Two-dimensional superconductor-insulator quantum phase transitions in an electron-doped cuprate

Abstract

We use an ionic liquid-assisted electric-field effect to tune the carrier density in an electron-doped cuprate ultrathin film and cause a two-dimensional superconductor-insulator transition (SIT). The low upper critical field in this system allows us to perform magnetic-field (B)-induced SIT in the liquid-gated superconducting film. Finite-size scaling analysis indicates that SITs induced both by electric and by magnetic fields are quantum phase transitions and the transitions are governed by percolation effects---quantum mechanical in the former and classical in the latter cases. Compared to the hole-doped cuprates, the SITs in the electron-doped system occur at critical sheet resistances $({R}_{c})$ much lower than the pair quantum resistance ${R}_{Q}=h/{(2e)}^{2}=6.45\phantom{\rule{0.16em}{0ex}}\mathrm{k}\mathrm{\ensuremath{\Omega}}$, suggesting the possible existence of fermionic excitations at finite temperatures at the insulating phase near the SITs.