title: Direct observation of lattice symmetry breaking at the hidden-order transition in URu2Si2.
abstract: Since the 1985 discovery of the phase transition at THO=17.5 K in the heavy-fermion metal URu2Si2, neither symmetry change in the crystal structure nor large magnetic moment that can account for the entropy change has been observed, which makes this hidden order enigmatic. Recent high-field experiments have suggested electronic nematicity that breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state in the absence of magnetic field. Here we report on the observation of lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure by high-resolution synchrotron X-ray diffraction measurements at zero field, which pins down the space symmetry of the order. Small orthorhombic symmetry-breaking distortion sets in at THO with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure samples, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice.
authors.affiliation: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Tonegawa
authors.affiliation: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Kasahara
authors.affiliation: 1] Quantum Beam Science Directorate, JAEA SPring-8, Sayo, Hyogo 679-5148, Japan [2] Materials Dynamics Laboratory, RIKEN SPring-8, Sayo, Hyogo 679-5148, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Fukuda
authors.affiliation: 1] Research and Utilization Division, JASRI SPring-8, Sayo, Hyogo 679-5198, Japan [2] Structural Materials Science Laboratory, RIKEN SPring-8, Sayo, Hyogo 679-5148, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Sugimoto
authors.affiliation: Research and Utilization Division, JASRI SPring-8, Sayo, Hyogo 679-5198, Japan.
authors.forename: N
authors.initials: N
authors.lastname: Yasuda
authors.affiliation: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Tsuruhara
authors.affiliation: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: D
authors.initials: D
authors.lastname: Watanabe
authors.affiliation: 1] Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan [2] Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Mizukami
authors.affiliation: Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Haga
authors.affiliation: 1] Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan [2] Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan.
authors.forename: T D
authors.initials: TD
authors.lastname: Matsuda
authors.affiliation: Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan.
authors.forename: E
authors.initials: E
authors.lastname: Yamamoto
authors.affiliation: 1] Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan [2] Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Onuki
authors.affiliation: 1] Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan [2] Department of Physics, Ritsumeikan University, Kusatsu 525-8577, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Ikeda
authors.affiliation: Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Matsuda
authors.affiliation: 1] Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan [2] Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8561, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Shibauchi