CRITICAL PROPERTIES OF THE ONE-DIMENSIONAL SPIN-1 2 ANTIFERROMAGNETICHEISENBERG-MODEL IN THE PRESENCE OF A UNIFORM-FIELD/

In the presence of a uniform field the one-dimensional spin-1/2 antife rromagnetic Heisenberg model develops zero frequency excitations at fi eld-dependent ''soft-mode'' momenta. We determine three types of criti cal quantities, which we extract from the finite-size dependence of th e lowest excitation energies, the singularities in the static structur e factors and the infrared singularities in the dynamical structure fa ctors at the soft mode momenta. We also compare our results with the p redictions of conformal field theory.