Medium/high field magnetoconductance in chaotic quantum dots

The magnetoconductance G in chaotic quantum dots at medium/high magnetic fluxes Phi is calculated by means of a tight binding Hamiltonian on a square lattice. Chaotic dots are simulated by introducing diagonal disorder on surface sites of L x L clusters. It is shown that when the ratio W/L is sufficiently large, W being the leads width, G increases steadily showing a maximum at a flux Phi_max ~ W. Bulk disordered ballistic cavities (with an amount of impurities proportional to L) does not show this effect. On the other hand, for magnetic fluxes larger than that for which the cyclotron radius is of the order of L/2, the average magnetoconductance inceases almost linearly with the flux with a slope proportional to W^2, shows a maximum and then decreases stepwise. These results closely follow a theory proposed by Beenakker and van Houten to explain the magnetoconductance of two point contacts in series.Comment: RevTeX including six postscript figure

Classical trajectories in quantum transport at the band center of bipartite lattices with or without vacancies

Here we report on several anomalies in quantum transport at the band center of a bipartite lattice with vacancies that are surely due to its chiral symmetry, namely: no weak localization effect shows up, and, when leads have a single channel the transmission is either one or zero. We propose that these are a consequence of both the chiral symmetry and the large number of states at the band center. The probability amplitude associated to the eigenstate that gives unit transmission ressembles a classical trajectory both with or without vacancies. The large number of states allows to build up trajectories that elude the blocking vacancies explaining the absence of weak localization.Comment: 5 pages, 5 figure

Effects of Fermi energy, dot size and leads width on weak localization in chaotic quantum dots

Magnetotransport in chaotic quantum dots at low magnetic fields is investigated by means of a tight binding Hamiltonian on L x L clusters of the square lattice. Chaoticity is induced by introducing L bulk vacancies. The dependence of weak localization on the Fermi energy, dot size and leads width is investigated in detail and the results compared with those of previous analyses, in particular with random matrix theory predictions. Our results indicate that the dependence of the critical flux Phi_c on the square root of the number of open modes, as predicted by random matrix theory, is obscured by the strong energy dependence of the proportionality constant. Instead, the size dependence of the critical flux predicted by Efetov and random matrix theory, namely, Phi_c ~ sqrt{1/L}, is clearly illustrated by the present results. Our numerical results do also show that the weak localization term significantly decreases as the leads width W approaches L. However, calculations for W=L indicate that the weak localization effect does not disappear as L increases.Comment: RevTeX, 8 postscript figures include

Recovery of the persistent current induced by the electron-electron interaction in mesoscopic metallic rings

Persistent currents in mesoscopic metallic rings induced by static magnetic fields are investigated by means of a Hamiltonian which incorporates diagonal disorder and the electron-electron interaction through a Hubbard term ($U$). Correlations are included up to second order perturbation theory which is shown to work accurately for $U$ of the order of the hopping integral. If disorder is not very strong, interactions increase the current up to near its value for a clean metal. Averaging over ring lengths eliminates the first Fourier component of the current and reduces its value, which remains low after interactions are included.Comment: uuencoded gzipped tar file containing the manuscript (tex file) and four figures (postscript files). Accepted for publication in Solid State Communications. Send e-mail to: [email protected]

Magnetic molecules created by hydrogenation of Polycyclic Aromatic Hydrocarbons

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 table

Hole Pairs in the Two-Dimensional Hubbard Model

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 page

Conductance scaling at the band center of wide wires with pure non--diagonal disorder

Kubo formula is used to get the scaling behavior of the static conductance distribution of wide wires showing pure non-diagonal disorder. Following recent works that point to unusual phenomena in some circumstances, scaling at the band center of wires of odd widths has been numerically investigated. While the conductance mean shows a decrease that is only proportional to the inverse square root of the wire length, the median of the distribution exponentially decreases as a function of the square root of the length. Actually, the whole distribution decays as the inverse square root of the length except close to G=0 where the distribution accumulates the weight lost at larger conductances. It accurately follows the theoretical prediction once the free parameter is correctly fitted. Moreover, when the number of channels equals the wire length but contacts are kept finite, the conductance distribution is still described by the previous model. It is shown that the common origin of this behavior is a simple Gaussian statistics followed by the logarithm of the E=0 wavefunction weight ratio of a system showing chiral symmetry. A finite value of the two-dimensional conductance mean is obtained in the infinite size limit. Both conductance and the wavefunction statistics distributions are given in this limit. This results are consistent with the 'critical' character of the E=0 wavefunction predicted in the literature.Comment: 10 pages, 9 figures, RevTeX macr

Dimensional effects in photoelectron spectra of Ag deposits on GaAs(110) surfaces

It is shown that the peak structure observed in angle-resolved photoelectron spectra of metallic deposits can only be unambiguously associated to single electronic states if the deposit has a two dimensional character (finite along one spatial direction). In one and zero dimensions the density of states shows peaks related to bunches of single electron states (the finer structure associated to the latter may not always be experimentally resolved). The characteristics of the peak structure strongly depend on the band dispersion in the energy region where they appear. Results for the density of states and photoemission yield for Ag crystallites on GaAs(110) are presented and compared with experimental photoelectron spectra.Comment: Uuencoded gz-compressed postcript file including text and three figures; Send comments to [email protected]

Blue carbon stocks in Baltic Sea eelgrass (Zostera marina) meadows

Although seagrasses cover only a minor fraction of the ocean seafloor, their carbon sink capacity accounts for nearly one-fifth of the total oceanic carbon burial and thus play a critical structural and functional role in many coastal ecosystems. We sampled 10 eelgrass (<i>Zostera marina</i>) meadows in Finland and 10 in Denmark to explore seagrass carbon stocks (C_org stock) and carbon accumulation rates (C_org accumulation) in the Baltic Sea area. The study sites represent a gradient from sheltered to exposed locations in both regions to reflect expected minimum and maximum stocks and accumulation. The C_org stock integrated over the top 25 cm of the sediment averaged 627 g C m⁻² in Finland, while in Denmark the average C_org stock was over 6 times higher (4324 g C m⁻²). A conservative estimate of the total organic carbon pool in the regions ranged between 6.98 and 44.9 t C ha⁻¹. Our results suggest that the Finnish eelgrass meadows are minor carbon sinks compared to the Danish meadows, and that majority of the C_org produced in the Finnish meadows is exported. Our analysis further showed that &gt; 40 % of the variation in the C_org stocks was explained by sediment characteristics, i.e. dry density, porosity and silt content. In addition, our analysis show that the root : shoot ratio of <i>Z. marina</i> explained &gt; 12 % and the contribution of <i>Z. marina</i> detritus to the sediment surface C_org pool explained &gt; 10 % of the variation in the C_org stocks. The mean monetary value for the present carbon storage and carbon sink capacity of eelgrass meadows in Finland and Denmark, were 281 and 1809 EUR ha⁻¹, respectively. For a more comprehensive picture of seagrass carbon storage capacity, we conclude that future blue carbon studies should, in a more integrative way, investigate the interactions between sediment biogeochemistry, seascape structure, plant species architecture and the hydrodynamic regime