  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.
