Neutron Scattering Studies of the Breathing Pyrochlore Antiferromagnet LiGaCr 4 O 8

We report neutron scattering measurements of the spinel oxide ${\mathrm{LiGaCr}}_{4}{\mathrm{O}}_{8}$, in which magnetic ions ${\mathrm{Cr}}^{3+}$ form a breathing pyrochlore lattice. Our experiments reveal the coexistence of a nearly dispersionless resonance mode and dispersive spin-wave excitations in the magnetically ordered state, which can be quantitatively described by a quantum spin model of hexagonal loops and linear spin-wave theory with the same set of exchange parameters, respectively. Comparison to other Cr spinel oxides reveals a linear relationship between the resonance energy and lattice constant across all these materials, which is in agreement with our hexagonal loop calculations. Our results suggest a unified picture for spin resonances in Cr spinel oxides.