hide
Free keywords:
-
Abstract:
In this thesis, we investigate the influence of electron-phonon interactions (EPI) on the band gap renormalization in crystalline solids at finite temperatures. The main goal is to identify the impact of the nuclear motion and the lattice thermal expansion on the band structure in a wide range of materials. For this purpose, the temperature influence on the EPI is calculated in the harmonic approximations by utilizing the stochastic sampling methodology and fully anharmonically, by performing ab initio molecular dynamics simulations (aiMD). The band gap at finite temperatures is extracted from the thermodynamically averaged spectra function, which is calculated using band-unfolding technique. While utilization of aiMD was already used for calculations of EPI the combination of aiMD and band-unfolding to treat the band gap renormalization was used only recently. In this thesis, we employed an improved band unfolding technique in order to effectively manage the calculations. This improved method incorporates several methodological innovations that serve to mitigate computational cost and minimize statistical noise in the final results. The updated method was thoroughly benchmarked, documented, and designed with a user-friendly interface. We present a comprehensive examination of the numerical aspects of thermodynamic averaging, the estimation of error bars, and the evaluation of convergence with respect to the size of the simulation supercell. Our established protocol enables the calculation of band gap renormalization at finite temperatures, which is in good agreement with prior theoretical studies and experimental data.
By utilizing aforementioned methodology, we evaluated the band gap renormalization in a collection of materials with varying structural types and levels of anharmonicity. This analysis contains an examination of the influence of quantum and classical treatments of the nuclear motion on the band gap renormalization. Furthermore, it enables an assessment of the impact of lattice dynamics anharmonicity on the band structure renormalization. The results indicate that materials with greater anharmonicities typically experience larger band gap renormalization at finite temperatures. These findings not only shed light on the underlying microscopic mechanisms of the band gap renormalization, but also have the potential to enable non-perturbative assessment of the charge transport coefficients. To illustrate this point, the electronic lifetimes of silicon were calculated. However, the convergence of the imaginary part of the electron-phonon self-energy with respect to the supercell size still presents a challenge and requires further methodological advancements.
