Phase separation in high-T c cuprates

Abstract

We develop a minimal non-BCS model for the CuO2 planes with the on-site Hilbert space reduced to only three effective valence centers CuO4 with different charge, conventional spin, and orbital symmetry, combined in a charge triplet, to describe the low-energy electron structure and the phase states of HTSC cuprates. Using the S = 1 pseudospin algebra we introduce an effective spin-pseudospin Hamiltonian which takes into account local and nonlocal correlations, one- and two-particle transport, and spin exchange. The T-n phase diagrams of the complete spin-pseudospin model for the CuO2 planes were reproduced by means of a site-dependent variational approach within effective field approximation typical for spin-magnetic systems. Limiting ourselves to two-sublattice approximation and nn-couplings we arrived at several Néel-like phases in CuO2 planes for parent and doped systems with a single nonzero local order parameter: antiferromagnetic insulator, charge order, glueless d-wave Bose superfluid phase, and unusual metallic phase. However, the Maxwell’s construction shows the global minimum of free energy is realized for phase separated states which are bounded by the third-order phase transition line T*(n), which is believed to be responsible for the onset of the pseudogap phenomenon.