Magnetic Excitations from the Exotic Ground State of the Quantum FCC Antiferromagnet Ba2YMoO6

We report on inelastic neutron scattering measurements on the geometrically frustrated double perovskite Ba2YMoO6. Previous experimental studies of this s=1/2 face-centered cubic system have revealed an absence of static long-range magnetic order and indicated a spin-singlet ground state. We have probed the spectrum of spin excitations, revealing low-Q scattering at 28 meV with a bandwidth of ~4 meV, and a continuum of states at intermediate energies. The intensity of the 28 meV feature decreases with temperature and disappears above ~125 K. This is strong evidence for weakly dispersive triplet excitations out of a singlet ground state, in combination with a magnetic continuum reminescent of spin polaron states within the singlet-triplet gap.