Symmetry Classes of Spin and Orbital Ordered States in a t_(2g) Hubbard Model on a Two-dimensional Square Lattice

This paper presents symmetry classes of the Hartree-Fock (HF) solutions of spin and orbital ordered states in a t_{2g} Hubbard model on a two-dimensional square lattice. Using a group theoretical bifurcation theory of the Hartree Fock equation, we obtained many types of broken symmetry solutions which bifurcate from the normal state through one step transition in cases of commensurate ordering vectors Q_0=(0,0), Q_1=(\pi,\pi), Q_2=(\pi,0) and Q_3=(0,\pi). Each broken symmetry state is characterized by the presence of local order parameters(LOP) at each lattice site: quadrupole moment Q=(Q_2^2,Q_{12},Q_{23},Q_{31}), orbital angular momentum l=(l_1,l_2,l_3), spin density s=(s^1,s^2,s^3), spin quadrupole moment Q^{\lambda}=(Q_2^{2\lambda}, Q_{12}^{\lambda},Q_{23}^{\lambda},Q_{31}^{\lambda}) and spin orbital angular momentum l^{\lambda}=(l_1^{\lambda},l_2^{\lambda},l_3^{\lambda}) where \lambda=1,2,3. We performed numerical calculations for some parameter sets. Then we have found that many types of non-collinear magnetic orbital ordered states having LOP:Q^{\lambda} and l^{\lambda} can be the ground state for these parameter sets.