and small layer separations.Comment: 5 pages, 5 figure

The microwave induced resistance response of a high mobility 2DEG from the quasi-classical limit to the quantum Hall regime

Microwave induced resistance oscillations (MIROs) were studied experimentally over a very wide range of frequencies ranging from ~20 GHz up to ~4 THz, and from the quasi-classical regime to the quantum Hall effect regime. At low frequencies regular MIROs were observed, with a periodicity determined by the ratio of the microwave to cyclotron frequencies. For frequencies below 150 GHz the magnetic field dependence of MIROs waveform is well described by a simplified version of an existing theoretical model, where the damping is controlled by the width of the Landau levels. In the THz frequency range MIROs vanish and only pronounced resistance changes are observed at the cyclotron resonance. The evolution of MIROs with frequency are presented and discussed.Comment: 4 pages, presented at EP2DS, to be published in Physica

Breakdown of Particle-Hole Symmetry in the Lowest Landau Level Revealed by Tunneling Spectroscopy

Tunneling measurements on 2D electron gases at high magnetic field reveal a qualitative difference between the two spin sublevels of the lowest Landau level. While the tunneling current-voltage characteristic at filling factor $\nu = 1/2$ is a single peak shifted from zero bias by a Coulomb pseudogap, the spectrum at $\nu=3/2$ shows a well-resolved double peak structure. This difference is present regardless of whether $\nu =1/2$ and $\nu = 3/2$ occur at the same or different magnetic fields. No analogous effect is seen at $\nu = 5/2$ and 7/2 in the first excited Landau level.Comment: 5 pages, 4 figure

Wigner crystalization about ν\nu=3

We measure a resonance in the frequency dependence of the real diagonal conductivity, Re[$\sigma_{xx}$], near integer filling factor, $\nu=3$. This resonance depends strongly on $\nu$, with peak frequency $f_{pk} \approx 1.7$ GHz at $\nu=3.04$ or 2.92 close to integer $\nu$, but $f_{pk} \approx$ 600 MHz at $\nu=3.19$ or 2.82, the extremes of where the resonance is visible. The dependence of $f_{pk}$ upon $n^*$, the density of electrons in the partially filled level, is discussed and compared with similar measurments by Chen {\it et al.}\cite{yong} about $\nu=1$ and 2. We interpret the resonance as due to a pinned Wigner crystal phase with density $n^*$ about the $\nu=3$ state.Comment: for proceedings of EP2DS-15 (Nara) to appear in Physica

Experimental Evidence for a Spin-Polarized Ground State in the \nu=5/2 Fractional Quantum Hall Effect

We study the \nu=5/2 even-denominator fractional quantum Hall effect (FQHE) over a wide range of magnetic (B) field in a heterojunction insulated gate field-effect transistor (HIGFET). The electron density can be tuned from n=0 to 7.6 \times 10^{11} cm^{-2} with a peak mobility \mu = 5.5 \times 10^6 cm^2/Vs. The \nu=5/2 state shows a strong minimum in diagonal resistance and a developing Hall plateau at magnetic fields as high as 12.6T. The strength of the energy gap varies smoothly with B-field. We interpret these observations as strong evidence for a spin-polarized ground state at \nu=5/2.Comment: new references adde