Electrical Transport and Magnetic Properties of High-Quality SrxBi2Se3 Single Crystals Obtained by Annealing
*Takumi Matsumae1, Sunseng Pyon1, Tsuyoshi Tamegai1

Topological insulator Bi2Se3 becomes topological superconductors by doping Cu or Sr [1,2]. These materials attract much interest because of the nematic state showing two-fold symmetry when magnetic field is applied parallel to ab-plane [3,4]. Although in-plane magnetic-field-angle dependence of resistivity has been measured to confirm nematicity, there had been only a few measurements of out-of-plane anisotropy. Out-of-plane anisotropy is also important because it represents dimensionality of superconductivity. Out-of-plane angle dependence of upper critical field (Hc2) is predicted by Tinkham model for 2D superconductivity and by anisotropic GL model for 3D. It is interesting which can be applied to doped Bi2Se3 because it has a layered crystal structure consisting of Bi-Se quintuple layers, which are bonded by van der Waals force.

In this study, single crystals of SrxBi2Se3 (0.1 ≦ x≦ 0.3) were made by the flux method. Since it is known that obtaining high-quality crystals with sharp transition width is difficult due to large inhomogeneity [5], we tried various temperature conditions to improve the quality of the crystal. Single crystals were grown by slow cooling from the maximum temperature 1,000 ℃ and quenched at 650 ℃ or 700 ℃. In addition, one of the samples was prepared by furnace cooling. The transition width of the sample obtained by furnace cooling was the sharpest among all the crystals and it became even sharper by additional annealing as shown in Fig. 1(a).

Using these high-quality single crystals, out-of-plane magnetic-field-angle dependence of resistivity was measured. The obtained results are as follows: Hc2ab = 2.2 T, Hc2c = 0.65 T, γ = 3.3 (values at 2.0 K). The angle dependence of Hc2 shown in Fig. 1(b) is well fitted by the anisotropic GL model rather than the Tinkham model.

We will also discuss Hall resistivity and magnetoresistance in SrxBi2Se3.

Fig. 1(a): Temperature dependence of normalized magnetization in Sr0.17Bi2Se3 obtained by furnace cooling followed by annealing at 560 ℃. (b) Out-of-plane magnetic-field-angle dependence of Hc2. θ = ±90° corresponds to c-axis. Hc20.9 is defined by ρ = ρ0 + 0.9 (ρn - ρ0) and Hc20.5 by ρ = ρ0 + 0.5 (ρn - ρ0). ρ0 is residual resistivity and ρn is resistivity in the normal state.

[1] Y. S. Hor et al., Phys. Rev. Lett. 104, 057001 (2010).
[2] Z . Liu, et al. J. Am. Chem. Soc. 137, 10512 (2015).
[3] K. Matano et al., Nat. Phys. 12, 852 (2016).
[4] Y. Pan et al., Sci. Rep. 6, 28632 (2016).
[5] J. A. Schneeloch et al., Phys. Rev. B 91, 144506 (2015).

Keywords: topological superconductor, nematic superconductor, Bi2Se3, electrical transport properties