Infrared properties of high T_c superconductors

Over the past several years a coherent phenomenology of the high Tc cuprate superconductors has begun to emerge. Infrared measurements have contributed several important ingredients to this picture including: 1) the inference of a scattering rate that is linear in frequency for ω>T, and of order ω, --2) a characteristic energy scale in the superconducting state of 500 cm-1 (60 meV), which can be interpreted as a superconducting pair excitation threshold or energy gap, and --3) evidence for very unusual temperature dependence in the vicinity of Tc. An attempt to describe these aspects of the data is presented here. © 1991.