Superconductivity with two-fold symmetry in topological superconductor Sr x Bi2Se3

Abstract

Topological superconductivity is the quantum condensate of paired electrons with an odd parity of the pairing function. One of the candidates is the triplet pairing superconductor derived from topological insulator Bi2Se3 by chemical doping. Theoretically it was predicted that a two-fold superconducting order parameter may exist in this kind of bulk topological superconductor. Earlier experiments by nuclear magnetic resonance and angle-resolved magnetocaloric measurements seem to support this picture. By using a Corbino-shape like electrode configuration, we measure the c-axis resistivity without the influence of the vortex motion and find clear evidence of two-fold superconductivity in the recently discovered topological superconductor SrxBi2Se3. The Laue diffraction measurements on these samples show that the maximum gap direction is either parallel or perpendicular to the main crystallographic axis.

Structural distortion behind the nematic superconductivity in Sr x Bi2Se3

“…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.…”

“…A few experiments used samples made by Bridgman method [19]. In most of the present experimental papers only powder x-ray diffraction (XRD) [10,14] and Laue [4,5,7] patterns are presented. Absence of high resolution single crystal XRD data is an indirect indicator that the crystal quality was not ultimately optimized.…”

Superconductivity with twofold symmetry in Bi 2 Te 3 /FeTe 0.55 Se 0.45 heterostructures

“…Theoretically, Fu ( 17 ) did a quantitative assessment by taking into account the spin-orbital coupling and the multi-orbital effect in M x Bi 2 Se 3 and predicted that it could be possible to have a fully gapped order parameter but with odd parity, namely, a twofold symmetry of the SC order parameter. Furthermore, the c axis resistivity also exhibits a twofold symmetry when the applied magnetic field is rotated within the basal plane in Sr x Bi 2 Se 3 ( 18 , 19 ). However, to extract the information of gap structure from these data, we inevitably need some models to relate the gap minimum with the crystalline lattice.…”