16.11.2016 | 13:10 | Age: 101 days

Field-induced spin-density wave beyond hidden order in URu2Si2

URu2Si2 is one of the most enigmatic strongly correlated electron systems and offers a fertile test ground for new concepts in condensedmatter science. In spite of more than thirty years of intense research, no consensus on the order parameter of its low-temperature hiddenorder phase exists.

Under a high magnetic field applied along c, a cascade of first-order phase transitions leads to a polarized paramagnetic regime above µ0H3 = 39 T. Here, thanks to a new cryomagnet (developed by the EMFL-Toulouse, the CEA-Grenoble, and the ILLGrenoble) allowing neutron diffraction up to 40 T, we have determined that URu2Si2 enters in a spin-density wave state in fields between 35 and 39 T. The transition to the spin-density wave represents a unique touchstone for understanding the hidden-order phase. The Figure shows the diffracted neutron intensities recorded in magnetic fields up to 40 T at the momentum transfers Q = (0.6 0 0) and (1.6 0 -1), which are satellites of wavevector k1 = Q – τ = (0.6

0 0) around the structural Bragg positions τ = (0 0 0) and (1 0 -1), respectively. The enhancement of the intensity at 2 K, absent at 18 K, shows that the spin-density wave with wavevector k1 is established at high field and low temperature. In an itinerant picture of magnetism, a spin-density wave can be related to a partial or complete nesting of two parts of the Fermi surface. In URu2Si2, our observation

of a spin-density wave in magnetic fields between 35 and 39 T will certainly push to develop models incorporating on equal basis the Fermi-surface topology and the magnetic interactions. To describe competing quantum instabilities between the hidden-order and long-range-ordered phases, such models will be a basis to solve the hidden-order puzzle.

(a) Time profile of a magnetic field pulsed up to 38 T, and corresponding time-dependence of the neutron-diffracted intensity at Q = (0.6 0 0) and Q = (1.6 0 -1). (b) Magnetization versus magnetic field at T = 1.5 K and magnetic-field-temperature phase diagram of URu2Si2. (c) Field dependence of the neutrondiffracted intensities in fields up to 40.5 T.

Field-induced spin-density wave beyond hidden order in URu2Si2 
W. Knafo, F. Duc, F. Bourdarot, K. Kuwahara, H. Nojiri, D. Aoki, J. Billette, P. Frings, X. Tonon, E. Lelièvre-Berna, J. Flouquet, and L.-P. Regnault, Nat. Commun. 7, 13075 (2016).