Reason for the formation of two superconducting phases in yttrium-barium cuprates

It is shown that the existence of two superconducting phases in the compound YBa2Cu3O7-ε is a consequence of alternating (as ε varies) dominant contributions to the BCS pairing from two branches of the effective vibrational frequencies of the apical O(4) atoms, which form a bistable sublattice. The concentration ranges for the existence of these phases are evaluated using percolation theory. The disparity between the critical superconducting transition temperature during cooling and the critical temperature for destruction of the superconducting state during heating of the compound from absolute zero is noted. 20 refs., 4 figs