Superconducting and normal-state anisotropy of the doped topological insulator Sr0.1Bi2Se3

Abstract

SrxBi2Se3 and the related compounds CuxBi2Se3 and NbxBi2Se3 have attracted considerable interest, as these materials may be realizations of unconventional topological superconductors. Superconductivity with Tc ~3 K in SrxBi2Se3 arises upon intercalation of Sr into the layered topological insulator Bi2Se3. Here we elucidate the anisotropy of the normal and superconducting state of Sr0.1Bi2Se3 with angular dependent magnetotransport and thermodynamic measurements. High resolution x-ray diffraction studies underline the high crystalline quality of the samples. We demonstrate that the normal state electronic and magnetic properties of Sr0.1Bi2Se3 are isotropic in the basal plane while we observe a large two-fold in-plane anisotropy of the upper critical field in the superconducting state. Our results support the recently proposed odd-parity nematic state characterized by a nodal gap of Eu symmetry in SrxBi2Se3.

Structural distortion behind the nematic superconductivity in Sr x Bi2Se3

“…One can see that the nematicity is almost insensitive to the current flow direction. This observation is in-line with previous magnetotransport studies [5] where current flow was perpendicular to basal plane as well as with thermodynamic studies (specific heat [3] and magnetometry [9]), and it was also recently confirmed in [6]. Interestingly, in all our samples, strongest SC direction (where H c2 is maximal) either roughly coincides or is perpendicular to crystallographical axis in the basal plane, indicated by arrows in figures 1, 2, 4.…”

“…The most studied topological insulator material, Bi 2 Se 3 , becomes superconductive being doped with Sr, Nb or Cu, with T c around 3 K and H c2 about a few Tesla [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. Nature of this superconductivity (topological or not) is in the focus of discussion during the last few years.…”

“…Recently this superconductivity (SC) was found to be nematic, i.e. superconducting properties depend strongly on the in-plane orientation of the magnetic field [2][3][4][5][6][7][8][9]. Critical magnetic fields, magnetization, resistivity, specific heat, and Knight shift have 180 • in-plane rotation symmetry, contrary to the trigonal (120 • ) crystal symmetry.…”

“…Nematicity of both thermodynamic and transport properties in these materials was found to be linked to crystallographical directions [2][3][4][5][6][7]. Apparently, real crystals never grow with perfect 3-fold symmetry, there should be some structural distortion.…”

“…The block structure of the grown crystals was almost not discussed at all. Only recently, XRD-data supported high enough structural quality Sr x Bi 2 Se 3 [6] and Nb x Bi 2 Se 3 [8,12] crystals were reported with ∼100% shielding fraction.…”