Abstract
Many correlated electron materials, such as high-temperature superconductors1, geometrically frustrated oxides2 and low-dimensional magnets3,4, are still objects of fruitful study because of the unique properties that arise owing to poorly understood many-body effects. Heavy-fermion metals5—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 parameters6. 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 0.03μB, 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.
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Acknowledgements
The authors would like to acknowledge helpful discussions with C. Broholm, C. D. Batista, A. Leggett, C. M. Varma, J. S. Gardner, J. S. Brooks, G. S. Boebinger and B. D. Gaulin. This work was made possible by support through the NSF and the state of Florida. L.B. acknowledges support from the NHMFL in house research program and Y.J. acknowledges support from the NHMFL Schuller program. G.M.L., G.J.M. and W.J.L.B. acknowledge support through NSERC and the CIAR. The authors are grateful to the local support staff at the NIST Center for Neutron Research. Data analysis was completed with DAVE, which can be obtained at http://www.ncnr.nist.gov/dave/. The work at NIST is supported in part by the National Science Foundation under Agreement No. DMR-0454672. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by NSF Cooperative Agreement No. DMR-0084173, by the State of Florida, and by the DOE.
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Scattering experiments were completed by C.R.W., J.A.J., G.J.M., H.D.Z., Y.Q., J.R.D.C., Z.Y. and W.J.L.B. The crystals were grown by J.D.G. and G.M.L. Specific heat measurements were made by Y.J.J. and L.B. Data analysis and writing of the paper was completed by C.R.W., J.A.J., G.J.M., G.M.L., L.B. and W.J.L.B.
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Wiebe, C., Janik, J., MacDougall, G. et al. Gapped itinerant spin excitations account for missing entropy in the hidden-order state of URu2Si2. Nature Phys 3, 96–99 (2007). https://doi.org/10.1038/nphys522
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DOI: https://doi.org/10.1038/nphys522
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