UCLA

Electrons are the major heat carriers in metals, as are phonons in semiconductors. The role of spin waves (magnons) in thermal transport problems has attracted attention in recent years with the discovery of spin Seebeck effects (SSE) in spintronics. Interactions among these particles or excitations are the origin of many fascinating phenomena and the focus of this work. Despite the computational cost, first-principles calculations use fewer approximations and no fitting parameters in comparison to semi-classical methods; therefore they produce more reliable results. Chapter 1 considers the theory of electron-electron, electron-phonon and phonon-magnon couplings from first principles. Chapter 2 reports first-principles calculations of electron-phonon coupling in bilayer graphene and the corresponding contribution to carrier scattering. At the phonon Γ point, electrons with energies less than 200 meV are scattered predominantly by LA′ and TA′ modes while higher- energy electron scattering is dominated by optical phonon modes. Based on a two-temperature model, heat transfer from electrons with an initial temperature of 2000 K to the lattice (phonons) with an initial temperature of 300 K is computed, and in the overall relaxation process, most of this energy scatters into K-point phonon optical modes due to their strong coupling with electrons and their high energies. A Drude model is used to calculate photoconductivity for bilayer graphene with different doping levels. Good agreement with prior experimental trends for both the real and imaginary components of photoconductivity confirms the model’s applicability. The effects of doping levels and electron-phonon scattering on photoconductiviy are analyzed. We also extract acoustic and optical deformation potentials from average scattering rates obtained from density functional theory (DFT) calculations and compare associated photoconductivity calculations with DFT results. The comparison indicates that momentum-dependent electron-phonon scattering potentials are required to provide accurate predictions. Chapter 3 combines first-principles calculations, spin-lattice dynamics and the non-equilibrium Green’s function (NEGF) method to compute thermal boundary conductance at a three-dimensional Co-Cu interface, considering spin-lattice interactions. Spin-lattice interactions are quantified through exchange interactions between spins, and the exchange constants are obtained from first principles. Equilibrium molecular dynamics (EMD) is used to calculate heat flux across the interface after the spin and lattice subsystems are in equilibrium. Because of the weak interaction between Co and Cu layers adjacent to the interface, spin-wave transmission is low. Spins are scattered by phonons inside the Co contact, and interfacial thermal conductance is reduced. We also compare the results to the NEGF method. Phonon and magnon scattering rates are incorporated into Buttiker probes attached to the device. NEGF results exhibit a similar trend in thermal boudary conductance with spins included. The Green’s function is solved recursively; therefore it can be applied to large devices. Chapter 4 investigates electronic and optical properties of single layer and bilayer armchair graphene nanoribbons using the first-principles method. An increase of nanoribbon width reduces the band gap and causes a redshift in photon absorption energy. We find that the 3n + 2 family nanoribbons have the smallest band gaps and lowest onset photon absorption energy among all three families considered due to the most π-conjugation indicated by the exciton wavefunctions. We also compare the bilayer α and β alignments of armchair graphene nanoribbons with their single-layer counterparts.The extra layer of these nanoribbons reduces the band gap and the onset photon absorption energy, and the difference between the α alignment and the single-layer configuration is more significant than that of the β alignment and the single layer. Our calculations indicate that the optical properties of graphene nanoribbons depend on the details of atomic structures, including nanoribbon width, edge alignment, and number of layers. Chapter 5 investigates the photo-thermal effect in the methane decomposition process. By calculating electronic transitions in polycyclic aromatic hydrocarbons from TDDFT, we extract the absorption coefficients. Further, the absorption coefficients are mapped to the experimental light intensity profile to predict the total absorption spectra. Temperature rise can be further induced by calculating heat capacities of the polycyclic aromatic hydrocarbons using frequencies of the vibrational modes.