The singlet-triplet pseudo-Jahn-Teller centers in copper oxides

One of the most exciting features of the hole centers CuO45- in doped cupra tes is an unusually complicated ground state which is the result of the ele ctronic quasi-degeneracy. An additional hole, doped to the basic CuO46- clu ster with the b(1g) hole can occupy both the same hybrid Cu3d-O2p orbital s tate resulting in a Zhang-Rice singlet (1)A(1g) and the purely oxygen e(u) molecular orbital resulting in a singlet or triplet E-1.3(u) term with the close energies. We present detailed analysis of the (pseudo)-Jahn-Teller (P JT) effect driven by the near - degeneracy within the ((1)A(1g), E-1.3(u))- manifold. (C) 1999 Elsevier Science Ltd. All rights reserved.