Two-dimensional phase-fluctuating superconductivity in bulk crystalline NdO0.5F0.5BiS2

We present a combined growth and transport study of superconducting single-crystalline NdO0.5F0.5BiS₂. 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 Tc (defined by resistivity ρ→0) increases with increasing disorder. (2) As T→Tc, 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 NdO0.5F0.5 layers and (super)conductivity in the BiS₂ layers is likely responsible for the two-dimensional phase fluctuations. As such, NdO0.5F0.5BiS₂ falls into the class of unconventional ``laminar'' bulk superconductors that include cuprate materials and 4Hb-TaS₂.