Abstract
An archetypical layered topological insulator Bi2Se3 becomes superconductive upon doping with Sr, Nb or Cu. Superconducting properties of these materials in the presence of in-plane magnetic field demonstrate spontaneous symmetry breaking: 180◦-rotation symmetry of superconductivity versus 120◦-rotation symmetry of the crystal. Such behavior brilliantly confirms nematic topological superconductivity. To what extent this nematicity is due to superconducting pairing in these materials, rather than due to crystal structure distortions? This question remains unanswered, because so far no visible deviations from the 3-fold crystal symmetry were resolved in these materials. To address this question we grow high quality single crystals of SrxBi2Se3, perform detailed x-ray diffraction and magnetotransport studies and reveal that the observed superconducting nematicity direction correlates with the direction of small structural distortions in these samples (∼0.02% elongation in one crystallographic direction). Additional anisotropy comes from orientation of the crystallite axes. 2-fold symmetry of magnetoresistance observed in the most uniform crystals well above the critical temperature demonstrates that these structural distortions are nevertheless strong enough. Our data in combination with strong sample-to-sample variation of the superconductive anisotropy parameter are indicative for significance of the structural factor in the apparent nematic superconductivity in SrxBi2Se3.
Highlights
The most studied topological insulator material, Bi2Se3, becomes superconductive being doped with Sr, Nb or Cu, with Tc around 3K and Hc2 about a few Tesla[1–19]
Later tunnel spectroscopy measurements[1] have clearly shown an s-wave pairing without any in-gap states
High purity elemental Bi, Se (99.999%) and Sr (99.95%) in the desired molar ratio were loaded in quartz ampoules inside inert atmosphere glove box
Summary
The most studied topological insulator material, Bi2Se3, becomes superconductive being doped with Sr, Nb or Cu, with Tc around 3K and Hc2 about a few Tesla[1–19]. Later tunnel spectroscopy measurements[1] have clearly shown an s-wave pairing without any in-gap states. This superconductivity(SC) was found to be nematic, i.e. superconducting properties depend strongly on the in-plane orientation of the magnetic field [2–9]. Magnetization, resistivity, specific heat, and Knight shift have 180◦ in-plane rotation symmetry, contrary to the trigonal (120◦) crystal symmetry. An explanation for such nematicity was again suggested within the topological superconductivity model with two component order parameter[23, 24], and most of the data brilliantly confirm this theoretical approach. Very recently nematicity of superconducting properties (distortion of Abrikosov vortices)in combination with the zero bias peak at the vortex cores were directly visualized using more thorough scanning tunneling spectroscopy[7]
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