Lattice vibrations are ubiquitous in condensed-matter systems and play a central role in determining material properties. Electron-phonon and spin-phonon interactions mediate energy transfer between atomic, electronic, and spin degrees of freedom, and govern phenomena ranging from the magnetoelectric effect to ultrafast magnetic switching and chiroptical responses. Despite major advances in first-principles electronic-structure methods, realistic microscopic modelling of finite-temperature magnetic and optical phenomena remains an outstanding challenge. This thesis combines Density Functional Perturbation Theory (DFPT), atomistic Spin-Lattice Dynamics (SLD), and Maximally Localized Wannier Function (MLWF) analysis into a first-principles-based computational framework to investigate phonon-driven phenomena in magnetic materials. This framework, summarized in Chapter 2, enabled us to address a number of problems posed by recent experiments. %The framework enables a unified treatment of lattice dynamics, spin interactions, and orbital magnetization across equilibrium and nonequilibrium regimes. Typically, Density Functional Theory is employed on low-temperature structures to describe magnetism and optical properties. At the same time, thermally excited phonons determine optical absorption in e.g. silicon solar cells that would be transparent at zero temperature. In Chapter 3 we demonstrate that thermal lattice fluctuations do not universally suppress magnetic exchange interactions, as commonly assumed. In NiO, thermal bond bending fluctuations weaken the dominant exchange coupling, consistent with conventional superexchange theory. In contrast, in Cr$_2$O$_3$ we predict an anomalous enhancement of the exchange interaction at elevated temperatures. First-principles analysis reveals that with increasing temperature, the Cr–O–Cr bond geometry distorts, increasing orbital overlap and producing an approximately 10\% enhancement of the exchange coupling near room temperature. A related phenomenon was observed in the $p$-orbital magnet CsO$_2$, where NMR experiments have reported an increase of $J/k_B$ from 100~K to 225~K. This thesis discusses an orbital ordering in CsO$_2$, calculates the parameters of the exchange Hamiltonian and compares the simulated neutron scattering spectra with experimental data. % this thesis calculates shows that thermal librations of the $\pi^\ast$ molecular orbitals generate additional superexchange pathways. The thesis further investigates ultrafast demagnetization dynamics in antiferromagnetic insulators Cr$_2$O$_3$ and FeBO$_3$. Despite having similar structures, following a laser pulse, the magnetic order parameter in Cr$_2$O$_3$ is reduced two orders of magnitude faster than in FeBO$_3$. The origin of this disparity is traced to differences in exchange-striction coupling constants, which determine the efficiency of energy transfer between spins and lattice vibrations. These results identify exchange-striction-mediated lattice-to-spin energy transfer as the dominant relaxation channel in the two insulating antiferromagnets studied here. Coupled spin-lattice simulations reproduce the observed hierarchy of relaxation times, while the distinct magnetic-ion networks and exchange pathways of Cr$_2$O$_3$ and FeBO$_3$ account for their different coupling strengths. Then, we study the giant Raman optical activity recently observed in Ni$_3$TeO$_6$. Contrary to previous interpretations invoking chiral phonons, the calculations demonstrate that the effect originates from conventional zone-center phonon modes that dynamically reshape the electronic band structure and induce large Berry curvature and orbital magnetization. In Chapter 7 we combine G$_0$W$_0$ quasiparticle calculations, BSE optical calculations, and frozen-phonon analysis to investigate low-energy momentum-forbidden excitonic features in bulk MoS$_2$ and related TMDs observed in near-field spectroscopy. These results establish that thermal phonons act not merely as a static backdrop but as an active dynamical subsystem that can enhance magnetic interactions, control ultrafast switching dynamics, and generate emergent magneto-optical functionalities.
Effects of Electron-Phonon Coupling in Magnetic Materials from First Principles
KAUSHIK, RAVI
2026-10-23
Abstract
Lattice vibrations are ubiquitous in condensed-matter systems and play a central role in determining material properties. Electron-phonon and spin-phonon interactions mediate energy transfer between atomic, electronic, and spin degrees of freedom, and govern phenomena ranging from the magnetoelectric effect to ultrafast magnetic switching and chiroptical responses. Despite major advances in first-principles electronic-structure methods, realistic microscopic modelling of finite-temperature magnetic and optical phenomena remains an outstanding challenge. This thesis combines Density Functional Perturbation Theory (DFPT), atomistic Spin-Lattice Dynamics (SLD), and Maximally Localized Wannier Function (MLWF) analysis into a first-principles-based computational framework to investigate phonon-driven phenomena in magnetic materials. This framework, summarized in Chapter 2, enabled us to address a number of problems posed by recent experiments. %The framework enables a unified treatment of lattice dynamics, spin interactions, and orbital magnetization across equilibrium and nonequilibrium regimes. Typically, Density Functional Theory is employed on low-temperature structures to describe magnetism and optical properties. At the same time, thermally excited phonons determine optical absorption in e.g. silicon solar cells that would be transparent at zero temperature. In Chapter 3 we demonstrate that thermal lattice fluctuations do not universally suppress magnetic exchange interactions, as commonly assumed. In NiO, thermal bond bending fluctuations weaken the dominant exchange coupling, consistent with conventional superexchange theory. In contrast, in Cr$_2$O$_3$ we predict an anomalous enhancement of the exchange interaction at elevated temperatures. First-principles analysis reveals that with increasing temperature, the Cr–O–Cr bond geometry distorts, increasing orbital overlap and producing an approximately 10\% enhancement of the exchange coupling near room temperature. A related phenomenon was observed in the $p$-orbital magnet CsO$_2$, where NMR experiments have reported an increase of $J/k_B$ from 100~K to 225~K. This thesis discusses an orbital ordering in CsO$_2$, calculates the parameters of the exchange Hamiltonian and compares the simulated neutron scattering spectra with experimental data. % this thesis calculates shows that thermal librations of the $\pi^\ast$ molecular orbitals generate additional superexchange pathways. The thesis further investigates ultrafast demagnetization dynamics in antiferromagnetic insulators Cr$_2$O$_3$ and FeBO$_3$. Despite having similar structures, following a laser pulse, the magnetic order parameter in Cr$_2$O$_3$ is reduced two orders of magnitude faster than in FeBO$_3$. The origin of this disparity is traced to differences in exchange-striction coupling constants, which determine the efficiency of energy transfer between spins and lattice vibrations. These results identify exchange-striction-mediated lattice-to-spin energy transfer as the dominant relaxation channel in the two insulating antiferromagnets studied here. Coupled spin-lattice simulations reproduce the observed hierarchy of relaxation times, while the distinct magnetic-ion networks and exchange pathways of Cr$_2$O$_3$ and FeBO$_3$ account for their different coupling strengths. Then, we study the giant Raman optical activity recently observed in Ni$_3$TeO$_6$. Contrary to previous interpretations invoking chiral phonons, the calculations demonstrate that the effect originates from conventional zone-center phonon modes that dynamically reshape the electronic band structure and induce large Berry curvature and orbital magnetization. In Chapter 7 we combine G$_0$W$_0$ quasiparticle calculations, BSE optical calculations, and frozen-phonon analysis to investigate low-energy momentum-forbidden excitonic features in bulk MoS$_2$ and related TMDs observed in near-field spectroscopy. These results establish that thermal phonons act not merely as a static backdrop but as an active dynamical subsystem that can enhance magnetic interactions, control ultrafast switching dynamics, and generate emergent magneto-optical functionalities.
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