Abstract
In contrast to magnetic order formed by electrons’ dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as “hidden order.” Previously, the hidden order in phase II was identified as primary antiferroquadrupolar and field-induced octupolar order. Here, we present a combined experimental and theoretical investigation of collective excitations in phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in a rotating field is calculated within a localized approach using the pseudospin representation for the Γ8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at a constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of intermultipolar interactions that stabilize hidden-order phases.6 MoreReceived 6 January 2020Revised 20 February 2020Accepted 5 March 2020DOI:https://doi.org/10.1103/PhysRevX.10.021010Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsHeavy-fermion systemsStrongly correlated systemsTechniquesInelastic neutron scatteringCondensed Matter, Materials & Applied Physics
Highlights
The high degeneracy and strong Coulomb repulsion of f electrons in lanthanide and actinide compounds can lead to exotic quantum matter states at low temperatures [1,2,3,4,5,6,7]
There is plenty of experimental evidence that RKKY-type exchange dominates the multipolar interactions in CeB6, as it was shown that the diffuse momentum-space distribution of quasielastic magnetic intensity in the high-temperature paramagnetic state nicely matches the static electronic response (Lindhard) function of the conduction electrons [41,42]
We propose an alternative approach to analyze the fingerprints of hidden order” (HO) in the magnetic excitation spectrum, which appears to be more promising in providing quantitative information while staying within the same theoretical framework
Summary
The high degeneracy and strong Coulomb repulsion of f electrons in lanthanide and actinide compounds can lead to exotic quantum matter states at low temperatures [1,2,3,4,5,6,7]. The nonmonotonic form factor of higher-order multipoles vanishes at Q 1⁄4 0 and starts to increase until reaching a maximum at some finite momentum transfer This result is established from both theory and experiment for elastic neutron scattering [3,49,50,51], but, to the best of our knowledge, the highly involved theory of INS beyond the dipolar approximation [52,53] was never successfully applied to calculate the dynamical response functions for any compound with a multipolar-ordered ground state. IV, where we argue that our results on CeB6 serve as a proof of principle for a new alternative way of obtaining quantitative information about hidden-order symmetry and interactions between multipolar degrees of freedom in f-electron materials with nondipolar order parameters This method should extend the conventional approach to the analysis of neutron-scattering data and may find applications in a much broader class of correlated-electron systems
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