Structures and optical properties of solid hydrogen at ultrahigh pressures

We have studied the electronic energy bands for the structures whose primitive cell contains up to four molecules, with full optimization of the structures, based on the GGA and LDA band calculations. Above 250 GPa, the eventual optimal structure obtained by the GGA or the LDA calculation is Cmca, which is a layered structure with the molecular bonds lying in planes, and has a metallic band structure with no band gaps. The metallic property of the band structure still remains unchanged even if the molecular bonds in the plane of the Cmca were inclined such that the atoms in the molecule escape from the plane. The electronic bands of the Cmca and those of some other candidate structures are discussed in the light of recent experimental result. Effects of the occupation of electronic states on the predicted optimal structures are also studied

Ab initio calculations of superconductivity in palladium under pressure

We have calculated the phonon dispersion and the electron-phonon interaction in palladium at ambient and under pressure using ab initio methods and evaluated superconducting transition temperature on the basis of the BCS theory. At ambient pressure, we have evaluated the electron-phonon coupling constant to be λ=0.377 by the use of the density-functional perturbation theory, and estimated the spin fluctuation effect in relation to the experimental data of specific heat. The vanishing of the superconducting transition at ambient pressure is resulted from the spin fluctuation effect. The spin fluctuation effect reduces with increasing pressure, and the superconductivity appears under pressure

Synthetic ferromagnetic nitrides: First-principles calculations of CaN and SrN

Encouraged by recent experimental facts of synthesizing materials which do not exist in nature, we introduce a type of ferromagnets, CaN and SrN, which have been proposed by first-principles calculations. These are half-metallic ferromagnets and they have magnetic moments of 1μB per chemical formula unit. Out of typical structures of binary compounds we have calculated, the rocksalt structure is the most stable form for both CaN and SrN. The majority of the magnetic moment of these compounds originates from the nitrogen sites since the p states of nitrogen are spin polarized. The mechanism of the ferromagnetism is discussed. Their formation energies have been calculated and the results show that it should be feasible to synthesize these materials. The structural stability of CaN has been confirmed by performing first-principles molecular dynamics simulations. We propose a synthesis process for CaN using the high pressure and temperature environment