Spin density wave and ferromagnetism in a quasi-one-dimensional organic polymer

Abstract

The spin density wave(SDW) — charge density wave(CDW) phase transition and the magnetic properties in a half-filled quasi-one-dimensional organic polymer are investigated by the world line Monte Carlo simulations. The itinerant π electrons moving along the polymer chain are coupled radically to localized unpaired d electrons, which are situated at every other site of the polymer chain. The results show that both ferromagnetic and anti-ferromagnetic radical couplings enhance the SDW phase and the ferromagnet order of the radical spins, but suppress the CDW phase. By finite size scaling, we are able to obtain the phase transition line in the parameter space. The ferromagnetic order of the radical spins are observed to coexist with the SDW phase. As compared to the system being free of the radical coupling, the phase transition line is shifted upward in the U-V parameter space in favor of larger V, where U is the on-site repulsion and V is the nearest-neighbor interaction between the π electrons. All of these findings can be understood qualitatively by a second-order perturbation theory starting from the classical state at zero temperature in the strong coupling limit. We also address the consequences of the radical coupling for the persistent current if the polymer chain is fabricated as a mesoscopic ring.