Finding missing entropy with neutrons in the “hidden order”

1. Finding missing entropy with neutrons in the “hidden order” state of the heavy-fermion superconductor URu2Si2 PI: Chris Wiebe, National High Magnetic Field Laboratory, Florida State University, NCNR/NIST, McMaster University, University of Maryland, Chalk River Labs.NSF Award Numbers: DMR-0084173, DMR -0454672. For nearly two decades, little progress has been made to identify the “hidden order” state in URu2Si2 (a precursor to the superconducting state) – until now. Combining work at the NHMFL and the NCNR at NIST, scientists have used a technique called neutron scattering to identify how the electrons interact with each other near this “hidden order” state. Neutrons show that at the phase transition, the change in the specific heat is predominantly due to the gapping of magnetic excitations. This discovery rules out many of the current theories of how the electrons organize themselves at the transition, and provides further clues about the enigmatic “hidden order.” Spin excitations in the hidden order state of URu2Si2. The inset shows how the gap develops at low temperatures. DCS spectrometer at NIST C. R. Wiebe et al., Nature Physics, 3, 96 (2007)