J. P. Bird

Nonlinear current-voltage characteristics of Pt nanowires and nanowire transistors fabricated by electron-beam deposition

We have fabricated Pt/C composite nanowires and nanowire transistors, using the technique of electron-beam-induced deposition. The current-voltage characteristics of the granular nanowires are strongly nonlinear at 4.2 K, and evidence for this nonlinearity is found to persist to room temperature. A voltage gap of order 0.1–0.2 V is observed at the lowest temperatures, and we suggest that this feature is consistent with single-electron tunneling via Pt nanocrystals that form in the wires during their fabrication. In order to further explore this possibility, we incorporate the nanowires into three-terminal transistor structures and find evidence for a gate-induced modulation of their voltage gap.

Classical and quantum transport in focused-ion-beam-deposited Pt nanointerconnects

We study the electrical properties of Pt nanointerconnects, formed on SiO2 substrates by focused-ion-beam deposition. Studies of their temperature-dependent resistivity reveal a small residual-resistivity ratio, and a Debye temperature that differs significantly from that of pure Pt, indicative of the disordered nature of the nanowires. Their magnetoresistance shows evidence for weak antilocalization at temperatures below 10 K, with a phase-breaking length of ∼100 nm, and a temperature dependence suggestive of quasi-one-dimensional interference.

Fano resonances in open quantum dots and their application as spin filters

We describe how a spin filter may be realized in open quantum-dot systems, by exploiting the Fano resonances that occur in their transmission characteristics. In quantum dots in which the spin degeneracy of carriers is lifted, we show that the Fano resonances may be used as an effective means to generate spin polarization of transmitted carriers, and that electrical detection of the resulting polarization should also be possible.

Large g-factor enhancement in high-mobility InAs/AlSb quantum wells

We discuss the growth by molecular-beam epitaxy, and studies of the low-temperature electrical properties, of undoped InAs/AlSb quantum wells. The two-dimensional electron gas realized in the wells exhibits high mobility at low temperatures, and an analysis of its Shubnikov–de Haas oscillations suggests this mobility is limited by scattering from remotely located unintentional dopants. Spin splitting of the oscillations is clearly resolved at 4.2 K, revealing a g-factor as large as −60 at high magnetic fields. The size of this enhancement increases with decreasing electron density, and is thought to reflect the associated increase in the strength of the effective Coulomb interaction.

Signatures of quantum transport in self-assembled epitaxialnickel silicide nanowires

We have measured the electrical properties of self-assembled epitaxial NiSi2 nanowires (NWs) formed on Si substrates. We find quantum corrections due to weak antilocalization and electron–electron interactions. Analysis of the magnetoresistance indicates that electron phase coherence in the NWs is limited by Nyquist dephasing below 10K, and by electron–phonon scattering at higher temperatures. The phase-breaking and spin–orbit scattering lengths are found to be ∼45nm and 3–7nm, at 4.2K, respectively, similar to reports for thin NiSi2 films.

Nonlocal resonant interaction between coupled quantum wires

We study the transport in a system of coupled quantum wires and show evidence for a resonant interaction that occurs whenever one of them is biased close to pinch off. Measuring the conductance of one of the wires, as the width of the other is varied, we observe a resonant peak in the conductance that is correlated to the point at which the other wire pinches off. The origin of this interaction remains undetermined at present, although its characteristics appear consistent with predictions that a correlated many-body state should form in narrow wires as their conductance vanishes.

Large effects due to electron-phonon-impurity interference in the resistivity of Pt/C-Ga composite nanowires

The temperature-dependent resistivity of highly disordered Pt/C-Ga composite nanowires is shown to be well described by the interference of electron–phonon scattering and elastic electron scattering from boundaries and defects. The strongly disordered nature of these wires, combined with a high value of their Debye temperature, are responsible for the pronounced nature of the interference effects in their resistivity.

The persistence of eigenstates in open quantum dots

We show that transport in open quantum dots can be mediated by single eigenstates, even when the dot leads support several propagating modes. The broadening of these few robust states, whose wave functions are generally localized within the interior of the dot, is found to be virtually independent of the lead width. Our results therefore indicate that a proper discussion of the specific nature of the individual eigenstates of the closed system is critical to determining their influence on transport through open dots.

Quantum-interference characteristics of a 25 nm trench-type InGaAs/InAlAs quantum-wire field-effect transistor

We study the quantum-interference characteristics of a 25 nm, trench-type, InGaAs quantum-wire field-effect transistor realized by selective epitaxy, and find very different behavior from that typically exhibited by disordered wires. The amplitude of the magnetoresistance fluctuations is exponentially suppressed at high fields, where evidence of an Aharonov–Bohm effect is observed. The exponential suppression appears to be consistent with theoretical predictions for the influence of magnetic field on the scattering rate in clean wires, while the Aharonov–Bohm effect points to an interference process in which the one-dimensional subbands of the wire themselves constitute well-resolved paths for electron interference.

Quantum transport in open mesoscopic cavities

Abstract In this review we describe the results of magneto-transport studies in open quantum dots, in which electronic motion is expected to be predominantly ballistic in nature. The devices themselves are realized in different semiconductor materials, using quite distinct fabrication techniques. Electron interference is an important process in determining the electrical properties of the devices at low temperatures and is manifested through the observation of periodic magneto-conductance fluctuations. These are found to result from selective excitation of discrete cavity eigenstates by incoming electrons, which are directed into a collimated beam by the input point contact. Under conditions of such restricted injection, quantum mechanical simulations reveal highly characteristic wavefunction scarring, associated with the remnants of a few classical orbits. The scarring is built up by interference between electrons, confined within the cavities over very long time scales, suggesting the underlying orbits are highly stable in nature. This characteristic is also confirmed by the results of experiment, which reveal the discrete components dominating the interference to be insensitive to changes in lead opening or temperature. The fluctuations decay with increasing temperature, although they can nonetheless still be resolved at a few degrees kelvin. This characteristic is confirmed by independent studies of devices, fabricated using very different techniques, further demonstrating the universal nature of the behavior we discuss here. These results therefore demonstrate that the correct description of electron interference in open quantum cavities, is one in which only a few discrete orbits are excited by the collimating action of the input lead, giving rise to striking wavefunction scarring with measurable magneto-transport results.