Table of contents

Preface

The conference titled, "Advances in Quantum Transport in Low Dimensional Systems (AQT2017)" was held at University College London, during September 4-5, 2017. The conference was organised by the Institute of Physics (IOP) Nanoscale Physics and Technology Group, and was sponsored by the Oxford Instruments (primary sponsor), National Instruments, Nanomagnetics Instruments, Zurich Instruments, University College London, Specs, IOP Nanoscale Physics and Technology Group and IOP Quantum Electronics and Photonics Group.

The idea of organising a conference dedicated to advances in the transport in low dimensional systems came from the fact that advances in fabrication of solid state systems have reached the limit in which quantum effects cannot be ignored anymore. This has opened new perspectives for the development of new fundamental physics and products for commercial applications. Due to the rapid development of quantum technologies active collaboration between theory, experiment and industry is essential to meet the future demands. In solid state quantum technologies, there are a number of challenges, at the level of both theory and experiments, which need to be discussed openly in a platform shared by theorists, experimentalists and industry people.

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

We report on an introductory study used to gauge the significance of random weak-edge disorder on the coherent transport properties of ultra-thin zig-zag nanoribbons (ZGNRs) beyond the simple (i.e., first nearest-neighbour) tight-binding approximation. Such extensions include up to third nearest-neighbour hopping in an extended tight-binding model, as well as a mean-field Hubbard-U. The effect of the random weak-edge disorder causes charge-carrier localization that reduces the conductance about the Fermi energy in all of the systems studied. In the non-interacting systems, the extended tight-binding model is found to be more robust against disorder due to the increased kinetic degrees of freedom. Localization effects from the random weak-edge disorder are found to compete with the mean-field Hubbard-U resulting in spin-dependent conductance properties.

We present transverse electron focusing measurements in the two dimensional electrons gas formed at the interface of a GaAs/AlGaAs heterostructure. The experimental arrangement consists of two orthogonal quantum point contacts (QPCs), one acting as injector and the other as detector of the collimated 1D electrons as a function of transverse magnetic field. The focusing spectrum shows anomalous behaviour, the first and third focusing peaks split into two sub-peaks while second peak remains as a single peak. The observed splitting, a signature of spin states, arises from the spin-orbit interaction when the 1D electrons are injected into the 2D regime, thus allowing us to manipulate the spin states within the 1D channel.

In the present work, we investigate the source-drain bias spectrum of a hybrid quantum device consisting of a dot-like QPC coupled to an electronic cavity. A singlet-triplet transition manifested as finite-bias anomaly is observed when the cavity is switched on. Besides, we noticed that the 0.7 conductance anomaly is not affected by the cavity, which provides a valuable insight on the origin of the 0.7 conductance anomaly.

An important requirement across a range of sensitive detectors is to determine accurately the energy deposited by the impact of a particle in a small volume. The particle may be anything from a visible photon through to an X-ray or massive charged particle. We have been developing nanobridge Josephson junctions based SQUIDs and nanoSQUID devices covering the entire range of particle detection energies from 1eV to MeV. In this paper we discuss some developments in nanobridge Josephson junctions fabrication using focussed ion beam (FIB) and how these developments impact future applications. We focus on tuning of the transition temperature of a superconducting thin-film absorber, with the aim to match the absorber T_c to the working temperature range of the SQUID and also on using a new Xe FIB to improve Josephson junction and superconducting film quality.

Recent reports of magnetotransport measurements of InSb/Al_1-xIn_xSb quantum well structures at low temperature (3 K) have shown the need for inclusion of a new scattering mechanism not present in traditional transport lifetime models. Observations and analysis of characteristic surface structures using differential interference contrast DIC (Nomarski) optical imaging have extracted representative average grain feature sizes for this surface structure and shown these features to be the limiting low temperature scattering mechanism. We have subsequently modelled the potential profile of these surface structures using Landauer-Büttiker tunnelling calculations and a combination of a Monte-Carlo simulation and Drude model for mobility. This model matches experimentally measured currents and mobilities at low temperatures, giving a range of possible barrier heights and widths, as well modelling the theoretical trend in mobility with temperature.

Mobilities and carrier densities of modulation doped Al_0.2Ga_0.8Sb/GaSb heterostructures are presented for the first time. The structures studied were grown by molecular beam epitaxy and consisted of a single heterojunction with Te compensation doping to reduce the intrinsic p-type background. Hall measurements were performed from 30–300 K, giving p-type mobilities peaking at 3240 cm²/Vs, a considerable improvement over previous reported bulk mobilities for samples with compensation doping. Growth trials on bulk material have also been carried out to investigate the optimum growth conditions for future structures, with the aim of minimising the occurrence of natural growth defects in GaSb, which act as acceptors. Together these measurements lay the ground work for (magneto)transport studies of two-dimensional charge-carriers in Al_xGa_1-xSb/GaSb heterostructures, which has not been previously reported.

We study the quantum transport of bosons through a quantum dot coupled to two macroscopic heat baths L and R, held at fixed temperatures T_L and T_R respectively. We manage to cast the particle as well as the heat current into the Landauer form. Following the correlation matrix approach, we compute the time-dependent mutual information of the dot with the baths. We find that mutual information goes logarithmically as the number of bosons, and at low temperatures, it is possible to set up the parameters in such a way that in steady-state, the mutual information goes quadratically as a function of current.

A suspended semiconductor quantum ring interferometer based on a GaAs/AlGaAs heterostructure with a two-dimensional electron gas (2DEG) is created and experimentally studied. The electron interference in suspended 2DEG is observed. The interference manifests itself as the Aharonov-Bohm oscillations of the interferometer magnetoresistance, clearly observed before as well as after suspension. The amplitude of the oscillations remains almost unchanged after suspension.