Theoretical study of the charge transport through C60-based single-molecule junctions

We present a theoretical study of the conductance and thermopower of single-molecule junctions based on C60 and C60-terminated molecules. We first analyze the transport properties of gold-C60-gold junctions and show that these junctions can be highly conductive (with conductances above 0.1G0, where G0 is the quantum of conductance). Moreover, we find that the thermopower in these junctions is negative due to the fact that the LUMO dominates the charge transport, and its magnitude can reach several tens of micro-V/K, depending on the contact geometry. On the other hand, we study the suitability of C60 as an anchoring group in single-molecule junctions. For this purpose, we analyze the transport through several dumbbell derivatives using C60 as anchors, and we compare the results with those obtained with thiol and amine groups. Our results show that the conductance of C60-terminated molecules is rather sensitive to the binding geometry. Moreover, the conductance of the molecules is typically reduced by the presence of the C60 anchors, which in turn makes the junctions more sensitive to the functionalization of the molecular core with appropriate side groups.Comment: 9 pages, 7 figure

The worldwide costs of marine protected areas

Declines in marine harvests, wildlife, and habitats have prompted calls at both the 2002 World Summit on Sustainable Development and the 2003 World Parks Congress for the establishment of a global system of marine protected areas (MPAs). MPAs that restrict fishing and other human activities conserve habitats and populations and, by exporting biomass, may sustain or increase yields of nearby fisheries. Here we provide an estimate of the costs of a global MPA network, based on a survey of the running costs of 83 MPAs worldwide. Annual running costs per unit area spanned six orders of magnitude, and were higher in MPAs that were smaller, closer to coasts, and in high-cost, developed countries. Models extrapolating these findings suggest that a global MPA network meeting the World Parks Congress target of conserving 20–30% of the world’s seas might cost between $5 billion and$19 billion annually to run and would probably create around one million jobs. Although substantial, gross network costs are less than current government expenditures on harmful subsidies to industrial fisheries. They also ignore potential private gains from improved fisheries and tourism and are dwarfed by likely social gains from increasing the sustainability of fisheries and securing vital ecosystem services

Conduction Channels of One-Atom Zinc Contacts

We have determined the transmission coefficients of atomic-sized Zn contacts using a new type of breakjunction which contains a whisker as a central bridge. We find that in the last conductance plateau the transport is unexpectedly dominated by a well-transmitting single conduction channel. We explain the experimental findings with the help of a tight-binding model which shows that in an one-atom Zn contact the current proceeds through the 4s and 4p orbitals of the central atom.Comment: revtex4, 5 pages, 5 figure

Electron-vibration interaction in transport through atomic gold wires

We calculate the effect of electron-vibration coupling on conduction through atomic gold wires, which was measured in the experiments of Agra\"it et al. [Phys. Rev. Lett. 88, 216803 (2002)]. The vibrational modes, the coupling constants, and the inelastic transport are all calculated using a tight-binding parametrization and the non-equilibrium Green function formalism. The electron-vibration coupling gives rise to small drops in the conductance at voltages corresponding to energies of some of the vibrational modes. We study systematically how the position and height of these steps vary as a linear wire is stretched and more atoms are added to it, and find a good agreement with the experiments. We also consider two different types of geometries, which are found to yield qualitatively similar results. In contrast to previous calculations, we find that typically there are several close-lying drops due to different longitudinal modes. In the experiments, only a single drop is usually visible, but its width is too large to be accounted for by temperature. Therefore, to explain the experimental results, we find it necessary to introduce a finite broadening to the vibrational modes, which makes the separate drops merge into a single, wide one. In addition, we predict how the signatures of vibrational modes in the conductance curves differ between linear and zigzag-type wires.Comment: 19 pages, 12 figure

Cluster-based density-functional approach to quantum transport through molecular and atomic contacts

We present a cluster-based density-functional approach to model charge transport through molecular and atomic contacts. The electronic structure of the contacts is determined in the framework of density functional theory, and the parameters needed to describe transport are extracted from finite clusters. A similar procedure, restricted to nearest-neighbor interactions in the electrodes, has been presented by Damle et al. [Chem. Phys. 281, 171 (2002)]. Here, we show how to systematically improve the description of the electrodes by extracting bulk parameters from sufficiently large metal clusters. In this way we avoid problems arising from the use of nonorthogonal basis functions. For demonstration we apply our method to electron transport through Au contacts with various atomic-chain configurations and to a single-atom contact of Al.Comment: 18 pages, 13 figure

Growth and mortality estimates of Sardinella brasiliensis in the southeastern Brazilian Bight

Length frequency analisys were applied for sardine data collected from commercial catches throughout the southeastern Brazilian coast in 1977-1987. Age/length keys were obtained by otolith ring countings and utilized to transform length into age composition. Growth parameters were estimated by the von Bertalanffy growth equation using age and length data for each year and for all the period of investigation. The mean growth parameters for the entire period were estimated as L∞ = 271 mm and K= 0.59 year-1. Instantaneous total mortality (Z) coefficients per year were estimated using catch curves and methods based on the mean length of the fish caught. Total mortality rate for the entire period was obtained through the average of the annual values. Natural mortality (M) was estimated using the Paul/s empirical equation (1980a), and fishing mortality (F) by the difference between total and natural mortality values.The results were Z = 3.6 year-1; M = 0.96 year-1; and F= 2.6 year-1.Estudo sobre o crescimento e a mortalidade de Sardinella brasiliensis, da costa sudeste do Brasil, foi realizado para o período 1977 a 1987. As análises foram efetuadas a partir de dados existentes sobre distribuições de freqüência de comprimento de amostras da captura comercial. Chaves idade/comprimento, construidas a partir de leitura de anéis de crescimento em otólitos, dentro do Programa Integrado de Estudos Biológicos sobre a Sardinha - PIEBS, foram utilizadas para transformar as distribuiçes de comprimento em idade. Os parâmetros de crescimento da equação de von Bertalanffy foram estimados, anualmente e para o período como um todo, a partir de métodos que utilizam dados de idade e comprimento. Os parâmetros médios encontrados para o período foram: L∞ = 271 mm e K= 0,59 ano-1. Os coeficientes instantâneos de mortalidade total (Z) anuais foram calculados pelas curvas de captura e por métodos baseados no comprimento médio dos indivíduos nas capturas. A mortalidade total para todo o período foi obtida pela média dos valores anuais. O coeficiente instantâneo de mortalidade natural (M) foi estimado pela equação empírica de Pauly (1980a), usando um fator de correção de 0,8 para clupeoides, e o coeficiente instantâneo de mortalidade por pesca (F), pela diferença entre a mortalidade total e a natural. Os valores encontrados foram: Z= 3,6 ano-1, M = 0,96 ano-1 e F= 2,6 ano-1

Probing two topological surface bands of Sb2Te3 by spin-polarized photoemission spectroscopy

Using high resolution spin- and angle-resolved photoemission spectroscopy, we map the electronic structure and spin texture of the surface states of the topological insulator Sb2Te3. In combination with density functional calculations (DFT), we directly show that Sb2Te3 exhibits a partially occupied, single spin-Dirac cone around the Fermi energy, which is topologically protected. DFT obtains a spin polarization of the occupied Dirac cone states of 80-90%, which is in reasonable agreement with the experimental data after careful background subtraction. Furthermore, we observe a strongly spin-orbit split surface band at lower energy. This state is found at 0.8eV below the Fermi level at the gamma-point, disperses upwards, and disappears at about 0.4eV below the Fermi level into two different bulk bands. Along the gamma-K direction, the band is located within a spin-orbit gap. According to an argument given by Pendry and Gurman in 1975, such a gap must contain a surface state, if it is located away from the high symmetry points of the Brillouin zone. Thus, the novel spin-split state is protected by symmetry, too.Comment: 8 pages, 10 figure

Experimental Characterization and Micromechanical Modeling of Woven Carbon/Copper Composites

The results of an extensive experimental characterization and a preliminary analytical modeling effort for the elastoplastic mechanical behavior of 8-harness satin weave carbon/copper (C/Cu) composites are presented. Previous experimental and modeling investigations of woven composites are discussed, as is the evolution of, and motivation for, the continuing research on C/Cu composites. Experimental results of monotonic and cyclic tension, compression, and Iosipescu shear tests, and combined tension-compression tests, are presented. With regard to the test results, emphasis is placed on the effect of strain gauge size and placement, the effect of alloying the copper matrix to improve fiber-matrix bonding, yield surface characterization, and failure mechanisms. The analytical methodology used in this investigation consists of an extension of the three-dimensional generalized method of cells (GMC-3D) micromechanics model, developed by Aboudi (1994), to include inhomogeneity and plasticity effects on the subcell level. The extension of the model allows prediction of the elastoplastic mechanical response of woven composites, as represented by a true repeating unit cell for the woven composite. The model is used to examine the effects of refining the representative geometry of the composite, altering the composite overall fiber volume fraction, changing the size and placement of the strain gauge with respect to the composite's reinforcement weave, and including porosity within the infiltrated fiber yarns on the in-plane elastoplastic tensile, compressive, and shear response of 8-harness satin C/Cu. The model predictions are also compared with the appropriate monotonic experimental results