  To describe the spin-orbital energy gap formation in the one-dimensional
symmetrically coupled spin-orbital model, we propose a simple mean field theory
based on an SU(4) constraint fermion representation of spins and orbitals. A
spin-orbital gapped phase is formed due to a marginally relevant spin-orbital
valence bond pairing interaction. The energy gap of the spin and orbital
excitations grows extremely slowly from the SU(4) symmetric point up to a
maximum value and then decreases rapidly. By calculating the spin, orbital, and
spin-orbital tensor static susceptibilities at zero temperature, we find a
crossover from coherent to incoherent magnetic excitations as the spin-orbital
coupling decreasing from large to small values.
