Determination of the condensate from optical techniques in unconventional superconductors

The optical properties of superconductors, in particular the real parts of the conductivity and the dielectric function, may he used to calculate the strength of the condensate. In systems where all the free carriers collapse into the condensate, this approach works well. However, in gapless systems, and in unconventional systems in which the superconducting energy gap is observed to have zeros at the Fermi surface. there is usually considerable amount of residual conductivity at low frequency. This is the case in many of the cuprate-based high-temperature superconductors. In particular. YBa2Cu3O7-delta is almost perfectly stoichiometric with little chemical disorder(delta approximate to 0), yet there is a. large amount of residual conductivity both in the copper-oxygen planes as well as perpendicular to the planes for T much less than T-c, due to the presumed unconventional nature of the energy gap. The assumption that the condensate dominates the optical response at low frequencies lends to optical estimates for the condensate which are too large. However, the microwave surface reactance depends only upon the condensate and is not affected by the presence of residual conductivity (which affects the surface resistance), thus allowing an unambiguous determination of the strength of the condensate. When optical techniques are used in conjunction with microwave techniques. a more complete physical picture emerges. This problem is examined and resolved for the oxygen 'overdoped' YBa2Cu3O6.99 material along the c axis. (C) 1998 Elsevier Science B.V.