Unusual vortex dynamics and phase transitions in mesoscopic superconducting islands

Granular mesoscopic niobium (Nb) islands provide a stage for strong confinement and pinning effects for vortex dynamics. We performed magnetotransport measurements on single Nb islands with varying diameters. We observed non-periodic oscillations in magnetoresistance along the superconducting transition which suggest non-trivial vortex configurations. Furthermore, we found strong indications of a quantum phase transition as a function of diameter. At large diameters (above 2 um) we observed an apparent crossing point in the magnetoresistance at the critical field (B$_{c}$) coupled with a magnetoresistance peak when the field is increased further. These signatures suggest a superconductor-to-metal transition and a quantum critical point at B$_{c}$, at resistances far below the quantum of resistance R$_{Q}$.