The band structure of double excited states for a linear chain

The energy band structure in the case of double excited states of finite sp in systems (s = 1/2) has been investigated. A geometrical construction base d on the Bethe Ansatz method for determining eigenstates has been proposed. The formula for energy spectrum in the center and at the border of Brillou in zone has been obtained. Classification of energy bands has been elaborat ed on and approximated dispersion law for bounded states given. Some proble ms with application of the Bethe Ansatz in the case of finite system has be en pointed out. (C) 2000 Elsevier Science B.V. All rights reserved.