Reentrant phase transitions of a coupled spin-electron model on doubly decorated planar lattices with two or three consecutive critical points

Abstract

The generalized decoration–iteration transformation is adapted for the exact study of a coupled spin-electron model on 2D lattices in which localized Ising spins reside on nodal lattice sites and mobile electrons are delocalized over pairs of decorating sites. The model takes into account a hopping term for mobile electrons, the Ising coupling between mobile electrons and localized spins as well as the Ising coupling between localized spins (J′). The ground state, spontaneous magnetization and specific heat are examined for both ferromagnetic (J′>0) as well as antiferromagnetic (J′<0) interaction between the localized spins. Several kinds of reentrant transitions between the paramagnetic (P), antiferromagnetic (AF) and ferromagnetic (F) phases have been found either with a single critical point, or with two consecutive critical points (P–AF/F–P) and three successive critical points AF/F–P–F/AF–P. Striking thermal variations of the spontaneous magnetization depict a strong reduction due to the interplay between annealed disorder and quantum fluctuations in addition to the aforementioned reentrance. It is shown that the specific heat displays diverse thermal dependencies including finite cusps at the critical temperatures.