OPTICAL STUDIES OF THE A-AXIS, B-AXIS, AND C-AXIS CHARGE DYNAMICS IN YBA2CU3O6+X

Optical and Raman-scattering studies of a-, b-, and c-axis YBa2Cu3O6+x between 0.025 and 5.5 eV are presented as a function of doping, and compared to the results of Bi2Sr2CaCU2O8 and La2-xSrxCuO4. Our doping-dependence studies show that the redistribution of spectral weight in the CuO2 planes of YBa2Cu3O6+x differs significantly from that seen in La2-xSrxCuO4. We also find that the redistribution of spectral weight in the cuprates is primarily responsible for the loss of two-magnon Raman-scattering intensity with doping. Finally, we show that bound-carrier contributions comprise a significantly larger fraction of the spectral weight below 1 eV in lower-T(c) cuprates such as the 2:1:4 compounds than in higher-T(c) cuprates such as YBa2Cu3O6+x and Bi2Sr2CaCu2O8. We suggest that the low-frequency conductivity (< 1 eV) in the 2:1:4 compounds is most appropriately described by a two-component picture, while that in YBa2Cu3O6+x and Bi2Sr2CaCu2Os is adequately described as a single component of strongly interacting carriers. In the metallic phase we find several interesting consequences of a single-component interpretation of the optical data in YBa2Cu3o6+x, such as a linear-in-omega frequency-dependent scattering rate and an increase in the interaction strength with decreased carrier density. Finally, we show that the c-axis optical response in YBa2Cu3O7 (T(c) approximately 90 K) is characterized by a c-axis polarized Raman continuum and a Drude conductivity arising from interbilayer charge transport along the c direction. With decreased doping, the c-axis Drude response decreases dramatically, indicating a decoupling of the CuO2 plane bilayers in YBa2Cu3O6+x. By comparison, the ab-plane optical response is not strongly influenced by interbilayer decoupling, suggesting that the unusual ab-plane charge dynamics in YBa2Cu3O6+x persist in nearly isolated CuO2 plane bilayers.