Suppression of Phase Separation and Enhanced Superconducting Transition Temperature of FeSe$_{1-x}$Te$_x$ Thin Films*

To clarify the mechanism of superconductivity of Fe-based superconductors, it is crucial to investigate superconductivity of FeSe$_{1-x}$Te$_x$, which has the simplest crystal structure. There is, however, a serious obstacle to the understanding of its superconductivity; phase separation by spinodal decomposition occurs in the region of $0.1 < x < 0.4$ and thus a whole phase diagram has not been available. A useful method to fabricate metastable materials is thin-film deposition because of its thermodynamically non-equilibrium growth. In the presentation we will report the first demonstration of the suppression of the phase separation of FeSe$_{1-x}$Te$_x$ thin films on CaF$_2$ substrates[1]. Surprisingly the optimal composition to achieve the highest superconducting transition temperature, $T_{\mathrm c}$, was found in this phase separation region; $T_{\mathrm c}^{\mathrm {onset}}$ reaches $\sim $23 K. A whole phase diagram we will present provides a new perspective for the superconductivity of this material. [1] F. Nabeshima $et\ al$., Appl. Phys. Lett. \textbf{103} (2013) 172602.