Two superconducting gaps and electron-phonon coupling in YBa2Cu4O8

Abstract

We report measurements of the superconducting gaps and the electron-phonon coupling in untwinned YBa2Cu4O8 (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 cm−1 (thus signaling a gap 2Δ2 such that 2Δ2/kTc≊6.5) is observed. We measure, in yy polarization only, another sharp decrease of the electronic scattering below 113 cm−1, indicating a second gap 2Δ1, such that 2Δ1/kTc≊2.3. The existence and the size of both gaps are confirmed by the observation of phonon anomalies at Tc for nearly all Ag phonons. We conjecture that the smaller gap Δ1 occurs at the chain bands and probably also exists in YBa2Cu3O7−δ (Y 1:2:3) but cannot be observed by Raman scattering. The larger gap Δ2 corresponds to the plane bands. 2Δ2 seems to be approximately the same as in Y 1:2:3, although Tc is 20% lower. Since, at low temperature, the electronic scattering intensity increases continuously from, at least, 35 cm−1 to 2Δ1 or 2Δ2, respectively, we suggest that both gap functions are anisotropic in k space. We draw three conclusions: First, for studies of superconducting Y 1:2:3 and Y 1:2:4, a gap in the chain-band Fermi surfaces must be taken into account. Second, values of 2Δ/kTc higher than the BCS value of 3.5 could be the result of gap anisotropy or multiple gaps rather than strong coupling. Third, the electron-phonon coupling observed by phonon anomalies at Tc is by no means specific of vibrations of the CuO2 plane oxygens; on the contrary, the effects of the Ba and chain copper Ag phonons are stronger in Y 1:2:4. We also demonstrate that the Raman scattering amplitude for the chain copper phonon has a different sign for xx and yy scattering, since the Fano line-shape coefficient q has the opposite sign in both cases. Finally, we show that the electronic Raman scattering intensity increases at the longer laser wavelengths, and we demonstrate that the opening of the superconducting gap can be directly observed in resonant Raman scattering as enhancements in the Raman efficiencies below Tc. We suggest an explanation for the unexpected dependence of the phonon intensities on temperature observed in the normal state based on a resonance between the phonon energies and the energy separation between the two chain bands.

Source-reported materials — not catalogue approval

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