An Introduction to Infrared Properties of High-Tc Superconductors

The essential element in the high Tc cuprate superconductors appears to be the CuO2 planes which, of course must reside in an electropositive environment (Tokura). Recognizing this, and focussing only on the properties of the CuO2 planes, as distinct from that of their environment, allows a relatively simple picture of the phenomenology of infrared properties to be obtained. In particular one finds that the normal state conductivity drops unusually slowly as a function of frequency, more like 1/ω0.8 than the conventional 1/ω2 and that it can be described in terms of a scattering rate that is linear in frequency. This linear frequency dependence appears to be fundamentally related to the linear temperature dependence of the resisitivity, and may also be related to a linear photoemission linewidth and a broad electronic background signal observed in Raman experiments. In the superconducting state a characteristic energy scale of -500 cm-1 (2Δ/kTc≃8) develops, and can be reasonably identified as a pair-excitation threshold (superconducting gap) at low temperature. Aspects of this feature which are similar to and different from expectations based on conventional BCS theory are discussed. In particular, one finds that the temperature dependence is quite unusual.