Non-semiclassical spin dynamics in the triangular lattice quantum antiferromagnet
e-mail: kawashima@issp.u-tokyo.ac.jp
We discuss our recent collaboration between theory and neutron scattering experiments on the equilateral triangular lattice Heisenberg antiferromagnet Ba3CoSb2O9 with the effective S = 1/2. The material is an ideal realization of this quintessential frustrated quantum spin model with small anisotropy, fairly good two-dimensionality, and the high-symmetric crystal structure precluding Dzyaloshinskii-Moriya interactions. We focus on spin dynamics in zero field [1] and in the 1/3 magnetization plateau phase [2]. We first demonstrate that nonlinear spin wave theory reproduces the spectrum in the plateau phase, thereby allowing us to determine model parameters. However, in zero field, nonlinear spin wave theory fails to explain intrinsic anomalous features in the spectrum, such as magnon line broadening throughout the whole Brillouin zone and the high intensity excitation continuum, even though the ground state is the conventional 120-degree ordered state. Finally, we discuss development of the 1/N expansion for the spectrum in magnetically ordered phases, a new framework alternative to the 1/S expansion, to study the zero-field spin dynamics [3].
Reference:
[1] J. Ma et al., Phys. Rev. Lett. 116, 087201 (2016)
[2] Y. Kamiya et al., Nature Communications 9, 2666 (2018)
[3] E. A. Ghioldi et al., Phys. Rev. B 98, 184403 (2018)