Temperature Dependence of the Anisotropy of Superconducting Properties in the Nematic Superconductor Sr_x Bi_2 Se_3

Nematic superconductors are characterized by an order parameter symmetry that spontaneously breaks the crystal symmetry in the superconducting state. This appears as the anisotropy of the upper critical field (Hc2) in the basal plane of the crystal. The symmetry breaking is usually attributed to the strain of the crystal lattice. The nature and the value of the strain are discussed. To clarify this issue, the anisotropy of Hc2 in the high-quality samples of the nematic superconductor SrxBi2Se3 is carefully measured. The measurements were carried out with mK precision temperature stabilization. It was observe that in all tested samples the anisotropy is practically constant when T<0,8Tc and smoothly decreases at higher temperatures without any sign of singularity. Such a behavior can be understood in the framework of the Ginzburg-Landau (GL) theory for the nematic superconductors assuming that the samples are appreciably deformed. The nature of the strain, the values of the GL parameters, and the effects of disorder near Tc are discussed. Such measurement can be used to study the symmetry breaking in the other nematic superconductors.