Weakly Coupled s=1/2 Quantum Spin Singlets in Ba₃Cr₂O₈

Using single crystal inelastic neutron scattering with and without the application of an external magnetic field and powder neutron diffraction, we have characterized magnetic interactions in Ba₃Cr₂O₈. Even without a field, we found that there exist three singlet-to-triplet excitation modes in the (h, h, l) scattering plane. Our complete analysis shows that the three modes are due to spatially anisotropic interdimer interactions that are induced by lattice distortions of the tetrahedron of oxygens surrounding the Jahn-Teller active Cr⁵⁺(3d¹). The strong intradimer coupling of J₀=2.38(2)  meV and weak interdimer interactions (|J_inter|≤0.52(2)  meV) makes Ba₃Cr₂O₈ a good model system for weakly coupled s=1/2 quantum spin dimers.