Superconducting Studies Of Several Typical Materials

In this paper, the electronic properties, lattice dynamics, and superconductivity of the several typical materials are extensively studied by the first principles based on density functional theory. It is found that all of these systems are potential high temperature superconduors, having a higher superconducting transition temperature Tc. Meanwhile, we obtain the following original results in three important systems:(1) Monatomic Metallic Hydrogen:Hydrogen, being the first element in the Periodic Table, has the simplest electronic structure than any other elements. It is the most abundant element in the solar system so far. And the hydrogen molecule includes the simplest covalent chemical bond. However, the system of hydrogen is poorly understood. Until 1935, Wigner and Huntigdon firstly predicted that the pressure induced moleculear dissociation of solid hydrogen and its resulting metallic state, such as alkaline metals. The studies related to hydrogen become a very important direction in the high pressure area. Particularly, the investigations of the superconductivity become a hot point. In this letter, we have firstly studied the final structure of the solid hydrogen at ultra-high pressure-monatomic metallic hydrogen (space group:I41/amd)-which has been identified recently. At high pressure we calcualte the lattice dynamics of monatomic metallic hydrogen. It is found that the structure is stable up to 802 GPa. Therefore, based on the conventional BCS theory, the electron-phonon coupling interaction has been studied. The results indicate the superconducting transition temperature is above the order of 102 K, becoming a potential room-temperature superconductor. Meanwhile, we also give a reasonable understand to its superconducting mechanism.(2) Hydrogen-rich Compounds——Sliane(SiH4):we firstly studies the lattice dynamics of the metallic phase of Silane, which is experimentally found. It is found that the phonon dispersive curves exists the imaginary frequencies in the whole Brillouin zone, indicating structural instability in P63 phase. It is possible that in-situ X-ray diffraction measurement is experimentally difficult to fix the accurate positin of the hydrogen atom in P63 structure. Subsequently, we predict a new structure (space group:Pbcn) by using the soften-mode phase transion theory, reveal its phyiscal mechanism, and give a possible path of phase transiton between the P63 phase and Pbcn structure. Through the analysis of the structure, it is found that the positons of Si atoms in Pbcn phase are the same as that of P63 structure. Only the postions of H atoms are changed. In additon, we systemically study electronic structure, lattice dynamics, and electron-phonon coupling properties of this new phase. It is found that this new structure is a metal, and has the higher superconducting transiton temperarure Tc, which is up to 16.5 K at 188 GPa, in excellent agreement with the experimental results (17.5 K).(3) P-type Diamond Thin Films:since the superconducting transiton temperature Tc (around 2.3-4 K) has been found in boron-doped diamond, it provides a new intersection point for the superconducting and superhard areas, and has been extensively studied by theorists and experimentalists. Subsequently,it is found that boron-doped diamond (100) and (111) thin films have a higher superconducting transition temperature Tc than that of bulk. Specially, the superconductivity of boron-doped diamond (111) thin films is much higher than that of boron-doped diamond (100) thin films. However, the theorectical understanding of superconducting mechanism remains imcomplete. Based on these reason, we calculate the electronic structure, phonon stability, and electron-phonon coupling properties of the diamond (100) and (111) thin films at different doping concentrations. By the analysis of the calculated results, superconducting mechanism of diamond (100) and (111) thin films is reasonably explained, while at different doping concentrations the values of the superconducting transition temperature Tc have been given approximately.