MAGNETIZATION PLATEAUS IN SPIN CHAINS - HALDANE-GAP FOR HALF-INTEGER SPINS SPINS

We discuss zero-temperature quantum spin chains in a uniform magnetic field, with axial symmetry. For integer or half-integer spin, S, the m agnetization curve can have plateaus and we argue that the magnetizati on per site m is topologically quantized as n(S-m)=integer at the plat eaus, where n is the period of the ground state. We also discuss condi tions for the presence of the plateau at those quantized values. For S =3/2 and m=1/2, we study several models and find two distinct types of massive phases at the plateau. One of them is argued to be a ''Haldan e gap phase'' for half-integer S.