Static and Dynamical Properties of the Spin-52 Nearly Ideal Triangular Lattice Antiferromagnet Ba3MnSb2O9

We study the ground state and spin excitations in Ba3MnSb2O9, an easy-plane S = 5/2 triangular lattice antiferromagnet. By combining single-crystal neutron scattering, electric spin resonance (ESR), and spin wave calculations, we determine the frustrated quasi-two-dimensional spin Hamiltonian parameters describing the material. While the material has a slight monoclinic structural distortion, which could allow for isoscelestriangular exchanges and biaxial anisotropy by symmetry, we observe no deviation from the behavior expected for spin waves in the in-plane 120 degrees state. Even the easy-plane anisotropy is so small that it can only be detected by ESR in our study. In conjunction with the quasitwo dimensionality, our study establishes that Ba3MnSb2O9 is a nearly ideal triangular lattice antiferromagnet with the quasiclassical spin S = 5/2, which suggests that it has the potential for an experimental study of Z- or Z2-vortex excitations.