Orbital-controlled magnetic transition between gapful and gapless phases in the Haldane system with t_2g-orbital degeneracy

In order to clarify a key role of the orbital degree of freedom in the spin S=1 Haldane system, we investigate ground-state properties of the t_2g-orbital degenerate Hubbard model on the linear chain by using numerical techniques. Increasing the Hund’s rule coupling in multiorbital systems, in general, there occurs a transition from an antiferromagnetic to a ferromagnetic phase. We find that the antiferromagnetic phase is described as the Haldane system with spin gap, while in the ferromagnetic phase, there exists the gapless excitation with respect to the orbital degree of freedom. Possible relevance of the present results to actual systems is also discussed.