Exact Kohn-Sham eigenstates versus quasiparticles in simple models of strongly correlated electrons

We present analytic expressions for the exact density functional and Kohn-Sham Hamiltonian of simple tight-binding models of correlated electrons. These are the single- and double-site versions of the Anderson, Hubbard and spinless fermion models. The exact exchange and correlation potentials are fully non-local. The analytic expressions allow to compare the Kohn-Sham eigenstates of exact density functional theory with the many-body quasi-particle states of these correlated-electron systems. The exact Kohn-Sham spectrum describes correctly many of the non-trivial features of the many-body quasi-particle spectrum, as for example the precursors of the Kondo peak. However, we find that some pieces of the quasi-particle spectrum are missing because the many-body phase-space for electron and hole excitations is richer

Universality in the transport response of molecular wires physisorbed onto graphene electrodes

We analyze the low-voltage transport response of large molecular wires bridging graphene electrodes, where the molecules are physisorbed onto the graphene sheets by planar anchor groups. In our study, the sheets are pulled away to vary the gap length and the relative atomic positions. The molecular wires are also translated in directions parallel and perpendicular to the sheets. We show that the energy position of the Breit-Wigner molecular resonances is universal for a given molecule, in the sense that it is independent of the details of the graphene edges, gaps lengths or of the molecule positions. We discuss the need to converge carefully the k-sampling to provide reasonable values of the conductance.Comment: 6 pages, 6 figure

Impact of edge shape on the functionalities of graphene-based single-molecule electronics devices

We present an ab-initio analysis of the impact of edge shape and graphene-molecule anchor coupling on the electronic and transport functionalities of graphene-based molecular electronics devices. We analyze how Fano-like resonances, spin filtering and negative differential resistance effects may or may not arise by modifying suitably the edge shapes and the terminating groups of simple organic molecules. We show that the spin filtering effect is a consequence of the magnetic behavior of zigzag-terminated edges, which is enhanced by furnishing these with a wedge shape. The negative differential resistance effect is originated by the presence of two degenerate electronic states localized at each of the atoms coupling the molecule to graphene which are strongly affected by a bias voltage. The effect could thus be tailored by a suitable choice of the molecule and contact atoms if edge shape could be controlled with atomic precision.Comment: 11 pages, 20 figure

Preferencias de hábitat, densidad y diversidad de las comunidades de aves en Tenerife (Islas Canarias)

Bird distribution and abundance are described and analyzed in Teneriffe (Canary Islands). Inter–habitat differences in density, diversity and species richness are shown in table 1. Figure 2 shows the main determinants of bird species richness in Teneriffe, and tables 2 and 3 and figure 3 show the species–specific patterns of spatial variation abundance (more detailed for Anthus berthelotii, Fringilla coelebs canariensis, Fringilla teydea, Parus caeruleus teneriffae, Phylloscopus canariensis, Regulus teneriffae, Serinus canarius and Turdus merula cabrerae). Deeply transformed environments due to human impact (urban habitats, agricultural mosaics, banana plantations) have high bird densities and species richness, even higher than those measured in native, unmodified habitats such as laurel forests or mature pinewoods. Urban environments in Teneriffe are very permeable to native bird fauna, as they have been occupied by many widespread endemic species/subspecies. Many of the endemic, well defined species or subspecies of island birds have high population densities within native, untransformed habitats. Density compensation and niche expansion is not a common phenomenon in the avifauna of Teneriffe. Nevertheless, all species/subspecies broadening the inter–habitat or altitudinal distribution are endemic of the Canary Islands.Mediante el empleo de transectos lineales, se describen las preferencias de hábitat, la distribución altitudinal y la abundancia de la avifauna reproductora de Tenerife (Islas Canarias). Los hábitats profundamente transformados debido a la acción humana (e.g., áreas urbanas, mosaicos agrícolas, plantaciones de plátanos) tienen elevadas densidades y riquezas de especies, que llegan a ser tan altas o mayores que las observadas en medios autóctonos no transformados como laurisilvas y pinares maduros. Muchas especies/subespecies taxonómicamente bien diferenciadas de las poblaciones continentales están distribuidas mayoritaria o exclusivamente en hábitats autóctonos poco degradados. Las hipótesis de la compensación de densidades y la expansión de nicho en poblaciones insulares no parecen cumplirse de modo generalizado en Tenerife. No obstante, todas las especies o subespecies que muestran una mayor amplitud de distribución en Tenerife son endémicas del archipiélago canario

Sesgos en la obtención de estimas de densidad obtenidas por medio de transectos lineales en ambientes estepáricos de las Islas Canarias

We studied bias in density estimations derived from strip transects in dry open–country in the Canary Islands. We also present some critical remarks on García–del–Rey’s (2005) paper regarding strip transects and the validity of comparisons based on population densities of birds in scrublands on Tenerife island using two different methods: territory mapping and strip transect sampling. Although strip transects with census belts of 25 m do not account for detectability, this method only slightly undervalues true density estimates, and allowed to detect more than 85% of birds present in poorly vegetated environments in the Canary Islands. Previously published works on distribution and abundance of terrestrial birds in the Canary Islands using the strip transect sampling with belts of 25 m on both sides of the observer, thus provide reliable information that only slightly underestimates true densities. Key words: Birds, Canary Islands, Census methods, Strip transects, Open–country environments, Population density, Territory mapping.We studied bias in density estimations derived from strip transects in dry open–country in the Canary Islands. We also present some critical remarks on García–del–Rey’s (2005) paper regarding strip transects and the validity of comparisons based on population densities of birds in scrublands on Tenerife island using two different methods: territory mapping and strip transect sampling. Although strip transects with census belts of 25 m do not account for detectability, this method only slightly undervalues true density estimates, and allowed to detect more than 85% of birds present in poorly vegetated environments in the Canary Islands. Previously published works on distribution and abundance of terrestrial birds in the Canary Islands using the strip transect sampling with belts of 25 m on both sides of the observer, thus provide reliable information that only slightly underestimates true densities. Key words: Birds, Canary Islands, Census methods, Strip transects, Open–country environments, Population density, Territory mapping.Se estudian los sesgos derivados del empleo del método del taxiado (transectos lineales con bandas de 25 m a cada lado del observador) para obtener densidades en ambientes estepáricos de las Islas Canarias. También se presentan algunos comentarios críticos al trabajo de García–del–Rey (2005) que compara estimas de densidad obtenidas en Tenerife utilizando dos métodos diferentes: mapeo de territorios y transecto lineal. Aunque el método del taxiado estima densidades relativas no corregidas por la detectabilidad de las especies, este método proporciona valores de densidad muy parecidos a los reales, ya que permite detectar en ambientes con poca cobertura vegetal a más del 85% de los individuos dentro de bandas de 25 m a cada lado del observador. Por tanto, los trabajos previamente publicados sobre densidades de aves en Canarias proporcionan estimas fiables sólo ligeramente infravaloradas. Palabras clave: Aves, Islas Canarias, Métodos de censo, Transecto lineal, Ambientes estepáricos, Densidad de población, Mapeo de territorios

What species-specific traits make a bird a better surrogate of native species richness? A test with insular avifauna

This is an electronic version of an article published in Biological Conservation. Carrascal, L.M. et al. What species-specific traits make a bird a better surrogate of native species richness? A test with insular avifauna. Biological Conservation 152 (2012): 204-21

Unconventional magnetism in small gold organic molecules

We present a theoretical study of the magnetic properties of dicyclopentadienyl metallocene and phthalocyanine molecules, that contain the transition metal atoms M = Fe, Co, Ni, Cu, Zn, Ir, Pt and Au. Our most important prediction is that gold and copper molecules are magnetic. We find that the magnetism of these molecules is fairly unconventional: the gold atom itself is weakly magnetic or even non-magnetic. Its role is rather to induce magnetism in the surrounding carbon and nitrogen atoms, producing a sort of spin density wave.Comment: 4 pages, 5 figure

Full analytical solution of finite-length armchair/zigzag nanoribbons

Finite-length armchair graphene nanoribbons can behave as one dimensional topological materials, that may show edge states in their zigzag-terminated edges, depending on their width and termination. We show here a full solution of Tight-Binding graphene rectangles of any length and width that can be seen as either finite-length armchair or zigzag ribbons. We find exact analytical expressions for both bulk and edge eigen-states and eigen-energies. We write down exact expressions for the Coulomb interactions among edge states and introduce a Hubbard-dimer model to analyse the emergence and features of different magnetic states at the edges, whose existence depends on the ribbon length. We find ample room for experimental testing of our predictions in N = 5 armchair ribbons. We compare the analytical results with ab initio simulations to benchmark the quality of the dimer model and to set its parameters. A further detailed analysis of the ab initio Hamiltonian allows us to identify those variations of the Tight-Binding parameters that affect the topological properties of the ribbons