Magnetotransport experiments at the LaAlO₃/SrTiO₃ interface

Cette thèse étudie la conduction électrique sous champ magnétique de l'interface entre un substrat de SrTiO3 et une mince couche cristalline de LaAlO3. Ce système combine les propriétés remarquables des gaz bidimensionnels et la richesse de la physique des oxydes complexes. Grâce à nos mesures, nous révélons le caractère multi-bande de ce système et mettons à jour une transition, induite par l'effet de champ, entre un régime dominé par des corrections quantiques et un régime semi-classique. Lorsque le peuplement des nouvelles sous-bandes est faible, nous montrons qu'il existe une sous-bande pour laquelle le couplage spin-orbite Rashba est si important que sa densité d'états est modifiée quand l'orientation du champ magnétique est variée. Nous mettons également à jour le comportement complexe des oscillations quantiques, lequel peut être relié au rôle primordial de l'interaction spin-orbite dans ce système. Pour finir, nous révélons une évolution subtile des oscillations quantiques avec l'effet de champ

Electron confinement at the LaAlO₃/SrTiO₃ interface

Physical and structural phenomena originating from polar discontinuities have generated enormous activity. In the last ten years, the oxide interface between polar LaAlO3 and non-polar SrTiO3, both band insulators, has attracted particular interest, as it hosts an electron liquid with remarkable properties: it superconducts, has a sizeable spin–orbit interaction and its properties are tunable by an electric field. The profile of the carrier density at the interface and the exact band structure are properties strongly linked and still objects of debate. Here we review the experimental findings on the origin and the extension of the electron liquid and discuss the theoretical models developed to describe the charge profile and the band structure. We also introduce a model to account for the effect of interface disorder which could modify the charge distribution

Ionic liquid gating of ultra-thin YBa₂Cu₃O_7−x films

In this paper, we present a detailed investigation of the self-field transport properties of an ionic liquid gated ultra-thin YBa2Cu3O7−x (YBCO) film. From the high temperature dynamic of the resistivity (>220 K), different scenarios pertaining to the interaction between the liquid and the thin film are proposed. From the low temperature evolution of J c and T c, a comparison between the behavior of our system and the standard properties of YBCO is drawn

Rashba induced magnetoconductance oscillations in the LaAlO₃-SrTiO₃ heterostructure

We report measurements of the normal state in-plane magnetoconductance in gated LaAlO$_3$-SrTiO$_3$ samples. As the orientation of the magnetic field changes within the plane of the interface, the signal displays periodic oscillations with respect to the angle between the field and the direction of the current. We show that in the underdoped to optimally doped range, a Fermi surface reconstruction takes place due to the Rashba spin-orbit term and that the oscillations are due to a magnetic field induced opening and closing of a gap at the $\Gamma$ point for those Ti out-of-plane orbitals having their band minimum close to the Fermi energy of the system.Comment: 5 pages, 4 figure

Systematic Study of the Contact Resistance Between REBCO Tapes: Pressure Dependence in the Case of No-Insulation, Metal Co-Winding and Metal-Insulation

No-insulation and partial-insulation are novel approaches for compact, high field rare-earth-Ba2Cu3O7-x-coated conductor magnets. They maximize the current density and overcome some of the coated conductors' intrinsic challenges such as the low normal zone propagation velocity and the resulting difficulties to quench REBCO magnets safely. However, this comes at a reduction of the field quality and an increase of the charging time. The performance of this type of coils is mainly determined by the contact resistance between the different layers of the winding. In the case of partially-insulated coils, the contact resistance can be adjusted by a tailored choice of the material used for the metal co-winding or for the metallic coating of the REBCO tape. We present here a systematic study of the impact of pressure on the contact resistance in the cases of direct contact (no-insulation) as well as metal co-winding and metal coating (partial-insulation). Different “insulation” metals as well as various “insulation” to REBCO tape ratios are tested at 77 K in self-field varying the contact pressure in the range of 10–200 MPa. The goal is to provide coil manufacturers with the means of achieving a controlled contact resistance to optimize the coil performance

Characterization of atomic force microscopy written conducting nanowires at LaAlO₃/SrTiO₃ interfaces

The realization of conducting nanostructures at the interface between LaAlO3 and SrTiO3 is an important step towards the realization of devices and the investigation of exotic physical regimes. We present here a detailed study of the conducting nanowires realized using the atomic force microscopy writing technique. By comparing experiments with numerical simulations, we show that these wires reproduce the ideal case of nanoconducting channels defined in an insulating background very well and that the tip bias is a powerful knob to modulate the size of these structures. We also discuss the role of the air humidity that is found to be a crucial parameter to set the size of the tip-sample effective interaction area

Influence of the growth conditions on the LaAlO₃/SrTiO₃ interface electronic properties

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Weak localization and spin-orbit interaction in side-gate field effect devices at the LaAlO₃/SrTiO₃ interface

Using field effect devices with side gates, we modulate the 2-dimensional electron gas hosted at the LaAlO3/SrTiO3 interface to study the temperature and doping evolution of the magnetotransport. The analysis of the data reveals different transport regimes depending on the interplay between the different (elastic, inelastic, and spin-orbit) scattering times and their temperature dependencies. We find that the spin-orbit interaction strongly affects the low-temperature transport in the normal state in a very large region of the phase diagram, extending beyond the superconducting dome

Rapid Synthesis of MgB₂ by Inductive Heating

The effect of a rapid internal magnesium diffusion synthesis on the electrical, magnetic, and microstructural properties of binary MgB2 bulk samples has been investigated. We have used a laboratory-made induction furnace to reduce the reaction time, typically of many hours, to few minutes. We report a comparison between two samples reacted at the same temperature with the induction furnace and with a commercial resistance oven. We have observed that the critical temperature and the width of the superconducting transition are not affected by the heat treatment. The induction heating leads to an increase of the critical current density at high fields, correlated with an enhancement of the upper critical field. This paper exposes the potentiality of the proposed fast process, which allows tuning the parameters of the heat treatment in ranges that would not be accessible with traditional ovens. The technique can be scaled for the preparation of wires and powders