Energy Scales in the High-T c Superconductor YBa2Cu3O6+x

The optical conductivity sum rule is used to examine the evolution of the spectral weight N(ω) in both the normal and superconducting states of optimally and underdoped YBa₂Cu₃O_6+x along the a axis. Differences in N(ω) above and below T _c allow the strength of the superconducting condensate ρ_s to be determined. In the optimally-doped material, ρ_s is fully formed at energies comparable to the full superconducting gap maximum (?0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (?0.6 eV). This difference is discussed in terms of normal-state properties.