Hidden order behaviour in URu₂Si₂ (A critical review of the status of hidden order in 2014)

Abstract

Throughout the past three decades, the hidden order (HO) problem in URu₂Si₂ 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 localized 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.

Keywords:

• strongly correlated electron systems
• heavy fermion physics
• URu₂Si₂
• hidden order

Acknowledgements

The Hidden Order Workshop held in November 2013 at the Lorentz Center Leiden, The Netherlands, was supported by the Dutch KNAW and ICAM. PMO acknowledges support from the Swedish Research Council (VR).