C. A. Marlow

Experimental investigation of the breakdown of the Onsager-Casimir relations.

We use magnetoconductance fluctuation measurements of phase-coherent semiconductor billiards to quantify the contributions to the nonlinear electric conductance that are asymmetric under reversal of magnetic field. We find that the average asymmetric contribution is linear in magnetic field (for magnetic flux much larger than 1 flux quantum) and that its magnitude depends on billiard geometry. In addition, we find an unexpected asymmetry in the power spectrum of the magnetoconductance with respect to reversal of magnetic field and bias voltage.

Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts

The strength of the Zeeman splitting induced by an applied magnetic field is an important factor for the realization of spin-resolved transport in mesoscopic devices. We measure the Zeeman splitting for a quantum point contact etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to the transport direction. We observe an enhancement of the Lande g-factor from |g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first subband, six times larger than in GaAs. We report subband spacings in excess of 10 meV, which facilitates quantum transport at higher temperatures.

A Review of Fractal Conductance Fluctuations in Ballistic Semiconductor Devices

The ability to scale device sizes to below a micro-meter has profound implications for electron conduction in semiconductor systems. For conventional circuits, the reduced component size offers the rewards of higher packing densities and faster operating speeds. However, in terms of the search for new classes of electronic devices, designed to replace the transistor as the basic component of electronic circuits, the prospect of current flow across sub-micron distances holds even greater potential. Traditional current flow concepts, in which electron propagation along the device’s length is modeled as a classical diffusion process, can no longer be applied. Both the classical and quantum mechanical transmission characteristics may ultimately be harnessed to produce revolutionary modes of device functionality. Intimately coupled to these objectives of applied physics, sub-micron devices also provide a novel environment for the study of a rich variety of fundamental semiconductor physics.

Is it the boundaries or disorder that dominates electron transport in semiconductor `billiards'?

Semiconductor billiards are often considered as ideal systems for studying dynamical chaos in the quantum mechanical limit. In the traditional picture, once the electrons mean free path, as determined by the mobility, becomes larger than the device, disorder is negligible and electron trajectories are shaped by specular reflection from the billiard walls alone. Experimental insight into the electron dynamics is normally obtained by magnetoconductance measurements. A number of recent experimental studies have shown these measurements to be largely independent of the billiards exact shape, and highly dependent on sample-to-sample variations in disorder. In this paper, we discuss these more recent findings within the full historical context of work on semiconductor billiards, and offer strong evidence that small-angle scattering at the sub-100 nm length-scale dominates transport in these devices. This has important implications for the role these devices can play for experimental tests of ideas in quantum chaos

Investigation of electron wave function hybridization in Ga0.25In0.75As/InP arrays

We present a measurement technique for quantifying coupling between semiconductor quantum dots in an array. This technique employs magnetoconductance fluctuations to probe the decrease in the average spacing of the quantum energy levels as the electron wave functions in the dots undergo hybridization. Focusing on Ga0.25In0.75As dots, we investigate hybridization as the coupling strength is varied and the number of dots in the array is increased. Our technique reveals a significant drop in the average energy level spacing for multiple dot arrays, which is strong evidence for wave function hybridization.

Carrier density saturation in a Ga0.25In0.75As/InPGa0.25In0.75As/InP heterostructure

We observe a strong saturation of the carrier density in the quantum well of a Ga0.25In0.75As/InPGa0.25In0.75As/InP MISFET at positive gate voltages. Using a self-consistent Schrodinger/Poisson solver, we model the band structure and find that the saturation is caused by the population of charge states between the gate and the quantum well. We discuss the impact of these charge states on the transport properties, and present a fabrication method that avoids parallel conduction in this heterostructure.

Carrier density saturation in a heterostructure

We observe a strong saturation of the carrier density in the quantum well of a Ga0.25In0.75As/InPGa0.25In0.75As/InP MISFET at positive gate voltages. Using a self-consistent Schrodinger/Poisson solver, we model the band structure and find that the saturation is caused by the population of charge states between the gate and the quantum well. We discuss the impact of these charge states on the transport properties, and present a fabrication method that avoids parallel conduction in this heterostructure.

Carrier density saturation in a Ga0.25In0.75As/InP heterostructure

We observe a strong saturation of the carrier density in the quantum well of a Ga0.25In0.75As/InPGa0.25In0.75As/InP MISFET at positive gate voltages. Using a self-consistent Schrodinger/Poisson solver, we model the band structure and find that the saturation is caused by the population of charge states between the gate and the quantum well. We discuss the impact of these charge states on the transport properties, and present a fabrication method that avoids parallel conduction in this heterostructure.

Nonlinear Effects on Quantum Interference in Electron Billiards

Magnetoconductance fluctuations are used to study the effect of an applied bias on an electron billiard. At lower bias, nonlinear effects can be well described by electron heating alone, while at higher bias (V > 2mV, ∼5% of the electron Fermi energy) non-equilibrium effects become significant. At high bias, we also observe that the spectral content of the MCF is sensitive to the nonequilibrium effects. Spectral behavior is consistent with a fractal scaling of the conductance fluctuations with magnetic field, resulting in the first observation of fractal conductance fluctuations outside of the linear regime of transport.

Preserved symmetries in nonlinear electric conduction

For the general case of a mesoscopic, two‐terminal device with no geometrical symmetry, conductance symmetries break down in the non‐linear regime. Using basic symmetry arguments, we predict and experimentally confirm a set of symmetry relations that are preserved for electric conductors with respect to bias voltage, V, and B in the non‐linear regime.