Triplet and in-gap magnetic states in the ground state of the quantum frustrated fcc antiferromagnet Ba₂YMoO₆

The geometrically frustrated double perovskite Ba₂YMoO ₆ 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 ∼4 meV, which vanishes above ∼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.