ANOMALOUS COMPOSITION DEPENDENCE OF T-C NEAR T-C(MAX) AND MICROSTRUCTURE OF YBA2CU3O7-DELTA

Accurate measurements of T-c(delta) and Delta C-p(delta)/T-c(delta) in the titled compound have shown sharp breaks in slope at or very near delta=delta(0)=0.125. This behavior is explained by a supercell model which is similar to one previously proposed to explain the formation of the low-temperature tetragonal phase centered on x=x(0)=0.125 in La-2_x(Ba,Sr)(x)CuO4 alloys. Supercell ordering of resonant pinning centers is essential to produce coherent electron-phonon interactions mediated by such centers. Thus these data provide evidence for the crucial element in the quantum percolation model of high-temperature superconductivity.