Emergence of highly degenerate excited states in frustrated magnet MgCr$_2$O$_4$

High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet MgCr$_2$O$_4$, which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like paramagnetic phase and a magnetically ordered phase with an extremely complex structure. Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems.