First-Principles Calculations Of New Superconductors

We use FP-LAPW method to study the superconductivity of two type new superconductors. In the first part, considering the randomly distributed of Al and Si atoms not only over the in-plane but also along the c direction,we chose the simplest double cell model to calculate the electronic band structure. Using zone-center frozen phonon method, and considering the anharmonicity of phonon frequency , we calculate the phonon frequencies and electron-phonon coupling constant of B1g mode. we obtain the phonon frequency of B_1g mode to be 110cm^-1, and the electron-phonon coupling constant between Ca d electron and B_1g phonon mode is 0.37. The calculated results are consistent with that by using the model of virtual-crystal aproximation, and proved that the superconductivity in CaAlSi can be well explained by the standard BCS theory in the moderate electron-phonon coupling strength.In the second part, we study the superconductivity and magnetic properties of new superconductor MgCNiwith antiperovskite structure. First, we calculate the electronic band structure and denstity of states (DOS) of MgCNi, and the calculated results show that the hybridization between C 2p and Ni 3d electron orbits lead to the plan characteristic of Ni 3d band. The Fermi level is located in the vicinity of the van Hove singularity(vHs) peak, which made MgCNi₃ to be a strong coupling superconductivity. Then, considering the local spin density approximation (LSDA) and using the model of virtual-crystal approximation, we study the variance of superconductivity and magnetism of Mg_1-xNa_xCNi₃ and Mg_1-xAl_xCNi₃at doping 0≤x≤0.5. The calculated results show that both hole doping and electron doping quench superconductivity, but the reason are differences. The former is that the increased magnetic...