A. Iyo

Octet-Line Node Structure of Superconducting Order Parameter in KFe2As2

An Eight-Noded Monster In superconductors, electrons are bound into pairs, and the exact form of that pairing and the resulting energy gap can vary, depending on the details of the electron-electron interaction and the band structure of the material. The energy gaps of the recently discovered iron-based superconductors exhibit a variety of pairing functions. KFe2As2 has been suggested to have a d-wave gap, similar to cuprate superconductors. Okazaki et al. (p. 1314) use laser-based angle-resolved photoemission spectroscopy (ARPES) to map out the superconducting gap on three Fermi surfaces (FS) of the compound. They find a different gap structure on each, with the middle FS gap vanishing at eight distinct positions (nodes). It appears that the gap respects the tetragonal symmetry of the crystal, indicating (although the details may vary) the all iron-based superconductors have an extended s-wave–symmetric pairing—a finding that will help understanding of unconventional superconductivity. Laser-based photoemission spectroscopy is used to map out the pairing gap of an iron-based superconductor. In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe2As2 has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe2As2 is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with “octet-line nodes” on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A1g superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.

Anisotropic Energy Gaps of Iron-Based Superconductivity from Intraband Quasiparticle Interference in LiFeAs

Uneven Gap Electron pairs that are responsible for the phenomenon of superconductivity can only be broken by investing a finite amount of energy, called the energy gap. The size of the gap may depend on the position on the Fermi surface; in cuprates, the gap completely disappears at certain points. What happens in the pnictide superconductors is still a subject of debate, not least because there appear to be differences between the different pnictide families. Allan et al. (p. 563) used scanning tunneling spectroscopy to study the compound LiFeAs. The gap was mapped on three of the five bands on which the Fermi surface resides and was found to be anisotropic in momentum space. The energy needed to break up electron pairs in a pnictide superconductor depends on position on the Fermi surface. If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δi(k⃗) should appear on the different electronic bands i. Here, we introduce intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δik⃗. in such materials, focusing on lithium iron arsenide (LiFeAs). We identify the three hole-like bands assigned previously as γ, α2, and α1, and we determine the anisotropy, magnitude, and relative orientations of their Δik⃗. These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.

Gap in KFe2As2studied by small-angle neutron scattering observations of the magnetic vortex lattice

We report the observation, by small-angle-neutron-scattering (SANS), of magnetic flux lines “vortices” in super-clean KFe2As2 single crystals. The results show clear Bragg spots from a well ordered vortex lattice, for the first time in a FeAs superconductor. These measurements can give important information about the pairing state in this material, because the spatial variation of magnetic field in the vortex lattice reflects this pairing. With field parallel to the fourfold c-axis, nearly isotropic hexagonal packing of vortices was observed without VL-symmetry transitions up to high fields, indicating rather small anisotropy of the superconducting properties around this axis. This rules out gap nodes parallel to the c-axis, and thus d-wave and also anisotropic s-wave pairing. The strong temperature-dependence of the scattered intensity down to T Tc further indicates either widely different full gaps on different Fermi surface sheets, or nodal lines perpendicular to the axis. PACS numbers: 74.25.Uv, 74.70.Xa, 74.20.Rp, 74.25.-q

Splitting of resonance excitations in nearly optimally doped Ba(Fe0.94Co0.06)2As2: an inelastic neutron scattering study with polarization analysis.

Magnetic excitations in Ba(Fe0.94Co0.06)2As2: are studied by polarized inelastic neutron scattering above and below the superconducting transition. In the superconducting state, we find clear evidence for two resonancelike excitations. At a higher energy of about 8 meV, there is an isotropic resonance mode with weak dispersion along the c direction. In addition, we find a lower excitation at 4 meV that appears only in the c-polarized channel and whose intensity strongly varies with the l component of the scattering vector. These resonance excitations behave remarkably similar to the gap modes in the antiferromagnetic phase of the parent compound BaFe2As2.

Normal-state charge dynamics in doped BaFe2As2: Roles of doping and necessary ingredients for superconductivity

In high-transition-temperature superconducting cuprates and iron arsenides, chemical doping plays an important role in inducing superconductivity. Whereas in the cuprate case, the dominant role of doping is to inject charge carriers, the role for the iron arsenides is complex owing to carrier multiplicity and the diversity of doping. Here, we present a comparative study of the in-plane resistivity and the optical spectrum of doped BaFe2As2, which allows for separation of coherent (itinerant) and incoherent (highly dissipative) charge dynamics. The coherence of the system is controlled by doping, and the doping evolution of the charge dynamics exhibits a distinct difference between electron and hole doping. It is found in common with any type of doping that superconductivity with high transition temperature emerges when the normal-state charge dynamics maintains incoherence and when the resistivity associated with the coherent channel exhibits dominant temperature-linear dependence.

Identifying the 'fingerprint' of antiferromagnetic spin fluctuations in iron pnictide superconductors

The mechanism holding Cooper pairs together in iron-based superconductors is highly debated. Finding the fingerprint of the pairing mechanism would be a leap forward. Cooper pairing in the iron-based high-Tc superconductors1,2,3 is often conjectured to involve bosonic fluctuations. Among the candidates are antiferromagnetic spin fluctuations1,4,5 and d-orbital fluctuations amplified by phonons6,7. Any such electron–boson interaction should alter the electron’s ‘self-energy’, and then become detectable through consequent modifications in the energy dependence of the electron’s momentum and lifetime8,9,10. Here we introduce a novel theoretical/experimental approach aimed at uniquely identifying the relevant fluctuations of iron-based superconductors by measuring effects of their self-energy. We use innovative quasiparticle interference (QPI) imaging11 techniques in LiFeAs to reveal strongly momentum-space anisotropic self-energy signatures that are focused along the Fe–Fe (interband scattering) direction, where the spin fluctuations of LiFeAs are concentrated. These effects coincide in energy with perturbations to the density of states N(ω) usually associated with the Cooper pairing interaction. We show that all the measured phenomena comprise the predicted QPI ‘fingerprint’ of a self-energy due to antiferromagnetic spin fluctuations, thereby distinguishing them as the predominant electron–boson interaction.

Two distinct superconducting states in KFe2As2 under high pressure

We report measurements of ac magnetic susceptibility

Universality of the dispersive spin-resonance mode in superconducting BaFe2As2.

chi_{ac}

Anisotropy of the superconducting gap in the iron-based superconductor BaFe2(As1-xPx)2

and de Haas-van Alphen (dHvA) oscillations in KFe

Thermodynamic Study of Nodal Structure and Multiband Superconductivity of KFe2As2

_2