Increase of critical current density in FeSe superconductor by strain effect

TL;DR

This work demonstrates that in-plane compressive strain, induced by the anisotropic contraction of a glass-fiber-reinforced plastic substrate, can substantially enhance the critical current density $J_c$ in FeSe without altering its superconducting transition temperature $T_c$. Using magnetization measurements and the Bean model, the authors report a fourfold increase in $J_c$ at zero field (from ~$2.3\times10^{4}$ to ~$8.7\times10^{4}$ A cm$^{-2}$ at 2 K) and an order-of-magnitude improvement at 5 T. Dew-Hughes analysis reveals a pinning mechanism shift from normal point pinning in unstrained FeSe to a coexistence of normal point and surface pinning under strain, indicating stronger and more versatile vortex pinning. This strain-engineering approach offers a practical, non-destructive route to boost current-carrying performance in FeSe and highlights its potential applicability to other iron-based superconductors where pinning optimization is critical.

Abstract

Conventional $J_c$-enhancement methods like doping and irradiation often introduce extrinsic elements or defects, altering intrinsic properties. Here, we report a significant $J_c$ enhancement in FeSe single crystals through compressive strain applied using a glass-fiber-reinforced plastic substrate with anisotropic thermal contraction during cooling. Under zero field at 2 K, $J_{\text{c}}$ increases by a factor of $\sim$4 from $\sim 2.3 \times 10^{4}$ to $\sim 8.7 \times 10^{4}$ A cm$^{-2}$; at 5 T, it achieves an order-of-magnitude enhancement, rising from $\sim 1.0 \times 10^{3}$ to $\sim 1.0 \times 10^{4}$ A cm$^{-2}$. Analysis based on the Dew-Hughes model of the $f_{\text{p}}$(h) relationship shows that strain strengthens vortex pinning, and shifts the pinning mechanism from point-like pinning to combined point and surface pinnings. This work offers an effective method to enhance FeSe's current-carrying limitation, deepens understanding of iron-based superconductors' pinning mechanisms, and highlights strain engineering's potential for optimizing superconducting performance.

Figures (4)

• Figure 1: (a) Schematic illustration of the contraction of the GFRP substrate and the strain measurement setup. The FeSe sample and the strain gauge were bonded using epoxy resin, with the tetragonal [100] direction of the FeSe crystal oriented parallel to the fibers of the GFRP substrate, and the sensitive grid of the strain gauge oriented perpendicular to the fibers. During the cooling process, the GFRP substrate contracts in the direction indicated by the red arrow. (b) Strain values of the 0.51 mm thick GFRP substrate as a function of temperature. The inset shows an enlarged view of the strain from 0 to 20 K. (c) Temperature dependence of the ZFC and FC magnetization of FeSe and FeSe-strain single crystals.
• Figure 2: The temperature dependence of MHLs for $\textit{H}\,||\,\textit{c}$ and the magnetic field dependence of the $J_{\text{c}}$ derived from the Bean model. ( a ) and ( b ) correspond to FeSe, while ( c ) and ( d ) correspond to FeSe-strain. In panels (b) and (d), the dashed lines represent the exponent $\alpha$ obtained from fitting the critical current density to the power-law relation $J_{\text{c}} \propto H^{-\alpha}$. The black dashed line indicates the exponent in the range of 5 -- 10 kOe, while the green dashed line corresponds to the exponent above 10 kOe.
• Figure 3: Scaled MHLs for (a) FeSe and (b) FeSe-strain, respectively, at different temperatures. (c) Scaled MHLs at 5 K for the pristine and strained crystals.
• Figure 4: The temperature variation of the normalized vortex pinning force ($f = F_{\text{p}} / F_{\text{pmax}}$) and reduced field ($h = H/H_{\text{max}}$) for FeSe and FeSe-strain.