Direct observation of lattice symmetry breaking at the hidden-order transition in URuSi

Tonegawa, Sho*; Kasahara, Shigeru*; Fukuda, Tatsuo  ; Sugimoto, Kunihisa*; Yasuda, Nobuhiro*; Tsuruhara, Yugo*; Watanabe, Daiki*; Mizukami, Yuta*; Haga, Yoshinori   ; Matsuda, Tatsuma*; Yamamoto, Etsuji  ; Onuki, Yoshichika; Ikeda, Hiroaki*; Matsuda, Yuji*; Shibauchi, Takasada*

We report on the observation of lattice symmetry breaking from the fourfold to twofold structure by high-resolution synchrotron X-ray diffraction measurements at zero field. Small orthorhombic symmetry-breaking distortion sets in at the transition temperature with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultra pure samples, implying a highly unusual coupling nature between the electronic pneumaticity and underlying lattice.