Formation of a Longitudinal Mode in One Dimensional Yb₂Pt₂Pb

The low energy magnetic excitations measured with neutron scattering (NS) in Yb₂Pt₂Pb are spinons on one dimensional chains, in good agreement with the expectations of the XXZ Hamiltonian for nearly isotropic S=±1/2 magnetic moments despite the large, rare earth Yb³⁺ moments that make up the system [1]. In applied magnetic field, we observe spinon confinement into bound states coinciding with a longitudinally polarized interchain mode [2]. New NS measurements probe the dispersion of this mode as a function of field in the vicinity of the confinement transition. We find weak modifications to the quantum continuum for fields approaching the critical field of 0.7 T, with the mode growing in intensity as field is increased through the transition. Interestingly, the antiferromagnetic order is strongly pinned with periodicity that does not evolve smoothly as a function of field as seen previously in a different orientation [2]. Rather, there is an abrupt transition to a weak, completely incommensurate order at 1.3 T with the mode correspondingly returning spectral weight to the continuum, reflecting strong low dimensional fluctuations even at fields near saturation.
[1] L. S. Wu, et al, Science 352, 1206 (2016).
[2] W. J. Gannon et al, submitted (2018).