Optical properties and electron-phonon interactions in superconducting transition metals

The infrared optical properties of various high-temperature superconductors are analyzed using a Drude form of the dielectric function ε(q,ω). By plotting the imaginary part ωε₂ vs ε₁, we identify the interband contributions, and estimate the plasmon damping Γ from the slope in the frequency range where the Drude form is justified. Thus we estimate the electron-phonon coupling parameter λ which enters the BCS expression for the superconducting transition temperature Tc. Independently we estimate λ from resistivity data, for temperatures $T>300$ ^∘{}K using the plasma frequency extracted from the optical data. For Nb the analysis gives good agreement for λ0.7 obtained from optical, resistance, and Tc data. For the A-15 compounds Nb₃Ge, Nb₃Sn, and V₃Si the λ values estimated from the optical data are in accord with the resistance measurements but generally are found to be much too small, typically λ<~0.2, to account for the high superconducting temperatures (Tc~20 ^∘{}K) of these materials.