2D Heisenberg Triangular Antiferromagnet in Ba$_3$CoSb$_2$O$_9$

We present inelastic neutron scattering (INS) experiments on the triangular antiferromagnet (TAF) Ba$_3$CoSb$_2$O$_9$. High energy INS measurements allowed the crystal field levels of Co$^{2+}$ ions to be resolved, and subsequently the terms relevant to its single ion Hamiltonian to be derived with the conclusion that the ions have a $J_\text{eff}=1/2$ doublet as their groundstate with relatively weak local trigonal distortion of CoO$_6$ octahedra. The result is a system which is a rare realisation of the canonical spin 1/2 Heisenberg TAF. Following this, low energy, high-resolution INS experiments have been performed which reveal the spin wave excitations emanating from the 120$^{\circ}$ ordered phase below $T_\text{N}=3.8$K. However, as will be seen, linear spin wave calculations are not sufficient to describe all the features of the data, and these anomalies hint at quantum dynamics beyond linear spin wave theory within this realisation of the canonical $S$=1/2 TAF system.