Evidence for a pseudogap in underdoped Bi₂Sr₂CaCu₂O_(8+δ) and YBa₂Cu₃O_(6.50) from in-plane optical conductivity measurements

The real part of the in-plane optical self-energy data in underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) and ortho II YBa$_{2}$Cu$_{3}$O$_{6.5}$ contains new and important information on the pseudogap. Using a theoretical model approach we find that the density of state lost below the pseudogap $\Delta_{pg}$ is accompanied with a pileup just above this energy. The pileup along with a sharp mode in the bosonic spectral function leads to an unusually rapid increase in the optical scattering rate and a characteristically sloped peak in the real part of the optical self-energy. These features are not found in optimally doped and overdoped samples and represent a clearest signature so far of the opening of a pseudogap in the in-plane optical conductivity.