First-Principles Study Of Phonons And Electron-phonon Coupling In Several Materials At High Pressures

High-pressure may change existing state of materials effectively and make materials show exotic behaviors that wouldn't appear at ambient pressure, thus, it opened up a new dimension for the research of Condensed Matter Physics.At high pressures, distances between the atoms in materials are expected to decrease significantly and electron orbits of adjacent atoms would overlap. These might induce electronic transformations and structural phase transitions. Accordingly, every aspect of materials'properties would change significantly. Among them, behaviors of phonons and related electron-phonon coupling (EPC) directly indicate the effects of lattice vibration on materials'properties and become a key breakthrough point of the research in high-pressure physics. Specifically, studies of phonons may help to understand materials'structures and transformations at high pressures, while EPC correlates closely to superconductivity. Therefore, my thesis has very considerable research value in further understanding properties of some materials at high pressures, seeking for new types of functional materials, and summarizing general physical principles that would be observed at high pressures.Due to moderate computational load, wide range of application, higher accuracy, and direct comparison with experimental results, the first-principle method based on density functional theory has become one of most important approaches in the field of computational materials. With this method, we systemically studied behaviors of phonons and EPC in materials at high pressures, obtaining original results in below four high-pressure systems:I. For the first time, from the viewpoint of phonon stability we explored the crystal structure for phase II of solid hydrogen that hasn't been determined in experiments, finding the ideal candidate—Pca21 structure.As sitting in V. L. Ginzburg's well known"list of the key problems in physics', studies of solid hydrogen at high pressures always attracted the most attentions. Through spectrum measurements carried out at high pressures, three phases of solid hydrogen have been verified. However, owing to big difficulties in performing in situ X-ray diffraction (XRD) measurement for hydrogen, the explicit crystal structure of orientationally ordered phase II has never been determined. On the theoretical side, many candidate structures were proposed, but because their energy difference is too small, until now the consistent conclusion hasn't been reached. The full phonon dispersion curves in the whole Brillouin zone for previously proposed candidate structures are examined. It is found that the energetically preferred Pca21 structure show exclusively dynamical stability in the pressure range of phase II (110~150 GPa) as indicated by the absence of phonon softening. A pressure-induced soft transverse acoustic (TA) phonon mode is identified and the TA mode completely softens at the zone boundary of Y point at ~151 GPa, which coincides with the experimentally observed transition pressure (150 GPa) from phase II to III. Moreover, the analysis of the eigenvector of the TA soft mode suggested that this phonon softening will result in enlarged intramolecular bond lengths. This fact might serve to explain the experimental observation of a sudden decrease in Raman and infrared active vibron frequencies during the phase II to III transition. The newly proposed P-3 structure with Pa3 type local order has been firstly explored. However, the calculated results of total energy, band structure, and phonon do not support the choice of P-3 structure.II. Through the phonon calculations of rutile-type magnesium hydride (rutile-MgH2) under compression, within the framework of the theory of soft-mode phase transitions, we predicted a new high-pressure phase for MgH2—CaCl2 structure, which hasn't been observed by experimenters.There has already for decades been considerable interest in MgH2, which is one of the promising base materials for hydrogen storage. In order to improve the performance of hydrogen storage, a full understanding of its behaviors under extreme conditions (temperature and pressure, etc.) is considered as essential. Because of the difficultyto accurately determine the H atomic positions in the XRDmeasurements due to their very low scattering cross section, the phasetransition sequence of MgH2 under compression are still under debate.While for most of rutile-type structured dioxides, the CaCl2-typestructure was demonstrated to be the first high-pressure phase. Thus inthe rutile-MgH2, it is of great interest to verify the possibility of atransformation to a CaCl2-phase under compression.We performed the phonon calculations for rutile-MgH2 withincreasing pressures. The Raman active BB1g optical mode was found tosoften, leading to the structural instability. Within the framework of thetheory of pressure-induced soft-mode phase transitions, we predicted anew high-pressure phase—CaCl2 structure. The transition of rutile- toCaCl2-phase was characterized as a second-order nature, driven by thesoftening of the Raman active B1g B mode, weakly coupling with theelastic shear modulus Cs. Good agreement was obtained between thecalculated equation of state and the available experimental datathrough the rutile-CaCl2 transition. The detailed analysis of thecalculated results of Gibbs free energies, Raman active phonons, andspontaneous strain further supported the phase transition. In addition,the predicted CaCl2-phase was shown to be dynamically stable. By allappearances, further experimental studies such as high-pressureRaman and neutron diffraction measurements are required to confirmthe presence of this CaCl2-type MgH2.III. We studied the origin of high superconductivity in calciumgraphite-intercalation compounds—CaC6 and reasonably explained the mechanism for the pressure-induced enhancement of superconductor performance.The superconducting behaviors in graphite-intercalation compounds (GICs) have been an exotic subject since the discovery of alkali-metal GICs superconductors of C8A (A = K, Rb, or Cs). Recently, it was found that the alkaline-earth-metal GICs, CaC6 exhibits superconductivity with the highest critical temperature Tc≈11.5 K in this class of materials. More interestingly, the very recent electrical resistivity measurements showed that Tc still increases linearly with pressure. Generally, two main mechanisms have been proposed to understand the superconductivity in CaC6, including the unconventional exciton- or plasmon-mediated pairing mechanism and conventional BCS phonon-mediated mechanism. To uncover the origin of the pressure-induced Tc enhancement in CaC6, J. S. Kim et al. suggested that the increased Tc is due to the softening of an optical phonon mode at zone center associated to in-plane Ca vibrations. However, a constant EPC matrix element was assumed and only zone center phonon modes were included to understand the elevated Tc with pressure in their work.We presented an explicit investigation of the pressure effects on the EPC in CaC6. Due to an uncompleted ionization of the intercalants Ca, the electronic states at Fermi level exhibit notable Ca 3d characteristic feature. These states, when coupling with the very soft in-plane Ca phonon modes, are found to be mainly responsible for superconductivity. With increasing pressures, the electronic stiffnessηincreases, which is caused by the increasing of the electron-phonon matrix element overcoming the reduction of the electronic density of states at Fermi energy N(EF). At the same time, the low-lying in-plane Ca phonon modes might soften by reason of pressure-induced charge transfering from intercalants Ca to the graphite layers. Consequently, the resulting EPC increases under compression, consistent with experimental observations. According to present calculations, we predict that codoping with transition-metals in CaC6 might be an effective route to obtain even higher Tc.IV. We first studied the superconductivity of the"molecular"metallic hydrogen at ultrahigh pressures, the results indicating that the superconducting transition temperature might be above 100 K.The investigation of the metallic hydrogen has attracted considerable interest for over seventy years since the pioneering work of Wigner and Huntington. It is widely assumed that metallization would occur either through a structural transformation to an atomic metallic phase, which involves dissociation of the hydrogen molecules, or through band-gap closure within the molecular phase itself, named as molecular metallic hydrogen. At the level of BCS theory, metallic hydrogen, either as a monatomic or paired metal, should be a good candidate for high temperature superconductor. Some theorists have performed the calculations of the EPC of metallic hydrogen in atomic phases, where a very high superconducting transition temperature Tc in the order of 102 K was suggested. However, the EPC study for the molecular metallic hydrogen with overlapping bands is less reported, partly due to the complexity of the studied systems. We presented the investigation of the EPC of the molecular metallic hydrogen with Cmca structure. This molecular metallic hydrogen with overlapping bands has an elastic instability at lower pressures (< 300 GPa), but stabilizes dynamically under further compression as indicated by the absence of phonon softening, thus, supporting the choice of Cmca structure as a good candidate for the metallic hydrogen. Within the conventional BCS theory, the predicted Tc is 107 K at 347 GPa, signifying a good candidate for high temperature superconductor. With increasing pressure, interestingly, the EPC parameterλ, hence, Tc increases resulting from the increased electronic density of states at the Fermi level.