Realization of epitaxial thin films of the superconductor K-doped BaFe$_\text{2}$As$_\text{2}$

TL;DR

This paper reports the successful growth of epitaxial K-doped BaFe2As2 superconducting thin films using molecular-beam epitaxy, demonstrating high critical current density and potential for property tuning via strain.

Contribution

It presents a novel method for synthesizing epitaxial Ba1-xKxFe2As2 thin films with high quality and superconducting properties, overcoming previous challenges related to potassium volatility.

Findings

Superconducting transition temperature of 36 K on CaF2 substrate.

Critical current density of 2.2 MA/cm2 at 4 K.

In-field Jc exceeds that of bulk crystals.

Abstract

The iron-based superconductor Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$ is emerging as a key material for high magnetic field applications owing to the recent developments in superconducting wires and bulk permanent magnets. Epitaxial thin films play important roles in investigating and artificially tuning physical properties; nevertheless, the synthesis of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ epitaxial thin films remained challenging because of the high volatility of K. Herein, we report the successful growth of epitaxial Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$ thin films by molecular-beam epitaxy with employing a combination of fluoride substrates (CaF$_\text{2}$, SrF$_\text{2}$, and BaF$_\text{2}$) and a low growth temperature (350$-$420$^\circ$C). Our epitaxial thin film grown on CaF$_\text{2}$ showed sharp superconducting transition at an onset critical temperature of 36 K, slightly lower than bulk crystals by ~2 K due presumably to the strain effect arising from the lattice and thermal expansion mismatch. Critical current density ($J$$_\text{c}$) determined by the magnetization hysteresis loop is as high as 2.2 MA/cm$^\text{2}$ at 4 K under self-field. In-field $J$$_\text{c}$ characteristics of the film are superior to the bulk crystals. The realization of epitaxial thin films opens opportunities for tuning superconducting properties by epitaxial strain and revealing intrinsic grain boundary transport of Ba$_{1-x}$K$_x$Fe$_\text{2}$As$_\text{2}$.