Evidence for enhanced phase fluctuations in nanostructured niobium thin films*

In a superconducting nanostructure, phase fluctuations are prominent and give rise to finite resistance below superconducting transition temperature T_c. By using a monolayer polymer/nanosphere hybrid we developed previously, we fabricated a large array of interconnected niobium (Nb) honeycomb lattices with the thinnest interconnected linewidth d ranging from 36 nm to 89 nm. The honeycomb cells form a highly ordered triangular lattice with more than 10⁸ unit cells extending over few mm² area, which enables the detailed transport study at nanometer scales. We found T_c gradually drops with decreasing d due to the phase-slip effect, while the critical field at lower temperature tends to follow that of a continuous Nb thin film. One likely scenario is to consider a model system of numerous superconducting islands interconnected by short phase-slip junctions, where the phase coherence is dictated by the phase slippage in the nanoconstriction. (PHYSICAL REVIEW B 96, 020506(R) (2017))