Far-infrared reflectivity of La2-xSrxCuO4

We have measured and modeled the far-infrared reflectivity of La2-xSrxCuO4 for x=0.175 in the normal and superconducting states in the frequency range 20 to 350 cm-1. We find a much stronger drop of the reflectivity with increasing frequency below 100 cm-1 than is implied by the dc conductivity. In addition, we observe a resonance of large spectral weight at 250 cm-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 cm-1, which can be accounted for in terms of the unusual normal-state properties of the material.