THE SINGLET-TRIPLET MAGNETISM AND INDUCED SPIN FLUCTUATIONS IN THE HIGH-T-C COPPER OXIDES

High-T-c cuprates such as La2-xSrxCuO4 and YBa2-xCu3O6+x are considere d as a system of the election and hole polar pseudo-Jahn-Teller CuO4 c enters [CuO47-](JT) and [CuO45-](JT), respectively, or a system of the local bosons moving in a lattice of the hole centers. Ground manifold of the polar centers includes three terms (1)A(1g) (Zhang-Rice single t). E-1(u), E-3(u) with different spin multiplicity, orbital degenerac y and parity that provides an unconventional multi-mode behavior of th e cuprates. The spin subsystem of the copper oxides within the polar J ohn-Teller CuO4 centers model is a two-component spin liquid and corre sponds to a singlet-triplet magnet with, in general, multiparametric n on-collinear spin configurations. In the framework of a modified mean field approximation some kinds of spin ordering are discussed includin g as a trivial singlet or triplet states as a pure quantum singlet-tri plet mixed state. A local boson movement is accompanied by a modulatio n of the spin density on the site resulting in the so-called induced s pin fluctuations. Some unconventional features of the induced spin flu ctuations are considered including an appearance of the induced longit udinal ferrimagnetism with an appropriate contribution to the spin sus ceptibility. a possibility to observe and examine the charge fluctuati ons with the help of the traditional magnetic methods such as the magn etic inelastic neutron scattering and the spin lattice relaxation expe riments. The suggested model, in comparison with the nearly antiferrom agnetic Fermi-liquid model. represents a new to the description of the spin system of the high-T-c cuprates. (C) 1998 Elsevier Science B.V.