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 $\left(R_c\right)$ much lower than the pair quantum resistance $R_Q=h/{\left(2e\right)}^2=6.45\mathrm{k}\mathrm{\Omega }$, suggesting the possible existence of fermionic excitations at finite temperatures at the insulating phase near the SITs.

• Received 21 May 2014
• Revised 3 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.020503