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Controlling the forbidden gap of graphene nano-ribbons (GNR) is a major challenge that has to be attained if this attractive material has to be used in micro- and nano-electronics. Using an unambiguous notation {m,n}-GNR, where m (n) is the number of six carbon rings in the arm-chair (zig-zag) directions, we investigate how varies the HOMO-LUMO gap when the size of the GNR is varied by increasing either m or n, while keeping the other variable fixed. It is shown that no matter whether charge- or spin-density-waves solutions are considered, the gap varies smoothly when n is kept fixed whereas it oscillates when the opposite is done, posing serious difficulties to the control of the gap. It is argued that the origin of this behavior is the fact that excess or defect charges or magnetic moments are mostly localized at zig-zag edges., Comment: 7 pages, 7 figures

Detailed electronic structure calculations of picene clusters doped by potassium modeling the crystalline K3picene structure show that while two electrons are completely transferred from potassium atoms to the LUMO of pristine picene, the third one remains closely attached to both material components. Multiconfigurational analysis is necessary to show that many structures of almost degenerate total energies compete to define the cluster ground state. Our results prove that the 4s orbital of potassium should be included in any interaction model describing the material. We propose a quarter filled two orbital model as the most simple model capable of describing the electronic structure of K-intercalated picene. Precise solutions obtained by a development of Lanczos method show low energy electronic excitations involving orbitals located at different positions. Consequently, metallic transport is possible in spite of the clear dominance of interaction over hopping., Comment: 12 pages, 10 figures (6 of them really heavy)

The Extended Hubbard Hamiltonian used by the Condensed Matter community is nothing but a simplified version of the Pariser, Parr and Pople Hamiltonian, well established in the Quantum Chemistry community as a powerful tool to describe the electronic structure of {\pi}-conjugated planar Polycyclic Aromatic Hydrocarbons (PAH). We show that whenever the interaction potential is non-local, unphysical charge inhomogeneities may show up in finite systems, provided that electrons are not neutralized by the ion charges. Increasing the system size does not solve the problem when the potential has an infinite range, and for finite range potentials these charge inhomogeneities become slowly less important as the potential range decreases and/or the system size increases. Dimensionality does also play a major role. Examples in bi-dimensional systems, such as planar PAH and graphene, are discussed to some extent.

Chemical adsorption of atomic hydrogen on a negatively charged single layer graphene sheet has been analyzed with ab-initio Density Functional Theory calculations. We have simulated both finite clusters and infinite periodic systems to investigate the effect of different ingredients of the theory, e.g. exchange and correlation potentials, basis sets, etc. Hydrogen's electron affinity dominates the energetic balance in the charged systems and the extra electron is predominantly attracted to a region nearby the chemisorbed atom. The main consequences are: (i) the cancellation of the unpaired spin resulting in a singlet ground-state, and (ii) a stronger interaction between hydrogen and the graphene sheet., Comment: 11 pages, 8 figures, to be published in PRB

Present routes to produce magnetic organic-based materials adopt a common strategy: the use of magnetic species (atoms, polyradicals, etc.) as building blocks. We explore an alternative approach which consists of selective hydrogenation of Polycyclic Aromatic Hydrocarbons. Self-Consistent-Field (SCF) (Hartree-Fock and DFT) and multi-configurational (CISD and MCSCF) calculations on coronene and corannulene, both hexa-hydrogenated, show that the formation of stable high spin species is possible. The spin of the ground states is discussed in terms of the Hund rule and Lieb's theorem for bipartite lattices (alternant hydrocarbons in this case). This proposal opens a new door to magnetism in the organic world., Comment: 6 pages, 4 figures and 2 tables

The single particle spectral-weight function (SWF) of the ionic Hubbard model at half filling is calculated in the cluster perturbation theory approximation. An abrupt change of regime in the low-energy region, near the chemical potential, is found at a critical value, $U_{c}$, of the coupling constant (Hubbard $U$). The SWF at the Fermi points $k_{F}$=$\pm{\pi}/2$ jumps, as $U$ increases, from a two-peak structure, the gap edges, to a four-peak structure accompanied by a (non-vanishing) minimum of the charge-gap. The two inner peaks of this structure show very small dispersion (flat bands) away from the Fermi points, whereas the outer peaks mark the edges of the Hubbard bands. No other signatures of abrupt change are detected in the SWF. The two regimes are physically realized in the angle-resolved photoelectron spectra of $(TaSe_{4})_{2}I$, and the blue-bronze $K_{0.3}MoO_{3}$, respectively., Comment: 13 pages, 7 figures

It is shown that recent experimental data on electronic transport through Pt nanocontacts in the presence of hydrogen admit an explanation in terms of topological and electron-electron correlation grounds. A model Hamiltonian, which incorporates two orbitals on Pt atoms, a single orbital on hydrogens, and the on-site Coulomb repulsion on the H atoms, is solved exactly, and connected to Pt leads described by a Bethe lattice. When two weakly coupled H atoms are placed between the Pt electrodes transversally to the transport direction (as recently suggested) a Kondo effect related to the symmetry of the Pt-H couplings, stabilizes the conductance around one quantum with a single channel contributing to the current, in agreement with the experiments., Comment: 4 pages, 5 figures, submitted to phys. rev. lett

Keldysh formalism is used to get the current-voltage characteristic of a small system of interacting electrons described by a Hubbard model coupled to metallic wires. The numerical procedure is checked recovering well-known results for an Anderson impurity. When larger interacting regions are considered quite different results are obtained depending on whether the Hubbard part is half-filled or not. At half-filling the existence of a gap charge manifests itself making current exponentially small as a function both of the number of interacting sites and the value of U. The behavior changes at large voltages above the gap energy when activated charge transport takes place. On the contrary, for filling factors other than half, current goes through the interacting system suffering just a small amount of scattering at both connections. Conductance depends slightly on U and much more on the filling factor but not on the length of the interacting region., Comment: 14 twocolumn pages including 15 figures, RevTex4, amended version

The internal structure of stripes in the two dimensional Hubbard model is studied by going beyond the Hartree-Fock approximation. Partially filled stripes, consistent with experimental observations, are stabilized by quantum fluctuations, included through the Configuration Interaction method. Hopping of short regions of the stripes in the transverse direction is comparable to the bare hopping element. The integrated value of $n_{\bf \vec{k}}$ compares well with experimental results., Comment: 4 pages

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 Ca$_2$CuO$_2$Cl$_2$ are analyzed within this context., Comment: New results and comparison with recent data added

The interactions between holes in the Hubbard model, in the low density, intermediate to strong coupling limit, are investigated by systematically improving mean field calculations. The Configuration Interaction basis set is constructed by applying to local Unrestricted Hartree-Fock configurations all lattice translations and rotations. It is shown that this technique reproduces, correctly, the properties of the Heisenberg model, in the limit of large U. Upon doping, dressed spin polarons in neighboring sites have an increased kinetic energy and an enhanced hopping rate. Both effects are of the order of the hopping integral and lead to an effective attraction at intermediate couplings., Comment: 17 pages. Corrected figures

We have used the Kubo formula to calculate the temperature dependence of the electrical conductance of the double exchange Hamiltonian. We average the conductance over an statistical ensemble of clusters, which are obtained by performing Monte Carlo simulations on the classical spin orientation of the double exchange Hamiltonian. We find that for electron concentrations bigger than 0.1, the system is metallic at all temperatures. In particular it is not observed any change in the temperature dependence of the resistivity near the magnetical critical temperature. The calculated resistivity near $T_c$ is around ten times smaller than the experimental value. We conclude that the double exchange model is not able to explain the metal to insulator transition which experimentally occurs at temperatures near the magnetic critical temperature., Comment: 6 pages, 5 figures included in the text

The interactions between holes in the Hubbard model, in the low density, intermediate to strong coupling limit, are investigated. Dressed spin polarons in neighboring sites have an increased kinetic energy and an enhanced hopping rate. Both effects are of the order of the hopping integral and lead to an effective attraction at intermediate couplings. Our results are derived by systematically improving mean field calculations. The method can also be used to derive known properties of isolated spin polarons., Comment: 4 pages