Hitose Nagara

Determining the Structure of Phosphorus in Phase IV

We explore the unknown structure of phosphorus in phase IV (P-IV phase) based on first-principles calculations using the metadynamics simulation method. Starting from the simple cubic structure, we find a new modulated structure of the monoclinic lattice. The modulation is crucial to the stability of the structure. Through refining the structure further by changing the modulation period, we find the structure whose x-ray powder diffraction pattern is in best agreement with the experimental pattern. We expect that the modulation period of the structure in the P-IV phase is very close to that found in this study and probably incommensurate.

Half-Metallic p-Electron Ferromagnetism in Ca and Sr Pnictides

We investigated the magnetism in Ca and Sr pnictides by using the first-principles calculations. These compounds are half-metallic and ferromagnetic (FM) when they assume the zinc-blende structure at the equilibrium lattice constant. Ferromagnetism is induced by the spin polarization of the p -orbitals of the pnictogen atoms; Ca and Sr atoms have no magnetic moments, which is different from that of CrAs or CrSb with a zinc-blende structure. To confirm the mechanism of the ferromagnetism, we have calculated a hypothetical crystal – fcc-As with two additional electrons – and have shown that fcc-As has the same magnetic moment as CaAs with a zinc-blende structure. This means that the role of Ca or Sr atoms is to provide electrons with As atoms at the fcc site and to sustain the distances between the As atoms and crystal symmetry. The FM exchange interactions between the pnictogen atoms are considered to exist in these lattices, which is briefly discussed.

Ab-initio Calculations of Lattice Dynamics and Superconductivity in FCC Lithium and Iodine and BCC Tellurium

We present the calculations of the phonon frequency and its line width over the Brillouin zone for simple substances based on ab-initio linear response theory and the evaluation of superconducting transition temperature. The substances we report on are FCC iodine, FCC lithium, and BCC tellurium at high pressures. In iodine the phonon dispersion reveals a phonon softening along the Σ-line (Γ–K direction) near the lower pressure boundary of the FCC phase. In lithium, the frequency softening is striking along the entire Λ-line (Γ–L direction) and along the Σ-line (Γ–K direction) in the upper pressure boundary of the FCC phase. The FCC structure shows a phonon instability around 40 GPa and a turnover of the T c before reaching this pressure. Similar phonon anomaly has been observed in BCC tellurium under high pressure. We discuss the origins of the soft phonon modes. The soft modes have much bigger line width than the other modes and contribute more to the electron phonon coupling constants. We have discussed...

Band structure and pressure-induced metallic transition in iodine – GW calculation

We have studied the band structure and the band gap closure in phase I of solid iodine under high pressure, using the methods based on the quasiparticle theory, i.e. GW approximation. Our calculations show that the band gap in the Cmca structure, which is the structure of the phase I of solid iodine, closes around 20 GPa. This pressure is near the upper boundary of phase I. We discuss the possible metallic transition in the molecular phase of solid iodine and the possible changes of the crystal structure.

Origin of the simple modulated structures and the pressure induced superconductivity

We have studied origins of the simple modulations of structures using first-principles calculations for group V, VI, and VII elements which have simple modulated structures. For the approximate structures which are defined by removing the modulations, we calculated phonon frequencies along the direction of the wave vectors of the modulation, and searched for possible phonon modes which may relate with the structural modulations. In phosphorus the Madelung energy works to destabilize the approximate monoclinic structure as well as the simple cubic and simple hexagonal structures, while it works to stabilize the approximate structures in other elements. Observing that the SC phase of phosphorus is stabilized by the band energies, we estimated the superconducting Tc in the SC phase. The calculated Tc remains at nearly same values in the SC phase. The electron phonon coupling increases with increasing pressure but the averaged phonon frequency decreases with increasing pressure, which is due to the increasing destabilizing effect of the Madelung energy.

Theoretical Evidences for Enhanced Superconducting Transition Temperature of CaSi2 in a High-Pressure AlB2 Phase

By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi2 at high pressure. Our simulation showed stability of the AlB2 structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron-phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB2 structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher Tc is expected in the AlB2 structure than the trigonal structures which are known so far.By means of first-principles calculations, we studied stable lattice structures and estimated superconducting transition temperature of CaSi 2 at high pressure. Our simulation shows stability of the AlB 2 structure in a pressure range above 17 GPa. In this structure, doubly degenerated optical phonon modes, in which the neighboring silicon atoms oscillate alternately in a silicon plane, show prominently strong interaction with the conduction electrons. In addition there exists a softened optical mode (out-of-plan motion of silicon atoms), whose strength of the electron–phonon interaction is nearly the same as the above mode. The density of states at the Fermi level in the AlB 2 structure is higher than that in the trigonal structure. These findings and the estimation of the transition temperature strongly suggest that higher T c is expected in the AlB 2 structure than the trigonal structures which are known so far.

Charge-density waves, incommensurate modulations and superconductivity in phosphorus and iodine

Modulated structures in phosphorous and iodine at high pressure have been studied by ab-initio calculations. In phosphorous the electrostatic interaction between the charge density, i.e. Ewald energy E ew and Hartree energy E h play an important role for the creation of the modulated structure, whereas such roles of E ew and E h are not observed in iodine. Results of a detailed study of the electronic properties are shown and discussed for phosphorous in comparison with those of iodine. Superconductivity in these materials has been discussed in relation to those properties.

Zero-Point Energy of the Proton Motions and Its Effect on the Pressure of Molecular Dissociation in Dense Hydrogen

Zero-point motion energy of the protons is evaluated in the quasi-harmonic approximation for both atomic and molecular phases of highly compressed hydrogen. The results are in good agreement with Kagan et al.s. In the calculation, the phonon frequencies all over the Brillouin zone are obtained using force constants calculated by the first-principles band theoretical treatments. In the atomic phase, the Cs-IV structure, which is one of the low energy structures in the tetragonal diamond family, is found to be stable with real frequencies all over the Brillouin zone, while the β-Sn structure is unstable with imaginary frequencies near zone-boundaries. In the molecular phase, taking the Cmca structure which is one of the candidate structures above ∼200 GPa, we have studied phonons and the stability of the lattice. We have evaluated the effect of the zero-point energy on the pressure of the molecular dissociation, assuming that it occurs between the Cs-IV and the Cmca structures. With inclusion of the zero-point energy the dissociation pressure is reduced by 90–120 GPa from that estimated by the static energy. The equation of state is in good agreement with the extrapolated one of Loubeyre et al.s.

Structural Expansion of the Ground-State Energy of Simple Metals in the Local-Density Approximation

On the basis of the local density approximation the structural expansion of the ground-state energy of simple metals is presented in the temperattare Greens function formalism, where the terms are evaluated up to the fourth-order energy in power series of inhomogeneous density. For cubic structures, the ground-state energies of metallic hydrogen are calculated and critically compared with the recent band-theoretical calculation. Nature of the expansion in local density approximation is also discussed in comparison with results from many-body theoretical treatment.

Stable phase of metallic hydrogens at high pressures

Abstract The stable phase of metallic hydrogens at zero temperatures is studied by the structural expansion. A number of structural transitions are predicted to occur with increase of the pressure.