arXiv subjects

Gregory W. Holloway

Publications and source records attributed to Gregory W. Holloway.

8 recordsLinked to original sources

arXiv2017

A double quantum dot memristor

Memristive systems are generalisations of memristors, which are resistors with memory. In this paper, we present a quantum description of memristive systems. Using this model we propose and experimentally demonstrate a simple and practical scheme for realising memristive systems with quantum dots. The approach harnesses a phenomenon that is commonly seen as a bane of nanoelectronics, i.e. switching of a trapped charge in the vicinity of the device. We show that quantum-dot memristive systems have hysteresis current-voltage characteristics and quantum jump induced stochastic behaviour. Realising such a quantum memristor completes the menu of components for quantum circuit design.

cond-mat.mes-hall↗

arXiv2016

Nb/InAs nanowire proximity junctions from Josephson to quantum dot regimes

The superconducting proximity effect is probed experimentally in Josephson junctions fabricated with InAs nanowires contacted by Nb leads. Contact transparencies $t \sim 0.7$ are observed. The electronic phase coherence length at low temperatures exceeds the channel length. However, the elastic scattering length is a few times shorter than the channel length. Electrical measurements reveal two regimes of quantum transport: (i) the Josephson regime, characterized by a dissipationless current up to $\sim 100$ nA, and (ii) the quantum dot regime, characterized by the formation of Andreev Bound States (ABS) associated with spontaneous quantum dots inside the nanowire channel. In regime (i), the behaviour of the critical current $I_c$ versus an axial magnetic field $B_{||}$ shows an unexpected modulation and persistence to fields $>2$ T. In the quantum dot regime, the ABS are modelled as the current-biased solutions of an Anderson-type model. The applicability of devices in both transport regimes to Majorana fermion experiments is discussed.

cond-mat.mes-hall↗

arXiv2016

Electrical characterization of chemical and dielectric passivation of InAs nanowires

The native oxide at the surface of III-V nanowires, such as InAs, can be a major source of charge noise and scattering in nanowire-based electronics, particularly for quantum devices operated at low temperatures. Surface passivation provides a means to remove the native oxide and prevent its regrowth. Here, we study the effects of surface passivation and conformal dielectric deposition by measuring electrical conductance through nanowire field effect transistors treated with a variety of surface preparations. By extracting field effect mobility, subthreshold swing, threshold shift with temperature, and the gate hysteresis for each device, we infer the relative effects of the different treatments on the factors influencing transport. It is found that a combination of chemical passivation followed by deposition of an aluminum oxide dielectric shell yields the best results compared to the other treatments, and comparable to untreated nanowires. Finally, it is shown that an entrenched, top-gated device using an optimally treated nanowire can successfully form a stable double quantum dot at low temperatures. The device has excellent electrostatic tunability owing to the conformal dielectric layer and the combination of local top gates and a global back gate.

cond-mat.mes-hall↗

arXiv2014

Magnetoconductance signatures of subband structure in semiconductor nanowires

The radial confining potential in a semiconductor nanowire plays a key role in determining its quantum transport properties. Previous reports have shown that an axial magnetic field induces flux-periodic conductance oscillations when the electronic states are confined to a shell. This effect is due to the coupling of orbital angular momentum to the magnetic flux. Here, we perform calculations of the energy level structure, and consequently the conductance, for more general cases ranging from a flat potential to strong surface band bending. The transverse states are not confined to a shell, but are distributed across the nanowire. It is found that, in general, the subband energy spectrum is aperiodic as a function of both gate voltage and magnetic field. In principle, this allows for precise identification of the occupied subbands from the magnetoconductance patterns of quasi-ballistic devices. The aperiodicity becomes more apparent as the potential flattens. A quantitative method is introduced for matching features in the conductance data to the subband structure resulting from a particular radial potential, where a functional form for the potential is used that depends on two free parameters. Finally, a short-channel InAs nanowire FET device is measured at low temperature in search of conductance features that reveal the subband structure. Features are identified and shown to be consistent with three specific subbands. The experiment is analyzed in the context of the weak localization regime, however, we find that the subband effects predicted for ballistic transport should remain visible when back scattering dominates over interband scattering, as is expected for this device.

cond-mat.mes-hall↗

arXiv2014

Electron Transport in InAs-InAlAs Core-Shell Nanowires

Evidence is given for the effectiveness of InAs surface passivation by the growth of an epitaxial InAlAs shell. The electron mobility is measured as a function of temperature for both core-shell and unpassivated nanowires, with the core-shell nanowires showing a monotonic increase in mobility as temperature is lowered, in contrast to a turnover in mobility seen for the unpassivated nanowires. We argue that this signifies a reduction in low temperature ionized impurity scattering for the passivated nanowires, implying a reduction in surface states.

cond-mat.mes-hall↗

arXiv2014

Josephson Interference due to Orbital States in a Nanowire Proximity Effect Junction

The Josephson supercurrent in a Nb-InAs nanowire-Nb junction was studied experimentally. The nanowire goes superconducting due to the proximity effect, and can sustain a phase coherent supercurrent. An unexpected modulation of the junction critical current in an axial magnetic field is observed, which we attribute to a novel form of Josephson interference, due to the multi-band nature of the nanowire. Andreev pairs occupying states of different orbital angular momentum acquire different superconducting phases, producing oscillations of the critical current versus magnetic flux. We develop a semi-classical multi-band model that reproduces the experimental data well. While spin-orbit and Zeeman effects are predicted to produce similar behaviour, the orbital effects are dominant in the device studied here. This interplay between orbital states and magnetic field should be accounted for in the study of multi-band nanowire Josephson junctions, in particular, regarding the search for signatures of topological superconductivity in such devices.

cond-mat.mes-hall↗

arXiv2013

Temperature-dependent electron mobility in InAs nanowires

Effective electron mobilities are obtained by transport measurements on InAs nanowire field-effect transistors at temperatures ranging from 10-200 K. The mobility increases with temperature below ~ 30 - 50 K, and then decreases with temperature above 50 K, consistent with other reports. The magnitude and temperature dependence of the observed mobility can be explained by Coulomb scattering from ionized surface states at typical densities. The behaviour above 50 K is ascribed to the thermally activated increase in the number of scatterers, although nanoscale confinement also plays a role as higher radial subbands are populated, leading to interband scattering and a shift of the carrier distribution closer to the surface. Scattering rate calculations using finite-element simulations of the nanowire transistor confirm that these mechanisms are able to explain the data.

cond-mat.mes-hall↗

arXiv2012

Trapped charge dynamics in InAs nanowires

We study random telegraph noise in the conductance of InAs nanowire field-effect transistors due to single electron trapping in defects. The electron capture and emission times are measured as functions of temperature and gate voltage for individual traps, and are consistent with traps residing in the few-nanometer-thick native oxide, with a Coulomb barrier to trapping. These results suggest that oxide removal from the nanowire surface, with proper passivation to prevent regrowth, should lead to the reduction or elimination of random telegraph noise, an important obstacle for sensitive experiments at the single electron level.

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