Massive triplet excitations in a magnetized anisotropic Haldane spin chain

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet $\mathrm{Ni}\left({\mathrm{C}}_5{\mathrm{D}}_{14}{\mathrm{N}}_2{)}_2{\mathrm{N}}_3\right({\mathrm{PF}}_6)$ are performed at mK temperatures in magnetic fields of almost twice the critical field $H_c$ applied perpendicular to the spin chains. Above $H_c$ a reopening of the spin gap is clearly observed. In the high-field Néel-ordered state the spectrum is dominated by three distinct excitation branches. A theoretical model consistently describing the experimental data is proposed.