Search

Although soft confinement of electrons in ballistic cavities gives rise to a power law of classical probability distributions, the behavior is limited to a narrow range of the variable if the potential is not substantially smooth. The presence of a magnetic field normal to the cavity extends the power-law regime by generating a hierarchical phase space structure. The change in dynamics induced by the magnetic field is confirmed experimentally through analysis of conductance fluctuations in quantum cavities defined by electrostatic gates on a high-mobility heterojunction. Nonfractal conductance fluctuations as a function of the gate bias at zero magnetic field are transformed to be fractal when the cyclotron radius is comparable to the cavity size.

We discuss the observation of an unusual type of localization in split-gate quantum dots and quantum-dot arrays. While no evidence for its existence is found prior to biasing the gates, the localization persists to conductance values as high as $50{e}^{2}/h$ and is not destroyed by the application of a weak magnetic field. The carrier density in the dots remains constant over the range of gate bias studied and these characteristics suggest that the localization is quite distinct to that studied previously in two-dimensional semiconductors. We suggest that a confinement-induced enhancement of the electron-electron interaction may be responsible for the localization and propose a simple functional form which allows us to account for its variation as a function of either temperature or source-drain voltage.

We report a clear and sudden period doubling in the Aharonov-Bohm oscillations (ABO), observed in the low-temperature magneto-resistance of a small quantum dot. The doubling occurs over a well-defined range of magnetic field, outside of which the ABO revert back to their single period. The precise nature of the doubling is reflected in the Fourier spectra of the oscillations, and suggests an interpretation in terms of the single-particle energy-level spectrum of the dot.

We study the effect of Coulomb blockade (CB) on the Aharonov-Bohm (AB) oscillations, observed in the magnetoresistance of small dots in both the quantum Hall and tunneling regimes. In the quantum Hall regime, where one or more edge states pass freely through the dot, the period of oscillation \ensuremath{\Delta}B is well correlated to the dot size, and the temperature-dependent decay of the oscillations is consistent with the expected form of the single-particle magnetospectrum. In the tunneling regime, however, \ensuremath{\Delta}B is enhanced by more than two orders of magnitude over the value expected from the size of the dot, and the resulting oscillations persist to above a degree Kelvin. This behavior can only be explained by considering the effect of CB on the magnetic depopulation of electrons from the isolated dot; these magneto-Coulomb oscillations are the magnetic analogue of the CB oscillations more conventionally observed using electrostatic techniques. These results demonstrate that by varying the coupling of the dot to the source and drain, the resulting AB oscillations can be used to separately probe both the single-particle and CB-related energy scales of small dots.

We measure a strong enhancement of the nonlinear differential conductance ($g=dI/dV$), the amplitude of which exceeds the universal quantum conductance ($2{e}^{2}/h$), under finite bias voltage in quantum point contacts (QPCs). By developing a spin-based model in the low-electron-density limit, we demonstrate that this resonance is an intrinsic nonequilibrium phenomenon that arises from many-body induced modifications to the QPC potential. A comparison with the linear conductance ($G=I/V$) shows that this phenomenon is driven by many-body dynamics within a single one-dimensional sub-band.

We study both the phase-breaking and the energy-relaxation times as a function of both temperature and bias current in arrays of coupled, open quantum dots. These times show a saturation at low temperature, and a decay at higher temperatures. Importantly, the variations of sample resistance as a function of temperature and current cannot be used to infer the electron temperature at a given bias current, which suggests that the energy relaxation process is likely to be phonon based.

We demonstrate a large (as much as 100%) and strongly hysteretic magnetoresistance (MR) in nominally nonferromagnetic silicide films and nanowires. This unusual MR is quenched above a few kelvins, where conventional behavior due to weak antilocalization is recovered. The dynamic characteristics of this effect are suggestive of weakly interacting, localized paramagnetic moments that form at the surface oxide of the silicide nanostructures, with dramatic consequences for transport when the system size is reduced to the nanoscale.

We investigate competition between one- and two-dimensional topological excitations---phase slips and vortices---in the formation of resistive states in quasi-two-dimensional superconductors in a wide temperature range below the mean-field transition temperature ${T}_{C0}$. The widths $w=100\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ of our ultrathin NbN samples are substantially larger than the Ginzburg-Landau coherence length $\ensuremath{\xi}=4\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, and the fluctuation resistivity above ${T}_{C0}$ has a two-dimensional character. However, our data show that the resistivity below ${T}_{C0}$ is produced by one-dimensional excitations---thermally activated phase slip strips (PSSs) overlapping the sample cross section. We also determine the scaling phase diagram, which shows that even in wider samples the PSS contribution dominates over vortices in a substantial region of current and/or temperature variations. Measuring the resistivity within 7 orders of magnitude, we find that the quantum phase slips can only be essential below this level.

We demonstrate a novel manifestation of dynamic Bragg reflection in artificial quantum-dot crystals that is driven by the application of a magnetic field. This backscattering is coherently cascaded as the number of dots in the structure is increased, causing a superlinear damping of the electron wave function and the appearance of a series of gaps in the miniband spectrum. The evolution of the dynamic miniband structure as the magnetic field is varied gives rise to behavior analogous to a metal-insulator transition, which is manifest as a dramatic resonance in the magneto-resistance of the structures.

We use a microwave cavity to investigate the influence of a movable absorbing center on the wave function of an open quantum dot. Our study shows that the absorber acts as a position-selective probe, which may be used to suppress those wave function states that exhibit an enhancement of their probability density near the region where the impurity is located. For an experimental probe of this wave function selection, we develop a technique that we refer to as scanning Fourier spectroscopy, which allows us to identify, and map out, the structure of the classical trajectories that are important for transmission through the cavity., 4 pages, 5 figures

We study the wave functions and transmission properties of an open microwave cavity in the frequency range of 1--17 GHz. The transmission of microwaves between the input and output antennas of the cavity is found to exhibit large fluctuations as a function of frequency, while the cavity wave functions are found to reveal the presence of periodically recurring scars, in close correspondence with specific peaks in the Fourier transform of the transmission fluctuations. This work, therefore, provides strong support for the results of recent studies of open quantum dots, which have revealed the presence of measurable transport results associated with specific wave function scars.