THE SPIN-1 LAI-SUTHERLAND MODEL WITH EXTERNAL AND INTERNAL FIELDS .1.THE PHASE-DIAGRAM

The one-dimensional S = 1 quantum spin chain with a linear and bilinea r nearest-neighbour interaction of equal strength is integrable by the nested Bethe ansatz. In this paper the model is studied in the presen ce of an external magnetic held and an internal crystal field. By solv ing the Bethe ansatz equations for chains up to length N = 360 we cons truct the complete phase diagram of the system. We discuss how the mag netization curves depend on the internal field. In the SU(3) phase, wh ere all three densities of atoms with Sc-component -1, 0, +1 are non-v anishing, lines of constant density can be approximately parametrized by modified hypocycloides.