Self-consistent mean-field theory for spin 1 and spin 1/2 ferrimagnetic chain

We use the self-consistent mean-field theory method to study the ground states of the quantum ferrimagnetic chain. This one-dimensional chain can be described by the Hamiltonian: H = Sigma(i) [(S(i)(.)s(j))(lambda) + (s(i)(.)s(j))(lambda)], where (S(i)(.)s(j))(lambda) = lambda(s(i)(x)s(j)(x) + S(i)(y)s(j)(y)) + S(i)(z)s(j)(z). At the Heisenberg point (lambda = 1), we observe two branches of the low lying excitation. We calculate the gap between the two excitation branches, the spin reduction and the spin fluctuation at T = 0 K. We also give the correlation length at T = 0 K. These results agree with the established numerical results quite well. We also calculate the ground-state energy and the excitation gap varying with the Ising anisotropy lambda. It agrees quite well with quantum Monte Carlo approaches and fourth-order perturbation approaches. (C) 2003 Published by Elsevier B.V.Physics, Condensed MatterSCI(E)EI0ARTICLE1-41-934