At temperatures below 17.5 K, the heavy-fermion uranium compound URu2Si2 exists in a mysterious 'hidden-order' phase that has eluded characterization for 25 years. In this study the authors have used various pieces of experimental evidence to establish the nature of the spontaneous phase transition that takes place at 17.5 K, and conclude that both single and double time-reversal symmetry occur, resulting in mixing of itinerant conduction electrons and localized 'Ising' states in the 5f2 orbital of uranium atoms. This is a fundamentally new type of order, which the authors dub hastatic (from the Latin hasta, a spear), and which they say could be a phenomenon that applies to other systems in which mixing with f-orbital states takes place.
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