Critical current density, vortex dynamics, and phase diagram of FeSe single crystal

TL;DR

This study investigates vortex pinning, dynamics, and the phase diagram of high-quality FeSe single crystals, revealing isotropic critical current density, collective creep behavior, and dominant strong point-like pinning mechanisms.

Contribution

It provides the first comprehensive analysis of vortex behavior and phase diagram in pure FeSe crystals, highlighting the nature of vortex pinning and creep.

Findings

Critical current density is nearly isotropic at ~3×10^4 A/cm^2 at 2 K.

Vortex creep shows a temperature-insensitive plateau, consistent with collective creep theory.

Vortex motion is dominated by sparse strong point-like pinning from nm-sized defects.

Abstract

We present a comprehensive study of the vortex pinning and dynamics in a high-quality FeSe single crystal, which is free from doping introduced inhomogeneities and charged quasi-particle-scattering because of its innate superconductivity. Critical current density, $J_c$, is found to be almost isotropic, and reaches a value $\sim$ 3 $\times$ 10$^4$ A/cm$^2$ at 2 K (self-field) for both $H$ $\|$ $c$ and $ab$. The normalized magnetic relaxation rate $S$ (= $\mid$dln$M$/dln$t$$\mid$) shows a temperature insensitive plateau behavior in the intermediate temperature range with a relatively high creep rate ($S$ $\sim$ 0.02 under zero field), which is interpreted in the framework of the collective creep theory. A crossover from the elastic to plastic creep is observed, while the fish-tail effect is absent for both $H$ $\|$ $c$ and $ab$. Based on this observation, the origin of the fish-tail effect is also discussed. Combining the results of $J_c$ and $S$, vortex motion in FeSe single crystal is found to be dominated by sparse strong point-like pinning from nm-sized defects or imperfections. The weak collective pinning is also observed and proved in the form of large bundles. Besides, the vortex phase diagram of FeSe is also constructed and discussed.