Tunneling Between Two-Dimensional Electron Gases in a Strong Magnetic Field

We have measured the tunneling between two two-dimensional electron gases at high magnetic fields $B$, when the carrier densities of the two electron layers are matched. For filling factors $\nu<1$, there is a gap in the current-voltage characteristics centered about $V=0$, followed by a tunneling peak at $\sim 6$~mV. Both features have been observed before and have been attributed to electron-electron interactions within a layer. We have measured high field tunneling peak positions and fitted gap parameters that are proportional to $B$, and independent of the carrier densities of the two layers. This suggests a different origin for the gap to that proposed by current theories, which predict a $\sqrt{B}$ dependence.Comment: 9 pages, cond-mat/yymmnn

Re-entrant resonant tunneling

We study the effect of electron-electron interactions on the resonant-tunneling spectroscopy of the localized states in a barrier. Using a simple model of three localized states, we show that, due to the Coulomb interactions, a single state can give rise to two resonant peaks in the conductance as a function of gate voltage, G(Vg). We also demonstrate that an additional higher-order resonance with Vg-position in between these two peaks becomes possibile when interactions are taken into account. The corresponding resonant-tunneling process involves two-electron transitions. We have observed both these effects in GaAs transistor microstructures by studying the time evolution of three adjacent G(Vg) peaks caused by fluctuating occupation of an isolated impurity (modulator). The heights of the two stronger peaks exibit in-phase fluctuations. The phase of fluctuations of the smaller middle peak is opposite. The two stronger peaks have their origin in the same localized state, and the third one corresponds to a co-tunneling process.Comment: 9 pages, REVTeX, 4 figure

An observation of spin-valve effects in a semiconductor field effect transistor: a novel spintronic device

We present the first spintronic semiconductor field effect transistor. The injector and collector contacts of this device were made from magnetic permalloy thin films with different coercive fields so that they could be magnetized either parallel or antiparallel to each other in different applied magnetic fields. The conducting medium was a two dimensional electron gas (2DEG) formed in an AlSb/InAs quantum well. Data from this device suggest that its resistance is controlled by two different types of spin-valve effect: the first occurring at the ferromagnet-2DEG interfaces; and the second occuring in direct propagation between contacts.Comment: 4 pages, 2 figure

The Fourier‐Kelvin Stellar Interferometer: A Concept for a Practical Interferometric Mission for Discovering and Investigating Extrasolar Giant Planets

The Fourier‐Kelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the near‐to‐mid‐infrared spectral region (3 – 8 μm). FKSI is conceived as a scientific and technological precursor to TPF. The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will answer key questions about extrasolar planets:• What are the characteristics of the known extrasolar giant planets?• What are the characteristics of the extrasolar zodiacal clouds around nearby stars?• Are there giant planets around classes of stars other than those already studied?We present preliminary results of a detailed design study of the FKSI. Using a nulling interferometer configuration, the optical system consists of two 0.5 m telescopes on a 12.5 m boom feeding a Mach‐Zender beam combiner with a fiber wavefront error reducer to produce a 0.01% null of the central starlight. With this system, planets around nearby stars can be detected and characterized using a combination of spectral and spatial resolution. © 2004 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87917/2/297_1.pd

Enhanced delivery and detection of terahertz frequency radiation from a quantum cascade laser within dilution refrigerator

We report on significant enhancements to the integration of terahertz (THz) quantum cascade lasers (QCL) and THz detection with a two-dimensional electron gas (2DEG) within a dilution refrigerator obtained by the inclusion of a multi-mesh 6 THz low-pass filter to block IR radiation, a Winston cone to focus light output, and gating the 2DEG for optimised sensitivity. We show that these improvements allow us to obtain a > 2.5 times reduced sample electron temperature (160 mK compared with 430 mK previously), during cyclotron resonance (CR) measurements of a 2DEG under QCL illumination. This opens up a route to performing sub-100 mK experiments using excitation by THz QCLs

Three Key Questions on Fractal Conductance Fluctuations: Dynamics, Quantization and Coherence

Recent investigations of fractal conductance fluctuations (FCF) in electron billiards reveal crucial discrepancies between experimental behavior and the semiclassical Landauer-Buttiker (SLB) theory that predicted their existence. In particular, the roles played by the billiard's geometry, potential profile and the resulting electron trajectory distribution are not well understood. We present measurements on two custom-made devices - a 'disrupted' billiard device and a 'bilayer' billiard device - designed to probe directly these three characteristics. Our results demonstrate that intricate processes beyond those proposed in the SLB theory are required to explain FCF.Comment: 17 pages, 4 figures, in press for Physical Review