  Uranium compounds can manifest a wide range of fascinating many-body
phenomena, and are often thought to be poised at a crossover between localized
and itinerant regimes for 5f electrons. The antiferromagnetic dipnictide USb2
has been of recent interest due to the discovery of rich proximate phase
diagrams and unusual quantum coherence phenomena. Here, linear-dichroic X-ray
absorption and elastic neutron scattering are used to characterize electronic
symmetries on uranium in USb2 and isostructural UBi2. Of these two materials,
only USb2 is found to enable strong Hund's rule alignment of local magnetic
degrees of freedom, and to undergo distinctive changes in local atomic
multiplet symmetry across the magnetic phase transition. Theoretical analysis
reveals that these and other anomalous properties of the material may be
understood by attributing it as the first known high temperature realization of
a singlet ground state magnet, in which magnetism occurs through a process that
resembles exciton condensation.
