Metallic ground state in an ion-gated two-dimensional superconductor

Ever since a discovery of insulator-superconductor transition in metallic thin films, the ground state of clean two-dimensional (2D) superconductors has been a long standing fundamental question. Recently emerging electric double layer transistors enabled researchers to realize electric-field-induced superconductivity in various substances, which provides us with new opportunities to investigate the ground state of 2D superconductivity. In this presentation, we report that the majority of the field-temperature phase diagram of electric-field-induced superconductivity in ZrNCl including T = 0 K is occupied by a metallic state with finite resistance due to the quantum tunneling of isolated vortices and flux flow caused by extreme two-dimensionality and weak pinning potential. These results imply that electric-field-induced superconductivity can be an ideal platform for accessing quantum vortex states in clean 2D superconductors.