Abstract
The frequencies of allB1u(z) phonon modes predicted by a group-theoretical analysis were measured and found to agree well with recent lattice dynamical calculations for this compound. We report also the determination of two superconducting gap values in YBa2Cu4O8 through phonon self-energy effects in the normal and superconducting conducting state. The gap-to-T c ratios obtained from an analysis of these effects are 2Δ1/kT c ≦2.5 and 5.8≦2Δ2/kT c ≦9.2. This coincides with previous results of both phononic and electronic Raman scattering where values of 2.1 and 6.3 were found. We further find anomalous softenings of two phonon modes ∼40 Kabove T c , which correlate with an observed deviation from the linear temperature dependence of the average plasma frequency ν p (T).
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