Showing papers by "Ove Jepsen published in 1995"

LDA energy bands, low-energy Hamiltonians, t', t'', t_{perp}(k), and J_{perp}

Abstract: We describe the LDA bandstructure of YBa_2Cu_3O_7 in the 2 eV range from the Fermi energy using orbital projections and compare with YBa_2Cu_4O_8. Then, the high-energy and chain-related degrees of freedom are integrated out and we arrive at two, nearest-neighbor, orthogonal, two-center, 8-band Hamiltonians, the even and odd bands of the bi-layer. Of the 8 orbitals, Cu{x2-y2}, O2x, O3y, and Cus have \sigma character and Cu{xz}, Cu{yz} O2z, and O3z have \pi character. The roles of the Cu_s orbital, which has some Cu{3z2-1} character, and the four \pi orbitals are as follows: Cu_s provides 2nd- and 3rd-nearest-neighbor (t' and t') intra-plane hopping, as well as hopping between planes (t_{perp}). The \pi -orbitals are responsible for bifurcation of the saddle-points for dimpled planes. The 4-\sigma-band Hamiltonian is generic for flat CuO_2 planes and we use it for analytical studies. The reduction of the \sigma-Hamiltonian to 3- and 1-band Hamiltonians is explicitly discussed and we point out that, in addition to the hoppings commonly included in many-body calculations, the 3-band Hamiltonian should include hopping between all 2nd-nearest-neighbor oxygens and that the 1-band Hamiltonian should include 3rd-nearest-neighbor hoppings. We calculate the single-particle hopping between the planes of a bi-layer. We show that the inclusion of t' is crucial for understanding ARPES for the anti-ferromagnetic insulator Sr_2CuO_2Cl_2. Finally, we estimate the value of the inter-plane exchange constant for an un-doped bi-layer in mean-field theory using different single-particle Hamiltonians.

Calculated electronic structure of the sandwichd 1 metals LaI2 and CeI2: Application of new LMTO techniques

Abstract: The electronic structures of the cubic layeredd1 metals LaI2 and CeI2 were calculated using local density-functional theory and the linear muffin-tin orbital method. Special care was taken in the sphere packing used for the atomic spheres approximation. the band structure and the bonding were analysed in terms of projections of the bands onto orthogonal orbitals. The conduction-band structure could be calculated with a down-folded two-orbital basis which then served for the construction of an analytical 2×2 orthogonal, two-center tight-binding Hamiltonian. The conduction band has almost pure Ln-Ln 5d egcharacter. Thex2−y2contribution dominates and is two-dimensional and short ranged. Strong hybridization with the 3z2−1 orbital occurs near the saddle point, which is thereby lowered in energy and bifurcated due to thekz-dispersion provided by the 3z2−1 orbital. This strengthens the metal-metal bonds and prevents the nesting instability of the Fermi surface of the half filledx2−y2band. Within the limited accuracy of the LDA, the band structure of CeI2 was found to be identical to that of LaI2. The conduction-band 4f hybridizationVdf2(0) was analysed and found to be several times smaller than in fcc γ-Ce, in qualitative agreement with recent photoemission results [1]. Of importance for this reduction seems to be that the conduction band is formed by essentially only one orbital,\(Ce 5d_{x^2 - y^2 } \),that the number of Ce nearest-neighbors is small, and that the Ce−Ce distance is relatively large.

Fermi surface, bonding, and pseudogap in MoSi2

Abstract: The electronic structure and Fermi surface of molybdenum disilicide has been calculated using local-density functional theory (LDA) and the linear muffin-tin orbital method (LMTO) The energy bands are analyzed in detail for their orbital character Our explanation for the presence of a pseudogap after the seventh band is that the two Si s bands lie low and that there are five Mo d-Si p pair bands The configuration is approximately Mo 4d5 Si2 3s2 3p25 An explanation in terms of directed bond-orbitals was not achieved The calculated angular dependence of the extremal Fermi surface cross-section areas are in good qualitative agreement with de Haas-van Alphen (dHvA) measurements However, in order to obtain quantitative agreement, the Mo dx2−y2 orbital energy has to be shifted upwards by 041 eV and the Mo dxy energy downwards by 020 eV This deficiency is ascribed to the use of a local exchange-correlation potential