Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu2Si2

Many correlated electron materials, such as high-temperature superconductors(1), geometrically frustrated oxides(2) and low-dimensional magnets(3,4), are still objects of fruitful study because of the unique properties that arise owing to poorly understood many-body effects. Heavy-fermion metals(5) - materials that have high effective electron masses due to those effects - represent a class of materials with exotic properties, ranging from unusual magnetism, unconventional superconductivity and 'hidden' order parameters(6). The heavy-fermion superconductor URu2Si2 has held the attention of physicists for the past two decades owing to the presence of a 'hidden-order' phase below 17.5 K. Neutron scattering measurements indicate that the ordered moment is 0.03 mu(B), much too small to account for the large heat-capacity anomaly at 17.5 K. We present recent neutron scattering experiments that unveil a new piece of this puzzle - the spin-excitation spectrum above 17.5 K exhibits well-correlated, itinerant-like spin excitations up to at least 10 meV, emanating from incommensurate wavevectors. The large entropy change associated with the presence of an energy gap in the excitations explains the reduction in the electronic specific heat through the transition.