Condensed Matter Theory Laboratory, RIKEN, Japan
Abstract: Spin wave theory is an established semi-classical framework for describing spin excitations in magnetically ordered materials in spatial dimensions d > 1 and is known to reproduce experiments quite well not only for large S but also in some cases of S = 1/2. Recently, an increasing amount of research interest is focused on identifying fractional excitations not describable as spin waves, such as spinons, induced by proximity to a liquid-like phase likely adjacent to magnetically ordered phases. I will present our recent joint work between theory and experiments on the equilateral triangular-lattice Heisenberg antiferromagnet (TLHAF) Ba3CoSb2O9, with effective S = 1/2 magnetic moments associated with the ground state Kramers doublet of Co2+. Ba3CoSb2O9 is an ideal realization of TLHAF with the nearly isotropic exchange and a rather small inter-layer coupling in the high-symmetric crystal structure (space group P63/mmc) essentially precluding Dzyaloshinskii-Moriya interactions. While Ba3CoSb2O9 magnetically orders into the 120-degree structure, it was found that the inelastic neutron scattering spectrum in the absence of a magnetic field exhibits a rather strong deviation from non-linear spin-wave theory, comprising a high-intensity excitation continuum and short-lived magnons . We also discuss spin dynamics in the up-up-down (UUD) phase in a magnetic field of > ~10T, in which quantum fluctuations are suppressed by the external magnetic field, providing a favorable condition for comparing against spin wave theory. Remarkably, it was found that the spectrum in the UUD phase was well reproduced by the same Hamiltonian as in Ref.  with almost identical coupling constants extended by the Zeeman coupling . This observation indicates that the anomalous zero-field excitation spectra indescribable with spin-wave theory must have an intrinsic quantum mechanical origin. If the time permits, I will also outline our on-going theoretical work based on an alternative theoretical approach, the 1/N expansion, to study the zero-field spin dynamics under the strong influence of quantum effects and geometric frustration.
 J. Ma et al., Phys. Rev. Lett. 116, 087201 (2016)
 Y. Kamiya et al., arXiv: 1701.07971
(*) This work has been done in collaboration with J. Ma [Shanghai Jiao Tong Univ. and Univ. of Tennessee, Knoxville (UTK)], L. Ge [Georgia Institute of Technology (GIT)], T. Hong [Oak Ridge National Laboratory (ORNL)], Y. Qui (National Institute of Standard and Technology and Univ. of Maryland), D. L. Quintero-Castro (Helmholtz-Zentrum Berlin für Materialien und Energie), H. B. Cao (ORNL), M. Matsuda (ORNL), C. D. Batista (UTK and ORNL), M. Mourigal (GIT), and H. D. Zhou (UTK and National High Magnetic Field Laboratory).
Host: Zi Yang Meng 9331