Superconductivity induced by In substitution into the topological crystalline insulator ${\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5}\mathrm{Te}$

Indium substitution turns the topological crystalline insulator (TCI) ${\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5}\mathrm{Te}$ into a possible topological superconductor. To investigate the effect of the indium concentration on the crystal structure and superconducting properties of $({\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5})$${}_{1-x}$${\mathrm{In}}_x\mathrm{Te}$, we have grown high-quality single crystals using a modified floating-zone method and have performed systematic studies for indium content in the range $0\le x\le 0.35$. We find that the single crystals retain the rocksalt structure up to the solubility limit of indium $\left(x\sim 0.30\right)$. Experimental dependencies of the superconducting transition temperature $\left(T_c\right)$ and the upper critical magnetic field $\left(H_{c2}\right)$ on the indium content $x$ have been measured. The maximum $T_c$ is determined to be 4.7 K at $x=0.30$, with ${\mu }_0{H}_{c2}(T=0)\approx 5$ T.

Figure 1

Scanning electron microscope (SEM) images for (a) 30% and (b) 35% In-substituted $({\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5})$${}_{1-x}$${\mathrm{In}}_x\mathrm{Te}$ crystals. The bright regions seen in (b) have a composition consistent with InTe; these are absent from (a).

Figure 2

(a) X-ray diffraction patterns for the $({\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5})$${}_{1-x}$${\mathrm{In}}_x\mathrm{Te}$ single crystals with composition $x=0$–0.35, with intensity plotted on a logarithmic scale. The data have been smoothed using the Savitzky-Golay algorithm, and the $K{\alpha }_2$ component has been removed [24]. The Miller indices of major peaks in the cubic phase have been identified. (b) Gaussian fitted curves for the (220) peak in the cubic phase. (c) Lattice parameters $a$ of the cubic phase as a function of the indium concentration $x$.

Figure 3

(a) Temperature dependence of the resistivity for $({\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5})$${}_{1-x}$${\mathrm{In}}_x\mathrm{Te}$ single crystals with indium contents $x=0$–0.30. (b) Temperature dependence of magnetic susceptibility for an optimally doped $({\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5})$${}_{1-x}$${\mathrm{In}}_x\mathrm{Te}$ $\left(x=0.30\right)$ single crystal measured under conditions of field cooling (FC, red squares) and zero-field cooling (ZFC, blue circles) in an applied field of 1 mT at a heating/cooling rate of 0.1 K/min.

Figure 4

Superconducting transition temperature as a function of indium concentration $x$ for $({\mathrm{Pb}}_{0.5}$${\mathrm{Sn}}_{0.5})$${}_{1-x}$${\mathrm{In}}_x\mathrm{Te}$ single-crystal samples, obtained from magnetization (solid circles) and resistivity (open squares) measurements. Horizontal error bars reflect the variation of indium concentration measured by EDS. The dashed lines are guides to the eye. Several samples with $x\le 0.1$ were obtained, but any superconducting critical temperatures were lower than the instrument limit of 1.75 K, represented by arrows.

Figure 5

(a) Field dependence of the resistivity for the $x=0.30$ crystal at fixed temperatures from 1.8 to 5 K. (b) Upper critical field $H_{c2}\left(T\right)$ determined from resistivity measurements on four single crystals.