First Principles Calculation of Electron-Phonon Coupling in Nonequilibrium Quantum Materials

Author

Chendi Xie, Clemson UniversityFollow

Date of Award

12-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics and Astronomy

Committee Chair/Advisor

Dr. Yao Wang

Committee Member

Dr. Apparao M. Rao

Committee Member

Dr. Feng Ding

Committee Member

Dr. Sumanta Tewari

Abstract

This dissertation combines first-principles computation and light-induced nonequilibrium analysis to understand and control electron–phonon coupling (EPC) and emergent many-body phenomena in quantum materials, in both equilibrium and nonequilibrium states.

Chapters 1 and 2 build the theoretical and computational foundation, beginning with electron and phonon self-energies, linewidths, and perturbation theory for EPC, followed by density functional theory (DFT) and density functional perturbation theory (DFPT), which serve as the first-principles workhorses used throughout. Chapter 3 reviews Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg theory, including the BCS gap - transition temperature (T_c) relation and the strong-coupling generalization, and delineates the practical validity bound T_c < 0.1ω_D that is especially relevant for hydrides.

Chapters 4 and 5 develop and apply a Floquet–DFPT workflow that directly incorporates steady-state light–matter effects into first-principles EPC calculations. With the Wannier-interpolated electronic Hamiltonians constructed, the light-matter interaction is introduced via Peierls substitution. The steady-state Floquet bandwidth renormalization is captured via Bessel-function factors, and the corresponding nonequilibrium EPC matrix elements and superconducting properties are calculated using Eliashberg theory. Applied to high-temperature hydride superconductors LaH₁₀ as an example, the calculations reveal that light-induced band compression increases the density of states at the Fermi level and strengthens EPC, yielding a systematic T_c enhancement potentially above room temperature.

Chapter 6 complements the theory with MeV ultrafast electron diffraction (UED) on Ta₂NiSe₅. Through quantitative extraction of photoinduced atomic displacements and comparison with time-resolved first-principles simulations, the results demonstrate that structural motion alone can largely account for the transient band-gap reduction, thereby clarifying the long-standing debate between excitonic and structural effects.

Chapter 7 expands the dissertation beyond EPC by introducing a first-principles–supported study of anomalous shot noise in nanojunctions of the “bad” metal β-Ta. First-principles calculations comparing PBE and r²SCAN functionals show a correlated suppression of the density of states near the Fermi level, providing microscopic support for non-Fermi-liquid behavior. Together with the sensitivity of transport to magnetic impurities, these findings point to short-range, valence-bond-like correlations and a possible Bose-liquid–like state of quasi-localized charge carriers in β-Ta.

Altogether, this dissertation unifies equilibrium EPC theory, nonequilibrium light-driven superconductivity, structural ultrafast dynamics, and correlated transport into a coherent first-principles framework. It provides predictive tools for engineering EPC and superconductivity with light, reveals how lattice motion controls electronic structure on ultrafast timescales, and demonstrates how first-principles analysis can illuminate correlation-driven phenomena in quantum materials. These results establish a broad, computationally grounded foundation for designing next-generation quantum materials in and out of equilibrium.

Recommended Citation

Xie, Chendi, "First Principles Calculation of Electron-Phonon Coupling in Nonequilibrium Quantum Materials" (2025). All Dissertations. 4118.
https://open.clemson.edu/all_dissertations/4118

Author ORCID Identifier

0000-0001-8938-2276