Structural distortion behind the nematic superconductivity in Sr _x Bi₂Se₃

Abstract

An archetypical layered topological insulator Bi 2 Se 3 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 Sr x Bi 2 Se 3 , 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 Sr x Bi 2 Se 3 .

Fermi-Bose Mixtures and BCS-BEC Crossover in High-Tc Superconductors

“…In conclusion let us stress that recently discovered superconductivity in bilayer twisted graphene possibly also corresponds to the regime of BCS-BEC crossover between local and extended pairs in the d-wave channel (similarly to underdoped cuprates) but with anomalous chiral superconductivity of d+id type [177] which is in agreement with the phase diagram for idealized bilayer graphene in AB modification at low doping levels [20,22,23]. The discovery of superconductivity in graphene with many properties resembling the cuprates helps us to build the bridge between low temperature topological superconductors based on graphene [177] and bismuth [178][179][180] and vanadium oxides [181][182][183][184] which are very promising for superconducting nanoelectronics and quantum calculations (for creation of topologically protected qubits). The band structure calculations and recent experimental results show the evidence for multigap superconductivity in pressurized H3S hydrides as well as in the recently discovered high-temperature superconductor 10 LaH [84,85,86] where the superconducting dome occurs around the Lifshitz transition driven by pressure.…”

The direction of the d-vector in a nematic triplet superconductor

“…We then ask how it could be realized by perturbations away from the ideal limit. One possible perturbation is the lattice distortion, which is indeed observed in Sr x Bi 2 Se 3 by x-ray diffraction [28]. By symmetry, the effect of the inplane lattice distortion on the hopping coefficients can be expressed as a = t t negative g , or relative compression of the lattice along x, the D x 4 state is the only stable one.…”

“…In real samples, there may be lattice distortions [28], e.g. in-plane one-axis elongation and c-axis inclination.…”

Visualizing pairing symmetry in nematic superconductors using spin-polarized spectroscopy of magnetic impurities

“…It is further suggested that the pairing potential is s-wave symmetric [13]. On the other hand, various experiments support that threefold rotation symmetry is broken and that these materials are nematic pairing [16], i.e., nuclear magnetic resonance [17,18], thermodynamics [19], upper critical field [19][20][21][22], penetration depth [23,24], torque magnetometry [25], superconducting gap versus in-plane magnetic field measurement [26], magnetoresistance [27], and transport experiment [28], etc. In addition to these 3-dimensional materials, recently few layer Bi 2 Te 3 grown on FeTe 0.55 Se 0.45 substrate is demonstrated to be nematic pairing by using the quasiparticle interference (QPI) and magnetic flux vortex measurements [29], etc.…”