THE AB-PLANE OPTICAL CONDUCTIVITY OF HIGH-T-C SUPERCONDUCTORS

There is anisotropy in the ab-plane optical properties of the high-tem perature superconductors, both in the normal state and in the supercon ducting state. In both states, two components appear in the optical co nductivity: a free carrier part and a ''midin-frared'' component. Belo w T-c, the free carriers form the superconducting condensate. In YBa2C U3O7-delta, the anisotropy of the penetration depth shows that the cha ins contribute strongly to this superfluid. In Bi2Sr2CaCu2O8, where ch ains are absent, there is still ab plane anisotropy. Below Te a finite absorption parallel b remains at frequencies as small as 20 meV. This anisotropy could be due to anisotropy either of the superconducting g ap or the midinfrared component.