Gapless excitations in the high-field phase of a Haldane antiferromagnet

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet NDMAP are performed in magnetic fields below and above the critical field $H_c$ at which the gap closes. Gapless excitations that have no analogue in conventional spin wave theory are observed $H>H_c$. The observed anomalous $q$-dependence of these excitations is consistent with the interpreted in the the framework of the free-fermion model.