High-field termination of a Cooper-pair insulator

Abstract

We conducted a systematic study of the disorder dependence of the termination of superconductivity, at high magnetic fields $(B)$, of amorphous indium oxide films. Our lower disorder films show conventional behavior where superconductivity is terminated with a transition to a metallic state at a well-defined critical field, ${B}_{\mathrm{c}2}$. Our higher-disorder samples undergo a $B$-induced transition into a strongly insulating state, which terminates at higher $B$'s forming an insulating peak. We demonstrate that the $B$ terminating this peak coincides with ${B}_{\mathrm{c}2}$ of the lower disorder samples. Additionally, we show that, beyond this field, these samples enter a different insulating state in which the magnetic field dependence of the resistance is weak. These results provide crucial evidence for the importance of Cooper-pairing in the insulating peak regime.