Magnetic excitations in a quantum spin dimer system Ba$_{3}$Cr$_{2}$O$_{8}$

We report our neutron scattering measurements on a powder sample of a new quantum spin system Ba3Cr2O8 in which Cr5+ (3d1, S = 1/2) ions form a double-layered triangular lattice. The system does not undergo any magnetic ordering down to 50 mK. Instead, bulk susceptibility data exhibit a broad peak around 16K. Our inelastic neutron scattering data at temperatures lower than 30 K show a prominent excitation at hw = 2.2 meV. Q- and temperature dependences of the integrated intensity of the excitation can be well accounted for by a model of weakly coupled quantum dimers. When an external magnetic field is applied, the excitation splits into three energies, which confirms the dimer model. We have investigated how the singlet-to-triplet excitations evolve with increasing the magnetic field up to 14.5 Tesla.