Doping-induced change of optical properties in underdoped cuprate superconductors

We report on the ab-plane optical reflectance measurements of single crysta ls of Y-doped Bi2Sr2CaCu2O8+delta and Pr-doped YBa2Cu3O7-delta over a wide frequency range from 80 to 40 000 cm(-1) (10 meV-5 eV) and at temperatures between 20 and :300 K. Y and Pr doping both decrease the hole concentration in the CuO2 planes. This has allowed us to investigate the evolution of ab -plane charge dynamics at doping levels ranging from heavily underdoped to nearly optimally doped. Our results of the low-frequency optical conductivi ty and spectral weight do not show any features associated with the normal- state pseudogap. Instead, one-component analysis for the optical conductivi ty shows the low-frequency depression in the scattering rate at T > T-c, si gnalling entry into the pseudogap state. Alternatively, no clear indication s of the normal-state pseudogap are detected in the temperature-dependent z ero-frequency free-carrier scattering rate by using two-component analysis. In the superconducting state, there is also no convincing evidence of supe rconducting gap absorption in all spectra. We find that there is a 'univers al correlation' between the numbers of carriers and the transition temperat ure. This correlation holds whether one considers the number of carriers in the superfluid or the total number of carriers.