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

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 sub(c)) much lower than the pair quantum resistance R sub(Q) = h/(2e) super(2) = 6.45 k[Omega], suggesting the possible existence of fermionic excitations at finite temperatures at the insulating phase near the SITs.