  The geometrically frustrated double perovskite Ba2YMoO6 is characterized by
quantum s=1/2 spins at the Mo5+ sites of an undistorted face-centered cubic
(FCC) lattice. Previous low-temperature characterization revealed an absence of
static long-range magnetic order and suggested a non-magnetic spin-singlet
ground state. We report new time-of-flight and triple-axis neutron spectroscopy
of Ba2YMoO6 that shows a 28 meV spin excitation with a bandwidth of ~4 meV,
which vanishes above ~125 K. We identify this as the singlet-triplet excitation
out of a singlet ground state, and further identify a weaker continuum of
magnetic states within the gap, reminiscent of spin-polaron states arising due
to weak disorder.
