Differences in the resistive and thermodynamic properties of the single crystalline chiral superconductor candidate SrPtAs

The locally noncentrosymmetric superconductor SrPtAs is proposed to host a topological chiral $d$-wave state, but experimental reports have been limited to polycrystalline samples. Here we report the synthesis of single crystalline SrPtAs grown from Pb flux. SrPtAs crystallizes in the hexagonal space group $P{6}_3/mmc$ with lattice parameters $a=4.2445\left(4\right)$ Å and $c=8.9513\left(18\right)$ Å. Magnetic susceptibility and electrical resistivity measurements reveal a superconducting transition at $T_c\sim 2.2$ K, in agreement with previous reports on polycrystalline samples. Surprisingly, heat capacity data show only a small bulk transition at 0.7 K. We discuss the possible origins of the discrepancy between the various measurements.

Figure 1

Crystal structure of SrPtAs ($P{6}_3/mmc$, No. 194), with Sr in purple, Pt in black, and As in yellow.

Figure 2

Experimental synchrotron powder x-ray diffraction pattern for SrPtAs collected at $\lambda =0.459$ Å under ambient conditions. Tick marks indicate Bragg peak positions for calculated SrPtAs (blue) and Pb (red) patterns. Pb impurity is less than $5\%$.

Figure 3

(a) Temperature-dependent magnetic susceptibility $\left[\chi \right(\mathrm{T}\left)\right]$ of SrPtAs with field ($H$ = 0.35 Oe) applied along the $c$ axis. Field-cooled (FC) data are shown with red triangles and zero-field-cooled (ZFC) data are shown with black squares. Inset shows field-dependent magnetization [$M$($H$) at $T$ = 1.6 K] in blue circles with the initial slope denoted with a red line. (b) Electrical resistivity $\left[\rho \right(T\left)\right]$ of SrPtAs at $H$ = 0. The bottom right inset shows the low temperature region (below 10 K), highlighting the superconducting transition of Pb at $\sim 7$ K. The top left inset shows resistivity below 3 K where the superconducting transition of SrPtAs occurs at 2.2 K.

Figure 4

(a) ${\chi }_{\mathrm{ac}}\left(T\right)$ with various fields applied perpendicular to the $c$ axis where the driving ac field $\sim 0.042$ Oe. (b) ${\chi }_{\mathrm{ac}}\left(T\right)$ with various fields applied along the $c$ axis where the driving ac field $\sim 0.042$ Oe. (c) Temperature-dependent resistivity $\left[\rho \right(T\left)\right]$ of SrPtAs with varying fields applied perpendicular to the $c$ axis. (d) Temperature dependence of the upper critical field $H_{c2}$. Data determined from ac susceptibility (resistivity) measurements are shown in closed black (open blue) symbols. Data collected with field applied parallel (perpendicular) to the $c$ axis are shown with triangles (squares).

Figure 5

Low temperature thermal expansion of SrPtAs along the $c$ axis from 0.1 to 7 K (black line). The shaded region indicates the detection threshold of the experimental setup.

Figure 6

(a) Heat capacity divided by temperature ($C/T$) shown as a function of temperature for SrPtAs. Inset shows zoomed low temperature region. (b) $C_p/T$ as a function of $T^2$. The red line denotes a fit to the data of $C/T=\gamma +\beta T^2$.