Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
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.
Nat Commun. 2014 Jun 19;5:4188. doi: 10.1038/ncomms5188.
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.
自 1985 年在重费米子金属 URu2Si2 中发现 THO=17.5 K 的相变以来，既没有观察到晶体结构中的对称性变化，也没有能够解释熵变化的大磁矩，这使得这种隐藏序变得神秘。最近的高场实验表明存在打破四重旋转对称性的电子向列性，但在没有磁场的情况下，其基态缺乏直接证据。在这里，我们通过在零场下进行高分辨率同步加速器 X 射线衍射测量，报告了晶格对称性从四重四方到二重复方结构的破坏，这确定了有序的空间对称性。小的正交对称性破坏在 THO 处发生跳跃，揭示了隐藏序跃迁的弱一级性质。这种畸变仅在超纯样品中观察到，这意味着电子向列性和基础晶格之间存在一种非常不寻常的耦合性质。
