Two-vibron bound states in the \beta--Fermi--Pasta--Ulam model

This paper studies the two-vibron bound states in the \beta --Fermi--Pasta--Ulam model by means of the number conserving approximation combined with the number state method. The results indicate that on-site, adjacent-site and mixed two-vibron bound states may exist in the model. Specially, wave number has a significant effect on such bound states, which may be considered as the quantum effects of the localized states in quantum systems.