Hidden Order Behaviour in URu2Si2 (A Critical Review of the Status of Hidden Order in 2014)

TL;DR

This review discusses experimental and theoretical progress in understanding the hidden order phase in URu2Si2, emphasizing electronic structure calculations and Fermi-surface nesting, while addressing controversial and exotic experimental findings.

Contribution

It synthesizes recent experimental results with density functional theory to provide a comprehensive understanding of the hidden order in URu2Si2 and evaluates various models against new data.

Findings

Consistent interpretation of ARPES, quantum oscillations, neutron scattering, RXD, optical spectroscopy, STM/STS within Fermi-surface nesting framework.

Identification of controversial experiments related to symmetry breaking and exotic order parameters.

Constraints on microscopic models based on localized 5f-U states and anisotropy.

Abstract

Throughout the past three decades the hidden order (HO) problem in URu$_2$Si$_2$ has remained a "hot topic" in the physics of strongly correlated electron systems with well over 600 publications related to this subject. Presently in 2014 there has been significant progress in combining various experimental results embedded within electronic structure calculations using density functional theory (DFT) to give a consistent description of the itinerant behaviour of the HO transition and its low temperature state. Here we review six different experiments: ARPES, quantum oscillations, neutron scattering, RXD, optical spectroscopy and STM/STS. We then establish the consistencies among these experiments when viewed through the Fermi-surface nesting, folding and gapping framework as predicted by DFT. We also discuss a group of other experiments (torque, cyclotron resonance, NMR and XRD) that are more controversial and are presently in a "transition" state regarding their interpretation as rotational symmetry breaking and dotriacontapole formation. There are also a series of recent "exotic" experiments (Raman scattering, polar Kerr effect and ultrasonics) that require verification, yet they offer new insights into the HO symmetry breaking and order parameter. We conclude with some constraining comments on the microscopic models that rely on localised $5f$-U states and strong Ising anisotropy {for explaining} the HO transition, and with an examination of different models in the light of recent experiments.