J. A. Vergés

Resistivity of Mixed-Phase Manganites.

The resistivity rho(dc) of manganites is studied using a random resistor-network, based on phase separation between metallic and insulating domains. When percolation occurs, both as chemical composition or temperature vary, results in good agreement with experiments are obtained. Similar conclusions are reached using quantum calculations and microscopic considerations. Above the Curie temperature, it is argued that ferromagnetic clusters should exist in Mn oxides. Small magnetic fields induce large rho(dc) changes and a bad-metal state with (disconnected) insulating domains.

First-Principles Phase-Coherent Transport in Metallic Nanotubes with Realistic Contacts

We present first-principles calculations of phase coherent electron transport in a carbon nanotube (CNT) with realistic contacts. We focus on the zero-bias response of open metallic CNTs considering two archetypal contact geometries (end and side) and three commonly used metals as electrodes (Al, Au, and Ti). Our ab initio electrical transport calculations make, for the first time, quantitative predictions on the contact transparency and the transport properties of finite metallic CNTs. Al and Au turn out to make poor contacts while Ti is the best option of the three.

First-principles approach to electrical transport in atomic-scale nanostructures

We present a first-principles numerical implementation of Landauer formalism for electrical transport in nanostructures characterized down to the atomic level. The novelty and interest of our method lie essentially on twofacts. First of all, it makes use of the versatile GAUSSIAN98 code, which is widely used within the quantum chemistry community. Second, it incorporates the semi-infinite electrodes in a very generic and efficient way by means of Bethe lattices. We name this method the Gaussian embedded cluster method (GECM). In order to make contact with other proposed implementations, we illustrate our technique by calculating the conductance in some well-studied systems such as metallic (Al and Au) nanocontacts and C-atom chains connected to metallic (Al and Au) electrodes. In the case of Al nanocontacts the conductance turns out to be quite dependent on the detailed atomic arrangement. In contrast, the conductance in Au nanocontacts presents quite universal features. In the case of C chains, where the self-consistency guarantees the local charge transfer and the correct alignment of the molecular and electrode levels, we find that the conductance oscillates with the number of atoms in the chain regardless of the type of electrode. However, for short chains and Al electrodes the even-odd periodicity is reversed at equilibrium bond distances.

Fullerene-based molecular nanobridges: A first-principles study

Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, Madrid 28049, Spain~Received 25 May 2001; published 24 August 2001!Building upon traditional quantum-chemistry calculations, we have implemented an ab initio method tostudy the electrical transport in nanocontacts. We illustrate our technique calculating the conductance of C

Strong covalent bonding between two graphene layers

Financed by CICYT under contracts MAT-2005-3866, MAT-2006-03741, FIS-2006-12117-C04-03,NAN-2004-09183-C10-08. We acknowledge the use of the Spanish Supercomputing Network and the CTI (CSIC).

Hydrogen on Graphene Under Stress: Molecular Dissociation and Gap Opening

Density-functional calculations are employed to study the molecular dissociation of hydrogen on graphene, the diffusion of chemisorbed atomic species, and the electronic properties of the resulting hydrogen on graphene system. Our results show that applying stress to the graphene substrate can lower the barrier to dissociation of molecular hydrogen by a factor of 6 and change the process from endothermic to exothermic. These values for the barrier and the heat of reaction, unlike the zero stress values, are compatible with the time scales observed in experiments. Diffusion, on the other hand, is not greatly modified by stress. We analyze the electronic structure for configurations relevant to molecular dissociation and adsorption of atomic hydrogen on a graphene single layer. An absolute band gap of 0.5 eV is found for the equilibrium optimum configuration for a narrow range of coverages

First-principles calculation of the effect of stress on the chemical activity of graphene

(\ensuremath{\theta}\ensuremath{\approx}0.25)

Crystal structure and electronic states of tripotassium picene

. This value is in good agreement with experiment [D. Elias et al., Science 323, 610 (2009)].

Ab initioelectronic and geometrical structures of tripotassium-intercalated phenanthrene

Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects σ and π bonds and the resulting change in the chemical activity. Stress affects more strongly π bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing 120° and 90°, an intermediate state between sp2 and sp3 bonding. We use ab initio density functional theory to study the adsorption of hydrogen on large clusters and two-dimensional periodic models for graphene. The influence of the exchange-correlation functional on the adsorption energy is discussed.

Electronic transport through C60 molecules

The crystal structure of potassium doped picene with an exact stoichiometry (K3picene) 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 structure of 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/ 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.