Suppression of phase separation and giant enhancement of superconducting transition temperature in FeSe_(1-x)Te_(x) thin films

We demonstrate the successful fabrication on CaF$_2$ substrates of FeSe$_{1-x}$Te$_{x}$ films with $0 \le x \le 1$, including the region of $0.1 \le x \le 0.4$, which is well known to be the "phase-separation region", via pulsed laser deposition which is a thermodynamically non-equilibrium method. In the resulting films, we observe a giant enhancement of the superconducting transition temperature, $T_\mathrm{c}$, in the region of $0.1 \le x \le 0.4$: the maximum value reaches 23 K, which is approximately 1.5 times as large as the values reported for bulk samples of FeSe$_{1-x}$Te$_{x}$. We present a complete phase diagram of FeSe$_{1-x}$Te$_{x}$ films. Surprisingly, a sudden suppression of $T_\mathrm{c}$ is observed at $0.1<x<0.2$, while $T_\mathrm{c}$ increases with decreasing $x$ for $0.2 \le x < 1$. Namely, there is a clear difference between superconductivity realized in $x=0-0.1$ and in $x \ge 0.2$. To obtain a film of FeSe$_{1-x}$Te$_{x}$ with high $T_\mathrm{c}$, the controls of the Te content $x$ and the in-plane lattice strain are found to be key factors.