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

Abstract

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 $\mathrm{Ti}{\mathrm{O}}_{x}$ films with different oxygen contents were grown on $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ 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 $\mathrm{Ti}{\mathrm{O}}_{x}$ films with increasing oxygen content and its critical sheet resistance is close to the quantum resistance $h/{(2e)}^{2}\ensuremath{\sim}6.45\phantom{\rule{0.16em}{0ex}}\mathrm{k}\mathrm{\ensuremath{\Omega}}$. The scaling analyses of magnetic field--tuned SITs show that the critical exponent products z\ensuremath{\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 $\mathrm{Ti}{\mathrm{O}}_{x}$ films was constructed.