This proposal is part of our research programme to explore the spin dynamics of strongly-frustrated quantum antiferromagnets. A canonical model system is the spin-1/2 triangular antiferromagnet for which many theoretical predictions such as quantum-fluctuations induced anomalous lineshape broadenings and roton-like minima in the dispersion have not been experimentally explored. In recent powder experiments we found that Ba3CoSb2O9 is a unique realization of such as system with i) a density of states consistent with an anomalous softening at the zone boundary as expected for roton-like minima and ii) strong scattering continuum at high energies indicating quantum dynamics beyond the linear spin-wave approximation. To explore such effects in detail we propose a single crystal experiment to map the excitations in the full Brillouin zone and compare with theories of quantum dynamics.
