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Guilhem Saiz : Multigap superconductivity at oxide interfaces
The LaAlO3/SrTiO3 (LAO/STO) interface is known to host a superconducting two-dimensional electron gas (2DEG). It has been a playground to study functional oxide materials in recent years since it harbors various physical phenomena, such as a strong Rashba spin-orbit coupling, superconductivity, multi-band conductivity, and disorder among others. The carrier density of the 2DEG at SrTiO3 interfaces can be controlled by electric field effect, which in turn tunes other properties such as the confinement potential, the Rashba spin-orbit coupling, and most famously, the superconducting critical temperature forming a dome upon doping. In this manuscript, we discuss the gate tunable transport properties of 2DEG formed at the interfaces in heterostructures involving the quantum paraelectric materials SrTiO3 or KTaO3, in different crystal orientations. Transport measurements at dc and radiofrequency have been performed in a dilution fridge down to 20mK under magnetic field and back gate voltage. In the first chapter, we report gate-tunable transport measurements such as the superconducting Tc, the upper critical magnetic field, and the carrier density of LAO/STO(110) and LAO/STO(111) interfaces. Then we describe resonant micro-wave measurements of the superfluid stiffness, giving us access to the fundamental superconducting properties of these 2DEGs. In LAO/STO(110), both the gate-tunable critical fields and the superfluid stiffness measurements show the existence of two-condensate superconductivity beyond a doping threshold which can be consistently analyzed by two independent models. We also show that in this system, the two condensates interact repulsively to form an exotic s±-wave superconducting state and that interband scattering strongly suppresses the Tc. Similar measurements of the critical field and superfluid stiffness in the LAO/STO(111) interfaces yielded contrasting results. Despite evidence for multiband transport in the normal state, no clear signature of multi-condensate superconductivity could be observed. In both orientations, the superfluid stiffness measurements are compared with predictions from the conventional BCS/Mattis-Bardeen theory. We conclude this part on the role of the substrate orientation on the band structure and the electron gas superconducting properties by comparing our results to the well-known properties of the conventional LAO/STO(001) interface. In a second chapter, we analyse the non-reciprocal transport of the in-plane anisotropic magnetoresistance of these 2DEGs (LAO/STO(110) and LAO/STO(111)) as a probe for spin-orbit interaction and as a way to qualitatively assess the Fermi surface evolution with back gate voltage. The quadratic and bilinear magnetoresistance derived from this non-reciprocal transport are compared to previous work at LAO/STO(001) interface, and an effective Rashba coupling constant is extracted from these measurements. We also show that the in-plane anisotropic magnetoresistance can be used as a tool to observe a Lifschitz transition in LAO/STO(110). In the last chapter, we discuss the properties of novel superconducting oxide interfaces. We first report gate tunable superconductivity at the AlOx/STO interface, which is a promising 2DEG substitute for future applications since its sputtering fabrication method is easier than the pulsed laser deposition of the LAO layer. Then, we present results on the recently discovered superconducting 2DEG at KTaO3(110) and KTaO3(111) oxide interface. Transport properties and superfluid stiffness measurement are studied in light of the Berezinskii Kosterlitz Thouless theory.