PAIR DENSITY AND GAPLESSNESS IN HIGH-T(C) CUPRATES

The results of muSR and heat capacity studies on high-T(c) superconducting cuprates, particularly in the overdoped regime, are presented, and it is shown that the pair density falls away to zero on either side of optimum doping. This is modelled in terms of a finite pair lifetime and depression of the condensate density through intrinsic pairbreaking interactions. This behavior appears to underlie the generic dependence of T(c) upon doped hole concentration. The emerging picture of gapless or near gapless superconductivity in the high-T(c) cuprates is confirmed from infrared studies both in our laboratory and elsewhere. muSR is also used to prove the rather high density of pairs which proximity-condense on the CuO chains in 123, 247, and 124. This is most clearly exposed through studies on the model system Yb1-xCaxBa1.6Sr0.4Cu3O7-delta which allows pair breaking on the chains and planes to be independently manipulated through x and delta.