Fabrication and Transport Properties of FeSe Thin Films on CaF$_2$ Substrates with Increased $T_{\rm c}$

Fe(Se,Te) has the simplest crystal structure among Fe-based superconductors. Superconducting transition temperature, $T_{\rm c}$, is strongly dependent on the applied pressure. Indeed, strained thin films of FeSe$_{0.5}$Te$_{0.5}$ have higher $T_{\rm c}$ than that of bulk crystals[1,2]. On the other hand, an end member, FeSe, shows large increase in $T_{\rm c}$ under pressure compared with Te-doped ones. However there is no report on increased $T_{\rm c}$ of FeSe thin films except for the interface-induced superconductivity[3]. In the presentation we will report on the first successful introduction of compressive strain in FeSe thin films using CaF$_2$ substrates. As a result, $T_{\rm c}^{\rm {zero}}$ reaches 11.4 K, which is about 1.5 times higher than that of bulk crystals[4]. We will also report on the transport properties of FeSe thin films on CaF$_2$ in the normal state including the THz conductivity and the Hall resistivity comparing them with the results of FeSe$_{0.5}$Te$_{0.5}$ films. [1] E. Bellingeri $et\ al$., APL {\bf 96} (2010) 102512. [2] I. Tsukada $et\ al$., APEX {\bf 4} (2011) 053101. [3] Q.-Y. Wang $et\ al$., Chin. Phys. Lett. {\bf 29} (2012) 037402. [4] F. Nabeshima $et\ al$., APL {\bf 103} (2013) 172602.