Experimental Evidence for a Collective Insulating State in Two-Dimensional Superconductors

We present the results of an experimental study of the current-voltage characteristics in a strong magnetic field ($B$) of disordered, superconducting, thin films of amorphous indium oxide. As the $B$ strength is increased superconductivity degrades, until a critical field ($B_c$) where the system is forced into an insulating state. We show that the differential conductance measured in the insulating phase vanishes abruptly below a well-defined temperature, resulting in a clear threshold for conduction. Our results indicate that a new collective state emerges in two-dimensional superconductors at high $B$.