  Recently emerging two-dimensional (2D) superconductors in atomically thin
layers and at heterogeneous interfaces are attracting growing interest in
condensed matter physics. Here, we report that ion-gated ZrNCl surface,
exhibiting a dome-shaped phase diagram with a maximum critical temperature of
14.8 kelvin, behaves as a superconductor persisting to the 2D limit. The
superconducting thickness estimated from the upper critical fields is ~ 1.8
nanometers, which is thinner than one-unit-cell. The majority of the vortex
phase diagram down to 2 kelvin is occupied by a metallic state with a finite
resistance, owing to the quantum creep of vortices caused by extremely weak
pinning and diminishing disorder. Our findings highlight the potential of
electric-field-induced superconductivity, establishing a new platform for
accessing quantum phases in clean 2D superconductors.
