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Description:

BaAl2O4 is a ferroelectric material that exhibits structural quantum criticality through chemical
composition tuning. Although theoretical calculations and several diffraction experiments have
suggested the involvement of a soft mode in its ferroelectric structural phase transition, direct
experimental verification is still lacking. In this study, we successfully observed two soft modes
of BaAl2O4 using x-ray inelastic scattering, providing direct experimental evidence for their role
in the structural phase transition. Furthermore, we reveal that the soft modes at the M and K
points are nearly degenerate in energy, indicating a delicate balance in which either mode could
potentially freeze. The K-point mode simultaneously softens toward the transition temperature
(TC) in a manner nearly identical to the M-point mode. However, the phase transition condenses
only at the M point, with the M-point mode stabilizing as an acoustic mode in the low-temperature
structure and the K-point mode hardening as temperature decreases.

Rights:

• Creative Commons BY Attribution 4.0 International
  

Keyword: phonons, soft modes, inelastic X-ray scattering

Date published: 2026-02-23

Publisher: American Physical Society (APS)

Journal:

• Physical Review B (ISSN: 24699950) vol. 113 issue. 6 064112

Funding:

• Japan Society for the Promotion of Science 23K17673
• Japan Synchrotron Radiation Research Institute 2020A1179

Manuscript type: Author's version (Accepted manuscript)

MDR DOI:

First published URL: https://doi.org/10.1103/9nxr-h9zl

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Updated at: 2026-02-24 10:44:01 +0900

Published on MDR: 2026-02-24 12:30:20 +0900