Chirality density wave of the 'hidden order' phase in URu$_2$Si$_2$

TL;DR

This study uses Raman spectroscopy to identify the symmetry-breaking and chiral properties of the hidden order phase in URu$_2$Si$_2$, revealing a chiral density wave as the ordered state.

Contribution

It provides the first symmetry-based identification of the hidden order parameter as a chiral density wave in URu$_2$Si$_2$.

Findings

HO parameter breaks vertical and diagonal reflection symmetries

Order involves chiral properties of uranium 5f orbitals

Results suggest a chiral density wave ground state

Abstract

A second-order phase transition is associated with emergence of an "order parameter" and a spontaneous symmetry breaking. For the heavy fermion superconductor URu$_2$Si$_2$, the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called "hidden order" (HO). Here we use polarization resolved Raman spectroscopy to specify the symmetry of the low energy excitations above and below the HO transition. These excitations involve transitions between interacting heavy uranium 5f orbitals, responsible for the broken symmetry in the HO phase. From the symmetry analysis of the collective mode, we determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state.