Elementary excitations of the symmetric spin-orbital model: The XY limit

The elementary excitations of the 1D, symmetric, spin-orbital model are stu died in two anisotropic cases: the pure XY and the dimerized XXZ models. Wh ile they preserve the symmetry between spin and orbital degrees of freedom, these models allow for a picture of the low-lying excitations. In the pure XY case, a phase separation takes place between two phases with free-fermi on-like gapless excitations. In the dimerized case the low-energy effective Hamiltonian reduces to the 1D Ising model with gapped excitations. All the elementary excitations involve simultaneous flips of the spin and orbital degrees of freedom: a clear indication of the breakdown of the traditional mean-field theory. [S0031-9007(99)08987-5].