Theab-plane optical conductivity of high-Tc superconductors

There is anisotropy in theab-plane optical properties of the high-temperature superconductors, both in the normal state and in the superconducting state. In both states, two components appear in the optical conductivity: a free carrier part and a midinfrared component. BelowT_c, the free carriers form the superconducting condensate. In YBa₂Cu₃O_7–, the anisotropy of the penetration depth shows that the chains contribute strongly to this superfluid. In Bi₂Sr₂CaCu₂O₈, where chains are absent, there is stillab plane anisotropy. BelowT_c a finite absorption parallelb remains at frequencies as small as 20 meV. This anisotropy could be due to anisotropy either of the superconducting gap or the midinfrared component.