Intrinsic and extrinsic pinning in NdFeAs(O,F): vortex trapping and lock-in by the layered structure

TL;DR

This study investigates the vortex pinning mechanisms in NdFeAs(O,F) superconductors, revealing how intrinsic layered structures influence vortex behavior and critical current density across various conditions.

Contribution

It provides the first comprehensive analysis of intrinsic and extrinsic pinning effects in NdFeAs(O,F), highlighting the role of layered structure in vortex dynamics.

Findings

Point-defect pinning dominates at high temperatures.

Surface pinning by planar defects prevails at low temperatures.

Intrinsic pinning causes a strong ab-plane peak in critical current density.

Abstract

The Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the $\it{RE}$FeAs(O,F) ($\it{RE}$1111, where $\it{RE}$ is a rare earth element) is the family with the highest critical temperature $T_c$ but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 ($T_c$ $\sim$ 55 K). Here we focus our attention on the pinning properties of the lower-$T_c$ Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density $J_c$ at high temperatures is dominated by point-defect pinning centres, whereas at low temperatures surface pinning by planar defects parallel to the $\it{c}$-axis and vortex shearing prevail. When the field approaches the $\it{ab}$-planes, two different regimes are observed at low temperatures as a consequence of the transition between 3D-Abrikosov and 2D-Josephson vortices: one is determined by the formation of a vortex staircase structure and one by lock-in of the vortices parallel to the layers. This is the first study on FBS showing this behaviour in a full temperature, field, and angular range and it demonstrates that, despite the lower $T_c$ and anisotropy of Nd1111 with respect to Sm1111, this compound is substantially affected by intrinsic pinning generating a strong $\it{ab}$-peak in $J_c$.