Enhancement of the Superconducting Transition Temperature in FeSe Epitaxial Thin Films by Anisotropic Compression

In order to investigate the effects of in-plane strain on the superconductivity of FeSe, epitaxial thin films of FeSe were fabricated on CaF${}_2$ substrates. The films are compressed along the a-axis and their superconducting transition temperatures $T_{\mathrm{c}}^{\mathrm{z}\mathrm{e}\mathrm{r}\mathrm{o}}$ reach 11.4 K, which is approximately 1.5 times higher than that of bulk crystals. The $T_{\mathrm{c}}$ values are weakly dependent on the ratio of the lattice constants, $c$ / $a$, compared to that of Fe(Se,Te). Our results indicate that even a binary system FeSe has room for improvement, and will open a new route for the application of Fe-based superconductors.