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We develop a simple analytical model to study the influence of different material systems on the operation of a quantum-point-contact spin filter. Such a device has been predicted to allow for local control of the spin polarization in a semiconductor, and for direct electrical detection of the induced spin polarization. Narrow band-gap semiconductors, such as InAs and InSb, are predicted to exhibit excellent spin-filter characteristics, due to their large g-factor values, and enhanced subband splittings. As a practical step towards the realization of such a spin filter, the electrical properties of InGaAs quantum wires are investigated.

We have found evidence of a transition from the semi-classical to quantum transport in quasi-ballistic narrow wires constructed in 2DEG system of a GaAs/AlGaAs heterojunction. By increasing the ratio of the mean free path to the wire length while keeping the wire width constant, we find a clear offset between the semi-classical and the quantum behaviors in the two-terminal measurement of the wire resisitance. We can explain such an offset by taking into account the transmission probability of electron waves propagating in the wire.

We study the properties of the logarithmic variation observed in the temperature-dependent conductance of open quantum-dot arrays. The magnitude of this variation is found to exhibit a well-defined scaling with system size, and is also found to be unaffected by the application of magnetic fields sufficient to break time-reversal symmetry. We suggest that these characteristics are consistent with the logarithmic variation resulting from a confinement-induced enhancement of electron interactions in the arrays. In particular, we note that the size-dependent scaling of this term is similar to that which we would expect for a system whose boundary interactions act to amplify the mutual repulsion of electrons trapped within it.

Ridge-type InGaAs/InAlAs quantum-wire field-effect transistors are realized by selective molecular-beam epitaxy and their transport characteristics are studied. An analysis of the depopulation of one-dimensional subbands in these structures reveals little evidence for sidewall depletion, and yields an estimate for the carrier density in good agreement with that found in two-dimensional InGaAs/InAlAs heterojunctions. Subband splittings as large as 7.4 meV are obtained in the wires, indicating their excellent one-dimensional transport properties.

We study the effect of mode coupling on the magnetoconductance fluctuations of a square ballistic quantum dot fabricated in AlGaAs/GaAs heterostructure material and defined by Schottky gates. Structures prepared allow for the independent variation of the number of propagating input and output modes and their coupling to the main cavity. While adjusting the opening of the input and output point contacts changes the number of modes allowed to propagate into the dot, the excitation from a quantum wire creates an additional selection rule. The autocorrelation and Fourier analyses of the fluctuations demonstrate that the formation of the quantum wire has a strong effect, not only on the average conductance of the dot but also on the excitation of the semiclassical orbits within it.

The influence of a magnetic field on the classical dynamics in ballistic cavities is investigated by analyzing conductance fluctuations that result from quantum interference effects. The magnetic field transforms the exponential decay of the probability distributions of the dwell time and the enclosed area to a power-law decay. The conductance fluctuations correspondingly exhibit a transition from a nonfractal to a fractal behavior. We also identify two additional states that take place when the magnetic field is weaker or stronger than that required to achieve the power-law probability distributions.

We studied quantum interference effects observed in the low-temperature magnetoresistance of corrugation gated wires on a two-dimensional electron gas system. We have also studied the transport change due to deep gate depletion by means of the controlling gate voltage. We analyzed the gate voltage and magnetic field dependences of the negative magnetoresistance and observed small oscillations in the low-field magnetoresistances. The results show that a boundary-related or geometrical effect should strongly affect the phase-breaking mechanism in corrugation gated wires, and that the electron wave propagation differs between a narrow wire and a dot array, depending on the gate voltage.

We have studied transport properties in the low‐temperature magnetoresistance through the ballistic narrow path restricted by a short width metallic gate on the 2 dimentional electron gas (2DEG) system. An alternate and systematic variation between a Lorentzian line fitting and a cusp‐like line fitting in the zero‐field peaks has been observed, as sweeping the gate voltage. It indicates a possibility of existence of chaotic and regular paths on the short gated ballistic/tunneling transport. We will discuss on the quantum chaos behavior on the systematic variation between the Lorentzian and the cusp‐like peakshape based on the disordered path system under the short gate.

Solid solutions of metals such as silver in arsenic trisulfide exhibit a variety of interesting characteristics, including the ability to bring the metal out of solution by electrodeposition. This allows voltage-controlled switching characteristics to be realized in two terminal devices which may be fabricated to nanoscale dimensions. In addition, surface electrodeposits may be formed and subsequently broken at grain boundaries to create structures that have adjustable current–voltage characteristics at low temperatures. This paper highlights some initial results of the characterization of nanoscale structures based on such solid solutions.

Analysing the conductance fluctuations observed in low-temperature magnetoresistance, we study electron wave interference effects in the quasi-ballistic transport regime of a narrow wire, defined by split metal gates, on the two-dimensional electron gas of a GaAs/AlGaAs heterojunction. There is a clear difference in the conductance fluctuations between short and long wires of 2 and 6 μm gate length, respectively. Such a difference can be explained by considering whether some correlated channels exist in the wire or not.

We study the resonant peaks in the low-temperature magneto-resistance measured in open quantum dot arrays at high magnetic field. The peaks appear at the field where the innermost edge channel passing through a dot array begins to be depopulated. From the gate voltage dependence, the peaks shift to lower magnetic field along the depopulation field at the point contact region in the dot array. We discuss the possible origin considering the charging effect in small puddles at the point contact region.

We have studied electron wave interference effects in wires and dots in ballistic and quasi-ballistic transport regimes, fabricated by using split gate, or wet etching techniques. We observe conductance fluctuations as a function of magnetic field, and we show they arise from the different origin of the scattering source for interference. It is shown experimentally that scatterings occur at the boundaries of the wall in the ballistic sample, while they occur at impurities in quasi-ballistic sample. This difference is clearly demonstrated in the temperature dependence of the fluctuation amplitude and the shape dependent negative magnetoresistance peak due to the weak localization effect. We discuss also the phase breaking time in a ballistic dot, which is very important for the interference.

We have studied the correlation between localisation and superconductivity in two-dimensional thin films of amorphous Si/Nb. Si/Nb superlattice films, which exhibit superconducting transitions below 2 K, and a Si/Nb/Si sandwich film, which exhibits no superconducting transition down to 0.3 K, were fabricated in this experiment. In order to clarify their mesoscopic properties, we have measured the low-temperature magnetoresistance and analysed this using the transport parameters of weak localisation theory. The analysis suggests that the onset of superconductivity occurs once the superconducting coherence length becomes shorter than the inelastic scattering length, and we deduce that the onset of superconductivity in the disordered films is controlled by these characteristic length scales.

We have studied transport properties in the magnetoresistance for coupled quantum-dot array systems fabricated by a split-metal-gate technique on GaAs/AlGaAs 2DEG layers. Edge-channel transport has been observed at high magnetic fields in the dot array system and edge state de-population has been analyzed in order to determine the effective g-value and the local carrier concentration at the quantum point contacts. The gate-voltage dependence of the Zeeman splitting is discussed for the lower Landau levels. It can be expected that a large variation of the g-value as a function of the gate-voltage will be interesting for spintronics device applications.

Magnetic characterization of a quantum dot array was carried out to extract the phase-breaking time as a function of temperature and current. These measurements indicate a saturation of the phase-breaking time at low temperature and low current. The phase-breaking time varies as 1 / T for temperatures above 1 K, which can be attributed to carrier–carrier scattering processes. Variation of the phase-breaking time with current yields an estimate for the electron temperature.

We compare open quantum dot magnetoconductance spectra from experiment and theory in the presence of environmental coupling and attributed broadening. Estimates of the phase-breaking time in experiment, and effective broadening in simulation, are determined independently. In a larger, more open dot, with a significantly shorter phase-breaking time, the observed spectrum is broadened, most noticeably about B=0. The required broadening in simulation is characterized by effective temperatures higher than estimates from experiment; however, without accounting for disorder, which will further broaden the spectrum, the agreement is reasonable.

Future VLSI scaling realization of gate lengths is expected to 70 nm and below. While we do not know all the underlying physics, we are beginning to understand some limiting factors, which include quantum transport, in these structures. The discrete nature of impurities, the fact that devices have critical lengths comparable to their coherence lengths, and size quantization will all be important in these structures. These phenomena will lead to pockets of charge, which will appear as coupled quantum dots in the device transport. We review some of the physics of these dots.

The mixed phase space in an open quantum dot array with a soft confining potential is studied in low and high magnetic fields. Chaos occurs due to the perturbations by the constrictions connecting the dots of the array. Regular orbits and the areas of Kalmogorov-Arnold-Moser islands in phase space become increasingly dominating with increasing magnetic field. The correspondence to the high-field quantum-mechanical picture in a 2D electron system with boundaries is discussed.

Simulating ballistic transport through square quantum dots, we find quasi-periodic conductance fluctuations and power spectra containing few frequencies, indicating few periodic orbits participate, in very good agreement with some recent experiments. We also find corresponding scar-like features in the wave functions. To reproduce these effects classically, the electrons must be restricted to a collimated beam which enters the dot at an angle .

We study impact-ionization-induced electroluminescence (EL) from a GaAs/AlGaAs heterostructure under high bias. In addition to k-space transfer (the Gunn effect), EL spectra indicate real-space (GaAs-to-AlGaAs) transfer. Microscopy shows strong EL near the anode.

Self-similarity ~the invariance of an intrinsic pattern under scaling ! in the fractal magnetotransport properties of mesoscopic billiards is highly topical. Employing three billiard geometries, we investigate the relationship between the two classes of observed scaling behavior—exact and statistical self-similarity. We employ a correlation analysis to determine the conditions required for their observation and to show that the two forms of self-similarity can be understood within a common framework. @S0163-1829~98!04934-0#

We have studied the low-temperature magnetotransport in the singly and doubly corrugated wires. The application of a sufficiently negative gate bias might result in the formation of weakly coupled dots. In the structures considered here the corrugations act as weak perturbations on the quantum wire. Compared with the experimental results and the numerical simulations of the transports in these perturbed quantum wires, we show that oscillatory magnetoresistance features, observed in the doubly corrugated wire, arise from back-scattered orbits. The observation of wave function scarring in the quantum simulations is given the open nature of the weakly perturbed quantum wires.

We have measured the magnetoresistance of sub-micron width GaAs/AlGaAs heterojunction wires at temperatures down to 0.04 K and in magnetic fields of up to 1.2 T. At low magnetic fields, universal conductance fluctuations and electron interaction effects are observed in the magnetoresistance. The analysis of these effects is found to be consistent with a temperature independent behaviour of the phase breaking and thermal diffusion lengths below a degree Kelvin. Surprisingly, this saturation occurs at length scales much shorter than the wire length and we speculate on the existence of some limiting length scale, which sets an upper limit on the spatial extent of quantum diffusion within the wire.

Recently, experimental studies of magnetotransport in nominally square quantum dots have revealed a high degree of periodicity in the conductance fluctuations, with only a few apparently harmonically related frequencies dominating the power spectra. Analysis of dots of different sizes has revealed that there are unresolved issues regarding the scaling of the dominant frequency of the fluctuations. The experimental data appear to suggest that the dominant frequency scales with , where A is the area of the dot. On the other hand, a semiclassical analysis of periodic orbits suggests that the scaling should be with area. In this paper, we attempt to resolve this issue by simulating dots of many different sizes using both quantum mechanical and classical approaches.

The role of the environment on the observation of specific transport properties is discussed for open ballistic quantum dots. The saturation of phase-breaking at temperatures below 1 K is found to be mediated by lead coupling, which is found to affect the nature of the observed zero-field resistance peak. This can generally be attributed to the lead-induced modification of the available density of states in the dot, which is related to the magnetoconductance spectrum. Simulations of the magnetoconductance spectrum demonstrate that averaging the spectrum produces widely varying zero-field-resistance peaks in the same system. The exact nature of the environmental coupling in quantum dots is required to study the non-universal character of transport in these systems.

We discuss the results of experimental studies of coherent electron transport in open quantum dot arrays. These studies suggest that the details of this transport remain highly phase coherent over much of the array, allowing features associated with the quantum mechanical coupling of the dots to be observed. Studies of magneto-conductance oscillations in the arrays appear to show the influence of this coupling through a change in frequency content as their gate voltage is varied. We speculate that this change arises as the contribution to interference of long orbits is suppressed by reducing the strength of the inter-dot coupling. Temperature-dependent studies of these devices reveal evidence for an unexpected `metal-insulator' transition, which is manifest as a logarithmic variation of the conductance at low temperatures, whose sign can correspond to either metallic or insulating behaviour. It is speculated that this logarithmic term provides a novel signature of electron-interaction effects in confined nanostructure devices.

The details of electron interference in open quantum-dot arrays are studied in experiment and numerical simulations. Reproducible fluctuations are observed in their low-temperature magnetoconductance and the characteristics of these are suggested to be consistent with a transition from multiple to single-dot interference, which occurs as the strength of the interdot coupling is varied. These results therefore reveal a nontrivial scaling of the conductance fluctuations in quantum-dot arrays, which is thought to arise due to the influence of the interdot coupling on energy hybridization.

A novel technique has been developed for the fabrication of Si quantum dot structures with controllable electron number through both top and side gates. We have tested devices ranging in size from 40 to 200 nm. By varying the density with the top gate, and controlling the input and output barriers of the dot with the side gates, conductance peaks are observed which map details of the energy level within the dot as well as the interaction of the electrons with one another.