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W. Weber

Publications and source records attributed to W. Weber.

25 records · Page 2Linked to original sources
arXiv2001

On the way to a Gutzwiller density functional theory

Multi-band Gutzwiller-correlated wave functions reconcile the contrasting concepts of itinerant band electrons versus electrons localized in partially filled atomic shells. The exact evaluation of these variational ground states in the limit of large coordination number allows the identification of quasi-particle band structures, and the calculation of a variational spinwave dispersion. The study of a generic two-band model elucidates the co-operation of the Coulomb repulsion and the Hund's-rule exchange for itinerant ferromagnetism. We present results of calculations for ferromagnetic nickel, using a realistic 18 spin-orbital basis of $4s$, $4p$ and $3d$ valence electrons. The quasi-particle energy bands agree much better with the photo-emission and Fermi surface data than the band structure obtained from spin-density functional theory (SDFT).

cond-mat.str-el↗
arXiv1999

Superexchange via Cluster States: Calculations of Spin-Phonon Coupling Constants for CuGeO3

Calculations for spin-phonon coupling constants in CuGeO3 are presented, applying fourth order superexchange perturbation theory to an extended Hubbard model. In our treatment, the intermediate oxygen ligand states are described by band-like cluster states, due to the presence of significant O(p)-O(p) hopping. We also include the effect of the Ge ions on the O ligand states. We find a considerable q-dependence of the spin-phonon coupling. For the pi-modes involved in the spin-Peierls transition, our results of the coupling constants are in fair agreement with the work of Werner, Gros, and Braden. Yet some discrepancies remain, especially concerning the coupling to the vibrations, which modulate the O(2)-O(2)-Ge angle delta. Our studies of the pressure dependence of the magnetic coupling constants J_1 and J_2 suggest, that relatively large non-linear effects are present, even at small pressure values, probably due to the existence of a `soft', van-der-Waals-type bond direction in CuGeO3.

cond-mat.str-el↗
arXiv1998

Interplay of Chemical Bonding and Magnetism in Fe_4N, Fe_3N, Fe_2N

Using spin density functional theory we have carried out a comparative study of chemical bonding and magnetism in Fe_4N, Fe_3N and Fe_2N. All of these compounds form close packed Fe lattices, while N occupies octahedral interstitial positions. High spin fcc Fe and hypothetical FeN with rock salt structure have been included in our study as reference systems. We find strong, covalent Fe-N bonds as a result of a substantial σ-type p-d hybridisation, with some charge transfer to N. Those Fe d orbitals which contribute to the p-d bonds, do no longer participate in the exchange splitting of the Fe d bands. Because of the large exchange fields, the majority spin d bands are always fully occupied, while the minority spin d bands are close to half-filling, thus optimizing the Fe d-d covalent bonding. As a consequence, in good approximation the individual Fe moments decrease in steps of 0.5 μ_B from fcc iron (2.7 μ_B) via Fe_4N (2.7 and 1.97 mu_B}), \chem{Fe_3N} (1.99 μ_B) to ζ- Fe_2N (1.43 μ_B).

cond-mat.mtrl-sci↗
arXiv1998

NaV_2O_5 as a quarter-filled ladder compound

A new X-ray diffraction study of the one-dimensional spin-Peierls compound α-NaV_2O_5 reveals a centrosymmetric (Pmmn) crystal structure with one type of V site, contrary to the previously postulated non-centrosymmetric P2_1mn structure with two types of V sites (V^{+4} and V^{+5}). Density functional calculations indicate that NaV_2O_5 is a quarter-filled ladder compound with the spins carried by V-O-V molecular orbitals on the rungs of the ladder. Estimates of the charge-transfer gap and the exchange coupling agree well with experiment and explain the insulating behavior of NaV_2O_5 and its magnetic properties.

cond-mat↗
arXiv1997

Multi-band Gutzwiller wave functions for general on-site interactions

We introduce Gutzwiller wave functions for multi-band models with general on-site Coulomb interactions. As these wave functions employ correlators for the exact atomic eigenstates they are exact both in the non-interacting and in the atomic limit. We evaluate them in infinite lattice dimensions for all interaction strengths without any restrictions on the structure of the Hamiltonian or the symmetry of the ground state. The results for the ground-state energy allow us to derive an effective one-electron Hamiltonian for Landau quasi-particles, applicable for finite temperatures and frequencies within the Fermi-liquid regime. As applications for a two-band model we study the Brinkman-Rice metal-to-insulator transition at half band-filling, and the transition to itinerant ferromagnetism for two specific fillings, at and close to a peak in the density of states of the non-interacting system. Our new results significantly differ from those for earlier Gutzwiller wave functions where only density-type interactions were included. When the correct spin symmetries for the two-electron states are taken into account, the importance of the Hund's-rule exchange interaction is even more pronounced and leads to paramagnetic metallic ground states with large local magnetic moments. Ferromagnetism requires fairly large interaction strengths, and the resulting ferromagnetic state is a strongly correlated metal.

cond-mat.str-el↗