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1. PRB Exciton-activated effective phonon magnetic moment in monolayer MoS2

   https://doi.org/10.5281/zenodo.11493104  

   He, Rui (June 2024, Zenodo)

   Optical excitation of chiral phonons plays a vital role in studying the phonon-driven magnetic phenomena in solids. Transition metal dichalcogenides host chiral phonons at high symmetry points of the Brillouin zone, providing an ideal platform to explore the interplay between chiral phonons and valley degree of freedom. Here, we investigate the helicity-resolved magneto-Raman response of monolayer MoS2 and identify a doubly degenerate Brillouin-zone-center chiral phonon mode at ∼270cm−1. Our wavelength- and temperature-dependent measurements show that this chiral phonon is activated through the resonant excitation of 𝐴 exciton. Under an out-of-plane magnetic field, the chiral phonon exhibits giant Zeeman splitting, which corresponds to an effective magnetic moment of ∼2.5⁢𝜇𝐵. Moreover, we carry out theoretical calculations based on the morphic effects in nonmagnetic crystals, which reproduce the linear Zeeman splitting and Raman cross section of the chiral phonon. Our study provides important insights into lifting the chiral phonon degeneracy in an achiral covalent material, paving a route to excite and control chiral phonons. 

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2. Spin–orbit exciton–induced phonon chirality in a quantum magnet

   https://doi.org/10.1073/pnas.2304360121  

   Lujan, David; Choe, Jeongheon; Chaudhary, Swati; Ye, Gaihua; Nnokwe, Cynthia; Rodriguez-Vega, Martin; He, Jiaming; Gao, Frank Y.; Nunley, T. Nathan; Baldini, Edoardo; et al (March 2024, Proceedings of the National Academy of Sciences)

   The interplay of charge, spin, lattice, and orbital degrees of freedom in correlated materials often leads to rich and exotic properties. Recent studies have brought new perspectives to bosonic collective excitations in correlated materials. For example, inelastic neutron scattering experiments revealed non-trivial band topology for magnons and spin–orbit excitons (SOEs) in a quantum magnet CoTiO₃(CTO). Here, we report phonon properties resulting from a combination of strong spin–orbit coupling, large crystal field splitting, and trigonal distortion in CTO. Specifically, the interaction between SOEs and phonons endows chirality to two $E_g$phonon modes and leads to large phonon magnetic moments observed in magneto-Raman spectra. The remarkably strong magneto-phononic effect originates from the hybridization of SOEs and phonons due to their close energy proximity. While chiral phonons have been associated with electronic topology in some materials, our work suggests opportunities may arise by exploring chiral phonons coupled to topological bosons. 

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3. Strong electron-phonon coupling driven pseudogap modulation and density-wave fluctuations in a correlated polar metal

   https://doi.org/10.1038/s41467-023-41460-x  

   Wang, Huaiyu; Xiong, Yihuang; Padma, Hari; Wang, Yi; Wang, Ziqi; Claes, Romain; Brunin, Guillaume; Min, Lujin; Zu, Rui; Wetherington, Maxwell T.; et al (December 2023, Nature Communications)

   Abstract There is tremendous interest in employing collective excitations of the lattice, spin, charge, and orbitals to tune strongly correlated electronic phenomena. We report such an effect in a ruthenate, Ca₃Ru₂O₇, where two phonons with strong electron-phonon coupling modulate the electronic pseudogap as well as mediate charge and spin density wave fluctuations. Combining temperature-dependent Raman spectroscopy with density functional theory reveals two phonons,B₂^PandB₂^M, that are strongly coupled to electrons and whose scattering intensities respectively dominate in the pseudogap versus the metallic phases. TheB₂^Psqueezes the octahedra along the out of planec-axis, while theB₂^Melongates it, thus modulating the Ru 4d orbital splitting and the bandwidth of the in-plane electron hopping; Thus,B₂^Popens the pseudogap, whileB₂^Mcloses it. Moreover, theB₂phonons mediate incoherent charge and spin density wave fluctuations, as evidenced by changes in the background electronic Raman scattering that exhibit unique symmetry signatures. The polar order breaks inversion symmetry, enabling infrared activity of these phonons, paving the way for coherent light-driven control of electronic transport. 

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4. Applying Unconventional Spectroscopies to the Single‐Molecule Magnets, Co(PPh ₃ ) ₂ X ₂ (X=Cl, Br, I): Unveiling Magnetic Transitions and Spin‐Phonon Coupling

   https://doi.org/10.1002/chem.202100705  

   Bone, Alexandria_N; Widener, Chelsea_N; Moseley, Duncan_H; Liu, Zhiming; Lu, Zhengguang; Cheng, Yongqiang; Daemen, Luke_L; Ozerov, Mykhaylo; Telser, Joshua; Thirunavukkuarasu, Komalavalli; et al (August 2021, Chemistry – A European Journal)

   Abstract Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single‐molecule magnets (SMMs). Spin‐phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin‐phonon coupling in molecules is challenging. We have found that far‐IR magnetic spectra (FIRMS) of Co(PPh₃)₂X₂(Co‐X; X=Cl, Br, I) reveal rarely observed spin‐phonon coupling as avoided crossings between magnetic andu‐symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero‐field split (ZFS) levels of theS=3/2 electronic ground state were probed by INS, high‐frequency and ‐field EPR (HFEPR), FIRMS, and frequency‐domain FT terahertz EPR (FD‐FT THz‐EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) andgvalues. Ligand‐field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities inCo‐X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin‐phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling. 

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5. Strongly correlated and topological states in [111] grown transition metal oxide thin films and heterostructures

   https://doi.org/10.1063/5.0009092  

   Chakhalian, Jak; Liu, Xiaoran; Fiete, Gregory_A (May 2020, APL Materials)

   We highlight recent advances in the theory, materials fabrication, and experimental characterization of strongly correlated and topological states in [111] oriented transition metal oxide thin films and heterostructures, which are notoriously difficult to realize compared to their [001] oriented counterparts. We focus on two classes of complex oxides, with the chemical formulas ABO3 and A2B2O7, where the B sites are occupied by an open-shell transition metal ion with a local moment and the A sites are typically a rare earth element. The [111] oriented quasi-two-dimensional lattices derived from these parent compound lattices can exhibit peculiar geometries and symmetries, namely, a buckled honeycomb lattice, as well as kagome and triangular lattices. These lattice motifs form the basis for emergent strongly correlated and topological states expressed in exotic magnetism, various forms of orbital ordering, topological insulators, topological semimetals, quantum anomalous Hall insulators, and quantum spin liquids. For transition metal ions with high atomic number, spin–orbit coupling plays a significant role and may give rise to additional topological features in the electronic band structure and in the spectrum of magnetic excitations. We conclude this perspective by articulating open challenges and opportunities in this actively developing field. 

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