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

Abstract

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 Tl 2Ba 2Ca 2Cu 3O 10 and normal incidence reflectometry for single crystals of La 1.85Sr 0.15CuO 4. In Tl 2Ba 2Ca 2Cu 3O 10 we observe no c-axis optical plasmon either above or below T c . For E// c in La 2− x Sr x CuO 4, 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 2 k B T c for T→0. The electronic contribution to σ(ω)≈7ω −1 cm −1 is independent of frequency up to several hundred cm −1, which implies that the optical scattering rate (τ c −1) is large compared to the c-axis plasma frequency (ω pc). We prove experimentally that h ̵ /τ c h ̵ ω pc⪢ 3.5k ET 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 δ-function at zero frequency). As a result the plasma edge in the superconducting state occurs at about 3.4(k BT c h ̵ /τ) 0.5 ω pc⪡3.5k BT c ⪡ h ̵ ω pc .