Plaquette Ground State in the Two-dimensional SU(4) Spin-Orbital Model
  In order to understand the properties of Mott insulators with strong ground state orbital fluctuations, we study the zero temperature properties of the SU(4) spin-orbital model on a square lattice. Exact diagonalizations of finite clusters suggest that the ground state is disordered with a singlet-multiplet gap and possibly low-lying SU(4) singlets in the gap. An interpretation in terms of plaquette SU(4) singlets is proposed. The implications for LiNiO_2 are discussed. 
Mathias van den Bossche (1), Fu-Chun Zhang (2) and Frederic Mila (1)   ((1) Universite Paul Sabatier - Toulouse, France, (2)...