Two superconducting gaps and electron-phonon coupling in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$

We report measurements of the superconducting gaps and the electron-phonon coupling in untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ (Y 1:2:4) single crystals by electronic and phononic Raman scattering. For xx- and, to a lesser extent, also for yy-polarized light, a scattering continuum that decreases in the superconducting state below 325 ${\mathrm{cm}}^{\mathrm{-}1}$ (thus signaling a gap 2${\mathrm{\Delta }}_2$ such that 2${\mathrm{\Delta }}_2$/${\mathit{kT}}_{\mathit{c}}$≊6.5) is observed. We measure, in yy polarization only, another sharp decrease of the electronic scattering below 113 ${\mathrm{cm}}^{\mathrm{-}1}$, indicating a second gap 2${\mathrm{\Delta }}_1$, such that 2${\mathrm{\Delta }}_1$/${\mathit{kT}}_{\mathit{c}}$≊2.3. The existence and the size of both gaps are confirmed by the observation of phonon anomalies at ${\mathit{T}}_{\mathit{c}}$ for nearly all ${\mathit{A}}_{\mathit{g}}$ phonons. We conjecture that the smaller gap ${\mathrm{\Delta }}_1$ occurs at the chain bands and probably also exists in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (Y 1:2:3) but cannot be observed by Raman scattering.