Suppression of phase separation and whole phase diagram in FeSe$_{1-x}$Te$_x$ thin films on LaAlO$_3$ substrates

In FeSe$_{1-x}$Te$_{x}$, it is known that the phase separation occurs at $x=0.1-0.4$ for bulk samples, and it prevents the complete understanding of this system. In previous meeting, we reported the fabrication of FeSe$_{1-x}$Te$_{x}$ films on CaF$_2$ with with whole range of $x$ by a pulsed laser deposition [1]. Surprisingly, the optimal composition was found in the phase-separation region; $T_c$ reaches 23 K at $x=0.2$. In this presentation, we report the successful fabrication of FeSe$_{1-x}$Te$_{x}$ films with $x=0-0.7$, which includes the phase-separation region, on LaAlO$_3$ substrates. The highest $T_c$ of the films on LaAlO$_3$ substrates reaches 19 K, which is also higher than that of bulk samples. The doping-temperature ($x-T$) phase diagram of FeSe$_{1-x}$Te$_{x}$ films clearly shows that $T_c$ suddenly changes at the Te content where the structural transition disappears, which is commonly observed for both films on LaAlO$_3$ and CaF$_2$[2,3]. Our result indicates that one of the key factors to realize a further increase of $T_c$ in iron chalcogenides is the control of the structural transition. $[1]$ Y. Imai $et$ $al.$, Proc. Natl. Acad. Sci. USA. 112 (2015) 1937. $[2]$ Y. Sawada $et$ $al.$, J. Phys. Soc. Jpn. 85 (2016) 073703. $[3]$ Y. Imai $et$ $al.$, $submitting$.