Bulletin of the American Physical Society
APS March Meeting 2024
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session N56: Novel Computational Approaches to Coupled PhononMagnon DynamicsInvited

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Sponsoring Units: DCOMP Chair: Di Xiao, University of Washington Room: 205AB 
Wednesday, March 6, 2024 11:30AM  12:06PM 
N56.00001: Chiral phononics: Controlling magnetic order with phonon angular momentum Invited Speaker: Dominik M Juraschek Chiral phonons are circularly polarized lattice vibrations, in which the atoms move along circular orbits around their equilibrium positions in a solid. In dielectric materials, the orbital motions of the ions generate atomistic charge currents and therefore a magnetic moment carried by the chiral phonon [1,2]. In this talk, I show that chiral phonons are able to induce giant effective magnetic fields in solids when driven with an ultrashort laser pulse, and I present two examples of how these fields can be utilized to control the magnetic order of materials. First, we demonstrate that chiral phonons in paramagnetic rareearth trihalides are able to create effective magnetic fields of potentially up to 100 T that polarize the spins in the material [3,4], a prediction that has been successfully verified by a recent experiment [5]. Second, we predict that the effective magnetic field produced by chiral phonons can rectify spin precession in antiferromagnets, leading to a canting of the spins and the generation of a quasistatic magnetization. As a result, a lightinduced weak ferromagnetic state is transiently created in the solid [6]. Our results represent a new way of inducing hidden spin configurations that are not accessible in the equilibrium. 
Wednesday, March 6, 2024 12:06PM  12:42PM 
N56.00002: Phononinduced magnetization and its reciprocity: effects of geometrical phase and nonlinearity Invited Speaker: Yafei Ren The interaction between phonons and magnetization can significantly influence both the phononic and magnetic properties of materials. Chiral phonons can induce magnetization by generating an effective magnetic field on electrons. Reciprocally, a nonzero magnetization can bestow chirality upon phonons via an effective magnetic field on phonons. 
Wednesday, March 6, 2024 12:42PM  1:18PM 
N56.00003: Intrinsic vibrational angular momentum as nonadiabatic effects in noncollinear magnetic molecules Invited Speaker: Francesco Mauri We show that in noncollinear magnetic molecules, nonadiabatic (dynamical) effects due to the electronvibron coupling are timereversal symmetry breaking interactions for the vibrational field. Because the electronic wave function cannot be chosen as real in these molecules, a nonzero geometric vector potential (Berry connection) arises. As a result, an intrinsic nonzero vibrational angular momentum occurs even for nondegenerate modes and in the absence of external probes. The vibronic modes can then be seen as elementary quantum particles carrying a sizeable angular momentum. As a proof of concept, we demonstrate the magnitude of this topological effect by performing nonadiabatic first principles calculations on platinum clusters and by showing that these molecules host sizeable intrinsic phonon angular momenta comparable to the orbital electronic ones in itinerant ferromagnets. 
Wednesday, March 6, 2024 1:18PM  1:54PM 
N56.00004: Adiabatic dynamics of coupled spins and phonons in magnetic insulators Invited Speaker: Shang Ren In this presentation, we discuss our recent advances in the methodology addressing the coupled adiabatic dynamics of phonons and magnons in magnetic insulators [1,2]. In conventional ab initio methodologies, phonons are calculated by solving equations of motion involving static interatomic force constants and atomic masses. The BornOppenheimer approximation, where all electronic degrees of freedom are assumed to adiabatically follow the nuclear dynamics, is also adopted. This approach does not fully account for the effects of broken timereversal symmetry in systems with magnetic order. Recent attempts to rectify this involve the inclusion of the velocity dependence of the interatomic forces in the equations of motion, which accounts for timereversal symmetry breaking, and can result in chiral phonon modes with nonzero angular momentum even at the zone center. However, since the energy ranges of phonons and magnons typically overlap, the spins cannot be treated as adiabatically following the lattice degrees of freedom. Instead, phonon and spins must be treated on a similar footing [2]. Focusing on zonecenter modes, we propose a method involving Hessian matrices and Berry curvature tensors in terms of both phonon and spin degrees of freedom, and describe a firstprinciples methodology for calculating these [1]. We then solve Lagrange's equations of motion to determine the energies and characters of the mixed excitations, allowing us to quantify, for example, the energy splittings between chiral pairs of phonons in CrI_{3}, and the degree of magnetically induced mixing between infrared and Raman modes in Cr_{2}O_{3}. The approach is general, and can be applied to determine the adiabatic dynamics of any mixed set of slow variables. We will also discuss the extension of our approach to finite momentum. 
Wednesday, March 6, 2024 1:54PM  2:30PM 
N56.00005: SpinOrbit induced linear magnonphonon coupling in 2D van der Waals magnets Invited Speaker: Tommaso Gorni Standard ab initio treatments of spin and lattice dynamics neglect the coupling between these two degrees of freedom, which turns out to be often a harsh approximation due to the energy overlapping of the respective normal modes. In the context of localizedspin models, the spinlattice coupling is normally modelled with a nonlinear term stemming from the dependence of the exchange couplings on the lattice positions whose treatment is demanding even in the case of model Hamiltonians and often restricted to the elastic regime only. However, it is known since the late 40es that a linear spinlattice coupling may arise in the presence of magnetic anisotropies, whose contribution may become dominant with respect to higher order terms. By means of a supercell approach within DensityFunctional Theory we provided, to the best of our knowledge, the first ab initio computation of the SOCinduced spinlattice couplings in a real material. Our calculations for the CrI3 van der Waals magnet a recentlysynthetized 2D material where a large spinorbit coupling stabilizes 2D longrange magnetic order and is argued to sustain topological magnetic excitations show that a polaritoniclike hybridization between the magnon and the phonon modes may induce ~meV gap in its magnetic excitation spectrum close to the K point of the Brillouin Zone. 
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