Structural transition and anisotropic properties of single-crystalline ${\text{SrFe}}_2{\text{As}}_2$

Platelike single crystals of ${\text{SrFe}}_2{\text{As}}_2$ as large as $3\times 3\times 0.5{\text{mm}}^3$ have been grown out of Sn flux. The ${\text{SrFe}}_2{\text{As}}_2$ single crystals show a structural phase transition from a high-temperature tetragonal phase to a low-temperature orthorhombic phase at $T_o=198\text{K}$, and do not show any sign of superconductivity down to 1.8 K. The structural transition is accompanied by an anomaly in the electrical resistivity, Hall resistivity, specific heat, and the anisotropic magnetic susceptibility. In an intermediate temperature range from 198 to 160 K, single-crystal x-ray diffraction suggests a coexistence of the high-temperature tetragonal and the low-temperature orthorhombic phases.

Figure 1

(Color online) Temperature dependent, anisotropic magnetization measured at a magnetic field of 50 kOe for ${\text{SrFe}}_2{\text{As}}_2$. The inset shows the photograph of a single crystal on a mm grid. The crystallographic $c$ axis is perpendicular to the plane of the plate. A few droplets of Sn flux can be seen on the surface.

Figure 2

(Color online) [(a) and (b)] Temperature dependence of normalized in-plane electrical resistivity in zero field and in an applied magnetic field of 140 kOe. The inset shows the field dependence of the magnetoresistivity $\Delta \rho /{\rho }_0$ measured at 2 K with magnetic field applied parallel and perpendicular to the $c$ axis. Note small features in the $H=0$ data at low temperatures are due to small amounts of Sn (flux) on the sample. (c) Temperature dependence of the Hall resistivity divided by applied magnetic field. Inset shows a close to linear magnetic-field dependence of the Hall resistivity at three salient temperatures: 300, 170, and 2 K.

Figure 3

(Color online) Temperature dependent specific heat of ${\text{SrFe}}_2{\text{As}}_2$. Upper inset: $C_p/T$ as a function of $T^2$ for low-temperature data. Lower inset: expanded view near the $T_o=198\text{K}$ phase transition.

Figure 4

X-ray diffraction scans along the (110) direction through the position of the tetragonal (1110) reflection for selected temperatures. The lines represent the fitted curves to obtain the reflection positions for the data shown in Fig. 5. The offset between every data set is 30 counts/s.

Figure 5

Lattice parameters for the tetragonal and orthorhombic phases as extracted from the data shown in Fig. 4 for the (1110) reflection and the (0010) reflection (not shown). To allow direct comparison of both phases, the lattice parameter $a$ of the tetragonal phase is given as $a_{HT}=\sqrt{2}a$. The inset shows the intensity of the reflections related to the lattice parameter $a$ in both phases. The error bars represent the relative precision, the absolute error is method related and significantly larger.