OPTICAL STUDIES OF UNTWINNED Y2BA4CU6+NO14+N(N = 1,2) - INTERPRETATION OF SUPERCONDUCTING GAPS

We have measured the polarized-reflectivity spectra of Y2Ba4Cu6+nO14+n (n = 1,2) at 6 and 300 K. These substances are not twinned because of thermally stable double CuO chains instead of single chains as in YBa2Cu3O7. The light was polarized along the two plane directions a and b. We have found a strong contribution of the chain (E parallel-to b) to the optical conductivity and thus an anisotropy of about 1:3. At 300 K, a flat plateau in the reflectance below 60 meV has been attributed to phonon interactions. As regards the interband transitions, the charge-transfer gap seems to be above 2.5 eV. The ir scattering rate of the charge carriers in the plane, sigma(a)(omega), is linear from 0.05 up to 0.8 eV. In the superconducting state, plasma edges have developed between 11 and 21 meV, respectively, which we assume to be determined rather by the density of superconducting carriers than by a gap. Good fits are obtained by a two-fluid model.