# Pressure-Induced Rotational Symmetry Breaking in URu$_2$Si$_2$

## TL;DR

This study reveals a pressure-induced rotational symmetry breaking in URu$_2$Si$_2$, indicating an orthorhombic phase and electronic nematic order that emerge above a critical pressure, challenging existing theories of its hidden order and antiferromagnetic phases.

## Contribution

It provides the first high-resolution x-ray diffraction evidence of pressure-induced nematic order in URu$_2$Si$_2$, adding a new symmetry breaking element to the understanding of its phases.

## Key findings

- No symmetry breaking below critical pressure $p_c$
- Emergence of orthorhombic phase above $p_c$
- Identification of electronic nematic order unrelated to hidden order

## Abstract

Phase transitions and symmetry are intimately linked. Melting of ice, for example, restores translation invariance. The mysterious hidden order (HO) phase of URu$_2$Si$_2$ has, despite relentless research efforts, kept its symmetry breaking element intangible. Here we present a high-resolution x-ray diffraction study of the URu$_2$Si$_2$ crystal structure as a function of hydrostatic pressure. Below a critical pressure threshold $p_c\approx3$ kbar, no tetragonal lattice symmetry breaking is observed even below the HO transition $T_{HO}=17.5$ K. For $p>p_c$, however, a pressure-induced rotational symmetry breaking is identified with an onset temperatures $T_{OR}\sim 100$ K. The emergence of an orthorhombic phase is found and discussed in terms of an electronic nematic order that appears unrelated to the HO, but with possible relevance for the pressure-induced antiferromagnetic (AF) phase. Existing theories describe the HO and AF phases through an adiabatic continuity of a complex order parameter. Since none of these theories predicts a pressure-induced nematic order, our finding adds an additional symmetry breaking element to this long-standing problem.