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Spontaneous symmetry breaking produces longitudinal amplitude (Higgs) mode as a well-defined collective excitation [1]. We investigate the emergence of the longitudinal mode in two-dimensional square-lattice antiferromagnet Ba₂FeSi₂O₇ –easy-plane antiferromagnet with effective S=1 near a continuous quantum phase transition into a quantum paramagnet [2,3]. Our inelastic neutron scattering measurements and extended spin wave theory analysis demonstrate its proximity to the quantum critical point (QCP), and reveal the presence of the longitudinal mode in the observed spectra. The longitudinal mode is visible thoughout the Brillouin zone. Noticeably, this mode is highly renormalized and broadened near the magnetic zone center, which though decay channels to the transverse mode. Finally, we discuss the necessary theoretical corrections to a generalized linear spin wave theory to accurately describe the decay of the longitudinal magnon in Ba₂FeSi₂O₇.

[1] D. Pekker and C. M. Varma, Annu. Rev. Condens. Matter Phys. 6 269 (2015)
[2] M. Matsumoto, Journal. of the Phys. Soc. of Jap. 83 084704 (2014)
[3] T.-H. Jang et al., (in preparation)