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-Gruneisen analysis of the resistivity yields a weak electron-phonon coupling constant lambda_ph ~ 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 omega_p,D ~ 7840 cm-1 (~ 0.972 eV) and scattering rate 1/tau_D ~ 125 cm-1 (~ 15 meV) just above Tc. The frequency-dependent scattering rate 1/tau(omega) 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 omega_p,S ~ 5200 cm-1 (lambda0 ~ 3000 Angstroms) 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 BCS dirty-limit superconductor in the weak-coupling limit. There are two energy scales for the superconductivity in the optical conductivity and photo-induced reflectivity at Delta1 ~ 3.1 +/- 0.2 meV and Delta2 ~ 7.4 +/- 0.3 meV. This corresponds to either the gaping 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.

Revised version (expanded discussion, additional references): 11 pages, one table and 8 figures

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• Infrared Measurement of the Pseudogap in P-Doped and Co-Doped BaFe2As2 High-Temperature Superconductors
• Optical conductivity of iron-based superconductors
• Anisotropic quasiparticle lifetimes in Fe-based superconductors
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• Unconventional Hall effect in pnictides from interband interactions
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• Extended Drude model and role of interband transitions in the mid-infrared spectra of pnictides
• Anomalous proximity effects at the interface of s and s+- superconductors
• Anomalous phonon redshift in K-doped BaFe2As2 iron pnictides
• Effective medium approximation and the complex optical properties of the inhomogeneous superconductor K_{0.8}Fe_{2-y}Se_2
• Coexistence of superconductivity and density wave in $\mathrm{B}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_2\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_4\mathrm{O}$: an optical spectroscopy study
• Infrared probe of the gap evolution across the phase diagram of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$
• Electron-phonon coupling in the superconducting single crystal Rb${}_{0.8}$Fe${}_{1.68}$Se${}_2$
• Role of electron-electron interactions in the charge dynamics of rare-earth-doped CaFe2As2
• Optical spectroscopy study of the collapsed tetragonal phase of CaFe${}_2$(As${}_{0.935}$P${}_{0.065}$)${}_2$ single crystals