The energy gap of the high Tc superconductor La1.85Sr0.15CuO4

We have measured the energy gap of a polycrystalline sample of La1.85Sr0.15CuO4 using far-infrared reflectance. The sample was prepared by sintering and has a superconducting transition at 36 ±1 K (mid point of a 2K wide region) both in resistance and magnetic susceptibility. The Meissner effect was incomplete and there was considerable absorption below the gap frequency in the superconducting state. The change in far infrared absorption on going to the superconducting state was fitted to the theory of Mattis and Bardeen, yielding an energy gap of (3.2 ± 0.3) kTc withTc= 36 K