Evidence of the pseudogap and superconducting gap in theab-plane electronic Raman continuum ofYBa2Cu4O8

Abstract

In contrast to a number of prior studies, our plane-polarized Raman measurements of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ single crystals display evidence of pseudogap and superconducting gap effects. With decreasing temperature in the normal state a continuous depletion of spectral weight from the electronic continuum is observed below 250 K, extending to around 1200 ${\mathrm{cm}}^{\ensuremath{-}1}.$ The normal state depletion is immediately reversed as the sample is cooled below ${T}_{c}$ and a very broad peak forms in the electronic continuum. A broad feature peaked at a Raman shift of 850 ${\mathrm{cm}}^{\ensuremath{-}1}$ also emerges around ${T}_{c},$ and appears to be linked with two-phonon Raman scattering. By application of a simple model, the phonons and pseudogap depletion are removed from the spectra revealing a normal-state electronic Raman continuum with a frequency and temperature dependence typical of a high-${T}_{c}$ cuprate superconductor.