The far-infrared conductivity of oxide superconductors

A number of open questions remain about the infrared response of high-T_c superconductors. Although there is clear evidence for the formation of a superconducting condensate, there is no convincing data showing a superconducting gap absorption in the far infrared spectrum. Most of their spectral weight of the free carriers goes into the superconducting condensate in the superconducting state. Most cuprates are orthorhombic crystals, so there is anisotropy in their transport and optical properties. In YBa₂Cu₃O_7-8, the anisotropy of the London penetration depth shows that the chains contribute strongly to the superfluid. In Bi₂Sr₂CaCu₂O₈, where chains are absent, there is still a definite anisotropy to the far-infrared absorption, with a finite absorption for E∥b down to -20 meV. This anisotropy of the ab plane could be due either to anisotropy of the superconducting gap or to anisotropy of the midinfrared component to the conductivity.