PhD Defense: Guenevere Prawiroatmodjo

Title: Attractive electron-electron interactions at the LaAlO3/SrTiO3 Interface
Subtitle: Towards nanoscale control of hybrid complex oxide devices
The conducting interface between the two insulating oxides LaAlO3 and SrTiO3 (LAO/STO) exhibits many intriguing properties such as high mobility, a gate-tunable superconducting phase, ferroelectricity and ferromagnetism. In this thesis, devices are fabricated at the LAO/STO interface using novel hard-mask patterning techniques with micrometer precision. The superconducting phase diagram is explored in terms of critical magnetic field Bc, temperature Tc and current Ic, and studied for varying carrier densities. From a finite shift between the superconducting Tc and Ic domes, evidence is found for a superconducting phase based on Josephson tunneling of supercurrent, possibly through a network of weak links. By patterning electrostatic gates in a split-gate geometry, a quantum dot is fabricated for which the dot occupation, tunnel coupling to the leads and size can be simultaneous controlled. A paired ground state is found, and transport characteristics are described to originate from attractive electron-electron interactions that result in a negative effective charging energy U. Further, the excitation spectrum is explored and compared to calculations based on a single-orbital Anderson model with negative U. We find that for strong tunnel coupling, the negative U gives rise to the charge Kondo effect, where an effective ’charge flip’ resonance at the pair tunneling resonance enhances conductivity at low temperature and magnetic field.