K. L. Campman

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.

Tunneling induced transparency: Fano interference in intersubband transitions

We report the observation of tunneling induced transparency in asymmetric double quantum well structures. Resonant tunneling through a thin barrier is used to coherently couple the two upper states in a three level system of electronic subbands in a GaAs/AlGaAs structure. This creates Fano-type interferences for the collective intersubband excitations in the absorption from the ground state, analogous to electromagnetically induced transparency (EIT) in atomic systems. We observe a 50% reduction in absorption between the subband resonances which can be explained by taking into account the coherent coupling of the upper states. We analyze the bias dependent absorption spectra and determine the relevant lifetime broadening and dephasing rates for the transitions.

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.

Gate-controlled spin-orbit quantum interference effects in lateral transport.

In situ control of spin-orbit coupling in coherent transport using a clean GaAs/AlGaAs two-dimensional electron gas is realized, leading to a gate-tunable crossover from weak localization to antilocalization. The necessary theory of 2D magnetotransport in the presence of spin-orbit coupling beyond the diffusive approximation is developed and used to analyze experimental data. With this theory the Rashba contribution and linear and cubic Dresselhaus contributions to spin-orbit coupling are separately estimated, allowing the angular dependence of spin-orbit precession to be extracted at various gate voltages.

CRYOGENIC SCANNING PROBE CHARACTERIZATION OF SEMICONDUCTOR NANOSTRUCTURES

We demonstrate the use of a scanned probe microscope (SPM) at 4 Kelvin to study electron transport through a ballistic point contact in the two‐dimensional electron gas inside a GaAs/AlGaAs heterostructure. The electron gas density profile is locally perturbed by the charged SPM tip providing information about the electron flow through the point contact. As the tip is scanned, one obtains a spatial image of the ballistic electron flux as well as the topographic profile of the structure. Calculations indicate the spatial resolution is comparable to the electron gas depth.

INTERFACE ROUGHNESS AND ALLOY-DISORDER SCATTERING CONTRIBUTIONS TO INTERSUBBAND TRANSITION LINEWIDTHS

We report measurements of intersubband absorption in single semiconductor quantum wells of different well widths and alloy compositions. The well width dependence of the intersubband absorption linewidth is consistent with broadening dominated by interface roughness. The linewidth, however, is found to be relatively unaffected by alloy composition in the quantum well, making alloying an effective tool in the design of quantum well optical devices relying on intersubband transitions.

Spontaneous coherence in a cold exciton gas

If bosonic particles are cooled down below the temperature of quantum degeneracy, they can spontaneously form a coherent state in which individual matter waves synchronize and combine. Spontaneous coherence of matter waves forms the basis of a number of fundamental phenomena in physics, including superconductivity, superfluidity and Bose–Einstein condensation. Spontaneous coherence is the key characteristic of condensation in momentum space. Excitons—bound pairs of electrons and holes—form a model system to explore the quantum physics of cold bosons in solids. Cold exciton gases can be realized in a system of indirect excitons, which can cool down below the temperature of quantum degeneracy owing to their long lifetimes. Here we report measurements of spontaneous coherence in a gas of indirect excitons. We found that spontaneous coherence of excitons emerges in the region of the macroscopically ordered exciton state and in the region of vortices of linear polarization. The coherence length in these regions is much larger than in a classical gas, indicating a coherent state with a much narrower than classical exciton distribution in momentum space, characteristic of a condensate. A pattern of extended spontaneous coherence is correlated with a pattern of spontaneous polarization, revealing the properties of a multicomponent coherent state. We also observed phase singularities in the coherent exciton gas. All these phenomena emerge when the exciton gas is cooled below a few kelvin.

STATISTICS OF COULOMB BLOCKADE PEAK SPACINGS

Distributions of Coulomb blockade peak spacings are reported for large ensembles of both unbroken (magnetic field

Photon‐assisted tunneling in a resonant tunneling diode: Stimulated emission and absorption in the THz range

B\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0

From spatially indirect excitons to momentum-space indirect excitons by an in-plane magnetic field

) and broken (