Crystal field excitations of new low energy spin-1/2 triangular antiferromagnets

Despite being frustrated, the triangular lattice antiferromagnet is expected to develop long-range magnetic order and spin-wave excitations at low temperatures. Theoretical studies suggest that the magnetic spectrum shows small deviations with respect to linear spin-wave theory with the band of excitations expected to be renormalized downward and broadened for the case of quantum spin-1/2. In contrast recent experiments on the spin-1/2 triangular antiferromagnet Ba3CoSb2O9 reveal major differences between the measured excitations and spin-wave theory, including 3 bands of excitations and a continuum that extends to much higher energies (6 times the exchange energy). Because of the unexpected nature of these results it is necessary to verify them in other materials, thus we plan to investigate the spectrum of the new effective spin-1/2 triangular lattice antiferromagnets Ba3CoNb2O9 and Ba3CoTa2O9. We also plan to investigate the crystal field levels of their magnetic Co2+ ions to determine their ground state wavefunction and identify the presence of any anisotropies.