Tripling of the Superconducting Critical Current Density in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Retained After Pressure Release

TL;DR

This study demonstrates that applying and releasing pressure on BaFe$_2$(As$_{1-x}$P$_x$)$_2$ superconductors significantly enhances their critical current density, with effects persisting after pressure release, offering a new method for performance improvement.

Contribution

We show that pressure cycling can irreversibly increase the critical current density in an iron-based superconductor, providing a novel approach to enhance superconducting performance.

Findings

Critical current density $J_c$ increases threefold after pressure cycles.

Pressure reduces vortex motion and alters pinning mechanisms.

Enhanced $J_c$ persists after pressure release, indicating irreversible effects.

Abstract

Superconducting performance is tunable not only via chemical modification or defect engineering, but also through external parameters such as pressure, though this method remains less readily accessible. In this work, we study how compression influences vortex dynamics and critical currents in an iron-based superconductor. Specifically, we perform magnetization measurements using an off-the-shelf pressure cell to investigate the effects of hydrostatic pressures up to 1.08 GPa on the magnetic properties of BaFe$_2$(As$_{0.62}$P$_{0.38}$)$_2$ crystals across a range of temperatures $T$ and magnetic fields $H$. Although these pressures minimally affect the superconducting critical temperature, they produce a clear increase in the critical current density $J_c(T,H)$, a pronounced reduction in the rate of thermally activated vortex motion $S(T,H)$, and can change the dominant vortex pinning mechanism. Furthermore, the effects of pressure are irreversible: after pressurization and subsequent release at room temperature, the crystals retain their enhanced critical current densities. The second magnetization peak vanishes at 22 K after the pressure cycle, which we attribute to a transition from predominantly $\delta \kappa$ pinning to a mixed mechanism of $\delta T_c$ and surface pinning. Lastly, a threefold increase in $J_c$ and more than 40\% reduction in $S$ at 8~K and 0.5~T was achieved after $1-2$ pressure cycles. These findings demonstrate the potential utility of pressure cycling for improving $J_c$, which may offer a simpler alternative to approaches such as chemical doping or the introduction of artificial pinning centers.