Two superconducting gaps and electron-phonon coupling in YBa2Cu4O8.

We report measurements of the superconducting gaps and the electron-phonon coupling in untwinned YBa{sub 2}Cu{sub 4}O{sub 8} (Y 1:2:4) single crystals by electronic and phononic Raman scattering. For {ital xx}- and, to a lesser extent, also for {ital yy}-polarized light, a scattering continuum that decreases in the superconducting state below 325 cm{sup {minus}1} (thus signaling a gap 2{Delta}{sub 2} such that 2{Delta}{sub 2}/{ital kT}{sub {ital c}}{approx}6.5) is observed. We measure, in {ital yy} polarization only, another sharp decrease of the electronic scattering below 113 cm{sup {minus}1}, indicating a second gap 2{Delta}{sub 1}, such that 2{Delta}{sub 1}/{ital kT}{sub {ital c}}{approx}2.3. The existence and the size of both gaps are confirmed by the observation of phonon anomalies at {ital T}{sub {ital c}} for nearly all {ital A}{sub {ital g}} phonons. We conjecture that the smaller gap {Delta}{sub 1} occurs at the chain bands and probably also exists in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (Y 1:2:3) but cannot be observed by Raman scattering.