Thermodynamic evidence for broken fourfold rotational symmetry in the hidden-order phase of URu2Si2

TL;DR

This study provides thermodynamic evidence that the hidden order phase in URu2Si2 breaks fourfold rotational symmetry, narrowing down the possible symmetry-breaking mechanisms involved.

Contribution

The paper presents sensitive torque measurements revealing symmetry breaking in URu2Si2, offering new insights into the nature of its hidden order phase.

Findings

Fourfold rotational symmetry is broken below the transition temperature.

Twofold oscillation in torque indicates symmetry reduction.

Results constrain the hidden order parameter to specific symmetry types.

Abstract

Despite more than a quarter century of research, the nature of the second-order phase transition in the heavy-fermion metal URu$_2$Si$_2$ remains enigmatic. The key question is which symmetry is being broken below this "hidden order" transition. We review the recent progress on this issue, particularly focusing on the thermodynamic evidence from very sensitive micro-cantilever magnetic torque measurements that the fourfold rotational symmetry of the underlying tetragonal crystal is broken. The angle dependence of the torque under in-plane field rotation exhibits the twofold oscillation term, which sets in just below the transition temperature. This observation restricts the symmetry of the hidden order parameter to the $E^{+}$- or $E^{-}$-type, depending on whether the time reversal symmetry is preserved or not.