The Superconducting Energy Gap in High Tc Materials

The optical properties of several high temperature superconductors, especially Ba sub 2 YCu sub 3 O sub (6+x), are beginning to provide some reproducible results which establish constraints on theoretical models. I will review spectroscopic measurements over a frequency range from a few meV to a few eV which have allowed the determination of the real and imaginary parts of the conductivity. Several groups, including ones at AT&T, Florida, IBM, McMaster, and Regensberg, have measured bulk cystals or highly-aligned films of sufficient quality to allow a reasonable consideration of possible intrinsic behavior. Results on improved materials now show a mid-infrared absorption, i.e., conductivity above that expected from a frequency-independent relaxation rate (contrary to initial claims by three groups). At the low energy edge of this region all groups find a knee in the reflectivity. This knee occurs at ~50meV, independent of T sub c, and is seen in the normal state. Some groups have argued that this feature is the superconducting energy gap (with 2DELTA-8ksub B T sub c), but others have found indications of an additional feature at lower energy (near or below 3.5 k sub B T sub c).