Spontaneous magnetic order and spin and charge entanglement of a coupled spin-electron model on a decorated square lattice composed from trigonal bipyramids

A coupled spin-electron model on a decorated square lattice formed by interconnected trigonal bipyramids is exactly solved by imposing two mobile electrons per each triangular plaquette with the help of a generalized decoration-iteration transformation, which establishes a precise mapping correspondence with the effective Ising model on a square lattice with temperature-dependent interaction. The investigated spin-electron model exhibits two different macroscopically degenerate ground states. The residual entropy of the first ground state, which shows a spontaneous ferromagnetic or ferrimagnetic long-range order depending on character of the exchange coupling, arises from chiral degrees of freedom of the mobile electrons. In contrast, the second ground state is disordered due to a kinetically driven frustration of the localized Ising spins triggered by the hopping term of the mobile electrons. The outstanding reentrant phase transitions connected with temperature induced formation of the spontaneous ferromagnetic or ferrimagnetic order can be found if the spin-electron model is driven sufficiently close to the ground-state phase boundary, but the disordered frustrated phase is the respective ground state. It is verified that the bipartite fermionic entanglement between two mobile electrons within the spontaneously ordered ferromagnetic or ferrimagnetic phase predominantly comes from their charge degrees of freedom, while the one within the disordered frustrated phase comes from both charge and spin degrees of freedom of these particles.