Direct observation of the superconducting energy gap in the optical conductivity of the iron pnictide superconductorBa(Fe0.9Co0.1<

Abstract

The temperature-dependent optical reflectivity and complex transmissivity of an epitaxially grown Ba͑Fe 0.9 Co 0.1 ͒ 2 As 2 thin film were measured over a wide frequency range ͑4 -35 000 cm −1 ͒. The opening of the superconducting gap 2⌬ 0 = ͑3.7Ϯ 0.3͒ meV is directly observed by vanishing optical conductivity at 30 cm −1 for T Ͻ T c = 20 K. While in this range the measured temperature-and frequency-dependent electrodynamic properties agree well with the BCS predictions of a nodeless order parameter, unexpectedly a strong quasiparticle absorption shows up below 1.5 meV. The spectral weight of the condensate 1.94ϫ 10 7 cm −2 corresponds to a penetration depth = 3600 Å.

Point Contact Spectroscopy Measurements of Ba(Fe0.96Co0.04)2As2Single Crystals

“…The gaps have two sizes -the small one with a strength up to the BCS weak coupling limit and the large one with a very strong coupling with 2∆/k B T c > 6-8. In the electron doped Ba(Fe 1−x Co x ) 2 As 2 systems contradicting results are published [4][5][6]. The most of the experiments including ours [4] find only a single gap with a strong coupling strength in optimally doped Ba(Fe 0.3 Co 0.7 ) 2 As 2 samples.…”

Highly anisotropic energy gap in superconductingBa(Fe0.9Co0.1)2

“…The low-frequency (terahertz-far-infrared) optical properties of the electrondoped 122 pnictides have recently been investigated by a few groups for doping levels near optimal. [27][28][29][30][31][32][33] There is, however, neither consensus in these reports about the gap sizes, nor about the number of gaps (although the majority of reports agrees on a two-gap picture). Obviously, the analysis of the optical spectra is challenged by the fact, that the gaps are likely to be different on different sheets of the FS.…”

Evolution of the optical spectrum with doping inBa(Fe1−xCox)2

“…4,5 Given that the low-energy spectrum contains some ambiguity due to experimental uncertainty and limitation in the energy region covered in the experiment, we cannot rule out a possibility that a second weak gap feature is hidden above the conductivity edge at ∼ 80 cm −127,28 or that finite conductivity remains in the region below 80 cm −1 which may show a gap feature indicative of a second smaller gap. 25,29 However, the steep drop of conductivity below 120 cm −1 toward 80 cm −1 is a robust result. So, if we take this as an s-wave gap edge, the gap magnitude would be 2∆ ∼ 80 cm −1 at T = 5 K for x = 0.06 corresponding to 2∆/k B T c ∼ 4.6 (probably, 2∆ for x = 0.08 is 50 cm −1 or lower).…”