Absence of Hole Confinement in Transition Metal Oxides with Orbital Degeneracy

The compounds with orbital degrees of freedom exhibit many possible scenarios for hole propagation which in most cases lead to hole localization [1]. Here we investigate the spectral properties of a hole moving in a two-dimensional Hubbard model for strongly correlated $t_{2g}$ electrons. Although superexchange interactions are Ising-like, a quasi-one-dimensional coherent hole motion arises due to effective three-site terms. This mechanism is fundamentally different either from the hole motion via quantum fluctuations in the conventional spin model with SU(2) symmetry or from the $e_g$ orbital model [2]. The present orbital model describes also propagation of a hole in some $e_g$ compounds [3], and we argue that orbital degeneracy alone does not lead to hole self-localization. [1] J. Zaanen and A.M. Ole\'{s}, Phys. Rev. B {\bf 48}, 7197 (1993). [2] J. van~den Brink, P. Horsch, and A.M. Ole\'{s}, Phys. Rev. Lett. {\bf 85}, 5174 (2000). [3] M. Daghofer, A.M. Ole\'s, and W. von der Linden, Phys. Rev. B {\bf 70}, 184430 (2004).