Damping mechanism of the strongly renormalized c-axis plasma frequency in high-Tc cuprates

We study the charge dynamics of high-T-c superconductors with the electric field perpendicular to the planes, using polarized oblique-incidence reflectometry for thin films of Tl2Ba2Ca2Cu(3)O(10) and normal incidence reflectometry for single crystals of La1.85Sr0.15CuO4. In Tl2Ba2Ca2Cu3O10 we observe no c-axis optical plasmon either above or below T-c. For E//c in La2-xSrxCuO4, no plasmon is observed in the normal state, but as soon as T drops below T-c, a plasma edge in the reflectivity occurs, which moves up to about 2k(B)T(c) for T --> 0. The electronic contribution to sigma(omega) approximate to 7 Ohm(-1)cm(-1) is independent of frequency up to several hundred cm(-1), which implies that the optical scattering rate (tau(c)(-1)) is large compared to the c-axis plasma frequency (omega(pc)) We prove experimentally that h/tau(c) > h omega(pc) >> 3.5k(B)T(c), hence the c-axis response is in a universality class different from the dirty (Mattis-Bardeen) limit, and the absence of a plasma edge in the normal state is due to overdamping. Recent experimental data of various groups show that this is a generic feature of high-T-c cuprates. There is no need to assume a large many-body reduction of the ab initio LDA-RPA plasma frequency along the c-axis. Yet the overdamping of the plasmon could be a many-body effect. The 'confinement' to the planes as was proposed by Anderson is therefore due to a strong scattering in the c-direction. In the dirty Limit only the relatively small amount of oscillator strength in the below-gap region is transferred to the pair-response (the delta-function at zero frequency). As a result the plasma edge in the superconducting state occurs at about 3.4(k(B)T(c)h/gamma)(0.5)omega(pc)