Superconducting and tetragonal-to-orthorhombic transitions in single crystals of FeSe1−xTex (0≤ x ≤0.61)

Single crystals of ${\mathrm{FeSe}}_{1\ensuremath{-}x}{\mathrm{Te}}_x$ were grown for $0\ensuremath{\le}x\ensuremath{\le}0.41$ by a flux method and at $x=0.61$ by chemical vapor transport in order to study the Te substitution effects on the quality of the single crystals and on their superconducting characteristics. From the in-plane resistivity data of the grown crystals, we found that the superconducting transition temperature ${T}_{\mathrm{c}}$ slightly decreases with increasing the doping level of $x$ up to around $x=0.2\phantom{\rule{0.16em}{0ex}}\text{--}\phantom{\rule{0.16em}{0ex}}0.3$, then begins to increase with $x$ at around $x=0.4$, and reaches a maximum around $x=0.6$. These experimental facts are contrasted with the results of thin films where ${T}_{\mathrm{c}}$ increases up to 23 K around $x=0.2$. We also observed an anomaly in the temperature dependence of the in-plane resistivity due to the tetragonal-to-orthorhombic structural transition ${T}_{\mathrm{s}}$, which decreases nearly linearly with increasing $x$ until finally disappearing near $x=0.5$. These features were not reported in previous studies performed with polycrystalline and thin film samples.