Dynamics of quantum spin liquid and spin solid phases in IPA-Cu Cl3 under an applied magnetic field studied with neutron scattering

Inelastic and elastic neutron scattering is used to study spin correlations in the quasi-one-dimensional quantum antiferromagnet IPA-Cu Cl3 in strong applied magnetic fields. A condensation of magnons and commensurate transverse long-range ordering is observe at a critical field Hc =9.5 T. The field dependencies of the energies and polarizations of all magnon branches are investigated both below and above the transition point. Their dispersion is measured across the entire one-dimensional Brillouin zone in magnetic fields up to 14 T. The critical wave vector of magnon spectrum truncation is found to shift from hc ≈0.35 at H< Hc to hc =0.25 for H> Hc. A drastic reduction of magnon bandwidths in the ordered phase is observed and studied in detail. New features of the spectrum, presumably related to this bandwidth collapse, are observed just above the transition field.