The diverse phenomena associated with the two-dimensional electron gas (2DEG) that occurs at oxide interfaces include, among others, exceptional carrier mobilities, magnetism and superconductivity. ...Although these have mostly been the focus of interest for potential future applications, they also offer an opportunity for studying more fundamental quantum many-body effects. Here, we examine the magnetic-field-driven quantum phase transition that occurs in electrostatically gated superconducting LaTiO3/SrTiO3 interfaces. Through a finite-size scaling analysis, we show that it belongs to the (2+1)D XY model universality class. The system can be described as a disordered array of superconducting puddles coupled by a 2DEG and, depending on its conductance, the observed critical behaviour is single (corresponding to the long-range phase coherence in the whole array) or double (one related to local phase coherence, the other one to the array). A phase diagram illustrating the dependence of the critical field on the 2DEG conductance is constructed, and shown to agree with theoretical proposals. Moreover, by retrieving the coherence-length critical exponent ν, we show that the quantum critical behaviour can be clean or dirty according to the Harris criterion, depending on whether the phase-coherence length is smaller or larger than the size of the puddles.
The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees ...of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LaAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements show that the Rashba coupling constant increases linearly with the interfacial electric field. Our results pave the way for the realisation of mesoscopic devices, where these two properties can be manipulated on a local scale by means of top-gates.
Abstract
In LaTiO
3
/SrTiO
3
and LaAlO
3
/SrTiO
3
heterostructures, the bending of the SrTiO
3
conduction band at the interface forms a quantum well that contains a superconducting two-dimensional ...electron gas (2-DEG). Its carrier density and electronic properties, such as superconductivity and Rashba spin-orbit coupling can be controlled by electrostatic gating. In this article we show that the Fermi energy lies intrinsically near the top of the quantum well. Beyond a filling threshold, electrons added by electrostatic gating escape from the well, hence limiting the possibility to reach a highly-doped regime. This leads to an irreversible doping regime where all the electronic properties of the 2-DEG, such as its resistivity and its superconducting transition temperature, saturate. The escape mechanism can be described by the simple analytical model we propose.
The VO2 metastable (B) phase is of interest for applications in temperature sensing, bolometry, and Li‐ion batteries. However, single‐phase growth of thin films of this metastable phase is a ...challenge because vanadium oxide exhibits many polymorphs and the VO2 stable (M1) phase is usually present as a secondary phase. Additionally, the phase transition at 350 °C in the (B) phase severely narrows the processing window for achieving phase‐pure films. Here, single‐phase growth of 5‐to 50‐nm thick VO2 (B) films on muscovite, mica, by pulsed direct‐current reactive magnetron sputtering at 400 °C is demonstrated. The films are phase‐pure and exhibit a high density of 4.05 g cm−3 and low resistivity of about 50 mΩcm at 30 °C. Increasing the film thickness to 100 nm results in a V2O5‐capped VO2 (B) film with a resistivity of 8000 mΩcm. These results indicate that the stability of the VO2 (B) phase is sensitive to in situ annealing during deposition. These findings should serve as a basis to design processes to exclusively obtain phase‐pure VO2 (B) films.
The VO2 metastable (B) phase is of interest for applications in temperature sensing, bolometry, and Li‐ion batteries, but its single‐phase growth is a challenge. This article shows single‐phase growth by sputter‐deposition of VO2 (B) films on mica, with phase stability sensitive to in situ annealing during deposition, and offers prospects for phase control in VO2 thin films.
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•The nature of etching agent modifies the surface chemistry of Ti3C2Tx MXene.•Combined XRD, Raman and XPS for an in-depth characterization of Ti3C2Tx MXene.•HER is a probe reaction ...for the characterization of Ti3C2Tx surface properties.•Various properties are obtained for Ti3C2Tx depending on the synthesis conditions.
MXenes are a new, and growing, family of 2D materials with very promising properties for a wide variety of applications. Obtained from the etching of MAX phases, numerous properties can be targeted thanks to the chemical richness of the precursors. Herein, we highlight how etching agents govern surface chemistries of Ti3C2Tx, the most widely studied MXene to date. By combining characterization tools such as X-ray diffraction, X-ray photoelectron, Raman and electron energy loss spectroscopies, scanning and transmission electron microscopies and a surface sensitive electrochemical reaction – the hydrogen evolution reaction, HER – we clearly demonstrate that the etching agent (HF, LiF/HCl or FeF3/HCl) strongly modifies the nature of surface terminal groups (F, OH and/or O), oxidation sensitivity, delamination ability, nature of the inserted species, interstratification, concentration of defects and size of flakes. Beyond showing how using these different characterization tools to analyze MXenes, this work highlights that the MXene synthesis routes can influence targeted applications.
Superconductivity at the LaTiO3/SrTiO3 interface is studied by low temperature and high magnetic field measurements as a function of a back-gate voltage. We show that it is intimately related to the ...appearance of a low density (a few 1012 cm−2) of high mobility carriers, in addition to low mobility ones always present in the system. These carriers form superconducting puddles coupled by a metallic two-dimensional electron gas, as revealed by the analysis of the phase transition driven by a perpendicular magnetic field. Two critical fields are evidenced, and a quantitative comparison with a recent theoretical model is made.
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•van der Waals epitaxy (VDWE) of non-layered stress-free VO2 films on mica.•Epitaxy with weak film-substrate bonding and complete interfacial stress relaxation.•Narrow ...domain-size-sensitive electrical-conductivity-temperature hysteresis.•Ab initio studies indicate that VDWE involves surface potassium atoms in the mica.
Realizing stress-free inorganic epitaxial films on weakly bonding substrates is of importance for applications that require film transfer onto surfaces that do not seed epitaxy. Film-substrate bonding is usually weakened by harnessing natural van der Waals layers (e.g., graphene) on substrate surfaces, but this is difficult to achieve in non-layered materials. Here, we demonstrate van der Waals epitaxy of stress-free films of a non-layered material VO2 on mica. The films exhibit out-of-plane 010 texture with three in-plane orientations inherited from the crystallographic domains of the substrate. The lattice parameters are invariant with film thickness, indicating weak film-substrate bonding and complete interfacial stress relaxation. The out-of-plane domain size scales monotonically with film thickness, but the in-plane domain size exhibits a minimum, indicating that the nucleation of large in-plane domains supports subsequent island growth. Complementary ab initio investigations suggest that VO2 nucleation and van der Waals epitaxy involves subtle polarization effects around, and the active participation of, surface potassium atoms on the mica surface. The VO2 films show a narrow domain-size-sensitive electrical-conductivity-temperature hysteresis. These results offer promise for tuning the properties of stress-free van der Waals epitaxial films of non-layered materials such as VO2 through microstructure control.
•Sputtered indium tin oxide films on polyimide substrate are polycrystalline.•Films were strained in biaxial tension with in situ diffraction measurements.•The four indium tin oxide films are found ...to have a slight elastic anisotropy.•Electrical measurements show crack onset strains ranging from 0.15 to 0.3%.•O2 flow during deposition delays crack onset as the temperature decrease does.
Four series of Indium Tin Oxide (ITO) thin films 600 nm thick were deposited on polyimide substrates by Xe ion beam sputtering under different deposition conditions: with or without O2 flow, and at room temperature or 100 °C heated substrates. Both electrical and mechanical properties of the four different films were investigated in situ by electrical resistance measurements and synchrotron x-ray diffraction during equibiaxial deformation tests. The ITO films are found to have a low elastically anisotropy, i.e. an elastic anisotropy index slightly smaller than 1. The elastic regime domain is quite small and is associated to a small effective negative gauge factor. During biaxial straining, electrical measurements show that all films are very brittle with crack onset strains corresponding to applied strains ranging from 0.15 to 0.3%. The introduction of oxygen flow during deposition delays the crack onset as the decrease of deposition temperature does.
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