Optical absorptivity of La1.87Sr0.13CuO4 below the superconducting plasma edge

We have directly measured the far-infrared absorptivity of single crystal L a1.87Sr0.13CuO4 at 2 K for radiation between 6 and 420 cm(-1) polarized alo ng the c axis. We have computed the dielectric functions epsilon(c)(omega) and conductivity sigma(c)(omega) using a Kramers-Kronig analysis. The exist ence of nonzero losses below the plasma edge and down to the lowest frequen cy measured is consistent with a model of a superconducting gap with nodes. From the calculation of epsilon(c)(omega), We deduce a value of 230 +/- 10 cm(-1) for the unscreened plasma frequency in the superconducting state. A small peak in the sigma(1)(c)(omega) is seen near 10 cm(-1). [S0163-1829(9 9)04701-3].