Dynamics of New and Old Triangular Antiferromagnets*

The dynamics of magnets stems from magnetic excitations above the ground state, yielding crucial insights into the microscopic mechanisms governing a material's physical properties. Our presentation centers on an old member of the antiferromagnetic family on triangular lattices from the 1970s. This member has garnered renewed attention in recent years, thanks to cutting-edge experimental instruments that allow high-precision measurement of its dynamics. Furthermore, this member reveals intriguing quantum effects, encompassing energy spectrum renormalization, quasiparticle instability, and the emergence of higher-order bound states, etc.. To elucidate these phenomena, we present theoretical methods designed to provide a comprehensive physical picture and a quantitative description. Significantly, the theoretical calculations successfully account for several recent experimental findings.