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1. Magnon-phonon hybridization in 2D antiferromagnet MnPSe ₃

   https://doi.org/10.1126/sciadv.abj3106  

   Mai, Thuc T.; Garrity, Kevin F.; McCreary, Amber; Argo, Joshua; Simpson, Jeffrey R.; Doan-Nguyen, Vicky; Aguilar, Rolando Valdés; Walker, Angela R. (October 2021, Science Advances)

   Magnetic excitations in van der Waals (vdW) materials, especially in the two-dimensional (2D) limit, are an exciting research topic from both the fundamental and applied perspectives. Using temperature-dependent, magneto-Raman spectroscopy, we identify the hybridization of two-magnon excitations with two phonons in manganese phosphorus triselenide (MnPSe 3 ), a magnetic vdW material that hosts in-plane antiferromagnetism. Results from first-principles calculations of the phonon and magnon spectra further support our identification. The Raman spectra’s rich temperature dependence through the magnetic transition displays an avoided crossing behavior in the phonons’ frequency and a concurrent decrease in their lifetimes. We construct a model based on the interaction between a discrete level and a continuum that reproduces these observations. Our results imply a strong hybridization between each phonon and a two-magnon continuum. This work demonstrates that the magnon-phonon interactions can be observed directly in Raman scattering and provides deep insight into these interactions in 2D magnetic materials. 

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2. Resonant thermal Hall effect of phonons coupled to dynamical defects

   https://doi.org/10.1073/pnas.2215141119  

   Guo, Haoyu; Joshi, Darshan G.; Sachdev, Subir (November 2022, Proceedings of the National Academy of Sciences)

   We present computations of the thermal Hall coefficient of phonons scattering off a defect with multiple energy levels. Using a microscopic formulation based on the Kubo formula, we find that the leading contribution perturbative in the phonon–defect coupling is proportional to the phonon lifetime and has a “side-jump” interpretation. Consequently, the thermal Hall angle is independent of the phonon lifetime. The contribution to the thermal Hall coefficient is at resonance when the phonon energy equals a defect-level spacing. Our results are obtained for three different defect models, which apply to different correlated electron materials. For the pseudogap regime of the cuprates, we propose a model of phonons coupled to an impurity quantum spin in the presence of quasistatic magnetic order with an isotropic Zeeman coupling to the applied field and without spin–orbit interaction. 

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3. Haldane topological spin-1 chains in a planar metal-organic framework

   https://doi.org/10.1038/s41467-023-41014-1  

   Tin, Pagnareach; Jenkins, Michael J.; Xing, Jie; Caci, Nils; Gai, Zheng; Jin, Rongyin; Wessel, Stefan; Krzystek, J.; Li, Cheng; Daemen, Luke L.; et al (December 2023, Nature Communications)

   Abstract Haldane topological materials contain unique antiferromagnetic chains with symmetry-protected energy gaps. Such materials have potential applications in spintronics and future quantum computers. Haldane topological solids typically consist of spin-1 chains embedded in extended three-dimensional (3D) crystal structures. Here, we demonstrate that [Ni(μ−4,4′-bipyridine)(μ-oxalate)]_n(NiBO) instead adopts a two-dimensional (2D) metal-organic framework (MOF) structure of Ni²⁺spin-1 chains weakly linked by 4,4′-bipyridine. NiBO exhibits Haldane topological properties with a gap between the singlet ground state and the triplet excited state. The latter is split by weak axial and rhombic anisotropies. Several experimental probes, including single-crystal X-ray diffraction, variable-temperature powder neutron diffraction (VT-PND), VT inelastic neutron scattering (VT-INS), DC susceptibility and specific heat measurements, high-field electron spin resonance, and unbiased quantum Monte Carlo simulations, provide a detailed, comprehensive characterization of NiBO. Vibrational (also known as phonon) properties of NiBO have been probed by INS and density-functional theory (DFT) calculations, indicating the absence of phonons near magnetic excitations in NiBO, suppressing spin-phonon coupling. The work here demonstrates that NiBO is indeed a rare 2D-MOF Haldane topological material. 

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4. Evolution of Phonon Spectral Energy Density in Superlattice Structures

   https://doi.org/10.3390/cryst15050446  

   Nasiri, Milad; Wang, Yan (May 2025, Crystals)

   Superlattices are a distinctive class of artificial nanostructures formed by the periodic stacking of two or more materials. The high density of interfaces in these structures often gives rise to exotic physical properties. In the context of thermal transport, it is well established that such interfaces can significantly scatter particle-like phonons while also inducing constructive or destructive interference in wave-like phonons, depending on the relationship between the phonons’ coherence lengths and the superlattice’s period thickness. In this work, we systematically investigate the effect of temperature on the spectral energy density of phonon modes in superlattices. Additionally, we examine how variations in superlattice period thickness influence phonon lifetimes and energy density. Our findings provide critical insights into the spectral phonon properties of superlattices, particularly in terms of their coherence and lifetimes. 

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5. Strong laser polarization control of coherent phonon excitation in van der Waals material Fe3GeTe2

   https://doi.org/10.1038/s41699-021-00275-4  

   Gong, Yu; Hu, Ming; Harris, Nico; Yang, Zhonghua; Xie, Ti; Teklu, Alem; Kuthirummal, Narayanan; koenemann, Jacob; Xu, Xianghan; Cheong, Sang-Wook; et al (December 2022, npj 2D Materials and Applications)

   Abstract Optical manipulation of coherent phonon frequency in two-dimensional (2D) materials could advance the development of ultrafast phononics in atomic-thin platforms. However, conventional approaches for such control are limited to doping, strain, structural or thermal engineering. Here, we report the experimental observation of strong laser-polarization control of coherent phonon frequency through time-resolved pump-probe spectroscopic study of van der Waals (vdW) materials Fe 3 GeTe 2 . When the polarization of the pumping laser with tilted incidence is swept between in-plane and out-of-plane orientations, the frequencies of excited phonons can be monotonically tuned by as large as 3% (~100 GHz). Our first-principles calculations suggest the strong planar and vertical inter-atomic interaction asymmetry in layered materials accounts for the observed polarization-dependent phonon frequencies, as in-plane/out-of-plane polarization modifies the restoring force of the lattice vibration differently. Our work provides insightful understanding of the coherent phonon dynamics in layered vdW materials and opens up new avenues to optically manipulating coherent phonons. 

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