The ${\mathrm{CuO}}_2$-plane optical reflectance of superconducting ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_4$ thin films (${\mathit{T}}_{\mathit{c}}$≃31 K) has been measured over a wide frequency and temperature range. The optical conductivity in the normal state is well described by a temperature-dependent weak-coupling (λ≊0.25) free-carrier term plus an overdamped, weakly temperature-dependent, midinfrared component. The free-carrier plasma frequency is nearly constant, ${\mathrm{\omega }}_{\mathit{p}\mathit{D}}$=6300 ${\mathrm{cm}}^{\mathrm{-}1}$, whereas the relaxation rate varies linearly with temperature above ${\mathit{T}}_{\mathit{c}}$. In the superconducting state, according to our two-component approach, most of the Drude oscillator strength condenses to a δ(ω) function. A two-fluid analysis gives a rapid drop in the quasiparticle damping rate below ${\mathit{T}}_{\mathit{c}}$. A reasonable estimate (∼2750 Å) for the ab-plane London penetration depth is obtained from the superfluid density. We observe that the midinfrared strength increases below ${\mathit{T}}_{\mathit{c}}$, suggesting that some (∼15%) of the free carriers do not condense into superconducting pairs and may have a strong interaction with pair-breaking excitations. Two absorption edges around 80 ${\mathrm{cm}}^{\mathrm{-}1}$ (3.7 ${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$) and 400 ${\mathrm{cm}}^{\mathrm{-}1}$ (18 ${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$) are seen but neither is assigned to the superconducting gap. Comparisons with a one-component picture described by a frequency-dependent scattering rate and effective mass are made and discussed. The far-infrared ab-plane phonons show systematic changes with temperature, which are associated with the structural transition near 250 K.

• Received 14 September 1992

DOI:https://doi.org/10.1103/PhysRevB.47.1036