Kondo physics in tunable semiconductor nanowire quantum dots

We have observed the Kondo effect in strongly coupled semiconducting nanowire quantum dots. The devices are made from indium arsenide nanowires, grown by molecular beam epitaxy, and contacted by titanium leads. The device transparency can be tuned by changing the potential on a gate electrode, and for increasing transparencies the effects dominating the transport changes from Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics appearing in the intermediate region.Comment: 4 pages, 4 figure

Shadow epitaxy for in-situ growth of generic semiconductor/superconductor devices

Uniform, defect-free crystal interfaces and surfaces are crucial ingredients for realizing high-performance nanoscale devices. A pertinent example is that advances in gate-tunable and topological superconductivity using semiconductor/superconductor electronic devices are currently built on the hard proximity-induced superconducting gap obtained from epitaxial indium arsenide/aluminium heterostructures. Fabrication of devices requires selective etch processes; these exist only for InAs/Al hybrids, precluding the use of other, potentially superior material combinations. We present a crystal growth platform -- based on three-dimensional structuring of growth substrates -- which enables synthesis of semiconductor nanowire hybrids with in-situ patterned superconductor shells. This platform eliminates the need for etching, thereby enabling full freedom in choice of hybrid constituents. We realise and characterise all the most frequently used architectures in superconducting hybrid devices, finding increased yield and electrostatic stability compared to etched devices, along with evidence of ballistic superconductivity. In addition to aluminium, we present hybrid devices based on tantalum, niobium and vanadium. This is the submitted version of the manuscript. The accepted, peer reviewed version is available from Advanced Materials: http://doi.org/10.1002/adma.201908411 Previous title: Shadow lithography for in-situ growth of generic semiconductor/superconductor device

Josephson effect in a few-hole quantum dot

We use a Ge-Si core-shell nanowire to realise a Josephson field-effect transistor with highly transparent contacts to superconducting leads. By changing the electric field we gain access to two distinct regimes not combined before in a single device: In the accumulation mode the device is highly transparent and the supercurrent is carried by multiple subbands, while near depletion supercurrent is carried by single-particle levels of a strongly coupled quantum dot operating in the few-hole regime. These results establish Ge-Si nanowires as an important platform for hybrid superconductor-semiconductor physics and Majorana fermions

Engineering Hybrid Epitaxial InAsSb/Al Nanowire Materials for Stronger Topological Protection

The combination of strong spin-orbit coupling, large $g$-factors, and the coupling to a superconductor can be used to create a topologically protected state in a semiconductor nanowire. Here we report on growth and characterization of hybrid epitaxial InAsSb/Al nanowires, with varying composition and crystal structure. We find the strongest spin-orbit interaction at intermediate compositions in zincblende InAs$_{1-x}$Sb$_{x}$ nanowires, exceeding that of both InAs and InSb materials, confirming recent theoretical studies \cite{winkler2016topological}. We show that the epitaxial InAsSb/Al interfaces allows for a hard induced superconducting gap and 2$e$ transport in Coulomb charging experiments, similar to experiments on InAs/Al and InSb/Al materials, and find measurements consistent with topological phase transitions at low magnetic fields due to large effective $g$-factors. Finally we present a method to grow pure wurtzite InAsSb nanowires which are predicted to exhibit even stronger spin-orbit coupling than the zincblende structure.Comment: 10 pages and 5 figure

Quantization of Hall Resistance at the Metallic Interface between an Oxide Insulator and SrTiO3_{3}

The two-dimensional metal forming at the interface between an oxide insulator and SrTiO3 provides new opportunities for oxide electronics. However, the quantum Hall effect, one of the most fascinating effects of electrons confined in two dimensions, remains underexplored at these complex oxide heterointerfaces. Here, we report the experimental observation of quantized Hall resistance in a SrTiO3 heterointerface based on the modulation-doped amorphous-LaAlO$_{3}$/SrTiO$_{3}$ heterostructure, which exhibits both high electron mobility exceeding 10000 cm$^{2}$/Vs and low carrier density on the order of ~10$^{12}$ cm$^{-2}$. Along with unambiguous Shubnikov-de Haas oscillations, the spacing of the quantized Hall resistance suggests that the interface is comprised of a single quantum well with ten parallel conducting two-dimensional subbands. This provides new insight into the electronic structure of conducting oxide interfaces and represents an important step towards designing and understanding advanced oxide devices

Diluted Oxide Interfaces with Tunable Ground States

The metallic interface between two oxide insulators, such as LaAlO3/SrTiO3 (LAO/STO), provides new opportunities for electronics and spintronics. However, due to the presence of multiple orbital populations, tailoring the interfacial properties such as the ground state and metal-insulator transitions remains challenging. Here, we report an unforeseen tunability of the phase diagram of LAO/STO by alloying LAO with a ferromagnetic LaMnO3 insulator without forming lattice disorder and at the same time without changing the polarity of the system. By increasing the Mn-doping level, x, of LaAl1-xMnxO3/STO, the interface undergoes a Lifshitz transition at x = 0.225 across a critical carrier density of nc= 2.8E13 cm-2, where a peak TSC =255 mK of superconducting transition temperature is observed. Moreover, the LaAl1-xMnxO3 turns ferromagnetic at x >=0.25. Remarkably, at x = 0.3, where the metallic interface is populated by only dxy electrons and just before it becomes insulating, we achieve reproducibly a same device with both signatures of superconductivity and clear anomalous Hall effect. This provides a unique and effective way to tailor oxide interfaces for designing on-demand electronic and spintronic devices.Comment: 18 pages and 6 figure

Fiscal measures to Reduce CO2 emissions from new passenger cars

The fuel efficiency of passenger cars is often emphasised as one of the most significant areas of action in terms of limiting the transport sector’s CO2 emission levels. This could be achieved either on the supply side through the technological development of cars, or through demand-side measures such as influencing first time buyers’ choice of car. The aim of this article is to present a model for analysis of the demand side, more specifically the relation between the CO2 emissions from new cars and the car taxation related to new car purchases in EU. The purpose of the model is twofold. Firstly, it should be used to analyse the CO2 efficiency of the car taxation in each member state. Secondly, results from model calculations should be used to illustrate what are the possibilities for the individual member states to reduce their CO2 emissions from new cars. The major conclusions from the study are: It is essential to apply a tax scheme, which is directly or indirectly CO2 related in order to provide for significant reductions in the average CO2 emissions from new cars. It is essential to differentiate the taxes in such a way that taxes for very energy effective cars are significantly lower than taxes for cars with poor energy efficiency. Fuel tax increases provide only very small reductions of the average CO2 emissions of new cars compared to vehicle taxes. The model is based on a revised and enhanced version of the Danish Car Choice model from 1997 combined with car characteristics, socio-economic data and car taxation data from the EU member states

Transport and excitations in a negative-U quantum dot at the LaAlO₃/SrTiO₃ interface

In a solid-state host, attractive electron-electron interactions can lead to the formation of local electron pairs which play an important role in the understanding of prominent phenomena such as high T c superconductivity and the pseudogap phase. Recently, evidence of a paired ground state without superconductivity was demonstrated at the level of single electrons in quantum dots at the interface of LaAlO3 and SrTiO3. Here, we present a detailed study of the excitation spectrum and transport processes of a gate-defined LaAlO3/SrTiO3 quantum dot exhibiting pairing at low temperatures. For weak tunneling, the spectrum agrees with calculations based on the Anderson model with a negative effective charging energy U, and exhibits an energy gap corresponding to the Zeeman energy of the magnetic pair-breaking field. In contrast, for strong coupling, low-bias conductance is enhanced with a characteristic dependence on temperature, magnetic field and chemical potential consistent with the charge Kondo effect

Superconducting proximity effect in interacting quantum dots revealed by shot noise

We study the full counting statistics of charge transport through a quantum dot tunnel-coupled to one normal and one superconducting lead with a large superconducting gap. As function of the level detuning, there is a crossover from a regime with strong superconducting correlations in the quantum dot to a regime in which the proximity effect on the quantum dot is suppressed. We analyze the current fluctuations of this crossover in the shot-noise regime. In particular, we predict that the full counting statistics changes from Poissonian with charge 2e, typical for Cooper pairs, to Poissonian with charge e, when the superconducting proximity effect is present. Thus, the onset of the superconducting proximity effect is revealed by the reduction of the Fano factor from 2 to 1.Comment: 5 pages, 3 figure