We investigate non-reversible transient electron transport and non-adiabatic decay of resistance in non-ferromagnetic GeTe wires/thin films and find both directional hysteresis and offset of its minimum away from B = 0 as a magnetic field B(t) is swept. Our study demonstrates interplay among Rashba spin-orbit coupling, Landau quantization and Zeeman splitting by employing quantum-Boltzmann equation and a non-adiabatic current. We further find non-reciprocal chemical potential fluctuations from quantum-capacitance coupling and show these extremely long-lived fluctuations decayed on the time scale of tens of milliseconds in the 1D case and over hundred seconds in the 2D case. Published by Elsevier B.V.

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and Sb_2 Te_3 (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb _2 Te _3 band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. In addition, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen et al. Sci. Rep. 6 , 27716 (2016)] where ETSs were seen to couple over thick layers.

Thermoelectric generators (TEGs) operate in the presence of a temperature gradient, where the constituent thermoelectric (TE) material converts heat into electricity via the Seebeck effect. However, TE materials are characterized by a thermoelectric figure of merit (ZT) and/or power factor (PF), which often has a strong dependence on temperature. Thus, a single TE material spanning a given temperature range is unlikely to have an optimal ZT or PF across the entire range, leading to inefficient TEG performance. Compositionally graded organic-inorganic nanocomposites are demonstrated, where the composition of the TE nanocomposite can be systematically tuned along the length of the TEG, in order to optimize the PF along the applied temperature gradient. The nanocomposite composition is dynamically tuned by an aerosol-jet printing method with controlled in situ mixing capability, thus enabling the realization of such compositionally graded thermoelectric composites (CG-TECs). It is shown how CG-TECs can be realized by varying the loading weight percentage of Bi2Te3 nanoparticles or Sb2Te3 nanoflakes within an organic conducting matrix using bespoke solution-processable inks. The enhanced energy harvesting capability of these CG-TECs from low-grade waste heat (<100 degrees C) is demonstrated, highlighting the improvement in output power over single-component TEGs.

Electron-electron interactions in topological p-n junctions consisting of vertically stacked topological insulators are investigated. n-type Bi2Te3 and p-type Sb2Te3 of varying relative thicknesses are deposited using molecular beam epitaxy and their electronic properties measured using low-temperature transport. The screening factor is observed to decrease with increasing sample thickness, a finding which is corroborated by semiclassical Boltzmann theory. The number of two-dimensional states determined from electron-electron interactions is larger compared to the number obtained from weak antilocalization, in line with earlier experiments using single layers.

Thermoelectric materials, capable of interconverting heat and electricity, are attractive for applications in thermal energy harvesting as a means to power wireless sensors, wearable devices, and portable electronics. However, traditional inorganic thermoelectric materials pose significant challenges due to high cost, toxicity, scarcity, and brittleness, particularly when it comes to applications requiring flexibility. Here, we investigate organic–inorganic nanocomposites that have been developed from bespoke inks which are printed via an aerosol jet printing method onto flexible substrates. For this purpose, a novel in situ aerosol mixing method has been developed to ensure uniform distribution of Bi2Te3/Sb2Te3 nanocrystals, fabricated by a scalable solvothermal synthesis method, within a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate matrix. The thermoelectric properties of the resulting printed nanocomposite structures have been evaluated as a function of composition, and the power factor was found to be maximum (∼30 μW/mK2) for a nominal loading fraction of 85 wt % Sb2Te3 nanoflakes. Importantly, the printed nanocomposites were found to be stable and robust upon repeated flexing to curvatures up to 300 m–1, making these hybrid materials particularly suitable for flexible thermoelectric applications.

This data is collected from the labs of Device Materials Group, Department of Materials Science and Metallurgy, University of Cambridge, in which the measurements were taken between October 2016 to March 2018. The data contains the EDX spectra, XRD pattern, thermoelectric properties measurements, flexing test measurements, and sample dimension measurements.

Aerosol-jet printing allows functional materials to be printed from inks with a wide range of viscosities and constituent particle sizes onto various substrates, including the printing of organic thermoelectric materials on flexible substrates for low-grade thermal energy harvesting. However, these materials typically suffer from relatively poor thermoelectric performance, compared to traditional inorganic counterparts, due to their low Seebeck coefficient, S, and electrical conductivity, σ. Here, we demonstrate a modified aerosol-jet printing technique that can simultaneously incorporate well-dispersed high-S Sb2Te3 nanoflakes and high-σ multi-walled carbon nanotubes (MWCNTs) providing good inter-particle connectivity to significantly enhance the thermoelectric performance of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate structures on flexible polyimide substrates. A nominal loading fraction of 85 wt. % yielded a power factor of ∼41 μW/mK2, which is among the highest for printed organic-based structures. Rigorous flexing and fatigue tests were performed to confirm the robustness and stability of these aerosol-jet printed MWCNT-based thermoelectric nanocomposites.

We report the room-temperature growth of vertically aligned ternary Bi2−xSbxTe3 nanowires of diameter ~200 nm and length ~12 µm, within flexible track-etched nanoporous polycarbonate (PC) templates via a one-step electrodeposition process. Bi2−xSbxTe3 nanowires with compositions spanning the entire range from pure Bi2Te3 (x = 0) to pure Sb2Te3 (x = 2) were systematically grown within the nanoporous channels of PC templates from a tartaric–nitric acid based electrolyte, at the end of which highly crystalline nanowires of uniform composition were obtained. Compositional analysis showed that the Sb concentration could be tuned by simply varying the electrolyte composition without any need for further annealing of the samples. Thermoelectric properties of the Bi2−xSbxTe3 nanowires were measured using a standardized bespoke setup while they were still embedded within the flexible PC templates.

We report the results of an investigation of ambipolar transport in a quantum well of 15 nm width in an undoped GaAs/AlGaAs structure, which was populated either by electrons or holes using positive or negative gate voltage Vtg, respectively. More attention was focussed on the low concentration of electrons n and holes p near the metal–insulator transition (MIT). It is shown that the electron mobility μe increases almost linearly with increase of n and is independent of temperature T in the interval 0.3 K–1.4 K, while the hole mobility μp depends non-monotonically on p and T. This difference is explained on the basis of the different effective masses of electrons and holes in GaAs. Intriguingly, we observe that at low p the source–drain current (ISD)–voltage (V) characteristics, which become non-linear beyond a certain ISD, exhibit a re-entrant linear regime at even higher ISD. We find, remarkably, that the departure and reappearance of linear behaviour are not due to non-linear response of the system, but due to an intrinsic mechanism by which there is a reduction in the net number of mobile carriers. This effect is interpreted as evidence of inhomogeneity of the conductive 2D layer in the vicinity of MIT and trapping of holes in ‘dead ends’ of insulating islands. Our results provide insights into transport mechanisms as well as the spatial structure of the 2D conducting medium near the 2D MIT.

By combining n-type Bi2Te3 and p-type Sb2Te3 topological insulators, vertically stacked p-n junctions can be formed, allowing to position the Fermi level into the bulk band gap and also tune between n-and p-type surface carriers. Here, we use low-temperature magnetotransport measurements to probe the surface and bulk transport modes in a range of vertical Bi2Te3/Sb2Te3 heterostructures with varying relative thicknesses of the top and bottom layers. With increasing thickness of the Sb2Te3 layer we observe a change from n-to p-type behavior via a specific thickness where the Hall signal is immeasurable. Assuming that the the bulk and surface states contribute in parallel, we can calculate and reproduce the dependence of the Hall and longitudinal components of resistivity on the film thickness. This highlights the role played by the bulk conduction channels which, importantly, cannot be probed using surface-sensitive spectroscopic techniques. Our calculations are then buttressed by a semiclassical Boltzmann transport theory which rigorously shows the vanishing of the Hall signal. Our results provide crucial experimental and theoretical insights into the relative roles of the surface and bulk in the vertical topological p-n junctions.

There is much current interest in combining superconductivity and spin-orbit coupling in order to induce the topological superconductor phase and associated Majorana-like quasiparticles which hold great promise towards fault-tolerant quantum computing. Experimentally these effects have been combined by the proximity-coupling of superconducting leads and high spin-orbit materials such as InSb and InAs, or by controlled Cu-doping of topological insulators such as Bi2Se3. However, for practical purposes, a single-phase material which intrinsically displays both these effects is highly desirable. Here we demonstrate coexisting superconducting correlations and spin-orbit coupling in molecular-beam-epitaxy-grown thin films of GeTe. The former is evidenced by a precipitous low-temperature drop in the electrical resistivity which is quelled by a magnetic field, and the latter manifests as a weak antilocalisation (WAL) cusp in the magnetotransport. Our studies reveal several other intriguing features such as the presence of two-dimensional rather than bulk transport channels below 2 K, possible signatures of topological superconductivity, and unexpected hysteresis in the magnetotransport. Our work demonstrates GeTe to be a potential host of topological SC and Majorana-like excitations, and to be a versatile platform to develop quantum information device architectures.

We review some of our recent experimental studies on low-carrier concentration, mesoscopic two-dimensional electron gases (m2DEGs). The m2DEGs show a range of striking characteristics, including a complete avoidance of the strongly localised regime even when the electrical resistivity rho>> h/e(2), giant thermoelectric response, and an apparent decoupling of charge and thermoelectric transport. We analyse the results and demonstrate that these observations can be explained based on the assumption that the charge carriers retain phase coherence over the m2DEG dimensions. Intriguingly, this would imply phase coherence on lengthscales of up to 10 mu m and temperature T up to 10 K, which is significantly greater than conventionally expected in GaAs-based 2DEGs. We critically assess this assumption and explore other possible explanations to the data. Such unprecedentedly large phase coherence lengths open up several possibilities in quantum information and computation schemes. (C) 2016 Academie des sciences. Published by Elsevier Masson SAS.

Mots-clés : Systèmes mésoscopiques Coefficient de Seebeck Cohérence de phase We review some of our recent experimental studies on low-carrier concentration, mesoscopic two-dimensional electron gases (m2DEGs). The m2DEGs show a range of striking characteristics, including a complete avoidance of the strongly localised regime even when the electrical resistivity ρ >> h/e2, giant thermoelectric response, and an apparent decoupling of charge and thermoelectric transport. We analyse the results and demonstrate that these observations can be explained based on the assumption that the charge carriers retain phase coherence over the m2DEG dimensions. Intriguingly, this would imply phase coherence on lengthscales of up to 10 μm and temperature T up to 10 K, which is significantly greater than conventionally expected in GaAs-based 2DEGs. We critically assess this assumption and explore other possible explanations to the data. Such unprecedentedly large phase coherence lengths open up several possibilities in quantum information and computation schemes. © 2016 Académie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

We investigate the electromechanical interactions in individual polyvinylidene fluoride-trifluoroethylene nanowires in response to localized electrical poling via a conducting atomic force microscope tip. Spatially resolved measurements of piezoelectric coefficients and elastic moduli before and after poling reveal a striking dependence on the polarity of the poling field, notably absent in thin films of the same composition. These observations are attributed to the unclamped nature of the nanowires and the inherent asymmetry in their chemical and electrical interactions with the tip and underlying substrate. Our findings provide insights into the mechanism of poling/switching in polymer nanowires critical to ferroelectric device performance.

Topological insulators (TIs) are bulk insulators with exotic ‘topologically protected’ surface conducting modes. It has recently been pointed out that when stacked together, interactions between surface modes can induce diverse phases including the TI, Dirac semimetal, and Weyl semimetal. However, currently a full experimental understanding of the conditions under which topological modes interact is lacking. Here, working with multilayers of the TI Sb 2 Te 3 and the band insulator GeTe, we provide experimental evidence of multiple topological modes in a single Sb 2 Te 3 -GeTe-Sb 2 Te 3 structure. Furthermore, we show that reducing the thickness of the GeTe layer induces a phase transition from a Dirac-like phase to a gapped phase. By comparing different multilayer structures we demonstrate that this transition occurs due to the hybridisation of states associated with different TI films. Our results demonstrate that the Sb 2 Te 3 -GeTe system offers strong potential towards manipulating topological states as well as towards controlledly inducing various topological phases.

We report the development of a simple and reliable, front-sided-only fabrication technique for n-type ohmic contacts to two-dimensional electron gases (2DEGs) in undoped GaAs/AlGaAs quantum wells. We have adapted the well-established recessed ohmic contacts/insulated metal gate technique for inducing a 2DEG in an undoped triangular well to also work reliably for undoped square quantum wells. Our adaptation involves a change in the procedure for etching the ohmic contact pits to optimise the etch side-wall profile and depth. As an application of our technique, we present a front-side-gated ambipolar field effect transistor (FET), where both 2D electron and hole gases can be induced in the same quantum well. We present results of low-temperature (0.3 K - 4 K) transport measurements of this device, including assessment of the n-type ohmic contact quality. On the basis of our findings, we discuss why the fabrication of these contacts is difficult and how our technique circumvents the challenges.

We investigate the striking absence of strong localisation observed in mesoscopic two-dimensional electron gases (2DEGs) (Baenninger et al 2008 Phys. Rev. Lett. 100 016805, Backes et al 2015 arXiv:1505.03444) even when their resistivity [Formula: see text]. In particular, we try to understand whether this phenomenon originates in quantum many-body effects, or simply percolative transport through a network of electron puddles. To test the latter scenario, we measure the low temperature (low-T) transport properties of long and narrow 2DEG devices in which percolation effects should be heavily suppressed in favour of Coulomb blockade. Strikingly we find no indication of Coulomb blockade and that the high-ρ, low-T transport is exactly similar to that previously reported in mesoscopic 2DEGs with different geometries. Remarkably, we are able to induce a 'metal'-insulator transition (MIT) by applying a perpendicular magnetic field B. We present a picture within which these observations fit into the more conventional framework of the 2D MIT.

We investigate mesoscopic Josephson-junction arrays created by patterning superconducting disks on monolayer graphene, concentrating on the high-T/T-c regime of these devices and the phenomena which contribute to the superconducting glass state in diffusive arrays. We observe features in the magnetoconductance at rational fractions of flux quanta per array unit cell, which we attribute to the formation of flux-quantized vortices. The applied fields at which the features occur are well described by Ginzburg-Landau simulations that take into account the number of unit cells in the array. We find that the mean conductance and universal conductance fluctuations are both enhanced below the critical temperature and field of the superconductor, with greater enhancement away from the graphene Dirac point.