Abstracts Category : Other

DC and AC transport in field-effect controlled LaAlO3/SrTiO3 interface : Transport DC et AC l'interface LaAlO3/SrTiO3 contrle par effet de champ

by Alexis Jouan

Cette thse est consacre l'tude des proprits de transport statique et dynamique du gaz d'lectrons bidimensionnel supraconducteur l'interface LaAlO3/SrTiO3. Dans un premier temps, nous tudions l'effet du dsordre microscopique induit par le dopage en Chrome, sur la supraconductivit et le couplage spin-orbite en fonction de la densit de porteur module par effet de champ. Dans une gomtrie de grille locale au-dessus du gaz, nous montrons le contrle lectrostatique de la transition supraconducteur-isolant. De mme, nous analysons l'ajustement du couplage spin-orbite contrl par effet de champ. A l'aide de mthodes de nanofabrication par lithographie lectronique, nous dmontrons la premire ralisation d'un point critique quantique dans LaAlO3/SrTiO3. En changeant le confinement latral et le niveau de Fermi par effet de champ, nous sommes capables de rgler le nombre de canaux conducteurs dans l'tat normal et de mesurer la quantification de la conductance. Enfin, nous prsentons des mesures radio-frquence qui donnent accs aux proprits dynamiques du gaz supraconducteur. L'volution de la conductivit en fonction de la densit de porteurs et de la temprature est compare avec la thorie standard BCS/Mattis-Bardeen d'une part, et avec la thorie BKT d'autre part. This thesis is devoted to the study of static and dynamical transport properties of the superconducting two-dimensional electron gas at the LaAlO3/SrTiO3 interface. Under strong 2D confinement, the degeneracy of the t2g bands of SrTiO3 is lifted at the interface, generating a rich and complex band structure. Starting from a free electron model, we derive numerically a self-consistent calculation of the potential well and the band structure (chapter 1). These simulations highlight the presence of two types of bands dxy and dxz/yz with very different transport properties. We investigate first the effect of microscopic disorder introduced by Cr doping, on superconductivity and spin-orbit coupling over a wide range of back-gate doping (chapter 3). We also describe the first implementation of a field-effect device where the superconductor-insulator transition could be continuously tuned with a top-gate. The presence of a strong spin-orbit coupling that could be controlled with the top-gate voltage is also demonstrated by analyzing the magneto-transport measurements. The gate dependence of the spin-splitting energy, of the order of a few meV, is found to be consistent with Rashba spin-orbit coupling. Going one step further in nanofabrication, we report on the first realization of a quantum point contact in LaAlO3/SrTiO3 using split gates (chapter 6). To go further in the understanding of the LaAlO3/SrTiO3 interface, we present high frequency measurements of the conductivity (chapter 5). This measurement gives us access to the superfluid stiffness and to the gap energy via the BCS theory. We show that the competition between these two energy scales controls the superconducting Tc in the phase diagram.Advisors/Committee Members: Lesueur, Jrme (thesis director), Bergeal, Nicolas (thesis director).