Scalable architectures for quantum computing using semiconductor quantum dots require interactions between qubits beyond adjacent quantum dots. Here, we propose using nanowires of electrons to mediate the interaction between two quantum dots. Virtual phonons in the linear chain of electrons can mediate an interaction that gives rise to effective spin-spin coupling of the electrons in distant quantum dots. We find coupling strengths of more than 30 MHz for experimentally realisable parameters in GaAs quantum dots.

Increasing the spin-orbit coupling in InGaAs quantum wells is desirable for applications involving spintronics and topological quantum computing. Digital alloying is an approach towards growing ternary quantum wells that enables asymmetric interfaces and compositional grading in the quantum well, which can potentially modify the spin-orbit coupling in the quantum well. The spin-orbit coupling of the quantum wells is extracted from beating patterns in the low magnetic field magnetoresistance. Digital alloying is found to modify the spin-orbit coupling by up to 138 meV. The changes induced in the spin-orbit coupling can be qualitatively understood as being due to modifications in the interfacial Rashba spin-orbit coupling.

Historically, in measurements of electron transport in disordered two-dimensional systems, an Arrhenius Hall carrier density has never been observed alongside an Arrhenius conductivity, when the Fermi level is below a mobility edge. This has long been an issue with respect to claiming observation of transport via activation to a mobility edge. In this work, an Arrhenius conductivity and Arrhenius Hall carrier density have been observed alongside one another in such a system. Measurements were made of a two-dimensional electron gas hosted in a gated GaAs/Al0.33Ga0.67As heterostructure. Furthermore, in the regime of Arrhenius conductivity and Arrhenius carrier density, the mobility is shown to be independent of the position of the Fermi level below the mobility edge. A transition between carrier density and mobility dominating the resistivity temperature dependence has been observed, as the Fermi level is varied.

We discuss the electrical properties of molecular beam epitaxy (MBE) grown, modulation doped, Ge1-x Sn- x quantum well devices. A consequence of the epitaxial growth process is that electronic disorder is introduced even in modulation doped quantum well structures and electrical transport properties that are characteristic of a high level of disorder are apparent. MBE growth of this material also results in the surface segregation of elemental beta-Sn in the way that has been observed utilizing other epitaxial growth methods. A thermally activated, p-type mobility is a clear feature of the electrical properties with generally temperature independent hole densities similar to 10(12) cm(-2) from the measured Hall effect and coming from the modulation doping. We present a discussion of Hall effect measurements in this disordered regime. The percolation carrier density in MBE modulation doped GeSn is in the region of similar to 1 x 10(12) cm(-2) although Hall measurements in this regime are difficult to quantify when the resistivity >(h/e( 2)). In this notation h is Planck's constant and e is the unit of charge. Conductivities (sigma) as low as similar to 0.028 x (e (2)/h) x square can be measured in the four-contact ac configuration and the temperature dependence indicates a mobility edge in these p-type devices below similar to 2 x 10(12) cm(-2). At lower temperatures (<similar to 1 K) the presence of a Coulomb gap can be determined using dc transport, constant voltage measurements where small ac current excitation is not available experimentally. This two-contact configuration can determine sigma down to similar to 10(-6) x (e (2)/h), deep into the localization regime, revealing a hopping conductivity dominated system. We discuss the relevance of these electrical properties for MBE grown GeSn devices.

Single Electron Pump (SEP) devices that can deliver single electrons at high frequencies are fabricated using two different gate geometries. The pump quantum dot (QD) can be defined either using two finger gates where the pump operates in what is known as the conventional pumping regime (CPR), or alternatively, the QD can be designed with the use of a single finger gate and split gate, where the pump operates in the long pumping regime (LPR). Here we investigate pump-maps produced in both the CPR and LPR under the effects of a low magnetic field (1 T to -1 T) and observe both the evolution of the pump-map and pump accuracy. Measurements were carried out in a cryogen free dilution refrigeration system with the use of a superconducting magnetic. Clear discrepancies between the behaviour of Med current in the different regimes can be seen indicating a fundamental difference in the operation of the pumps in the two different pumping regimes.

We present the first detailed study of the effect of a strong magnetic field on single-electron pumping in a device utilising a finger-gate split-gate configuration. In the quantum Hall regime, we demonstrate electron pumping from Landau levels in the leads, where the measurements exhibit pronounced oscillations in the lengths of the pumping plateaus with the magnetic field, reminiscent of Shubnikov-de Haas oscillations. This similarity indicates that the pumping process is dependent on the density of states of the 2D electron gas over a narrow energy window. Based on these observations, we develop a new theoretical description of the operation of single-electron pumps which for the first time allows for the determination of the physical parameters of the experiment; such as the capture energy of the electrons, the broadening of the quantised Landau levels in the leads, and the quantum lifetime of the electrons.

Porosity significantly influences lithium-ion battery performance, impacting cell capacity, voltage, and specific power - critical to designers. Terahertz technology enables non-destructive, remote porosity assessment, addressing limitations in current measurement methods.

High In content InGaAs quantum wells (In $\geq$ 75%) are potentially useful for topological quantum computing and spintronics applications. In high mobility InGaAs quantum wells, alloy disorder scattering is a limiting factor. In this report, we demonstrate that by growing the InGaAs quantum wells as a digital alloy, or a short period superlattice, we can reduce the alloy disorder scattering within the quantum well and increase the peak 2 K electron mobility to 545,000 cm^2/V s, which is the highest reported mobility for high In content InGaAs quantum wells to the best of the authors' knowledge. Our results demonstrate that the digital alloy approach can be used to increase the mobility of quantum wells in random alloy ternary materials.

The need to pump single electrons with a high degree of accuracy and fidelity has led to the development of a range of different pump and turnstile designs. Previous pumping mechanisms have all demonstrated that pumping more than one electron per cycle degrades the quantisation of the measured current. This unreliable delivery of multiple electrons per cycle has limited the use of on-demand single electron sources in electron quantum optic experiments. We present highly quantised current with multiple electrons pumped per cycle. We experimentally demonstrate that in our pumps an increase in electron throughput per cycle does not lead to an appreciable degradation in the accuracy of the produced current. Our pump is realised in an aluminium gallium arsenide two-dimensional electron gas, where electrons are pumped through a one-dimensional split-gate confinement potential under the influence of an applied source-drain voltage V_SD , and where the pump is driven by a trapezoidal arbitrary waveform. This combination of a split-gate potential, V_SD bias and trapezoidal wave form has led to the observation of robust quantised plateaus where not just a single electron, but a multiple integer number of electrons are pumped per cycle with a high degree of robustness and without the need of a magnetic field. For seven electrons per cycle, we report an increase of over two orders of magnitude in pumping accuracy from 2.72 × 10^-2 in devices operating in the conventional pumping regime, to 1.64 × 10^-4 in pumps operating in what we call the long plateau regime, a regime accessed under a change in a split-gate pumps applied V_SD voltage. This pump will find direct use in quantum transport measurements where the metrological accuracy of single electrons pumped per cycle is not required and the low throughput per cycle of electrons is limiting.

In this Letter, we report a systematic study of a structure found in zero magnetic field at or near 0.2 ×(e2/h) in In0.75Ga0.25As heterostructures, where e is the fundamental unit of charge and h is Planck's constant. This structure has been observed in many samples and stays at near constant conductance despite a large range of external potential changes, the stability indicating a quantum state. We have also studied the structure in the presence of high in-plane magnetic fields and find an anisotropy which can be related to the Rashba spin–orbit interaction and agrees with a recent theory based on the formation of coherent back-scattering. A possible state with conductance at 0.25 ×(e2/h) has also been found. The quantum states described here will help with the fundamental understanding of low-dimensional electronic systems with strong spin–orbit coupling and may offer new perspectives for future applications in quantum information schemes.

Illumination is performed at low temperature on dopant-free two-dimensional electron gases (2DEGs) of varying depths, under unbiased (gates grounded) and biased (gates at a positive or negative voltage) conditions. Unbiased illuminations in 2DEGs located more than 70 nm away from the surface result in a gain in mobility at a given electron density, primarily driven by the reduction of background impurities. In 2DEGs closer to the surface, unbiased illuminations result in a mobility loss, driven by an increase in surface charge density. Biased illuminations performed with positive applied gate voltages result in a mobility gain, whereas those performed with negative applied voltages result in a mobility loss. The magnitude of the mobility gain (loss) weakens with 2DEG depth, and is likely driven by a reduction (increase) in surface charge density. Remarkably, this mobility gain/loss is fully reversible by performing another biased illumination with the appropriate gate voltage, provided both $n$-type and $p$-type Ohmic contacts are present. Experimental results are modeled with Boltzmann transport theory, and possible mechanisms are discussed.

A variable range hopping (VRH) transport mechanism can be induced in molecular beam epitaxial, n-type doped InSb wafers with focussed Ga + ion beam damage. This technique allows areas of wafer to be selectively damaged and then subsequently processed into gated metal–insulator–semiconductor (MIS) devices where a disordered, two-dimensional (2D) device can be established. At high levels of damage (dose >10 16 Ga + ions cm −2 ) amorphous crystalline behavior results with activated conductivity characteristic of a three-dimensional system with VRH below 150 K. At lower doses (10 14 –10 16 Ga + ions cm −2 ) a thermally activated conductivity is induced at ∼0.9 K, characteristic of Mott phonon-assisted VRH. At 1 K the devices either conduct with conductivity >∼( e 2 / h ) where e is the fundamental charge and h is Planck’s constant, or are thermally activated depending on the dose level. The lightly damaged devices show weak antilocalization signals with conductivity characteristic of a 2D electronic system. As the Ga + dose increases, the measured phase coherence length reduces from ∼500 nm to ∼100 nm. This provides a region of VRH transport where phase-coherent transport processes can be studied in the hopping regime with the dimensionality controlled by a gate voltage in an MIS-device.

Localization behaviour is a characteristic feature of the p -type GeSn quantum well (QW) system in a metal–insulator–semiconductor device. The transition to strongly localized behaviour is abrupt with thermally activated conductivity and a high temperature intercept of 0.12 × e 2 ħ −1 at a hole carrier density 1.55 × 10 11 cm −2 . The activation energy for the conductivity in the localized state is 0.40 ± 0.05 meV compared to an activation energy of ∼0.1 meV for conductivity activation to a mobility edge at carrier densities >1.55 × 10 11 cm −2 . Insulating behaviour can occur from a system that behaves as though it is in a minimum metallic state, albeit at high temperature, or from a conductivity greater than a minimum metallic state behaviour showing that local disorder conditions with local differences in the density of states are important for the onset of localization. In the presence of a high magnetic field, thermally activated conductivity is present down to Landau level filling factor < 1 / 2 but without a magnetic-field-dependent carrier density or a variable range hopping (VRH) transport behaviour developing even with conductivity ≪ e 2 h −1 . In the localized transport regime in p -type doped Ge 0.92 Sn 0.08 QWs the VRH mechanism is suppressed at temperatures >100 mK and this makes this two-dimensional system ideal for future many body localization studies in disordered hole gases that can be thermally isolated from a temperature reservoir.

In this Perspective article, we present recent developments on interaction effects on the carrier transport properties of one-dimensional (1D) semiconductor quantum wires fabricated using the GaAs/AlGaAs system, particularly the emergence of the long predicted fractional quantization of conductance in the absence of a magnetic field. Over three decades ago, it was shown that transport through a 1D system leads to integer quantized conductance given by N·2e2/h, where N is the number of allowed energy levels (N = 1, 2, 3, …). Recent experiments have shown that a weaker confinement potential and low carrier concentration provide a testbed for electrons strongly interacting. The consequence leads to a reconfiguration of the electron distribution into a zigzag assembly which, unexpectedly, was found to exhibit quantization of conductance predominantly at 1/6, 2/5, 1/4, and 1/2 in units of e2/h. These fractional states may appear similar to the fractional states seen in the Fractional Quantum Hall Effect; however, the system does not possess a filling factor and they differ in the nature of their physical causes. The states may have promise for the emergent topological quantum computing schemes as they are controllable by gate voltages with a distinct identity.

We present results on electron transport in quasi-one dimensional (1D) quantum wires in GaAs/AlGaAs heterostructures obtained using an asymmetric confinement potential. The variation of the energy levels of the spatially quantized states is followed from strong confinement through weak confinement to the onset of two-dimensionality. An anticrossing of the initial ground and first excited states is found as the asymmetry of the potential is varied giving rise to two anticrossing events which occur on either side of symmetric confinement. We present results analysing this behaviour and showing how it can be affected by the inhomogeneity in background potential. The use of an enhanced source-drain voltage to alter the energy levels is shown to be a significant validation of the analysis by showing the formation of double rows of electrons which correlate with the anticrossing.

In this chapter, we present developments in quantum transport in one-dimensional (1D) semiconductor nanostructures during the last decade or so where several important results were reported, including observation of the incipient Wigner lattice, interaction-induced spin polarisation and fractional quantised states in the absence of a magnetic field in weakly confined quasi-1D quantum wires.

Future quantum based electronic systems will demand robust and highly accurate on-demand sources of current. The ultimate limit of quantized current sources is a highly controllable device that manipulates individual electrons. We present a GaAs single-electron pump, where electrons are pumped through a one-dimensional split-gate saddle point confinement potential, which show quantized plateaus with length and width that can be independently tuned with the application of a source-drain bias and RF amplitude. The plateaus can be over two orders of magnitude longer than conventional pumps, and flatness improves with the application of a source-drain bias.

$G{e}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ quantum wells can be incorporated into $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$--based structures with low-carrier effective masses, high mobilities, and the possibility of direct band-gap devices with x \ensuremath{\sim} 0.1. However, the electrical properties of p-type $G{e}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ devices are dominated by a thermally activated mobility and metallic behavior. At 30 mK the transport measurements indicate localization with a mobility of $380\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{2}$/Vs, which is thermally activated with a temperature-independent carrier density of 4 \ifmmode\times\else\texttimes\fi{} ${10}^{11}\phantom{\rule{0.1em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. This weakly disordered system with conductivity, \ensuremath{\sigma} \ensuremath{\sim} ${e}^{2}$/h, where e is the fundamental charge and h is Planck's constant, is a result of negatively charged ``Sn-vacancy'' complex states in the barrier layers that act as hole traps. A measured hole effective mass of 0.090 \ifmmode\pm\else\textpm\fi{} $0.005{m}_{e}$ from the Shubnikov-de Haas effect, where ${m}_{e}$ is the free electron mass shows that the valence band is heavy hole dominated and is similar to p-type $\mathrm{Ge}$ with the compressive strain playing the role of quenching the spin-orbit coupling and shifting the unoccupied light-hole states to higher hole energies. The $G{e}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ devices have a high quantum mobility of approximately 36 000 ${\mathrm{cm}}^{2}$/Vs that is not thermally activated. The ratio of transport-to-quantum mobility of approximately 0.01 in $G{e}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ devices is unusual and points to several competing scattering mechanisms in the different experimental regimes.

Experimental evidence of resistance anomalies in the high-mobility two-dimensional electron gas (2DEG) formed in the GaAs/AlGaAs heterostructure, in the integer and fractional quantized Hall regime, is shown. The data complement to a good approximation the semianalytic calculations used to describe the formation of integral and fractional incompressible strips. The widths of current-carrying channels were calculated by incorporating the screening properties of the 2DEG and the effect of a magnetic field in the perpendicular mode. The manybody effects of the composite fermions are taken into consideration for the energy gap for the fractional states. It is shown that incompressible strips at the edges for both integer and fractional filling factors coexist in their evanescent phase for a particular range of magnetic fields, resulting in overshoot effects at the Hall resistance. Specifically, anomalous Hall resistances were noticed for filling factors v = 4/3, 3/2, 5/3, 8/3, 3, 10/3, 7/2, and 5. This effect is explained and discussed using the screening theory.