[Paper Review] Symmetry and correlations underlying Hidden Order in URu2Si2

Nicholas P. Butch, Michael E. Manley+7 more|OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information)|Dec 26, 2012

Rare-earth and actinide compounds3 citations

TL;DR

This study investigates the hidden order (HO) phase in URu2Si2 using inelastic neutron and x-ray scattering, revealing that the HO phase preserves the body-centered tetragonal symmetry of the high-temperature paramagnetic phase. Despite the emergence of HO, no reduction in spatial symmetry is observed, and magnetic excitations track the Fermi surface, indicating a non-local, symmetry-preserving order parameter inconsistent with conventional density waves or local order models.

ABSTRACT

We experimentally investigate the symmetry in the Hidden Order (HO) phase of intermetallic URu2Si2 by mapping the lattice and magnetic excitations via inelastic neutron and x-ray scattering measurements in the HO and high-temperature paramagnetic phases. At all temperatures the excitations respect the zone edges of the body-centered tetragonal paramagnetic phase, showing no signs of reduced spatial symmetry, even in the HO phase. The magnetic excitations originate from transitions between hybridized bands and track the Fermi surface, whose feature are corroborated by the phonon measurements. Due to a large hybridization energy scale, a full uranium moment persists in the HO phase, consistent with a lack of observed crystal-field-split states. Our results are inconsistent with local order parameter models and the behavior of typical density waves. We suggest that an order parameter that does not break spatial symmetry would naturally explain these characteristics.

Motivation & Objective

• To determine whether the hidden order (HO) phase in URu2Si2 breaks spatial symmetry, as predicted by some theoretical models.
• To identify the origin of low-energy excitations in the HO phase and distinguish between magnetic and phononic character.
• To probe the role of electron correlations and hybridization in stabilizing the HO phase.
• To test whether the observed excitations are consistent with conventional density wave or local order parameter models.
• To establish whether the full uranium moment persists in the HO phase, indicating the absence of crystal-field splitting.

Proposed method

• Inelastic neutron scattering (INS) was used to map lattice and magnetic excitations in both the high-temperature paramagnetic and hidden order phases.
• Inelastic x-ray scattering (IXS) provided complementary data on phonon and electronic excitations, with high momentum resolution.
• Measurements were performed across the entire Brillouin zone to assess symmetry breaking, focusing on zone-edge features.
• Fermi surface reconstruction was probed via the dispersion of magnetic excitations, which were compared to band structure calculations.
• The hybridization energy scale between uranium 5f and ligand states was estimated from the energy of the magnetic excitations.
• Comparison of observed modes with theoretical predictions for phonons and magnons allowed identification of magnetic character in previously misclassified soft modes.

Experimental results

Research questions

• RQ1 Does the hidden order phase in URu2Si2 break the spatial symmetry of the body-centered tetragonal crystal structure?
• RQ2 Are the low-energy excitations in the HO phase magnetic in nature, or are they phononic as previously suggested?
• RQ3 Does the full uranium magnetic moment persist in the hidden order phase, indicating no crystal-field splitting?
• RQ4 Do the observed magnetic excitations track the Fermi surface, suggesting a coherent electronic origin?
• RQ5 Can the data be explained by conventional density wave or local order parameter models?

Key findings

• The lattice and magnetic excitations in both the paramagnetic and hidden order phases respect the full body-centered tetragonal symmetry, with no evidence of reduced spatial symmetry.
• The low-energy modes previously identified as soft phonons are instead found to be magnetic in character, originating from transitions between hybridized bands.
• Magnetic excitations track the Fermi surface, confirming a coherent electronic origin and strong coupling to the electronic structure.
• A large hybridization energy scale (~100 meV) is inferred, consistent with a full uranium 5f moment and the absence of crystal-field-split states.
• The results are inconsistent with local order parameter models and conventional density wave scenarios, which would break spatial symmetry.
• The data support an order parameter that does not break spatial symmetry, suggesting a novel, non-local form of electronic order.