Triplet And In-Gap Magnetic States In The Ground State Of The Quantum Frustrated Fcc Antiferromagnet Ba2ymoo6

The geometrically frustrated double perovskite Ba2YMoO6 is characterized by quantum s = 1/2 spins at the Mo5+ sites of an undistorted fcc lattice. Previous low-temperature characterization revealed an absence of static long-range magnetic order and suggested a nonmagnetic spin-singlet ground state. We report unique time-of-flight and triple-axis neutron spectroscopy of Ba2YMoO6 that shows a 28 meV spin excitation with a bandwidth of similar to 4 meV, which vanishes above similar to 125 K. We identify this as the singlet-triplet excitation that arises 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.