Superconducting and tetragonal-to-orthorhombic transitions in single crystals of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ $(0\le$ $x$ $\le 0.61)$

Single crystals of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ were grown for $0\le x\le 0.41$ 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 $x=0.2\text{–}0.3$, 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.

Figure 1

(a) Schematic image of the temperature distribution in the horizontal tube furnace for single-crystal growth of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ by the flux method. The crystallization takes place at the cold end of the furnace. Temperature pairs of ${\mathit{T}}_{\mathrm{hot}}$ and ${\mathit{T}}_{\mathrm{cold}}$ are selected depending on $x$. (b) Photograph of as-grown single crystals of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ nominally with $x=0.3$ after removing the flux.

Figure 2

(a) X-ray diffraction patterns of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ ($x=0$, 0.04, 0.19, 0.30, $0.3{}^{*}$, 0.41) single crystals; (b) and (c) the magnified plots at around the (003) and (004) reflections. Except for the bottom $x=0.41$ data, the intensities are offset for clarity. (d) Substitution $x$ dependence of the $c$-axis lattice parameters of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ ($x=0–0.41$). The dashed line is a linear fit to the data. (e) SEM image and elemental mappings of (f) Fe, (g) Se, and (h) Te measured on the sample with $x=0.30$. (i) SEM image and elemental mappings of (j) Fe, (k) Se, and (l) Te measured on the sample with $x=0.3{}^{*}$, which contains an inhomogeneous distribution of Se and Te, as can be seen clearly in (k) and (l).

Figure 3

(a)Temperature dependence of in-plane resistivity of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ single crystals for $x=0$, 0.04, 0.19, 0.30, 0.41, 0.61, normalized by its respective value at 290 K. Black arrows indicate the anomalies due to structural transitions, whose ${\mathit{T}}_{\mathrm{s}}$ values are determined accurately by examining the derivative curves, $dR/dT$. (b) Magnified plots of the normalized resistivity curves shown in (a) for 5 K $\le T\le$ 20 K. ${\mathit{T}}_{\mathrm{c}}^{\mathrm{onset}}$ is defined as the temperature of the crossing point of the linear fit to the resistivity curve in the normal state and the slope curve just above the superconducting transition. The superconducting transition temperature ${\mathit{T}}_{\mathrm{c}}$ is defined to be the highest temperature at which the resistivity is zero.

Figure 4

Temperature phase diagram of ${\mathrm{FeSe}}_{1-x}{\mathrm{Te}}_x$ single crystals as a function of $x$. The red solid circles and the green solid squares represents $T_{\mathrm{c}}$ and $T_{\mathrm{s}}$, respectively. Those values are deduced from our resistivity measurements. The green dashed line is a least-squares fit to the $T_{\mathrm{s}}\left(x\right)$ data. On the other hand, red open circles for $T_{\mathrm{c}}$ and a brown open square for $T_{\mathrm{N}}$ are values taken from a previous report [16]. Vertical bars of $T_{\mathrm{c}}$ denote the onset of the transition temperature, and horizontal bars denote the standard deviation of $x$ estimated by EPMA. $T_{\mathrm{c}}$ values for thin films [21] are plotted by the open black circles with a dashed curve that is an eye guide.