An ongoing challenge in the study of quantum materials, is to reveal and explain collective quantum effects in spin systems where interactions between different mode types are important. This talk will focus on this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an S=1 easy-planar quantum magnet, Ba₂FeSi₂O₇ [1-3]. Our inelastic neutron scattering measurements show that Ba₂FeSi₂O₇ is a quasi-two-dimensional antiferromagnet with large single-ion anisotropy (D>5J), and the extended spin wave theory analysis demonstrates its close proximity to a quantum critical point. The measured spectrumreveals the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. The decay of the longitudinal mode is particularly pronounced at the zone center, where the decay channels to the transverse modes are most active. To account for the many-body effects of the interacting transverse and longitudinal modes in Ba₂FeSi₂O₇, we used a generalization of the spin-wave theory for SU(3) spin representation. The measured mode decay and renormalization are reproduced by including one-loop corrections, where the kinematic conditions for the interactions are satisfied. Finally, the kinematic condition becomes violated in the magnetic field. Our field inelastic neutron scattering measurement reveals that the longitudinal mode evolves into a long-lived quasi-particle in magnetic fields, demonstrating that this quantum many-body effect is tunable [4].

[1] S.-H. Do et al., Nat. Commun. 12 5331 (2021)

[2] T.-H. Jang, S.-H. Do et al., PRB 104 214434 (2021)

[3] S.-H. Do et al., arXiv:2205.11770 (2022)

Host : Dr. Dietrich Wulferding