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Terahertz lattice imprint of a hidden multipolar ordering transition in Gd2Ru2O7

Nicolas M. Kawahala, Rafael L. Sabainsk*, Esteban Marulanda, Rafael S. Freitas, and Felix G. G. Hernandez†

  • *Present address: Institute of Physics, Goethe University Frankfurt, Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany.
  • †Contact author: felixggh@if.usp.br
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Phys. Rev. Research 8, 043114 – Published 2 November, 2026

DOI: https://doi.org/10.1103/m693-z3kk

Abstract

Magnetic order carried by higher-rank multipoles produces no net moment and couples only weakly to conventional dipolar probes. Recently, the lattice has been proposed as an alternative detection channel for the associated ordering transitions. Here, we report magneto-terahertz time-domain spectroscopy of the pyrochlore ruthenate Gd2Ru2O7, in which a single spectrum resolves an exchange-split Gd3+ transition and an infrared-active optical phonon. The temperature and field dependence of the magnetic mode give two independent determinations of the internal exchange field at the rare-earth site, which yield a magnitude of 5–6 T and a free-ion g factor. The optical phonon, sensitive to spin-disorder scattering rather than to the static exchange field, exhibits a sharp anomaly in frequency and lifetime at 10 K that shifts to 30 K under 7 T, which we identify as an ordering transition of the Gd sublattice hidden from bulk thermodynamic probes. These results identify spin-phonon coupling as a route to rare-earth ordering transitions in multipolar magnets, one that is applicable to polycrystalline samples.

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Phys. Rev. Research 8, 043114– Published 2 November, 2026
  • Received 16 July 2026
  • Accepted 19 October 2026
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