Studies of the infrared properties of the high T c superconductors are reviewed, with particular emphasis on attempts to determine the energy gap by far infrared spectroscopy and on the properties of the strong absorption that occurs in the mid infrared. We argue that this mid-infrared absorption is a direct particle-hole excitation rather than a Holstein emission process. In addition, we conclude that although the energy gap is not easily observed, several recent experiments place it in the weak to moderate strong coupling range.
Preparation, characterization, and far-infrared study of ceramic high-Tc superconductors