Effect of Hund's rule coupling on SU(4) spin-orbital system
  We investigate the ground-state property of a one-dimensional two-orbital Hubbard model at quarter filling by numerical techniques such as the density-matrix renormalization group method and the exact diagonalization. When the Hund's rule coupling $J$ is zero, the model is SU(4) symmetric. In fact, both spin and orbital correlations have a peak at $q=\pi/2$, indicating an SU(4) singlet state with a four-site periodicity. On the other hand, with increasing $J$, it is found that the peak position of the orbital correlation changes to $q=\pi$, while that of the spin correlation remains at $q=\pi/2$. We briefly discuss how the SU(4) symmetry is broken by $J$. 
Hiroaki Onishi and Takashi Hotta