Charge doping and large lattice expansion in oxygen-deficient heteroepitaxial WO3

Tungsten trioxide is a versatile material with widespread applications ranging from electrochromic and optoelectronic devices to water splitting and catalysis of chemical reactions. For technological applications, thin films of WO3 are particularly appealing, taking advantage from high surface-to-volume ratio and tunable physical properties. However, the growth of stoichiometric, crystalline thin films is challenging because the deposition conditions are very sensitive to the formation of oxygen vacancies. In this work, we show how background oxygen pressure during pulsed laser deposition can be used to tune the structural and electronic properties of WO3 thin films. By performing X-ray diffraction and low-temperature transport measurements, we find changes in WO3 lattice volume up to 10%, concomitantly with an insulator-to-metal transition as a function of increased level of electron doping. We use advanced ab initio calculations to describe in detail the properties of the oxygen vacancy defect states, and their evolution in terms of excess charge concentration. Our results depict an intriguing scenario where structural, electronic, optical, and transport properties of WO3 single-crystal thin films can all be purposely tuned by a suited control of oxygen vacancies formation during growth

Nanoscale Electrostatic Control of Oxide Interfaces

We develop a robust and versatile platform to define nanostructures at oxide interfaces via patterned top gates. Using LaAlO$_3$/SrTiO$_3$ as a model system, we demonstrate controllable electrostatic confinement of electrons to nanoscale regions in the conducting interface. The excellent gate response, ultra-low leakage currents, and long term stability of these gates allow us to perform a variety of studies in different device geometries from room temperature down to 50 mK. Using a split-gate device we demonstrate the formation of a narrow conducting channel whose width can be controllably reduced via the application of appropriate gate voltages. We also show that a single narrow gate can be used to induce locally a superconducting to insulating transition. Furthermore, in the superconducting regime we see indications of a gate-voltage controlled Josephson effect.Comment: Version after peer review; includes additional data on superconductivit

Growth-induced electron mobility enhancement at the LaAlO3_3/SrTiO3_3 interface

We have studied the electronic properties of the 2D electron liquid present at the LaAlO$_3$/SrTiO$_3$ interface in series of samples prepared at different growth temperatures. We observe that interfaces fabricated at 650{\deg}C exhibit the highest low temperature mobility ($\approx 10000 \textrm{ cm}^2/\textrm{Vs}$) and the lowest sheet carrier density ($\approx 5\times 10^{12} \textrm{ cm}^{-2}$). These samples show metallic behavior and Shubnikov-de Haas oscillations in their magnetoresistance. Samples grown at higher temperatures (800-900{\deg}C) display carrier densities in the range of $\approx 2-5 \times 10^{13} \textrm{ cm}^{-2}$ and mobilities of $\approx 1000 \textrm{ cm}^2/\textrm{Vs}$ at 4K. Reducing their carrier density by field effect to $8\times 10^{12} \textrm{ cm}^{-2}$ lowers their mobilites to $\approx 50 \textrm{ cm}^2/\textrm{Vs}$ bringing the conductance to the weak-localization regime

Superconducting quantum point contact with split gates in the two dimensional LaAlO3/SrTiO3 superfluid

One of the hallmark experiments of quantum transport is the observation of the quantized resistance in a point contact formed with split gates in GaAs/AlGaAs heterostructures. Being carried out on a single material, they represent in an ideal manner equilibrium reservoirs which are connected only through a few electron mode channel with certain transmission coefficients. It has been a long standing goal to achieve similar experimental conditions also in superconductors, only reached in atomic scale mechanically tunable break junctions of conventional superconducting metals, but here the Fermi wavelength is so short that it leads to a mixing of quantum transport with atomic orbital physics. Here we demonstrate for the first time the formation of a superconducting quantum point contact (SQPC) with split gate technology in a superconductor, utilizing the unique gate tunability of the two dimensional superfluid at the LaAlO3/SrTiO3 (LAO/STO) interface. When the constriction is tuned through the action of metallic split gates we identify three regimes of transport: (i) SQPC for which the supercurrent is carried only by a few quantum transport channels. (ii) Superconducting island strongly coupled to the equilibrium reservoirs. (iii) Charge island with a discrete spectrum weakly coupled to the reservoirs. Our experiments demonstrate the feasibility of a new generation of mesoscopic all-superconductor quantum transport devices.Comment: 18 page

Epitaxial growth and thermodynamic stability of SrIrO3/SrTiO3 heterostructures

Obtaining high-quality thin films of 5d transition metal oxides is essential to explore the exotic semimetallic and topological phases predicted to arise from the combination of strong electron correlations and spin-orbit coupling. Here, we show that the transport properties of SrIrO3 thin films, grown by pulsed laser deposition, can be optimized by considering the effect of laser-induced modification of the SrIrO3 target surface. We further demonstrate that bare SrIrO3 thin films are subject to degradation in air and are highly sensitive to lithographic processing. A crystalline SrTiO3 cap layer deposited in-situ is effective in preserving the film quality, allowing us to measure metallic transport behavior in films with thicknesses down to 4 unit cells. In addition, the SrTiO3 encapsulation enables the fabrication of devices such as Hall bars without altering the film properties, allowing precise (magneto)transport measurements on micro- and nanoscale devices.Comment: 5 pages, 3 figure

Tunable Rashba spin-orbit interaction at oxide interfaces

The quasi-two-dimensional electron gas found at the LaAlO3/SrTiO3 interface offers exciting new functionalities, such as tunable superconductivity, and has been proposed as a new nanoelectronics fabrication platform. Here we lay out a new example of an electronic property arising from the interfacial breaking of inversion symmetry, namely a large Rashba spin-orbit interaction, whose magnitude can be modulated by the application of an external electric field. By means of magnetotransport experiments we explore the evolution of the spin-orbit coupling across the phase diagram of the system. We uncover a steep rise in Rashba interaction occurring around the doping level where a quantum critical point separates the insulating and superconducting ground states of the system

Optimización de las osteotomías de tibia mediante cálculos matemáticos

El objetivo de este trabajo es aclarar con cálculos matemáticos las dificultades que existen en cuanto a la relación entre la cuña a resecar con la desviación a corregir y los diferentes alargamientos según que las osteotomías sean cuneiformes o bien arciformes. Así hemos llegado a las siguientes conclusiones: 1) Se ha establecido una fórmula matemática que nos permite obtener una relación exacta entre la cuña a resecar y la desviación a corregir; 2) Se demuestra el importante rol que juega el ancho de la metáfisis como factor relevante en la cuña a resecar; 3) Las osteotomías cuneiformes producen un acortamiento real del hueso, que se ve compensado con la corrección del eje del mismo; 4) Las osteotomías arciformes no modifican la longitud real del hueso, pero producen un alargamiento aparente del miembro al corregir el eje.This work was aimed at evaluating, by means of a mathematical approachtint, the relationship between the resected bone wedge and the deformity to be corrected, and second the different tibial lengthening induced by close wedge or archiform osteotomy. We drew the following conclusions: 1) a mathematical equation was defined expresing an exact relationship between the bone wedge and the angular correction; 2) the metaphysical widening has been found to be a relevant factor conditioning the angle of the bone wedge; 3) core wedge osteotomies induce a shortening of the tibia which is compensated by the correction of the axis; 4) Archiform osteotomies do not modify tibial length but produce an apparent limb lengthening when correcting the axis of the extremity

Seebeck effect in the conducting LaAlO_{3}/SrTiO_{3} interface

The observation of metallic behavior at the interface between insulating oxides has triggered worldwide efforts to shed light on the physics of these systems and clarify some still open issues, among which the dimensional character of the conducting system. In order to address this issue, we measure electrical transport (Seebeck effect, Hall effect and conductivity) in LaAlO_{3}/SrTiO_{3} interfaces and, for comparison, in a doped SrTiO_{3} bulk single crystal. In these experiments, the carrier concentration is tuned, using the field effect in a back gate geometry. The combined analysis of all experimental data at 77 K indicates that the thickness of the conducting layer is ~7 nm and that the Seebeck effect data are well described by a two-dimensional (2D) density of states. We find that the back gate voltage is effective in varying not only the charge density, but also the thickness of the conducting layer, which is found to change by a factor of ~2, using an electric field between -4 and +4MV/m at 77K. No enhancement of the Seebeck effect due to the electronic confinement and no evidence for two-dimensional quantization steps are observed at the interfaces.Comment: 15 pages, 5 figure

Design of Operational Transconductance Amplifiers for voltage to current conversion in gas sensing applications

This paper presents a study of Operational Transconductance Amplifiers (OTAs) for voltage to current conversion circuits. The paper includes a comparative analysis of three OTA architectures implemented in 0.35\u3bcm CMOS AMS Technology under \ub1 1.65V power supply voltage. The impact of the OTA topology has been investigated by simulation. The designed OTAs managed to deliver large current values of 10mA and 1mA to the load with a worst-case error of 0.02% under worst-case power supply and temperature conditions and a worst percentage error of 0.12% under process variation for both Miller Compensated and Capacitor Multiplier Compensated OTA. \ua9 2016 AEIT