Far infrared reflectivity of La sub (2-x) Sr sub x CuO sub 4.

We have measured and modelled the far-infrared reflectivity of La sub (2-x) Sr sub x CuO sub 4 for x = 0.175 in the normal and superconducting states in the frequency range 20 cm sup (-1) to 350 cm sup (-1). We find a much stronger drop of the reflectivity with increasing frequency below 100 cm sup (-1) than is implied by the dc conductivity. In addition, we observe a resonance of large spectral weight at omega approx 250 cm sup (-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 ~ 80 cm sup (-1), which can be accounted for in terms of the unusual normal state properties of the material.