Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films

In Hyeok Choi; Seung Gyo Jeong2; Jae Hyuck Lee; San Kang; Sreejith Nair; Changyoung Kim; Dirk Wulferding; Bharat Jalan; Jong Seok Lee

arXiv · 2509.23969

Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films

In Hyeok Choi·Seung Gyo Jeong2·Jae Hyuck Lee·San Kang·Sreejith Nair·Changyoung Kim·Dirk Wulferding·Bharat Jalan·Jong Seok Lee

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Abstract

Magnetic order parameters in altermagnets can couple to quantized lattice vibration via both piezomagnetic and magnetoelastic effects, leading to the renormalization of phonon dispersion. Here, we demonstrate photo-induced dynamic frequency modulation of THz phonons excited in anisotropically-strained epitaxial RuO2 thin films using ultrafast coherent phonon spectroscopy and time-resolved magneto-optic Kerr effect measurement. A coherent oscillation of a transverse acoustic phonon appears in the sub-THz range with increasing film thickness above 4 nm due to local dislocation arising from the anisotropic strain relaxation, which hosts large non-zero shear strain. Interestingly, this phonon mode exhibits a time-varying mode hardening below ~ 500 K. Furthermore, an optical phonon oscillation emerges in magnetization dynamics of the photo-induced non-equilibrium state, and it becomes significantly softened near the critical temperature, while there is no observable magneto-optic signal in fully-strain-relaxed films. Such notable dynamic frequency modulations in acoustic and optical phonons offer an opportunity to manipulate phonons in the THz range through the spin-phonon coupling controlled by epitaxial design, which can inspire the new class of altermagnetic applications in the ultrafast quantum opto-spintronics.

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In Hyeok Choi, Seung Gyo Jeong2, Jae Hyuck Lee, San Kang, Sreejith Nair, Changyoung Kim, Dirk Wulferding, Bharat Jalan, Jong Seok Lee. 2025-09-28. Strain-induced Dynamic Spin-Phonon Coupling in Epitaxial RuO2 Films. https://arxiv.org/abs/2509.23969

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