Superconducting fluctuation induced conductance corrections near a pair-breaking quantum phase transition in doubly connected ultrathin cylinders of Al

We report measurements on ultrathin, doubly connected superconducting cylinders of Al that exhibit a destructive regime, which refers to the loss of superconductivity in a doubly connected superconductor near applied half flux quanta due to the sample topology and the small size of the sample. A depairing quantum phase transition (QPT) between a superconducting and metallic state tuned by the magnetic flux enclosed in the quasi one-dimensional (1D) cylinder was found at the onset of the destructive regime. Results on magnetic flux and temperature dependent sample resistance as well as current-voltage characteristics revealed the presence of both thermally activated and quantum phase slips near the depairing QPT. On the superconducting side of the QPT, thermally activated phase slips as described by the Langer-Ambegaokar and McCumber-Halperin (LAMH) theory were found to describe the sample resistance as the system was pushed towards the QPT by a magnetic field applied along the cylinder axis. However, deviation from this behavior was found at low temperatures, signaling the presence of the quantum phase slips. Most importantly, we observed a highly unusual negative slope in the resistance versus temperature curves on the metallic side of the QPT as predicted by the diagrammatic calculation of the dc conductivities in a 1D system near a depairing QPT. Our work suggests that fluctuations from both the phase and the amplitude of the superconducting order parameter are important for the superconductor-to-metal depairing QPT.