Quantum superconductor-insulator transition in titanium monoxide thin films with a wide range of oxygen contents

The superconductor-insulator transition (SIT), one of the most fascinating quantum phase transitions, is closely related to the competition between superconductivity and carrier localization in disordered thin films. Here, superconducting TiOx films with different oxygen contents were grown on Al2O3 substrates by a pulsed laser deposition technique. The increasing oxygen content leads to an increase of disorder, a reduction of carrier density, an enhancement of carrier localization, and therefore a decrease of superconducting transition temperature. A fascinating SIT emerges in cubic TiOx films with increasing oxygen content and its critical sheet resistance is close to the quantum resistance h/(2e)(2) similar to 6.45 k Omega. The scaling analyses of magnetic field-tuned SITs show that the critical exponent products z nu increase from 1.02 to 1.31 with increasing disorder. Based on the results, the SIT can be described by the "dirty boson" model, and a schematic phase diagram for TiOx films was constructed.