Eliashberg approach to infrared anomalies induced by the superconducting state of Ba0.68K0.32Fe

Abstract

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.

Effect of heavy-ion irradiation on superconductivity in Ba0.6K0.4Fe2Salovich1, Kim2, Ghosh3 et al. 2013Phys. Rev. B182210View full textAdd to dashboardCite

“…In each case the BCS expression provided a superior fit to a power law. These gap values are in good agreement with STM [15] and ellipsometry [19] measurements that report with an s +− order parameter for which interband scattering produces midgap states, but not for an s ++ pairing state [4,5]. However, interband scattering (unless it is in the pure unitary limit) should also suppress C…”

On the structure of the superconducting order parameter in high-temperature Fe-based superconductors

“…This is because for s-wave superconductor at the temperatures below TC the reflectance approaches unity at energies ħ< 2Δ. As a result, a peak at ~ 160 cm −1 (19.8 meV) correlates with the magnitude of the larger superconducting gap L ≈ 10 meV [25][26][27]47]. The smaller gap is beyond the frequency range of our IR measurements.…”

“…No model has got an unambiguous experimental confirmation yet, thus making the main issues still unanswered. For example, the experimentally determined BCS ratio in Ba-122 family varies by a factor of six [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], for a review, see 12,34]. A possible reason is the strong out-of-plane gap anisotropy in k-space which seems to "smear" the gap values obtained in bulk probes, and a sensitivity-to-surface of the superconducting properties.…”

Quasiparticle interference in multiband superconductors with strong coupling

“…The maximum of the spectra is sf = 144 cm −1 , which determines the natural energy scale [28]. This spectrum gives a rather good description of thermodynamical [58] and optical [59,60] properties in the SC as well as normal states [61]. Moreover, we will use all temperatures and energies, expressed below, in the units of inverse cm (i.e., cm −1 In the strong-coupling approach, as opposed to the weakcoupling limit, the gap functions are complex and frequency dependentφ α =φ α (ω).…”