a-Axis optical conductivity of detwinned ortho-II YBa2Cu3O6.50

The a-axis optical properties of a detwinned single crystal of YBa2Cu3O6.50 in the ortho-II phase (Ortho-II Y123, T-c=59 K) were determined from reflectance data over a wide frequency range (70-42 000 cm(-1)) for nine temperature values between 28 and 295 K. Above 200 K the spectra are dominated by a broad background of scattering that extends to 1 eV. Below 200 K a shoulder in the reflectance appears and signals the onset of scattering at 400 cm(-1). In this temperature range we also observe a peak in the optical conductivity at 177 cm(-1). Below 59 K, the superconducting transition temperature, the spectra change dramatically with the appearance of the superconducting condensate. Its spectral weight is consistent, to within experimental error, with the Ferrell-Glover-Tinkham (FGT) sum rule. We also compare our data with magnetic neutron scattering on samples from the same source that show a strong resonance at 31 meV. We find that the scattering rates can be modeled as the combined effect of the neutron resonance and a bosonic background in the presence of a density of states with a pseudogap. The model shows that the decreasing amplitude of the neutron resonance with temperature is compensated for by an increasing of the bosonic background yielding a net temperature-independent scattering rate at high frequencies. This is in agreement with the experiments.