Controlled two-dimensional ground states of a superconducting oxide interface [electronic resource]
Keywords: Applied Physics.

Quantum ground states which arise at atomically controlled oxide interfaces provide an opportunity to address key questions in condensed matter physics, including the nature of two-dimensional (2D) metallic behaviour adjacent to superconductivity. This quantum metal behavior has been observed in a wide range of two-dimensional superconductors, suggesting a quantum phase transition. In the 2D superconductor at the LaAlO3/SrTiO3 interface, a metallic ground state emerges upon the collapse of superconductivity with field-effect back gating. Strikingly, such metallicity is accompanied with a pseudogap, in analogy to the cuprates. Here, I utilize independent control of carrier density and disorder of the interfacial superconductor using dual electrostatic gates, which is demonstrated in Chapter 3. This enables the comprehensive examination of the electronic phase diagram approaching zero temperature. In Chapter 4, I show that that the pseudogap corresponds to precursor pairing, and the onset of long-range phase coherence forms the superconducting dome. The gate-tuned superconductor-metal transition (SMT) approaching zero temperature is driven by macroscopic phase fluctuations of Josephson coupled superconducting puddles. In Chapter 5, I further identify a phenomenological switching from zero to positive linear magnetoresistance at the gate-, and magnetic field-tuned SMT approaching zero temperature. Temperature-tuned SMT shares qualitatively same phenomenon. By spanning the gate, field, and temperature axes, I construct a phase diagram crossing from the superconductor, the anomalous metal, the vortex liquid, to the Drude metal states, based on combined longitudinal and Hall resistivity measurements. These results demonstrate an exemplary system for SMT and suggest the need for a unifying picture for transitions tuned by gate, field, and temperature. In Chapter 6, I analyze a dual gate device with much lower disorder and realize a sharper transition from the Drude state to the superconducting states via a top gate. By decreasing back gate, the sample abruptly transit from the superconducting state to the high-resistivity state with pronounced positive magnetoresistance indicating signature of Cooper pairs. Therefore, four distinct states are observed approaching zero temperature with dual-gate electrostatic tuning: 1) superconducting state, 2) Drude metal state, 3) insulating state dominated by Cooper pairs, 4) insulating state dominated by electrons. Interestingly, the abrupt transition via back gate is nearly independent of the top gate voltage. By Poisson-tight binding calculations incorporating spin-orbit coupling, I find that the observed independence is consistent with a Lifshitz transition picture, which suggests the dominant superconducting subband to be the lowest lying subbands. In Chapter 7, I switch topics and demonstrate the synthesis of freestanding single-crystal YBa2Cu3O7-x (YBCO) thin film. On SrTiO3 substrate, a heteroepitaxial water-soluble sacrificial layer Sr3Al2O6 followed by the YBCO film is grown by pulsed laser deposition. By thorough water etching of the sacrificial layer, we obtain millimeter-size freestanding YBCO thin films with thickness range from 10 nm to 150 nm. The high quality of the freestanding YBCO thin films is subsequently shown with four-probe transport and X-ray diffraction measurement.