Far-infrared measurement of the gap of the high-Tc superconductor La1.85Sr0.15CuO4-x

We report on far infrared measurements of the energy gap of the high-temperature superconductor La1.85Sr0.15CuO4-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 cm-1 and a gaplike depression below 140 cm-1. The strong edge seen in reflectance at 50 cm-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 ε1(ω) caused by the strong phonons.