STRONG DAMPING OF THE C-AXIS PLASMON IN HIGH-T-C CUPRATE SUPERCONDUCTORS

We analyze the infrared reflectivity of La1.85Sr0.15CuO4 single crystals with E parallel to the c-axis. The plasma edge at around 6 meV (50 cm(-1)), which occurs only for T <T-c, is due to Cooper-pair tunneling. This low value of the plasma edge is shown to be consistent with the c-axis plasma frequency obtained from LDA band structure calculations (> 0.1 eV) if we take into account that the single-particle charge transport along the c-axis is strongly incoherent in the normal state, and remains so in the superconducting state. From a comparison of the optical conductivity with model calculations based on s- and d-wave weak-coupling theory in the dirty limit, we find no evidence for a reduction of the c-axis quasi-particle scattering rate below T-c. The c-axis scattering rate, the normal-state c-axis plasma frequency, and T-c obey h gamma > h nu(p) >> 3.5k(B)T(c), which is exactly opposite to the clean limit.