Date of Award
2025
Degree Type
Dissertation
Degree Name
Doctor of Philosophy in Mechanical Engineering and Applied Mechanics
Department
Mechanical, Industrial and Systems Engineering
First Advisor
Ashutosh Giri
Abstract
Understanding and controlling the spatiotemporal dynamics of energy and charge carriers in solids is central to advancing technologies ranging from microelectronics to plasmonics and energy materials. This dissertation presents a comprehensive investigation into the microscopic mechanisms governing hot carrier and thermal transport in metals and polar insulators, combining ultrafast optical experiments with first-principles simulations to uncover new physical phenomena.
Using a custom-built scanning ultrafast thermoreflectance microscope, we directly measure the ballistic and diffusive mean free paths of hot electrons in laser-excited noble metals with unprecedented spatial and temporal resolution. In gold, we observe ballistic transport lengths exceeding 150 nm and diffusive mean free paths reaching up to 45 nm - both significantly larger than previously reported or predicted. First-principles calculations based on density functional perturbation theory (DFPT) reveal that these enhanced transport lengths stem from a sharp reduction in electron-phonon coupling following interband excitation, a consequence of lattice stiffening and altered phonon populations.
Further, we explore a novel regime of negative thermal diffusion, where electronic temperature profiles contract spatially rather than expand, following energy transfer to a colder lattice. This counterintuitive effect is shown to be universal in materials composed of thermodynamically coupled subsystems with different diffusivities and becomes more prominent in systems with weak electron-phonon coupling.
To understand transport in extreme nonequilibrium conditions, we perform parameter-free calculations of temperature-dependent electron-phonon interactions and thermal conductivities in noble metals at electronic temperatures up to ~60,000 K. We find that although all three metals (Au, Ag, Cu) exhibit similar band structures, their electron-phonon coupling strengths diverge significantly with rising electronic temperatures, leading to a peak in thermal conductivity at ~6,000 K, followed by a sharp decline driven by strong electron-electron scattering.
The thesis addresses the implications of size effects and external pressure on thermal transport in metallic interconnects used in microelectronics. We demonstrate that pressure-induced enhancements in phonon-mediated conductivity in refractory metals such as tungsten provide a promising route to mitigate the size-dependent thermal resistivity that plagues ultra-thin interconnects.
Finally, beyond metals, we investigate the formation and tunability of polarons in polar insulators, particularly lithium halides. First-principles calculations show that hydrostatic pressure can dramatically increase the spatial extent of hole polarons, especially in LiBr, transitioning from a small polaron (~2 Å) to a large polaron (~17 Å) regime. This transition is attributed to changes in the valence band curvature and dominant coupling with low-wave vector acoustic modes, offering a pathway to enhance charge mobility through pressure engineering.
Overall, this work provides new experimental and theoretical insights into the ultrafast, non-equilibrium dynamics of hot carriers and quasiparticles, offering guiding principles for the design of next-generation electronic, optoelectronic, and energy-conversion devices.
Recommended Citation
Karna, Pravin, "UNDERSTANDING ELECTRON-PHONON COUPLING UNDER EXTREME CONDITIONS: IMPLICATIONS TO THE THERMAL CONDUCTIVITY AND CARRIER MOBILITIES IN METALS AND ULTRA-WIDE BAND GAP MATERIALS" (2025). Open Access Dissertations. Paper 4501.
https://digitalcommons.uri.edu/oa_diss/4501