Negative differential resistance state in the free-flux-flow regime of driven vortices in a single crystal of 2H-NbS$_2$

TL;DR

This study investigates the negative differential resistance transition in driven vortices within 2H-NbS$_2$ crystals, revealing how magnetic field and vortex state preparation influence vortex dynamics and stability.

Contribution

It provides new insights into the statistical nature and conditions of NDR transitions in vortex matter, highlighting the role of vortex viscosity drops and dynamical instabilities.

Findings

NDR transition probability increases with magnetic field in weak pinning regimes.

High Ic vortex state is uniquely accessible and not via conventional routes.

NDR occurs in high dissipation regimes with vortex viscosity drops.

Abstract

Time series measurements in 2H-NbS$_2$ crystal had unravelled a drive induced transition wherein the critical current (Ic) changes from a low to a high Ic jammed vortex state, via a negative differential resistance (NDR) transition. Here, using multiple current (I) - voltage (V) measurement cycles, we explore the statistical nature of observing the NDR transition in the free-flux-flow (FF) regime in a single crystal of 2H-NbS$_2$. The probability of observing the NDR transition always remains finite for a vortex state created with either fast or slow rate of magnetic field. 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 B-independent. We show that the higher Ic state is unique and cannot be accessed via any conventional route. While the I-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 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-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 Ic.