Probing the Difference Between Amorphous and Granular Superconducting Nanowires in Transport Measurements

Superconducting non-granular quasi-one-dimensional (1D) NbN nanowires and relatively wide granular wires of the same material exhibit similar magneto-transport behavior arising from different physical origin. Both types of wires exhibit a broad transition into the superconducting state with non-vanishing resistance well below T_c, and negative magnetoresistance (nMR) decreasing in magnitude with temperature. A distinct behavior between the two wires is revealed in their response to increasing current. In V-I measurements, the 1D wires exhibit finite initial slope, i.e., zero critical current, at all temperatures below the transition, while the granular wires exhibit a nonzero critical current that depends on temperature. Also, the two wires differ from each other in the current dependence of the nMR. In the 1D wires, at low temperature, the nMR decreases monotonically with the current, while in the granular wires the nMR initially increases with the current. The different current response of the two types of wires indicates the different physical origin of their behavior: That of the 1D wires is attributed to fluctuations of the order parameter, while that of the granular wires reflects the response of an inhomogeneous chain of Josephson junctions.