High $J_{\rm c}$ and low anisotropy of hydrogen doped NdFeAsO superconducting thin film

TL;DR

Hydrogen doping in NdFeAsO thin films enhances critical current density and reduces anisotropy, maintaining high superconducting transition temperature, thus promising for high-field magnet applications.

Contribution

This study demonstrates that hydrogen substitution in NdFeAsO thin films improves $J_{c}$ and lowers anisotropy without reducing $T_{c}$, a novel approach for superconductor optimization.

Findings

High $J_{c}$ over 10 MA/cm$^2$ at 5 K.

Decreasing anisotropy parameter $ heta_{J}$ with lower temperature.

Hydrogen doping yields lower anisotropy than fluorine doping.

Abstract

The recent realisations of hydrogen doped $Ln$FeAsO ($Ln$=Nd and Sm) superconducting epitaxial thin films call for further investigation of their structural and electrical transport properties. Here, we report on the microstructure of a NdFeAs(O,H) epitaxial thin film and its temperature, field, and orientation dependencies of the resistivity and the critical current density $J_{\rm c}$. The superconducting transition temperature $T_{\rm c}$ is comparable to NdFeAs(O,F). Transmission electron microscopy investigation supported that hydrogen is homogenously substituted for oxygen. A high self-field $J_{\rm c}$ of over 10 MA/cm$^2$ was recorded at 5 K, which is likely to be caused by a short London penetration depth. The anisotropic Ginzburg-Landau scaling for the angle dependence of $J_{\rm c}$ yielded temperature-dependent scaling parameters $\gamma_{\rm J}$ that decreased from 1.6 at 30 K to 1.3 at 5 K. This is opposite to the behaviour of NdFeAs(O,F). Additionally, $\gamma_{\rm J}$ of NdFeAs(O,H) is smaller than that of NdFeAs(O,F). Our results indicate that heavily electron doping by means of hydrogen substitution for oxygen in $Ln$FeAsO is highly beneficial for achieving high $J_{\rm c}$ with low anisotropy without compromising $T_{\rm c}$, which is favourable for high-field magnet applications.