GAPLESS SPECTRUM IN A CLASS OF S=1 EXCHANGE MODELS WITH LONG-RANGE INTERACTIONS

We present evidence for the absence of a gap in a class of S = 1 antif erromagnetic exchange models. The spin exchange is long ranged, of the type -(-1)(i-j)/\i - j\(alpha) where 1 < alpha < 3. We have shown pre viously that without the alternating factor the model for alpha = 2 (t he S = 1 Haldane-Shastry model) has a gap, exponentially decaying corr elation functions and exponentially small susceptibility at very low t emperatures. In the case of the alternating interaction the stabilizin g next-nearest-neighbour ferromagnetic interaction changes the behavio ur of the system qualitatively. We have studied the ground state and f irst excited state using a modified Lanczos algorithm for system sizes up to 16 sites. Also, we performed exact diagonalization for systems up to 8 sites and obtained the thermodynamics. The correlation functio ns decay with distance like a power law. These models define a new cla ss of integer spin chains that do not show a Haldane gap. The results may be relevant for describing impurity spins coupled by a RKKY intera ction through a half-filled conduction electron band.