Slave boson mean field theory for the 2-D Peierls-Hubbard system

Abstract

Adapting the Kotliar-Ruckenstein slave boson approach to the Hubbard model with an additional local electronphonon interaction, we have studied the stability of the ground state against (π, π) and (π, 0) Peierls distortions on a square lattice. The theory is evaluated within an adiabatic two-sublattice saddle-point approximation restricted to symmetry broken states compatible with the underlying bipartite lattice, i.e. para-, ferro-, ferri- and antiferromagnetic states with and without static lattice displacement. At half-filling the Holstein coupling leads to a stable paramagnetic phase with an on-site frozen in breathing mode accompanied by a long-range charge density wave below a critical Hubbard interactionU. Away from half-filling, we distinguish between two possible phase diagrams of the Peierls-Hubbard model. One of them is obtained in the usual way of comparing the relative stability of several homogeneous phases, the other more complete one allows for heterogeneous mixing of phases. In this case we found phase separated regions with an without ‘dimerization’, homogeneous ‘dimerized’ para- and ferrimagnetic states, and the pure para-and ferromagnetic phases. Upon doping the local electron-phonon coupling can induce a magnetic ordered state.