Shanghai Jiao Tong University
We discuss my recent joint work between theory and experiments on the equilateral triangular-lattice Heisenberg antiferromagnet (TLHAF) Ba3CoSb2O9 with effective S = ½ magnetic moments. The material is an ideal realization of TLHAF with small exchange anisotropy, a fairly good two-dimensionality, and the high-symmetric crystal structure essentially precluding Dzyaloshinskii-Moriya interactions. We focus on the spin dynamics in the 1/3 magnetization plateau phase  and also in zero field . We show that nonlinear spin wave theory reproduces the experiments in the plateau phase, thereby allowing us to determine model parameters. However, spin wave theory cannot explain intrinsic features in the spectrum in zero field, such as the magnon-line broadening throughout the Brillouin zone and the high-intensity continuum at high frequencies, even though the ground state is the conventional 120-degree structure. Finally, we also discuss our alternative theoretical approach, the 1/N expansion for the ordered phase, to study the zero-field spin dynamics under the strong influence of quantum fluctuations .
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