S = 1 ANTIFERROMAGNETIC HEISENBERG CHAIN IN A MAGNETIC-FIELD

A one-dimensional S = 1 Heisenberg antiferromagnet in a magnetic field H (parallel-to z axis) at T = 0 is studied by numerical diagonalizations up to N = 16. We give the magnetization curve at the thermodynamic limit, and derive an anomaly at H(c)1 (= DELTA), where DELTA is the Haldane gap. It is also found that the transverse spin correlation has the asymptotic form <S(o)xS(r)x> approximately (- 1)(r)r-eta, and the transverse staggered susceptibility chi-st(xx) diverges between H(c1) and H(c2) (= 4). The exponent-eta has a minimum (eta-congruent-to 0.3) at magnetization m congruent-to 1/3 and eta congruent-to 0.5 at H(c1) and H(c2). If the system is quasi-one-dimensional, even small interchain couplings can create a canted Neel order, within a mean-field approximation for interchain interactions. This is consistent with a recent NMR measurement for Ni(C2H8N2)2NO2(ClO4) (NENP) at low temperature.