Indirect exchange pairing: a possible key to high-temperature superconductivity

Quantitative agreement with experiment provided by a recently proposed indirect-exchange mechanism of pairing in high-Tc superconductors is illustrated for several specific properties. Within this pairing mechanism, conduction electrons couple due to an indirect-exchange interaction between them, via closed-shell oxygen anions. A striking prediction of the theory is the occurrence of a maximum in Tc with increasing number of CuO2 layers. In the same framework the recently observed steep decrease in Tc in Bi2(Ln1-xCexO)2Sr2Cu2O8 (Ln = Sm, Eu or Gd) and BaPb1-xSbxO3, relative to Bi2CaSr2Cu2O8 and BaPb1-xBixO3, respectively, can be readily understood. The theory accounts quantitatively for the observed Tc of 60 K in the new double-layer superconductor La1.60Sr0.40CaCu2O6. The calculated pressure gradient of Tc in (La, Ba)-Cu-O is in agreement with experiment and some hole-doped superconductors are predicted to have negative pressure gradients of Tc. In Bi2(Ln1-xCexO)2Sr2Cu2O8 the value of d ln Tc/dP is predicted to be 0.6·10-2kbar-1