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 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.

Lattice dynamics of the heavy-fermion compoundURu2Si2

“…After almost three decades 1 of intensive experimental and theoretical research, the nature of the ordered phase found in the Kondo system URu 2 Si 2 at temperature below T 0 =17.5 K remains to be unraveled 2,3 . Whereas appearing clearly in the thermodynamic and transport quantities 1,4,5 , the order parameter of this electronic hidden order (HO) state could not be determined by any usual or sophisticated experimental techniques 6 .…”

Role of band filling in tuning the high-field phases of URu2Si2

“…Amongst the f-electron intermetallics, URu 2 Si 2 continues to attract interest because it hosts an unidentified ordered state ("hidden order") and unconventional superconductivity at temperatures below T 0 = 17.6 K and T c = 1.5 K, respectively [1][2][3][4][5]. These phenomena occur within a strongly hybridized f-electron lattice that is superficially similar to that of related systems with magnetically ordered ground states [6,7].…”

“…Despite this parallel, various measurements (e.g., neutron scattering) have revealed that the ordered state does not have an intrinsic magnetic moment [8]. A multitude of theories have been proposed to describe hidden order, where a distinguishing factor is the assumed degree of f-electron localization, but no consensus has been reached regarding their applicability [4,5].…”

Unusual magnetic field-dependence of a possible hidden order phase

“…Due to the absence of a definitive identification of the symmetries that are broken at the HO transition, the nature of the HO phase remains an outstanding issue in heavy-fermion physics. 35,36 Here, we report an unusual magnetic field dependence of the properties found for a novel phase of the underscreened Anderson lattice (UAL) model 37 that was recently proposed to describe the HO phase of URu 2 Si 2 . The choice of an itinerant model is motivated by the absence of evidence of well-defined crystal field levels in inelastic neutron scattering data 38 and highresolution inelastic X-ray scattering experiments, which indicate that the widths of any crystal field excitations are simply too large to be considered as excitations of a localized system.…”