Superconductors are classified by their pairing mechanism and the coupling strength, measured as the ratio of the energy gap, 2∆, to the critical temperature, T c . We present an extensive comparison of the 2∆/k B T c ratios among many single-and multiband superconductors from simple metals to high-T c cuprates and iron pnictides. Contrary to the recently suggested universality of this ratio in Fe-based superconductors, we find that the coupling in pnictides ranges from weak, near the BCS limit, to strong, as in cuprates, bridging the gap between these two extremes. Moreover, for Fe-and Cu-based materials, our analysis reveals a universal correlation between the gap ratio and T c , which is not found in conventional superconductors and therefore supports a common unconventional pairing mechanism in both families. An important consequence of this result for ferropnictides is that the separation in energy between the excitonic spin-resonance mode and the particle-hole continuum, which determines the resonance damping, no longer appears independent of T c .

We report the full complex dielectric function of high-purity Ba 0.68 K 0.32 Fe 2 As 2 single crystals with T c = 38.5 K determined by wideband spectroscopic ellipsometry at temperatures 10 T 300 K. We discuss the microscopic origin of superconductivity-induced infrared optical anomalies in the framework of a multiband Eliashberg theory with two distinct superconducting gap energies, 2 A ≈ 6 k B T c and 2 B ≈ 2.2 k B T c . The observed unusual suppression of the optical conductivity in the superconducting state at energies up to 14 k B T c can be ascribed to spin-fluctuation-assisted processes in the clean limit of the strong-coupling regime.

We studied phase separation in the single-crystalline antiferromagnetic superconductor Rb(2)Fe(4)Se(5) (RFS) using a combination of scattering-type scanning near-field optical microscopy and low-energy muon spin rotation (LE-μSR). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches 10  μm. By means of LE-μSR we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

No abstract

We investigated the ab-plane optical properties of single crystals of WTe2 for light polarized parallel and perpendicular to the W-chain axis over a broad range of frequency and temperature. At far-infrared frequencies, we observed a striking dependence of the reflectance edge on light polarization, corresponding to anisotropy of the carrier effective masses. We quantitatively studied the temperature dependence of the plasma frequency, revealing a modest increase of the effective mass anisotropy in the ab-plane upon cooling. We also found strongly anisotropic interband transitions persisting to high photon energies. These results were analyzed by comparison with ab initio calculations. The calculated and measured plasma frequencies agree to within 10% for both polarizations, while the calculated interband conductivity shows excellent agreement with experiment.