Critical comparison of electrode models in density functional theory based quantum transport calculations

We study the performance of two different electrode models in quantum transport calculations based on density functional theory: Parametrized Bethe lattices and quasi-one dimensional wires or nanowires. A detailed account of implementation details in both cases is given. From the systematic study of nanocontacts made of representative metallic elements, we can conclude that parametrized electrode models represent an excellent compromise between computational cost and electronic structure definition as long as the aim is to compare with experiments where the precise atomic structure of the electrodes is not relevant or defined with precision. The results obtained using parametrized Bethe lattices are essentially similar to the ones obtained with quasi one dimensional electrodes for large enough sections of these, adding a natural smearing to the transmission curves that mimics the true nature of polycrystalline electrodes. The latter are more demanding from the computational point of view, but present the advantage of expanding the range of applicability of transport calculations to situations where the electrodes have a well-defined atomic structure, as is case for carbon nanotubes, graphene nanoribbons or semiconducting nanowires. All the analysis is done with the help of codes developed by the authors which can be found in the quantum transport toolbox Alacant and are publicly available.Comment: 17 pages, 12 figure

Modelling colossal magnetoresistance manganites

I briefly survey here attempts to model the rich and strange behaviour of colossal magnetoresistance manganites, after outlining some of the phenomena observed in them, and describing the three relevant strong local interactions of the e_g electrons (in two different orbital states at each site), namely with Jahn-Teller phonon modes (strength g), with resident t_2g spins (ferromagnetic Hund's rule coupling J_H) and amongst each other (the Mott Hubbard correlation U) . A new two fluid model of nearly localized l polarons and band (b) electrons for low energy behaviour emerges for large g; some of its applications are mentioned here. I describe some results of strong coupling U, J_H calculations in single site DMFT (Dynamical Mean Field Theory), and show that in the wide orbital liquid regime many characteristic manganite phenomena such as an insulating ferromagnetic ground state, thermal insulator metal transition, colossal magnetoresistance (cmr), materials systematics and the observed low effective carrier density can all be understood qualitatively and quantitatively. We also discuss the two 'phase' coexistence frequently found in these systems, and show that electrostatic coulomb interactions mute lb phase separation into nanoscale electronic inhomogeneity with l regions and b puddles. Finally, some problems of current interest as well as general ones arising, eg polarons and the physics of large electron phonon coupling g in the adiabatic regime, are mentioned

A 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 lies essentially on two facts. First of all, it makes use of the versatile Gaussian98 code, which is widely used within the quantum chemistry community. Secondly, 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. On the contrary, 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.Comment: 14 pages, two-column format, submitted to PR

Topological superconductivity in lead nanowires

Superconductors with an odd number of bands crossing the Fermi energy have topologically protected Andreev states at interfaces, including Majorana states in one dimensional geometries. Superconductivity, a low number of 1D channels, large spin orbit coupling, and a sizeable Zeeman energy, are present in lead nanowires produced by nanoindentation of a Pb tip on a Pb substrate, in magnetic fields higher than the Pb bulk critical field. A number of such devices have been analyzed. In some of them, the dependence of the critical current on magnetic field, and the Multiple Andreev Reflections observed at finite voltages, are compatible with the existence of topological superconductivity