Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu"2Si"2

Many correlated electron materials, such as high-temperature superconductors, geometrically frustrated oxides and low-dimensional magnets, are still objects of fruitful study because of the unique properties that arise owing to poorly understood many-body effects. Heavy-fermion metalsmaterials that have high effective electron masses due to those effectsrepresent a class of materials with exotic properties, ranging from unusual magnetism, unconventional superconductivity and hidden order parameters. The heavy-fermion superconductor URu"2Si"2 has held the attention of physicists for the past two decades owing to the presence of a hidden-order phase below 17.5K. Neutron scattering measurements indicate that the ordered moment is 0.03B, much too small to account for the large heat-capacity anomaly at 17.5K. We present recent neutron scattering experiments that unveil a new piece of this puzzlethe spin-excitation spectrum above 17.5K exhibits well-correlated, itinerant-like spin excitations up to at least 10meV, 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 thetransition.