Mesoscopic Quantum Dot Research Articles

Insulating state in open quantum dots and quantum dot arrays

AbstractWe describe the observation of novel localization in mesoscopic quantum dots and quantum dot arrays, which are realized in high mobility GaAs/AlGaAs heterojunctions using the split‐gate technique. With a sufficient gate voltage applied to form the devices, their resistance diverges as the temperature is lowered below a degree Kelvin, behavior which we attribute to localization. Evidence for the localization is found over the entire range of gate voltage for which the dots are defined, persisting to conductances higher than 50e2/h.

Localization effect in mesoscopic quantum dots and quantum-dot arrays

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.

Superfluid, Superconducting, and Magnetic Ordering in Mesoscopic Quantum Dots

The nature of superfluid, superconducting, and magnetic ordering is elucidated for mesoscopic systems in which the single-particle level spacing is much larger than both the temperature and the critical temperature of ordering. Ordering is defined as a spontaneous breaking of symmetry, the gauge invariance and time reversal being by definition symmetries broken in superfluidity (superconductivity) and magnetism contexts, respectively. Superfluidity and superconductivity are realized in thermodynamic equilibrium states with a nonintegral average number of particles and are accompanied by the spontaneous breaking of time homogeneity. In Fermi systems, two types of superfluidity and superconductivity are possible which are characterized by the presence of pair or single-particle “condensates.” The latter is remarkable in that spontaneous breaking of fundamental symmetries such as spatial 2π rotation and double time reversal takes place. Possible experiments on metallic nanoparticles and ultracold atomic gases in magnetic traps are discussed.

Conductance fluctuations in a ballistic in-plane gated InGaAs quantum dot

Recent experiments have examined the magnetoconductance properties of mesoscopic quantum dots, defined in high mobility GaAs/AlGaAs heterojunctions utilizing split-gate techniques. Here, we report on a submicron, square quantum dot defined in a pseudomorphic InGaAs/GaAs single quantum well through an in-plane gate (IPG) technique. The high mobility of the sample ensures that the mean free path is larger than the characteristic dimensions of the quantum dot; hence transport through the structure is ballistic. These structures exhibit reproducible conductance fluctuations and weak localization. We discuss the results of low temperature studies of fluctuations observed in the low field magnetoconductanct. These fluctuations appear to be regular and quasiperiodic, containing only a few frequencies in the Fourier spectrum.

Eigenfunctions of electrons in weakly disordered quantum dots: Crossover between orthogonal and unitary symmetries.

A one-parameter random-matrix model is proposed for describing the statistics of the local amplitudes and phases of electron eigenfunctions in a mesoscopic quantum dot in an arbitrary magnetic field. Comparison of the statistics obtained with recent results derived from first principles within the framework of supersymmetry technique allows us to identify a transition parameter with real microscopic characteristics of the problem. The random-matrix model is applied to the statistics of the height of the resonance conductance of a quantum dot in the regime of the crossover between orthogonal and unitary symmetry classes. \textcopyright{} 1996 The American Physical Society.

Magnetization of disordered ballistic quantum billards

AbstractWe study the magnetic response of mesoscopic quantum dots in the ballistic regime where the mean free path le is larger that the size L of the sample, yet smaller than L(KFL)d−1. In this regime, disorder plays an important role. Employing a semiclassical picture we calculate the contribution of long tranjectories which are strongly affected by static disorder and which differ sharply from those of clean systems. In the case of a magnetic field, they give rise to a large linear paramagnetic susceptibility (which is disorder independent), whose magnitude is in agreement with recent experimental results. In the case of a Aharonov‐Bohm flux, the susceptibility is disorder dependent and is proportional to the mean free path as in the diffusive regime. We also discuss the corresponding non‐linear susceptibilities.

Energy and current correlations in mesoscopic rings and quantum dots.

With a view to some of the late developments in the thermodynamics of mesoscopic systems, we examine the problem of level correlations in moderately disorderd (diffusive) conductors. We briefly review the thermodynamic relations, employed within perturbation theory. We calculate correlation functions among single electron levels, and magnetic-field (magnetic-flux) derivatives thereof. Specifically, we evaluate both single-level current and single-level curvature correlations, and use them to examine the typical current, the paramagnetic susceptibility, and the energy ``power spectrum'' of Aharonov-Bohm geometries, derived previously. We stress the role of spectral rigidity, and present comparison with simply connected systems subject to a magnetic field.