Having spins on a triangular or tetragonal lattice causes geometric frustration, as the opposing interactions between neighbouring spins cannot be accommodated. This frustration can lead to the formation of ground states with exotic spin correlations and excitations. One example is the spinel oxide LiGaCr4O8, where the magnetic ions form alternating arrays of small and large tetrahedra: a structure known as a breathing pyrochlore lattice.
Using inelastic neutron scattering on Merlin, researchers were able to investigate the spin excitations in LiGaCr4O8. They found that both a dispersionless resonance mode and dispersive spin-wave excitations were present in the magnetically ordered state. The origin of the resonance modes was found to be related to the Cr-Cr bond distance in the lattice.
By establishing the correct quantum spin model, and detemining the dominant magentic interactions, this study provides the basis from which similar systems could also be understood.
Image right: The sketch of a hexagonal spin loop on the lattice of LiGaCr4O8. The arrows marked by different colors stand for the spins belonging to different loops.
Related publication: Neutron Scattering Studies of the Breathing Pyrochlore Antiferromagnet LiGaCr4O8, Phys. Rev. Lett. 127, 147205