Optimizing the superconducting transition temperature and upper critical field of Sn${}_{1-x}$In${}_x$Te

Sn${}_{1-x}$In${}_x$Te is a possible candidate for topological superconductivity. Previous work has shown that substitution of In for Sn in the topological crystalline insulator SnTe results in superconductivity, with the transition temperature, $T_c$, growing with In concentration. We have performed a systematic investigation of Sn${}_{1-x}$In${}_x$Te for a broad range of $x$, synthesizing single crystals (by a modified floating-zone method) as well as polycrystalline samples. The samples have been characterized by x-ray diffraction, resistivity, and magnetization. For the single crystals, the maximum $T_c$ is obtained at $x=0.45$ with a value of 4.5 K, as determined by the onset of diamagnetism.

Figure 1

Superconducting transition temperature as a function of indium concentration $x$ for both polycrystalline (blue squares) and single-crystal samples (red triangles), obtained from magnetization measurements, and for single crystals (brown circles) from resistivity measurements. For each concentration, different parts of the as-grown crystal rod were measured to give an average value of $T_c$. Data for indium concentrations less than 10$\%$ are taken from Erickson et al.[14]

Figure 2

X-ray diffraction spectra for Sn${}_{1-x}$In${}_x$Te (In$x$) polycrystalline samples with nominal compositions $x=0$–1.0.

Figure 3

(a) X-ray diffraction measurements for the single crystals with composition $x=0.1$–0.5, with intensity plotted on a logarithmic scale. (b) Lattice parameters $a$ derived from Rietveld analysis of XRD patterns of the cubic phases in polycrystalline (blue squares) and single crystal (red triangles) samples as a function of the nominal indium concentration $x$.

Figure 4

(a) Temperature dependence of magnetic susceptibility for Sn${}_{0.55}$In${}_{0.45}$Te single crystals measured under conditions of FC (ZFC) in an applied field of 1 mT at a cooling (heating) rate of 0.1 K/min. (b) Hysteresis loop of a Sn${}_{0.55}$In${}_{0.45}$Te single crystal measured at 1.75 K. The inset shows the initial $M-H$ behavior at a field less than 10 mT and 1.75 K. For magnetization measurements, samples are cut into roughly cubiclike small chunks. The demagnetization factor can be taken as 1 considering that the crystal structure is isotropic.

Figure 5

Temperature dependence of the resistivity for Sn${}_{1-x}$In${}_x$Te single crystals.

Figure 6

(a) Field dependence of the resistivity for the $x=0.45$ crystal at fixed temperatures from 1.8 to 4.6 K. (b) Upper critical field $H_{c2}\left(T\right)$ determined from resistivity measurements on four single crystals.