514 research outputs found
Crystal structure and electronic states of tripotassium picene
The crystal structure of potassium doped picene with an exact stoichiometry
(K3C22H14, K3picene from here onwards) has been theoretically determined within
Density Functional Theory allowing complete variational freedom of the crystal
structure parameters and the molecular atomic positions. A modified herringbone
lattice is obtained in which potassium atoms are intercalated between two
paired picene molecules displaying the two possible orientations in the
crystal.Along the c-axis, organic molecules alternate with chains formed by
three potassium atoms. The electronic structureof the doped material resembles
pristine picene, except that now the bottom of the conduction band is occupied
by six electrons coming from the ionized K atoms (six per unit cell).
Wavefunctions remain based mainly on picene molecular orbitals getting their
dispersion from intralayer edge to face CH/pi bonding, while eigenenergies have
been modified by the change in the electrostatic potential. The small
dispersion along the c-axis is assigned to small H-H overlap. From the
calculated electronic density of states we expect metallic behavior for
potassium doped picene.Comment: Published version: 8 twocolumn pages, 7 color figures, 2 structural
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Lattice-Spin Mechanism in Colossal Magnetoresistant Manganites
We present a single-orbital double-exchange model, coupled with cooperative
phonons (the so called breathing-modes of the oxygen octahedra in manganites).
The model is studied with Monte Carlo simulations. For a finite range of doping
and coupling constants, a first-order Metal-Insulator phase transition is
found, that coincides with the Paramagnetic-Ferromagnetic phase transition. The
insulating state is due to the self-trapping of every carrier within an oxygen
octahedron distortion.Comment: 4 pages, 5 figures, ReVTeX macro, accepted for publication in PR
Dynamics of Holes and Universality Class of the Antiferromagnetic Transition in the Two Dimensional Hubbard Model
The dynamics of a single hole (or electron) in the two dimensional Hubbard
model is investigated. The antiferromagnetic background is described by a
N\`eel state, and the hopping of the carrier is analyzed within a configuration
interaction approach. Results are in agreement with other methods and with
experimental data when available. All data are compatible with the opening of a
mean field gap in a Fermi liquid of spin polarons, the so called Slater type of
transition. In particular, this hypothesis explains the unusual dispersion
relation of the quasiparticle bands near the transition. Recent photoemission
data for CaCuOCl are analyzed within this context.Comment: New results and comparison with recent data adde
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