Vortex depinning as a nonequilibrium phase transition phenomenon: Scaling of current-voltage curves near the low and the high critical-current states in 2H-NbS2 single crystals

摘要

The vortex depinning phenomenon in single crystals of 2H-NbS2 superconductors is used as a prototype for investigating properties of the nonequilibrium (NEQ) depinning phase transition. The 2H-NbS2 is a unique system as it exhibits two distinct depinning thresholds, viz., a lower critical current I-c(l) and a higher one I-c(h). While I-c(l) is related to depinning of a conventional, static (pinned) vortex state, the state with I-c(h) is achieved via a negative differential resistance (NDR) transition where the velocity abruptly drops. Using a generalized finite-temperature scaling ansatz, we study the scaling of current (I)-voltage (V) curves measured across I-c(l) and I-c(h). Our analysis shows that for I I-c(l), the moving vortex state exhibits Arrhenius-like thermally activated flow behavior. This feature persists up to a current value where an inflexion in the IV curves is encountered. While past measurements have often reported similar inflexion, our analysis shows that the inflexion is a signature of a NEQ phase transformation from a thermally activated moving vortex phase to a free flowing phase. Beyond this inflection in IV, a large vortex velocity flow regime is encountered in the 2H-NbS2 system, wherein the Bardeen-Stephen flux flow limit is crossed. In this regime the NDR transition is encountered, leading to the high I-c(h) state. The IV curves above I-c(h) we show do not obey the generalized finite-temperature scaling ansatz (as obeyed near I-c(l)). Instead, they scale according to the Fisher's scaling form [Fisher, Phys. Rev. B 31, 1396 (1985)] where we show thermal fluctuations do not affect the vortex flow, unlike that found for depinning near I-c(l).