Superconductor to weak-insulator transitions in disordered tantalum nitride films

We study the two-dimensional superconductor-insulator transition (SIT) in thin films of tantalum nitride. At zero magnetic field, films can be disorder-tuned across the SIT by adjusting thickness and film stoichiometry; insulating films exhibit classical hopping transport. Superconducting films exhibit a magnetic-field-tuned SIT, whose insulating ground state at high field appears to be a quantum-corrected metal. Scaling behavior at the field-tuned SIT shows classical percolation critical exponents $z\nu \approx 1.3$, with a corresponding critical field $H_c\ll H_{c2}$, the upper critical field. The Hall effect exhibits a crossing point near $H_c$, but with a nonuniversal critical value ${\rho }_{xy}^c$ comparable to the normal-state Hall resistivity. We propose that high-carrier-density metals will always exhibit this pattern of behavior at the boundary between superconducting and (trivially) insulating ground states.