Far-infrared reflectivity ofLa2−xSrxCuO4

We have measured and modeled the far-infrared reflectivity of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ for x=0.175 in the normal and superconducting states in the frequency range 20 to 350 ${\mathrm{cm}}^{\mathrm{-}1}$. We find a much stronger drop of the reflectivity with increasing frequency below 100 ${\mathrm{cm}}^{\mathrm{-}1}$ than is implied by the dc conductivity. In addition, we observe a resonance of large spectral weight at ω≊250 ${\mathrm{cm}}^{\mathrm{-}1}$ which is absent or much weaker in the insulating x=0 compound. Below the superconducting temperature of the x=0.175 sample we observe an additional sharp feature at ≊80 ${\mathrm{cm}}^{\mathrm{-}1}$, which can be accounted for in terms of the unusual normal-state properties of the material.