Hidden Order in $URu_2Si_2$

TL;DR

This paper reviews the hidden order in URu2Si2, discussing experimental evidence for dual order parameters and proposing experiments to determine the nature of the primary order parameter, especially its time-reversal symmetry.

Contribution

It synthesizes current experimental and theoretical understanding of the hidden order and suggests specific experiments to clarify its fundamental nature.

Findings

Evidence for two coexisting order parameters.

Classification of theories based on time-reversal symmetry.

Proposed experiments to test the primary order's properties.

Abstract

We review current attempts to characterize the underlying nature of the hidden order in $URu_2Si_2$. A wide variety of experiments point to the existence of two order parameters: a large primary order parameter of unknown character which co-exists with secondary antiferromagnetic order. Current theories can be divided into two groups determined by whether or not the primary order parameter breaks time-reversal symmetry. We propose a series of experiments designed to test the time-reversal nature of the underlying primary order in $URu_2Si_2$ and to characterize its local single-ion physics.