Observation of quantum dissipation in the vortex state of a highly disordered superconducting thin film

We present transport measurements on a highly disordered, amorphous thin film superconductor that shows a crossover from activated to temperature independent resistance at low temperatures in the presence of a magnetic field. We interpret the data in terms of quantum tunneling of vortices, and find that within this interpretation the tunneling objects are not single vortices but rather dislocations and antidislocations of the vortex lattice, which nucleate quantum mechanically as unbound pairs. We note, however, that other interpretations of the data may be possible.