Showing papers by "Ove Jepsen published in 2004"

Band-structure calculations of the half-metallic ferromagnetism and structural stability of full- and half-Heusler phases

Abstract: Self-consistent first principle calculations within the local density approximation were used to study the effect of lattice distortions on the electronic and magnetic properties of half $(\mathrm{NiMnSb})$ and full-Heusler $({\mathrm{Co}}_{2}\mathrm{CrAl})$ compounds. We find that $\mathrm{NiMnSb}$ is half-metallic from $\ensuremath{-}2%$ to $+3%$ uniform strain, whereas ${\mathrm{Co}}_{2}\mathrm{CrAl}$ is half-metallic from $\ensuremath{-}1%$ to $+3%$. The loss of the half-metallic character is primarily due to a shift in the density of states relative to the Fermi energy. Under tetragonal distortions, high spin polarization at the Fermi level is maintained over a larger range, but the gap width is strongly affected. Therefore, only nonuniform strain should not be the expected reason for the poor performance of Heusler compounds in spintronic devices. The impacts of these results on spintronic devices are discussed.

Comment on "First-principles calculation of the superconducting transition in MgB2 within the anisotropic Eliashberg formalism"

Abstract: Choi et al [Phys Rev B 66, 020513 (2002)] recently presented first-principles calculations of the electron-phonon coupling and superconductivity in MgB2, emphasizing the importance of anisotropy and anharmonicity We point out that (1) variation of the superconducting gap inside the sigma or the pi bands can hardly be observed in real samples, and (2) taking the anisotropy of the Coulomb repulsion into account influences the size of the small gap, Deltapi

An inorganic double helix sheathing alkali metal cations: ANb2P2S12 (A=K, Rb, Cs), a series of thiophosphates close to the metal-nonmetal boundary--chalcogenide analogues of transition-metal phosphate bronzes?

Abstract: The new quaternary niobium thiophosphates ANb 2 P 2 S 1 2 (A=K, Rb, Cs) have been prepared and characterized. The title compounds were synthesized by reacting Nb metal, A 2 S, P 2 S 5 , and S at 600-700°C in evacuated silica tubes. They crystallize as "stuffed" variants of the tetragonal TaPS 6 structure type in the tetragonal space group I42d with eight formula units per unit cell and lattice constants a=15.923(2) and c=13.238(3) A for CsNb 2 P 2 S 1 2 , a= 15.887(3) and c=13.132(3) A for RbNb 2 P 2 S 1 2 , and a=15.850(2) and c= 13.119(3) A for KNb 2 P 2 S 1 2 . Their structures are based on double helices formed from interpenetrating, noninteracting spiral chains of binuclear [Nb 2 S 1 2 ] cluster units and [PS 4 ] thiophosphate groups. The cavities and tunnels, which are formed by the helical chains, are filled with A + ions. Temperature-dependent conductivity studies reveal thermally activated electrical transport behavior. This result is consistent with the observation of a temperature-dependent contribution to the 3 1 P MAS-NMR shift, suggesting that the delocalized s-electron spin density increases with increasing temperature. These findings are supported by the results of tight-binding band structure calculations which reveal that the unusual electrical transport behavior of ANb 2 P 2 S 1 2 is a consequence of the structure symmetry. Therefore, CsNb 2 P 2 S 1 2 may be considered a chalcogenide analogue of metal phosphate bronzes.