Negative differential resistance state in the free-flux-flow regime of driven vortices in a single crystal of 2H−NbS2

Abstract

Time series measurements [G. Shaw et al., Phys. Rev. B 85, 174517 (2012); B. Bag et al., Sci. Rep. 7, 5531 (2017)] in $2H\text{\ensuremath{-}}{\mathrm{NbS}}_{2}$ crystal had unraveled a drive-induced transition wherein the critical current $({I}_{c})$ changes from a low to a high ${I}_{c}$ jammed vortex state, via a negative differential resistance (NDR) transition. Here, using multiple current-voltage $(I\text{\ensuremath{-}}V)$ measurement cycles, we explore the statistical nature of observing the NDR (or a quasi-NDR in reversing $I$ measurements) transition in the free-flux-flow (FF) regime in a single crystal of $2H\text{\ensuremath{-}}{\mathrm{NbS}}_{2}$. Prior to the occurrence of the NDR transition, the pristine full $I\text{\ensuremath{-}}V$ curve exhibits a featureless smooth depinning from the low ${I}_{c}$ state. With subsequent current cycling, the NDR transition appears in the $I\text{\ensuremath{-}}V$ curve. Post-NDR, the full $I\text{\ensuremath{-}}V$ curve is seen to be noisy with depinning commencing from the higher ${I}_{c}$ state. The probability of observing the NDR transition always remains finite for a vortex state created with either fast or slow rate of magnetic field, $\stackrel{\ifmmode \dot{}\else \.{}\fi{}}{B}$. The probability of observing the NDR transition in the FF regime is found to systematically increase with magnetic field $(B)$ in weak collective pinning regime. In the strong pinning regime, the said probability becomes field independent. Retaining of a nonzero probability for the occurrence of the NDR transition under all conditions, the observed new data shows that the $I\text{\ensuremath{-}}V$ branch with higher ${I}_{c}$ is the more stable compared to the lower ${I}_{c}$ branch. We show that the higher ${I}_{c}$ state, generated via the NDR transition, is unique and cannot be accessed via any conventional route, in particular, by preparing the static vortex state with a different thermomagnetic history. While the $I\text{\ensuremath{-}}V$ curves do not distinguish between zero field cooled (ZFC) and field cooled (FC) modes of preparing the vortex state, the probability for observing an NDR transition has different $B$ dependences for the vortex matter prepared in the ZFC and FC modes. We find that the NDR transition occurs in a high dissipation regime, where the flow resistivity is well above the theoretical value expected in the FF regime. We understand our results on the basis of a rapid drop in vortex viscosity at high drives in $2H\text{\ensuremath{-}}{\mathrm{NbS}}_{2}$, which triggers a rapid increase in the vortex velocity and reorganization in the moving vortex matter leading to a dynamical unstable vortex flow. This dynamical instability leads to the NDR transition into a high entropy vortex state with high ${I}_{c}$.