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Band Electronic Structure of One- and Two-Dimensional Pentacene Molecular Crystals
We report EHT calculations of the band electronic structure of substituted pentacene derivatives and the polymorphs of the parent compound. The results show that there are wide disparities among the bandwidths and electronic dimensionalities of these compounds. The parent pentacene polymorphs are 2-dimensional in their band electronic structure with moderate dispersions; the bandwidths in the 14.1 Ã… d-spacing polymorph are noticeably larger than for the 14.5 Ã… d-spacing polymorph, reported by Campbell. Whereas the parent pentacene polymorphs adopt the well-known herringbone packing, the new, substituted pentacenes are noticeably different in their solid state structures and this is reflected in the band electronic structures. TMS adopts a highly 1-dimensional structure that leads to a large bandwidth along the stacking direction; TIPS also adopts a stacked structure, but because the molecules are laterally interleaved in the fashion of bricks in a wall, this compound is strongly 2-dimensional.
Infinite Layer LaNiO(2): Ni(1+)is not Cu(2+)
The Ni ion in LaNiO has the same formal ionic configuration as
does Cu in isostructural CaCuO, but it is reported to be nonmagnetic and
probably metallic whereas CaCuO is a magnetic insulator. From ab initio
calculations we trace its individualistic behavior to (1) reduced
mixing due to an increase of the separation of site energies () of at least 2 eV, and (2) important Ni mixing with
La states that leads to Fermi surface pockets of La
character that hole-dope the Ni 3d band.Correlation effects do not appear to be
large in LaNiO. However, ad hoc increase of the intraatomic repulsion on
the Ni site (using the LDA+U method) is found to lead to a novel correlated
state: (i) the transition metal and states undergo
consecutive Mott transitions, (ii) their moments are antialigned leading
(ideally) to a "singlet" ion in which there are two polarized orbitals, and
(iii) mixing of the upper Hubbard band with the La
states leaves considerable transition metal 3d character in a band pinned to
the Fermi level. The magnetic configuration is more indicative of a Ni
ion in this limit, although the actual charge changes little with U.Comment: 7 pages, 8 figure
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