Hastatic Order

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Science  08 Feb 2013:
Vol. 339, Issue 6120, pp. 628-629
DOI: 10.1126/science.339.6120.628-d

Phase transitions are usually associated with the breaking of symmetry; the colder phase is generally more ordered than the hotter one. Normally, it is easy to deduce which symmetry is broken. A rare exception to that rule is the heavy fermion compound URu2Si2, which forms a mysterious phase below 17.5 K, known as "hidden order," that is yet to reveal its true nature in spite of intense experimental and theoretical efforts. Inspired by recent experimental findings, Chandra et al. propose that the exotic state breaks not only the time reversal symmetry (as is the case in magnetism), but also the doubly applied time reversal symmetry. This hastatic order is achieved through the hybridization of conduction electrons with the f-orbital states of uranium, which leads to the mixing of states of integer and half-integer spin, the latter ones causing the peculiar behavior. The theory makes testable predictions, and whether or not future experiments confirm that this is the correct description of the hidden order in URu2Si2, it describes an attractive mechanism that may be at work in other similar compounds.

Nature 493, 621 (2013).

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