Theory for charge and orbital density-wave states in manganite La0.5Sr1.5MnO4

We investigate the high-temperature phase of layered manganites, and demonstrate that the charge-orbital phase transition without magnetic order in La_0.5Sr_1.5MnO₄ can be understood in terms of the density-wave instability. The orbital ordering is found to be induced by the nesting between segments of Fermi surface with different orbital characters. The simultaneous charge and orbital orderings are elaborated with a mean-field theory. The ordered orbitals are shown to be d_x²-y²±d_3z²-r².