Quantum criticality tuned by magnetic field in optimally electron-doped cuprate thin films

Antiferromagnetic (AF) spin fluctuations are commonly believed to play a key role in electron pairing of cuprate superconductors. In electron-doped cuprates, a paradox still exists about the interplay among different electronic states in quantum perturbations, especially between superconducting and magnetic states. Here, we report a systematic transport study of cation-optimized ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta}}$ $(x=0.10)$ thin films in high magnetic fields. We find an AF quantum phase transition near 60 T, where the Hall number jumps from ${n}_{\mathrm{H}}=\ensuremath{-}x$ to ${n}_{\mathrm{H}}=1\ensuremath{-}x$, resembling the change in ${n}_{\mathrm{H}}$ at the AF boundary $({x}_{\mathrm{AF}}=0.14)$ tuned by Ce doping. In the AF region a spin-dependent state manifesting anomalous positive magnetoresistance is observed, which is closely related to superconductivity. Once the AF state is suppressed by magnetic field, a polarized ferromagnetic state is predicted, reminiscent of the recently reported ferromagnetic state at the quantum end point of the superconducting dome by Ce doping. The magnetic field that drives phase transitions in a manner similar to but distinct from doping thereby provides a unique perspective to understand the quantum criticality of electron-doped cuprates.