We have measured the conductance and shot noise of superconductor-normal metal (S-N) junctions between a niobium (Nb) film and a two-dimensional electron gas (2DEG), formed in an InAs-based semiconductor heterostructure. Adjacent to the junction, the 2DEG is shaped into a sub-micrometer beam splitter. The current shot noise measured through one arm of the beam splitter is found to be enhanced due to Andreev reflection. Both noise and conductance measurements indicate that the Nb-2DEG interface is of high quality with a transparency approaching $\ensuremath{\approx}60\text{\char21{}}70\phantom{\rule{0.2em}{0ex}}%$. The present device can be seen as a quasi-ballistic S-N beam-splitter junction.

We report tunneling spectroscopy measurements of the Zeeman spin splitting in InAs few-electron quantum dots. The dots are formed between two InP barriers in InAs nanowires with a wurtzite crystal structure grown by chemical beam epitaxy. The values of the electron g-factors of the first few electrons entering the dot are found to strongly depend on dot size and range from close to the InAs bulk value in large dots |g^*|=13 down to |g^*|=2.3 for the smallest dots. These findings are discussed in view of a simple model.

During the last 5 years the potential for applications of semiconductor nanowires has grown rapidly via the development of methods for catalytically induced nanowire growth using the, so-called vapor–liquid–solid (VLS) growth mode. The VLS method offers a high degree of control of parameters such as position, diameter, length and composition, including the realization of atomically abrupt heterostructure interfaces inside a nanowire. In this review, we summarize the progress and the standing of our research from the point of view of controlled growth, structural and electronic properties and in terms of different families of devices which have been possible to realize.

Semiconductor nanowires are grown using chemical beam epitaxy and metal organic vapor phase epitaxy from size-selected gold nanoparticles acting as catalysts. By changing materials during the growth it is possible to form heterostructures both along the length of the nanowires but also in a core-shell fashion. In particular, incorporation of pairs of InP tunnel barriers in InAs nanowires has been used to fabricate single-electron transistors and resonant tunneling diodes.

Nanowires in the InAs/InP material system are grown with catalyst-assisted chemical beam epitaxy. Ohmic contacts are then fabricated to selected wires, allowing electron transport measurements to be carried out at room-temperature as well as at low T. InAs nanowires show strong quantum confinement effects, where thin wires (<30 nm) are depleted from carriers. Measurements on InAs wires with a quantum point contact configuration indicate a scattering length in the order of 100 nm. Heterostructure barriers of InP are also incorporated into InAs wires to produce resonant tunneling diodes and single-electron transistors (SETs) with different dot lengths. Wires containing dots with a length of 100 nm function as ideal SETs, whereas the transport in wires with 15 nm long dots is strongly governed by quantum confinement and resonant tunneling. For the smaller dots it is possible to observe electron transport through excited states.

We demonstrate transport spectroscopy on bottom-up grown few-electron quantum dots in semiconductor nanowires. The dots are defined by InP double barrier heterostructures in InAs nanowires catalytically grown from nanoparticles. By changing the dot size, we can design devices ranging from single-electron transistors to few-electron quantum dots. In the latter case, electrons can be added one by one to the dots from 0 to similar to50 electrons while maintaining an almost constant charging energy, with addition spectra of the devices displaying shell structures as a result of spin and orbital degeneracies. The reduced dimensionality of the nanowire emitter gives rise to pronounced resonant tunneling peaks, where a gate can be used to control the peak positions.

We present detailed experimental studies of the temperature dependence of the plateau conductance of GaAs quantum point contacts in the temperature range from 0.3 K to 10 K. Due to a strong lateral confinement produced by a shallow-etching technique we are able to observe the following unexpected feature: a linear temperature dependence of the measured mid-plateau conductance. We discuss an interpretation in terms of a temperature dependent, intrinsic series resistance, due to non-ballistic effects in the 2D-1D transition region. These results have been reproduced in several samples from different GaAs/GaAlAs heterostructures and observed in different experimental set-ups.

Detailed experimental studies of the conductance of mesoscopic GaAs devices in the few-mode regime reveal a novel thermal effect: for temperatures up to at least 10 K the measured gate characteristics, i.e. conductance $G$ versus gate voltage $V_g$, exhibit a systematic downward shift in gate voltage with increasing temperature. The effect is 'universal', in the sense that it is observed in different modulation doped GaAs/GaAlAs heterostructures, in different device geometries, and using different measurement setups. Our observations indicate that the effect originates in the surrounding 2D electron gas and not in the mesoscopic devices themselves.

We study electron decoherence by measuring the temperature dependence of Aharonov–Bohm (AB) magnetoconductance oscillations in quasi-1D rings, etched in a high-mobility GaAs/GaAlAs heterostructure. The oscillation amplitude, which is quantified by Fast Fourier Transform (FFT) amplitudes, is influenced both by phase-breaking and by thermal averaging. FFT components of frequency up to six times the fundamental h/e component are observed and used in the analysis. It is crucial to study not only the h/e AB amplitude, but also the higher h/ne components, in order to distinguish the effects of phase-breaking and thermal averaging. For the phase-breaking, it is demonstrated that the damping of the oscillation amplitude is proportional to the length of the interfering paths. For temperatures T from 0.3 to 4K we find the phase coherence length Lφ∝T−1. The phase coherence length does also seem to depend on sample geometry. Recently, the T−1 dependence has been predicted theoretically for AB rings by Seelig and Büttiker (Phys. Rev. B 64 (2001) 245313).

We study electron decoherence by measuring the temperature dependence of Aharonov-Bohm (AB) oscillations in quasi-one-dimensional rings, etched in a high-mobility GaAs/GaxAl1-xAs heterostructure. The oscillation amplitude is influenced both by phase breaking and by thermal averaging. Thermal averaging is important when the temperature approaches the energy scale on which the AB oscillations shift their phase. For the phase breaking, it is demonstrated that the damping of the oscillation amplitude is proportional to the length of the interfering paths, For temperatures T from 0.3 to 4 K we find the phase-coherence length L(phi)proportional toT(-1), close to what has been reported for open quantum dots. This might indicate that the T-1 decoherence rate is a general property of open and ballistic mesoscopic systems.

The 07(2e(2)/h) conductance anomaly is studied in etched, GaAs/GaAlAs quantum point contacts by measuring the the differential conductance, G = dI/dV(sd), as a function af source-drain bias voltage, V-sd, and gate-source voltage, Vg.,, as well as a function of temperature T. The conductance suppression at the 0.7(2e(2)/h) conductance anomaly shows activated behaviour as a function of T, When a finite sd is applied the 0.7 conductance anomaly evolves smoothly into a well-defined conductance plateau at G = 0.85(2e(2)/h) and further into another plateau at G = 1.3(2e(2)/h). These observations are compared to a recently proposed model, in terms of an anomalus ID subband edge, split off from each normal ID subband below the chemical potential.

The 0.7 (2e(2)/h) conductance anomaly is studied in strongly confined, etched GaAs/GaAlAs quantum point contacts, by measuring the differential conductance as a function of source-drain and gate bias as well as a function of temperature. We investigate in detail how, fur a given gate voltage, the differential conductance depends on the finite bias voltage and find a so-called self-gating effect, which we correct for. The 0.7 anomaly at zero bias is found to evolve smoothly into a conductance plateau at 0.85 (2e(2)/h) at finite bias. On varying the gate voltage the transition between the 1.0 and 0.85 (2e(2)/h) plateaus occurs for definite bias voltages, which define a gate-voltage-dependent energy difference Delta. This energy difference is compared with the activation temperature T-a extracted from the experimentally observed activated behavior of the 0.7 anomaly at low bias. We find Delta = k(B)T(a), which lends support to the idea that the conductance anomaly is due to transmission through two conduction channels, of which the one with its subband edge Delta below the chemical potential becomes thermally depopulated as the temperature is increased.

We have investigated the Aharonov-Bohm effect in a one-dimensional GaAs/GaAlAs ring at low magnetic fields. The oscillatory magnetoconductance of these systems are for the first time systematically studied as a function of density. We observe phase-shifts of $\pi$ in the magnetoconductance oscillations, and halving of the fundamental $h/e$ period, as the density is varied. Theoretically we find agreement with the experiment, by introducing an asymmetry between the two arms of the ring.

We have investigated the Aharonov-Bohm effect in mesoscopic semiconductor GaAs/GaAlAs rings in low magnetic fields. The oscillatory magnetoconductance of these systems is systematically studied as a function of electron density. We observe phase shifts of pi in the magnetoconductance oscillations, and halving of the fundamental h/e period, as the density is varied. Theoretically, we find agreement with the experiment, by introducing an asymmetry between the two arms of the ring. (C) 2000 Elsevier Science B.V. All rights reserved.

The relationship between anisotropic electronic circular dichroism (CD) and the molecular structure for planar cis-butadiene and for 1,3-cyclohexadiene and some of its allylic methyl derivatives is studied through calculation and graphical display of the rotatory strength tensor for the lowest pi --> pi* excitation in these systems. Also included is cis-butadiene in chiral structures mimicking the diene units in two of the cyclohexadiene systems. The calculations are done ab initio in the random phase approximation using an aug-cc-pVTZ atomic basis set chosen from a systematic basis set study for chiral cis-butadiene. For planar cis-butadiene this study provides the first calculation of anisotropic CD of an achiral molecule and predicts a CD intensity distribution exhibiting two numerically equal, but oppositely signed, lobes along mutually orthogonal directions perpendicular to the C-2 axis for the system. The CD intensities for the chiral molecules cyclohexadiene and its allylic derivatives exhibit two large and oppositely signed major lobes, echoing the CD of cis-butadiene, in addition to a nonvanishing CD intensity along the C-2 axis of the diene unit. The chirality of the ring conformation and of the arrangement of the substituents is reflected in a difference between the magnitudes of the two major lobes in the CD response and in the variation in sign and magnitude of the CD intensity along the C-2 axis. More specifically, the effects of the allylic methyl groups follow a quadrant rule and are almost additive, the effects being significantly larger for axial than for equatorial substituents. The helical twisting of the diene chromophore is of minor importance. These trends are found also for the isotropic CD, confirming earlier results. The analysis of the anisotropic CD suggests that the CD intensity along the uniaxial C-2 direction of the diene may serve as a sensitive chiral indicator for these systems.

Aharonov–Bohm magnetoconductance oscillations are investigated in shallow etched, ring shaped GaAs/GaAlAs devices. The devices are operated with a few propagating transverse modes in the arms of the ring. The magnetoconductance oscillations, which have a period ΔB=3.3 mT, are studied around zero magnetic field. At millikelvin temperature the amplitude of the oscillations is as large as 10% of the conductance, and they are observed at temperatures up to 8 K. The magnetoconductance oscillations shows phase-shifts and halving of the fundamental h/e periodicity as the electron density is varied globally by means of a Ti/Au topgate electrode. The findings are interpreted in terms of an asymmetry between the two arms of the ring. We demonstrate that the phase and shape of the AB oscillations is very sensitive to an asymmetry.

Magneto-conductance measurements of a micron sized GaAI 0.3 As 0.7 /GaAs Aharonov–Bohm devicehas been performed at temperatures above T= 4.2 K , inthe regime where only a few transverse modes are occupied. Wefind that the Aharonov–Bohm oscillations are still visible atthese relatively high temperatures. The electron density ofthe Aharonov–Bohm device was during the measurementscontrolled via a gate voltage applied to a small stub whichwas attached to one arm of the ring. Due to this highlyasymmetric gate configuration the electron density in the ringis strongly asymmetric, and hence an oscillating conductanceas a function of gate voltage is expected—in analogy withthe Mach–Zender interferometer.

We examine particle trajectories in capillary waves formed on a water surface subject to vertical vibrations. We focus on the role of a distinct length scale present in our experiment, namely, the wavelength $\ensuremath{\lambda}$ of the surface waves. We observe non-Brownian particle trajectories with a fractal dimension $D$ different from the random walk value $D\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2$. A crossover is observed from one anomalous behavior at length scales below $\ensuremath{\lambda}$, to another at larger length scales. Data collapse is shown to be feasible, and scaling functions characterizing the crossover are identified. Our results are compared to those obtained from observations of drifters in the upper ocean. The distinct length scale $\ensuremath{\lambda}$ allows us to divide the particle trajectories into flights and traps. The distribution of flight times shows a power-law behavior with an exponent between 2.3 and 3.