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.

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 two facts. 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.

We show that two graphene layers stacked directly on top of each other ($AA$ stacking) form strong chemical bonds when the distance between planes is $0.156\mathrm{nm}$. Simultaneously, C–C in-plane bonds are considerably weakened from partial double bond $\left(0.141\mathrm{nm}\right)$ to single bond $\left(0.154\mathrm{nm}\right)$. This polymorphic form of graphene bilayer is metastable with an activation energy of $0.16\mathrm{eV}∕\text{cell}$ with respect to the standard configuration bound by van der Waals forces at a larger separation between planes $\left(0.335\mathrm{nm}\right)$. Carbon atoms form four single bonds in a geometry mixing 90° and 120° angles, intermediate between the usual $s{p}^2$ and $s{p}^3$, but similar to the one found in molecules like the cubane, pentaprismane, or hexaprismane. Under an in-plane stress of $9\mathrm{GPa}$, this carbon allotrope becomes the global energy minimum. As a function of the separation between layers, the electronic band structure goes through different regimes: It is a semimetal at van der Waals-like distances, a wide gap semiconductor at covalentlike distances, and in between it displays metallic behavior.

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 CNT’s 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 CNT’s. Al and Au turn out to make poor contacts while Ti is the best option of the three.

Building upon traditional quantum-chemistry calculations, we have implemented an ab initio method to study the electrical transport in nanocontacts. We illustrate our technique calculating the conductance of ${\mathrm{C}}_{60}$ molecules connected in various ways to Al electrodes characterized at the atomic level. Central to a correct estimate of the electrical current is a precise knowledge of the local charge transfer between molecule and metal which, in turn, guarantees the correct positioning of the Fermi level with respect to the molecular orbitals. Contrary to our expectations, ballistic transport seems to occur in this system.

Based on the results of density functional theory calculations, a novel mechanism for the diffusion of water dimers on metal surfaces is proposed, which relies on the ability of H bonds to rearrange through quantum tunneling. The mechanism involves quasifree rotation of the dimer and exchange of H-bond donor and acceptor molecules. At appropriate temperatures, water dimers diffuse more rapidly than water monomers, thus providing a physical explanation for the experimentally measured high diffusivity of water dimers on Pd{111} [Mitsui et al., Science 297, 1850 (2002)].