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.