Strongly anisotropic s-wave gaps in exotic superconductors

Abstract

The exotic superconductors, defined as those which follow the phenomenological trend of Uemura (T c approximately µ l L -2 ), presently constitute the most broad and general class of superconductors which can reasonably be considered "similar to the high-T c cuprates". It is therefore of much interest to determine the forms of their pairing gap functions. We examine evidence for the gap forms in non-cuprate exotics, and demonstrate some general features. The cubic materials often have highly anisotropic gaps. The planar materials tend to have even stronger gap anisotropy, to the extent that they often have gap nodes, but nevertheless they usually have an s-wave-like gap symmetry. There is good evidence for the latter even in the controversial cases of planar organics and nickel borocarbides. Exceptions to these generalizations are also pointed out and discussed. §1. INTRODUCTIONThere are now many unusual superconductors which are recognized to share some of the novel features of the high-T c cuprate superconductors. The possibility of further similarity to the cuprates, in particular the possibility of pairing by exchange of a virtual spin fluctuation or paramagnon, has motivated suggestions of d-wave gap forms in some of these other superconductors, in cases where there is evidence for gap nodes. This argument has been applied especially to the planar organics and sometimes also to the 2 nickel borocarbides, in addition to the well known cases of heavy-fermion materials. This line of reasoning calls for a careful examination of the evidence for the gap forms in these and other unusual superconductors.In this paper we focus on the exotic superconductors of Uemura and co-workers. 1 These exotics include a number of the unusual superconductors of earlier (pre-cuprate) interest, as well as the cuprates themselves and others discovered after the cuprates, and they include most of the materials previously suggested as being similar to the cuprates. 2 As discussed below, they now constitute the most broad and general class of superconductors that can reasonably be considered similar to the cuprates. (The variety of crystal structures and material chemistries in this class of materials is remarkably extensive.2 ) It is therefore very reasonable to focus on these materials. We now examine many of the non-cuprate exotic materials and material families, those for which there is reasonable gap-form evidence, in order to establish two general conclusions about their superconducting phenomenology: (1) In the cubic and nearly-cubic materials, which are conventionally expected to have nearly isotropic gaps, there is often evidence of very strong gap anisotropy. Sometimes there even appear to be gap nodes, although the gap symmetry remains s-wave-like. (2) In the planar (quasi-two-dimensional) materials there are often gap nodes, but nevertheless they typically still have an s-wave-like gap symmetry.[Here s-wave-like means having the full point-group symmetry of the crystal structure, a condition which does allow the p...

s-wave pairing in the optimally doped LaO0.5F0.5BiSLamura1, Shiroka2, Bonfà3 et al. 2013Phys. Rev. B

“…We should recall that in extreme type-II superconductors with highly diluted superfluid density the presence of some anisotropy in the gap function is quite common. 32 Finally, it is worth noticing that our λ ab (0) value is comparable with those found in alkali-metal organicsolvent intercalated iron-selenide superconductors. 33 By considering an interlayer distance equal to the c-axis parameter 4 we can estimate a 2D superfluid density d/λ 2 ab (0) ∼ 0.0056 µm −1 .…”

d-Wave Superconductivity and s-Wave Charge Density Waves: Coexistence between Order Parameters of Different Origin and Symmetry

“…Patterns for both order parameters differ substantially. Since T c0 > T d0 in the whole relevant interval Equation (19), the critical temperature T c = T c0 is not affected by the dielectric gapping. The parameter ∆(0) rapidly decreases with σ 0 , since the upper part of the ∆(T )-dependence in Figure 17(b) is effectively cut away in comparison with typical theoretical or observed BCS-like curves, so that a conventional increase of ∆(T ) at low T is arrested for this set of problem parameters.…”

Angle-resolved photoemission observation of the superconducting-gap minimum and its relation to the nesting vector in the phonon-mediated superconductorYNi2B2C

“…On the other hand, regarded as phonon-mediated superconductors, 3-7 borocarbide YNi 2 B 2 C and LuNi 2 B 2 C have turned out to have large SC gap anisotropy with nodes. [8][9][10][11][12] Since such an extremely large gap anisotropy cannot be obtained from a simple s-wave paring wave function that is assumed for a phonon-mediated superconductor, it is surprising and a challenge of condensed matter physics 13 as to what kind of interactions play an essential role and how such an extremely large anisotropy is produced.For understanding the origin of the large SC gap anisotropy, it has been considered essential to determine the direction and type of nodal structure and to study its relation to nesting vector 14,15 reported in borocarbides. However, results are controversial: the magnetic field orientation dependence of thermal conductivity, 16 ultrasonic attenuation, 17 and scanning tunneling microscopy/spectroscopy in the vortex state 18 have reported point nodes located along ͓100͔ and ͓010͔ directions while field-angle-dependent heat capacity suggested a linelike nodal structure.…”

“…On the other hand, regarded as phonon-mediated superconductors, [3][4][5][6][7] borocarbide YNi 2 B 2 C and LuNi 2 B 2 C have turned out to have large SC gap anisotropy with nodes. [8][9][10][11][12] Since such an extremely large gap anisotropy cannot be obtained from a simple s-wave paring wave function that is assumed for a phonon-mediated superconductor, it is surprising and a challenge of condensed matter physics 13 as to what kind of interactions play an essential role and how such an extremely large anisotropy is produced.…”