Seung-Hwan Do, Materials Science and Technology Division, Oak Ridge National Laboratory, Fri, June 17, 11:00am – 12:30pm
One of the strongest signatures of collective quantum behavior is the spontaneous quasiparticle decay in interacting bosonic systems [1]. Revealing and explaining the quantum many-body effects arising between different mode types is an ongoing challenge in thestudy of quantum materials. This talk will focus on this problem through a combined experimental and theoretical study of interacting transverse and longitudinal modes in an S=1easy-planar quantum magnet, Ba2FeSi2O7 [2-4]. Our inelastic neutron scattering measurements show that Ba2FeSi2O7is 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. As the result, the measured spectrum reveals the emergence, decay, and renormalization of a longitudinal mode throughout the Brillouin zone. In particular, the decay of the longitudinal mode is pronounced at the zone center where the decay channels to the transverse modes are most active. A generalization of the standard spin-wave theory is used to account for the many-body effects of the interacting transverse and longitudinal modes in Ba2FeSi2O7. The measured mode decay and renormalization are reproduced by including one-loop corrections, where the kinematic conditions for the interactions are satisfied. Finally, these kinematic conditions can be violated by the Zeeman energy of the magnetic field. Our magnetic field inelastic neutron scattering measurements observe that the longitudinal modes evolve into long-lived quasi-particles above 2 T, demonstrating that this quantum many body effect is tunable [5].
[1] M. E. Zhitomirsky, Rev. of Mod. Phys. 85. 219 (2013)
[2] S.-H. Do, H. Zhang, et al., Nat. Commun. 12 5331 (2021)
[3] T.-H. Jang, S.-H. Do et al., PRB 104 214434 (2021)
[4] S.-H. Do, H. Zhang, et al., arXiv:2205.11770 (2022)
[5] S.-H. Do et al., (in preparation)