Superconductivity in a two dimensional 5d electron gas

Abstract

This thesis is an experimental investigation of the superconducting state of the two dimensional electron gas formed at the interface between amorphous LaAlO3 and (111) KTaO3. The superconducting phase diagram of (111) KTaO3 is explored by means of electrical transport measurements both above and far below the resistive transition applying small magnetic fields as well as some of the highest available on the planet.

In Chapter 1 we give a brief introduction to the physics of interfacial oxide superconductors.
In Chapter 2 we investigate the extreme resilience of the superconducting state to planar magnetic fields and find the highest critical fields ever reported at oxide interfaces. By considering paramagnetic and orbital pair breaking on equal footing, we find that the magnitude of the critical fields is enabled by the cooperation of an atomically thin superconducting layer and strong spin-orbit scattering.

In Chapter 3 we study the quantum corrections to conductivity that affect the resistive transitions at (111) LaAlO3/KTaO3 interfaces. We find that, thanks to electron-phonon scattering, electrons experience significant decoherence which enables sizeable conductivity contributions from quasiparticle-interference and Maki-Thompson superconducting fluctuations.

Finally in Chapter 4 we discover that the very low temperature switching behavior of our samples are compatible with the spontaneous formation of a Josephson junction array at this interface. Investigating the resistive switching in time-reversal-symmetric conditions we find that the superconducting patches can couple through non-harmonic current-phase relations and sizeable capacitive contributions. This results in a stochastic distribution of switching currents. In the presence of an externally applied magnetic field, we observe non reciprocal responses that affect both the average and width of the switching currents distributions. We propose a simple model in which phase frustration and a non-harmonic CPR can rectify thermally activated fluctuations in the array.