Critical properties of the one-dimensional spin-1/2 antiferromagnetic Heisenberg model in the presence of a uniform field.

In the presence of a uniform field the one-dimensional spin-1 2 antiferromagnetic Heisenberg model develops zero frequency excitations at field-dependent 'soft mode' momenta. We determine three types of critical quantities, which we extract from the finite-size dependence of the lowest excitation energies, the singularities in the static structure factors and the infrared singularities in the dynam-ical structure factors at the soft mode momenta. We also compare our results with the predictions of conformal field theory.