# Promoting superconductivity in FeSe films via fine manipulation of crystal lattice

## TL;DR

This study demonstrates that fine manipulation of the crystal lattice in FeSe thin films can enhance their superconducting transition temperature up to 15 K, with stability in ambient conditions, by controlling deposition parameters and stoichiometry.

## Contribution

The paper introduces a method to tune Tc in FeSe films through substrate choice and stoichiometry, revealing lattice parameter dependence and strain effects.

## Key findings

- Maximum Tc of 15 K on CaF2 substrate.
- Tc adjustable from 15 K to below 2 K by stoichiometry tuning.
- Tc decreases with film thickness below 10 nm.

## Abstract

Stabilized FeSe thin films in ambient pressure with tunable superconductivity would be a promising candidate for superconducting electronic devices yet its superconducting transition temperature (Tc) is below 10 K in bulk materials. By carefully controlling the depositions on twelve kinds of substrates using pulsed laser deposition technique, high quality single crystalline FeSe samples were fabricated with full width of half maximum 0.515? in the rocking curve and clear four-fold symmetry in phi-scan from x-ray diractions. The films have a maximum Tc 15 K on the CaF2 substrate and do not show obvious decay in the air for more than half a year. Slightly tuning the stoichiometry of the FeSe targets, the Tc becomes adjustable from 15 to < 2 K with quite narrow transition widths less than 2 K, and shows a positive relation with the out-of-plane (c-axis) lattice parameter of the films. However, there is no clear relation between the Tc and the surface atomic distance of the substrates. By reducing the thickness of the films, the Tc decreases and fades away in samples of less than 10 nm, suggesting that the strain effect is not responsible for the enhancement of Tc in our experiments.