Optical properties of the iron arsenic superconductor BaFe1.85Co0.15As2

Abstract

The transport and complex optical properties of the electron-doped iron-arsenic superconductor BaFe1.85Co0.15As2 with Tc=25 K have been examined in the Fe-As planes above and below Tc. A Bloch-Grüneisen analysis of the resistivity yields a weak electron-phonon coupling constant λph≃0.2. The low-frequency optical response in the normal state appears to be dominated by the electron pocket and may be described by a weakly interacting Fermi liquid with a Drude plasma frequency of ωp,D≃7840 cm−1 (≃0.972eV) and scattering rate 1/τD≃126 cm−1 (≃15 meV) just above Tc. The frequency-dependent scattering rate 1/τ(ω) has kinks at ≃12 and 55 meV that appear to be related to bosonic excitations. Below Tc the majority of the superconducting plasma frequency originates from the electron pocket and is estimated to be ωp,S≃5200 cm−1 (λ0≃3000 Å) for T⪡Tc, indicating that less than half the free carriers in the normal state have collapsed into the condensate, suggesting that this material is not in the clean limit. Supporting this finding is the observation that this material falls close to the universal scaling line for a Bardeen, Cooper, and Schrieffer dirty-limit superconductor in the weak-coupling limit. There are two energy scales for the superconductivity in the optical conductivity and photoinduced reflectivity at Δ1(0)≃3.1±0.2 meV and Δ2(0)≃7.4±0.3 meV. This corresponds to either the gapping of the electron and hole pockets, respectively, or an anisotropic s-wave gap on the electron pocket; both views are consistent with the s± model.

Source-reported materials — not catalogue approval

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