Explorations in the realm of phonon-mediated superconductors and energy conversion materials by first-principles calculations
| Autor: | Duc-Long Nguyen, 阮德龍 |
|---|---|
| Rok vydání: | 2019 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 107 In this thesis, the electronic and vibrational properties of selected phonon-mediated superconductors are investigated using the first-principles calculations. First, the superconductivity of various bulk phases of Gallium are studied. Based on structural predictions using \textit{ab-initio} random structure searching with the \textit{object} (RSSWO) concept, we have obtained eight lowest energy structures of Ga. RSSWO not only captures accurately the global minima ground state $\alpha$-Ga and other metastable structures reported in the literature but also reveals the unknown crystalline \textit{Imma}-Ga. Subsequently, the electronic structures and electron-phonon coupling calculations of these structures were carried out to explain the large variation in superconducting transition temperatures of Ga phases. We found that The T$_c$s were separated into two categories for different phases: T$_c$ > 5K, and T$_c$ $\leq$ 1K. Such major distinction is found owing to the structural feature. Some of the higher-T$_c$ structures Ga, which are experimentally identified as $\beta$ and $\gamma$ phase, do not contain Ga dimers. However, the low T$_c$ phases, which contains Ga dimers with the partial covalent bonding, significantly decreases the density of state at Fermi level. This weakens the electron-phonon coupling strength, leading to a considerably lower T$_c$. Second, we explore the superconductivity in a two-dimensional lattice, the single layer thick CoO$_2$ as well as its bulk and bilayer counterpart. We show that the monolayer CoO$_2$ sheets have a metallic ferromagnetic ground state. The non-spinpolarized calculation shows that this 2D material possesses a phonon-mediated superconductivity at 25-28 K. The strong electron-phonon coupling in monolayer CoO$_2$ is mainly driven by the acoustic phonons making CoO$_2$ one of the highest-temperature superconductor in existing 2D materials. In addition, CoO$_2$ sheets can be synthesized by exfoliating bulk because of the relatively small binding energy in the interlayer while maintaining their stability under normal experimental conditions. We then present first-principles calculations for Al(100), Al(110), and Al(111) to study the oscillatory quantum size effects (QSE) exhibited in the surface energy, work function, electron-phonon coupling constant, and superconductivity transition temperature $T_c$. These physical characteristics are found to have significant oscillatory QSE that are associated with the thickness dependence of the energies of confined electrons. A damped sinusoidal function with the periodicity determined by one Fermi Wave vector along the [111] direction can well fit the surface energy and work function of Al(111) films as a function of film thickness while it is required for the case of Al(110) films a combination of three Fermi wave vectors over the direction [110]. To describe these QSE quantitatively, a full consideration of the crystal band structure is necessary. While the main part of the thesis relates to phonon-mediated superconductors, the final chapter discusses of the work carried out in the field of energy conversion material, including thermoelectric and metal-organic perovskite solar cells. We study the temperature dependence of band structure in SnSe whose lattice constant is determined from experiments. The indirect-direct band gap transition was found as a function of temperature in the \textit{Cmcm} phase of SnSe, which gives new perspective into the understanding of this record-breaking thermoelectric material. Finally, the agreement between STM simulation and experimental work on the metal-organic perovskite solar cell MAPbBr$_3$ crystal is then presented in the last part of chapter 6. The theoretical calculations may shed some light on the underlying mechanism of illumination-induced organic cation molecule dipole orientation in this emergent material. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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