Direct observation of lattice symmetry breaking at the hidden-order transition in URu2Si2

TL;DR

This study provides direct evidence of lattice symmetry breaking at the hidden-order transition in URu2Si2, revealing a weakly first-order transition and suggesting a coupling between electronic nematicity and lattice structure.

Contribution

First direct observation of lattice symmetry breaking at the hidden-order transition in URu2Si2 using high-resolution X-ray diffraction.

Findings

Lattice symmetry changes from tetragonal to orthorhombic at T_HO

The transition is weakly first-order with a jump in distortion

Symmetry breaking is observed only in ultrapure samples

Abstract

Since the 1985 discovery of the phase transition at $T_{\rm HO}=17.5$ K in the heavy-fermion metal URu$_2$Si$_2$, neither symmetry change in the crystal structure nor magnetic ordering have been observed, which makes this "hidden order" enigmatic. Some high-field experiments have suggested electronic nematicity which breaks fourfold rotational symmetry, but direct evidence has been lacking for its ground state at zero 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 $T_{\rm HO}$ with a jump, uncovering the weakly first-order nature of the hidden-order transition. This distortion is observed only in ultrapure sample, implying a highly unusual coupling nature between the electronic nematicity and underlying lattice.