Far-infrared measurement of the gap of the high-$T_c$ superconductor ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{x}}$

We report on far infrared measurements of the energy gap of the high-temperature superconductor ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{x}}$. From the reflectance, measured over a wide range of frequencies, we calculate the optical conductivity. It shows phononlike lines at 239, 355, and 494 ${\mathrm{cm}}^{\mathrm{-}1}$ and a gaplike depression below 140 ${\mathrm{cm}}^{\mathrm{-}1}$. The strong edge seen in reflectance at 50 ${\mathrm{cm}}^{\mathrm{-}1}$, and misidentified by a number of investigators as the energy gap, does not appear in the conductivity. It is due to a zero crossing of ${\epsilon }_1$(ω) caused by the strong phonons.