Arthur C. Gossard

An Adiabatic Quantum Electron Pump

A quantum pumping mechanism that produces dc current or voltage in response to a cyclic deformation of the confining potential in an open quantum dot is reported. The voltage produced at zero current bias is sinusoidal in the phase difference between the two ac voltages deforming the potential and shows random fluctuations in amplitude and direction with small changes in external parameters such as magnetic field. The amplitude of the pumping response increases linearly with the frequency of the deformation. Dependencies of pumping on the strength of the deformations, temperature, and breaking of time-reversal symmetry were also investigated.

The Coulomb Blockade in Coupled Quantum Dots

Individual quantum dots are often referred to as “artificial atoms.” Two tunnel-coupled quantum dots can be considered an “artificial molecule.” Low-temperature measurements were made on a series double quantum dot with adjustable interdot tunnel conductance that was fabricated in a gallium arsenide-aluminum gallium arsenide heterostructure. The Coulomb blockade was used to determine the ground-state charge configuration within the “molecule” as a function of the total charge on the double dot and the interdot polarization induced by electrostatic gates. As the tunnel conductance between the two dots is increased from near zero to 2e2/h (where e is the electron charge and h is Plancks constant), the measured conductance peaks of the double dot exhibit pronounced changes in agreement with many-body theory.

GaAs/AlGaAs self-sensing cantilevers for low temperature scanning probe microscopy

We have fabricated scanning probe microscope cantilevers with dimensions 65×11.4×0.25 μm3 and 3×2×0.129 μm3 from GaAs/Al0.3Ga0.7As heterostructures containing two-dimensional electron gases. Deflection is measured by an integrated field-effect transistor (FET) that senses strain via the piezoelectric effect and provides a low noise, low power displacement readout. We present images of a 200 nm mica grating taken with the large cantilever having a deflection (force) noise 10 A/√Hz (19 pN/√Hz) at T=2.2 K. The small cantilever has a resonant frequency of 11 MHz, a FET gate charge noise of 0.001 e/√Hz, and is projected to have a deflection (force) noise of 0.002 A/√Hz (1 pN/√Hz) at T=4.2 K.

Some effects of a longitudinal electric field on the photoluminescence of p‐doped GaAs‐AlxGa1−xAs quantum well heterostructures

Some effects of a longitudinal electric field on the intrinsic and extrinsic photoluminescence of Be‐doped GaAs‐AlxGa1−xAs multiquantum well samples grown by molecular beam epitaxy have been investigated. The principal sample exhibits four photoluminescence peaks whose relative intensities are observed to change in a very nonuniform manner with forward bias. As the forward bias is reduced (i.e., electric field increased) luminescence is shifted progressively from the free n=1 heavy hole exciton peak to the n=1 heavy hole exciton bound to Be0 peak to an n=1 confined electron‐Be0 peak. Changes in the integrated photoluminescence by more than two orders of magnitude are observed over the bias region studied. Shifts in the energy positions of the various peaks with bias are at most of the order of 1 meV.

Self-assembled ErAs islands in GaAs for THz applications

This paper concerns self-assembled ErAs islands in GaAs grown by molecular beam epitaxy. The nucleation of ErAs on GaAs occurs in an island growth mode leading to spontaneous formation of nanometer-sized islands. Pump–probe measurements indicate that the ErAs islands capture photogenerated carriers on a subpicosecond time scale. This together with the high resitivity of the material allows us to use it as a fast photoconductor. The performance of photomixer devices made from this material is discussed.

Extrinsic photoluminescence from GaAs quantum wells

Abstract Extrinsic photoluminescence has been studied from both undoped and p-type doped single and multiquantum well (MQW) GaAsAl x Ga 1−x As heterostructures grown by molecular beam epitaxy. For undoped MQW samples this luminescence is attributed to n = 1 electrons recombining with shallow neutral acceptors (probably carbon) near the well center whereas for single well samples the acceptors (probably carbon) are frequently concentrated in a thin GaAs layer at the first grown heterointerface. Ionization energies obtained from these data are in reasonable agreement with the calculations of Bastard. With Be-doped samples one can also see luminescence due to ground state heavy hole excitons bound to Be neutral acceptors. These data provide estimates of the dissociation energy of the heavy hole exciton neutral acceptor complex.

Observation of resonant tunneling through a compositionally graded parabolic quantum well

We report the first observation of electron resonant tunneling through parabolic quantum wells, compositionally graded by means of short-period (15A) Al x Ga 1-x As/GaAs superlattices grown by molecular beam epitaxy. In one structure, comprising a 300A wide well compositionally graded from AlAs to GaAs, five equally spaced resonances are observed in the current voltage characteristic (I-V), in good agreement with the theory. In another structure with 432A wide wehs graded from Al 0.30 Ga 0.70 As to GaAs, up to sixteen resonances are observed in the I-V. The first ten correspond to resonant tunneling through the quasi-bound-states of the double barrier while the other ones are ascribed to electron interference effects associated with virtual levels in the quasi-continuum energy range above the collector barrier.