By using bosonization technique and quantum self-consistent theory, we study the low-energy excitations of a spin-Peierls system. The ground state, one-particle excited state, two-particle bound state are calculated, and it is shown, as the frustration increases, the energy of the ground state decreases while the energy gap of the one- and two-particle states increases. By analysing the asymototic behavior of the longitudinal spin correlation function, we obtain that the two-particle bound state has similar spin structures as those of the singlet ground state. Thus, we regard the bound state as a singlet state, which exists between the ground state and the two-particle continuous excited state. Our results are qualitatively in agreement with the experimental phenomena observed by Ain et al. It is shown that the existence of bound state is one of the spin-Peierls system's characteristic features.
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