Two-Dimensional Phase-Fluctuating Superconductivity in Bulk-Crystalline NdO${}_{0.5}$F${}_{0.5}$BiS${}_2$

Abstract

We present a combined growth and transport study of superconducting single-crystalline NdO${}_{0.5}$F${}_{0.5}$BiS${}_2$. Evidence of two-dimensional superconductivity with significant phase fluctuations of preformed Cooper pairs preceding the superconducting transition is reported. This result is based on three key observations. (1) The resistive superconducting transition temperature $T_c$ (defined by resistivity $\ddot{I}\to 0$) increases with increasing disorder. (2) As $T\to T_c$, the conductivity diverges significantly faster than what is expected from Gaussian fluctuations in two and three dimensions. (3) Non-Ohmic resistance behavior is observed in the superconducting state. Altogether, our observations are consistent with a temperature regime of phase-fluctuating superconductivity. The crystal structure with magnetic ordering tendencies in the NdO${}_{0.5}$F${}_{0.5}$ layers and (super)conductivity in the BiS${}_2$ layers is likely responsible for the two-dimensional phase fluctuations. As such, NdO${}_{0.5}$F${}_{0.5}$BiS${}_2$ falls into the class of unconventional ``laminar" bulk superconductors that include cuprate materials and 4Hb-TaS${}_2$.