Pulsed laser deposition conditions and superconductivity of FeSe thin films

We report the effects of growth conditions on the superconducting properties of FeSe films epitaxially grown on LaAlO3 substrates by pulsed laser deposition (PLD). Customary materials characterization techniques [X-ray diffraction (XRD), in-situ X-ray photoelectron spectroscopy (XPS), in-situ ultra-violet photoelectron spectroscopy (UPS), and scanning electron microscopy (SEM)] revealed the films had a c-axis oriented tetragonal structure with lattice constants dependent on the growth temperature (varied from 100 to 600°C). The standard four-point probe method was used to measure the resistivity and superconducting transitions. Films grown at 400–550°C showed a clear superconducting onset but no zero resistance down to 2 K. The highest superconducting onset temperature (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$T_{\mathrm{c}}^{\mathrm{onset}}$\end{document}) of 8 K was observed in films grown at 500°C and the onset temperature was clearly correlated to the ratio of the lattice constants (c/a). As the thickness of the FeSe films increased from 27 nm to 480 nm, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$T_{\mathrm{c}}^{\mathrm{onset}}$\end{document} also increased as the strain in the system was relaxed.