  A central issue in material science is to obtain understanding of the
electronic correlations that control complex materials. Such electronic
correlations frequently arise due to the competition of localized and itinerant
electronic degrees of freedom. While the respective limits of well-localized or
entirely itinerant ground states are well-understood, the intermediate regime
that controls the functional properties of complex materials continues to
challenge theoretical understanding. We have used neutron spectroscopy to
investigate plutonium, which is a prototypical material at the brink between
bonding and non-bonding configurations. Our study reveals that the ground state
of plutonium is governed by valence fluctuations, that is, a quantum-mechanical
superposition of localized and itinerant electronic configurations as recently
predicted by dynamical mean field theory. Our results not only resolve the
long-standing controversy between experiment and theory on plutonium's
magnetism, but also suggest an improved understanding of the effects of such
electronic dichotomy in complex materials.
