Comparison of in- and out-of-plane charge dynamics in YBa₂Cu₃O_(6.95)

The in-plane optical conductivity has been successfully employed to obtain information about the coupling of the charge carriers to the spin degrees of freedom in the high T_c oxides. We investigate how this inelastic scattering affects the out-of-plane charge dynamics. We consider both coherent (in-plane momentum is conserved) and incoherent (no momentum conservation) c-axis transfers and find that the two cases give quite a distinct c-axis conductivity as a function of energy $\omega$. Comparison of our theoretical calculations with the available data does not allow a definitive conclusion, but a momentum dependent coherent matrix element characteristic of the CuO_2 chemistry is favored with the possibility of a subdominant incoherent contribution.