  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 $T_{\mathrm c}^{\mathrm {zero}}$ 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.

Superconductivity