ON THE INSTABILITY AND PHASE-SEPARATION OF A DOPED 2D ANTIFERROMAGNET

A mean-field decoupling for the Schwinger bosons and slave fermions in a t-J model shows that at small dopings the thermodynamic potential (with fixed chemical potential) of a doped 2D antiferromagnet is higher than that of an undoped one. At zero temperature the threshold doping beyond which doped states have lower potentials is delta(t) is-approximately-equal-to 0.3 for t/J = 3-4. The violation of the particle number constraint at the mean-field level is further improved via a generalized Gutzwiller approach in which a two-site reduced density matrix is employed. This substantially modifies the energy, the thermodynamic and the chemical potentials, but delta(t) remains roughly the same. Our result may be related to the instability and phase separation of the system discussed in the literature.