Ap. Saiko et Ve. Gusakov, ON FORMATION OF 2 SUPERCONDUCTING PHASES WITH VARYING OXYGEN NONSTOICHIOMETRY IN Y-BA CUPRATES, Fizika nizkih temperatur, 21(8), 1995, pp. 821-824
The existence of two superconducting phases in YBa2Cu3O7-delta is show
n to be due to the alternating (on variation in delta) dominant contri
bution to the BCS pairing from two branches of the effective oscillati
on frequency of apical atoms O(4) that form a bistable sublattice. Con
centration ranges of the existence of the above phases are estimated b
y using the percolation theory.