GRAZING-INCIDENCE INFRARED REFLECTIVITY OF LA1.85SR0.15CUO4 AND NBN

Infrared reflectivity measurements, using p-polarized light at a grazi ng angle of incidence, show an increased sensitivity to the optical co nductivity of highly reflecting superconducting materials. We demonstr ate that when this measurement technique is applied to the conventiona l s-wave superconductor NbN, the results are in perfect agreement with BCS theory. For the in-plane response of a La1.85Sr0.15CUO4 Single cr ystal, in the superconducting state, we find a reduction of the optica l conductivity in the frequency range below 20 meV. The observed frequ ency dependence excludes an isotropic s-wave gap, but agrees well with model calculations assuming a d-wave order parameter.