Rotational symmetry breaking in the topological superconductor SrxBi2Se3 probed by upper-critical field experiments.

Y Pan,Y K Huang,Y Matsushita,T Naka,G K Araizi,A M Nikitin,A De Visser

doi:10.1038/srep28632

Y Pan, Y K Huang + Show 5 more

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https://doi.org/10.1038/srep28632

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Recently it was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi2Se3. Topological superconductors are predicted to be unconventional with an odd-parity pairing symmetry. An adequate probe to test for unconventional superconductivity is the upper critical field, Bc2. For a standard BCS layered superconductor Bc2 shows an anisotropy when the magnetic field is applied parallel and perpendicular to the layers, but is isotropic when the field is rotated in the plane of the layers. Here we report measurements of the upper critical field of superconducting SrxBi2Se3 crystals (Tc = 3.0 K). Surprisingly, field-angle dependent magnetotransport measurements reveal a large anisotropy of Bc2 when the magnet field is rotated in the basal plane. The large two-fold anisotropy, while six-fold is anticipated, cannot be explained with the Ginzburg-Landau anisotropic effective mass model or flux flow induced by the Lorentz force. The rotational symmetry breaking of Bc2 indicates unconventional superconductivity with odd-parity spin-triplet Cooper pairs (Δ4-pairing) recently proposed for rhombohedral topological superconductors, or might have a structural nature, such as self-organized stripe ordering of Sr atoms.

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It was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi2Se3
The data follow a sin θ dependence, which tells us the variation is due to the classical magnetoresistance related to the Lorentz force FL =BI sin θ, where I is the transport current that flows in the basal plane
Having conclusively established the two-fold anisotropy of Bc2 in the basal plane, we turn to possible explanations

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Introduction

It was demonstrated that Sr intercalation provides a new route to induce superconductivity in the topological insulator Bi2Se3. The gapless surface states have a Dirac-type energy dispersion with the spin locked to the momentum and are protected by symmetry This makes TIs promising materials for applications in fields like spintronics and magnetoelectrics[1,2]. By evaluating the topological invariants of the Fermi surface, CuxBi2Se3 is expected to be a time-reversal invariant fully-gapped odd-parity topological superconductor[6,7]. This was put on a firmer footing by a two-orbital pairing potential model where odd-parity superconductivity is favoured by strong spin-orbit coupling[18]. The issue of topological superconductivity in CuxBi2Se3 has not been settled and further experiments are required, as well as new materials

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