Disorder, critical currents, and vortex pinning energies in isovalently substituted BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$

TL;DR

This study investigates vortex pinning, critical currents, and energies in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, revealing how P-content influences pinning properties and vortex configurations in this archetypical clean superconductor.

Contribution

It provides a detailed analysis of vortex pinning mechanisms and their dependence on P-content, highlighting differences from charge-doped counterparts and linking pinning characteristics to superfluid density.

Findings

Pinning force and energy distributions depend on P-content.

Mean distance between pinning centers increases with P-content.

Vortex configurations show chain-like arrangements and a wide Meissner belt.

Abstract

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$, a material that qualifies as an archetypical clean superconductor, containing only sparse strong point-like pins [in the sense of C.J. van der Beek {\em et al.}, Phys. Rev. B {\bf 66}, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals. Analysis reveals that the pinning force and -energy distributions depend on the P-content $x$. However, they are always much narrower than in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching $T_{c}$ in BaFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers $\bar{\mathcal L} \sim 90$ nm, increasing with increasing P-content $x$. This evolution can be understood as being the consequence of the P-dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chain-like vortex arrangements, observed at all levels of P-substitution.