Direct observation of lattice symmetry breaking at the hidden-order transition in URu_2Si_2.

In the heavy-fermion metal URu2Si2, direct evidence of symmetry change in the crystal structure at the hidden-order transition has been missing. Using synchrotron X-ray diffraction measurements, Tonegawa et al. observe lattice symmetry breaking from the fourfold tetragonal to twofold orthorhombic structure.