Theoretical Study of Spin-dependent Electron Transport in Atomic Fe Nanocontacts

We present theoretical predictions of spintronic transport phenomena that should be observable in ferromagnetic Fe nanocontacts bridged by chains of Fe atoms. We develop appropriate model Hamiltonians based on semi-empirical considerations and the known electronic structure of bulk Fe derived from ab initio density functional calculations. Our model is shown to provide a satisfactory description of the surface properties of Fe nano-clusters as well as bulk properties. Lippmann-Schwinger and Green's function techniques are used together with Landauer theory to predict the current, magneto-resistance, and spin polarization of the current in Fe nanocontacts bridged by atomic chains under applied bias. Unusual device characteristics are predicted including negative magneto-resistance and spin polarization of the current, as well as spin polarization of the current for anti-parallel magnetization of the Fe nanocontacts under moderate applied bias. We explore the effects that stretching the atomic chain has on the magneto-resistance and spin polarization and predict a cross-over regime in which the spin polarization of the current for parallel magnetization of the contacts switches from negative to positive. We find resonant transmission due to dangling bond formation on tip atoms as the chain is stretched through its breaking point to play an important role in spin-dependent transport in this regime. The physical mechanisms underlying the predicted phenomena are discussed.Comment: 13 pages, 6 figures, Accepted for publication in Physical Review

Even-odd parity effects in conductance and shot noise of metal-atomic wire-metal(superconducting) junctions

In this paper, we study the conductance and shot noise in transport through a multi-site system in a two terminal configuration. The dependence of the transport on the number of atoms in the atomic wire is investigated using a tight-binding Hamiltonian and the nonequilibrium Green's function method. In addition to reproducing the even-odd behavior in the transmission probability at the Fermi energy or the linear response conductance in the normal-atomic wire-normal metallic(NAN) junctions, we find the following: (i) The shot noise is larger in the even-numbered atomic wire than in the odd-numbered wire. (ii) The Andreev conductance displays the same even-odd parity effects in the normal-atomic wire-superconducting(NAS) junctions. In general, the conductance is higher in the odd-numbered atomic wire than in the even-numbered wire. When the number of sites ($N$) is odd and the atomic wire is mirror symmetric with respect to the center of the atomic wire, the conductance does not depend on the details of the hopping matrices in the atomic wire, but is solely determined by the coupling strength to the two leads. When $N$ is even, the conductance is sensitive to the values of the hopping matrices.Comment: 12 pages, 9 figure

Electron transport across a quantum wire in the presence of electron leakage to a substrate

We investigate electron transport through a mono-atomic wire which is tunnel coupled to two electrodes and also to the underlying substrate. The setup is modeled by a tight-binding Hamiltonian and can be realized with a scanning tunnel microscope (STM). The transmission of the wire is obtained from the corresponding Green's function. If the wire is scanned by the contacting STM tip, the conductance as a function of the tip position exhibits oscillations which may change significantly upon increasing the number of wire atoms. Our numerical studies reveal that the conductance depends strongly on whether or not the substrate electrons are localized. As a further ubiquitous feature, we observe the formation of charge oscillations.Comment: 7 pages, 7 figure

Los mayores y las nuevas tecnologías de la comunicación

[Resumen] Fundamentos: En una sociedad envejecida es necesario establecer nuevas alternativas que de alguna manera traten de satisfacer las necesidades de las personas mayores, a la vez que incrementen su calidad de vida percibida. En este sentido, las nuevas tecnologías, como la informática, se han convertido en una herramienta básica de nuestra sociedad, de la que los mayores, sin ninguna duda, podrán beneficiarse. El objetivo del presente trabajo es conocer la opinión de los mayores acerca de temas relacionados con la informática e Internet, así como su capacidad de acceso a las nuevas tecnologías y los conocimientos con los que cuentan. Métodos: Se realiza una entrevista estandarizada, de diseño propio, a una muestra de 105 personas mayores con una edad media de 73 años, residentes en la ciudad de A Coruña. Resultados: De los resultados obtenidos se constata que aunque la mayoría de las personas mayores no cuenta con ordenador personal ni ha trabajado con aplicaciones informáticas, les gustaría aprender más sobre este campo. Además, cuando se conectan a Internet lo hacen desde centros sociales y/o educativos, no encontrando excesivos problemas en su manejo y navegación. Conclusiones: Es necesario fomentar el uso de la informática por parte de los mayores, ya que en un futuro puede constituir una herramienta imprescindible en la intervención sobre problemas inherentes al envejecimiento como son el aislamiento o la soledad entre otros.[Abstract] new alternatives, that somehow try to satisfy the needs of elderly people and that at the same time increase their quality of life. On this sense, the new technologies, such as computer science, have become a basic tool of our society, and the elderly, without no doubt will be able to benefit from it. The objective of the present work is to know what the elderly think about themes related to computer science and internet, as well as their capacity of accessing to new technologies, and the knowledge that they have about them. Methods: A standarized interview has been made, it´s self designed, to a sample of 105 elderly people living in the city of A Coruña. Results: Although most elderly people don´t have a personal computer neither have worked with a computer application, would like to learn more about this field. When they are connected to internet they do it from social and/or educative centres, not having great problems neither in its handling or navigation. Conclusions: It´s necessary to promote the use of computer science on the elderly people, since in a future it can constitute an essential tool in the intervention on inherent problems to aging such as isolation or loneliness among others

Microscopic Theory of Josephson Mesoscopic Constrictions

We present a microscopic theory for the d.c. Josephson effect in model mesoscopic constrictions. Our method is based on a non-equilibrium Green function formalism which allows for a self-consistent determination of the order parameter profile along the constriction. The various regimes defined by the different length scales (Fermi wavelength $\lambda_F$, coherence length $\xi_0$ and constriction length $L_C$) can be analyzed, including the case where all these lengths are comparable. For the case $\lambda_F \tilde{<} (L_C,\xi_0)$ phase oscillations with spatial period $\lambda_F/2$ can be observed. In the case of $L_C>\xi_0$ solutions with a phase-slip center inside the constriction can be found, in agreement with previous phenomenological theories.Comment: 4 pages (RevTex 3.0), 3 postscript figures available upon request, 312456-C

ab initio modeling of open systems: charge transfer, electron conduction, and molecular switching of a C_{60} device

We present an {\it ab initio} analysis of electron conduction through a $C_{60}$ molecular device. Charge transfer from the device electrodes to the molecular region is found to play a crucial role in aligning the lowest unoccupied molecular orbital (LUMO) of the $C_{60}$ to the Fermi level of the electrodes. This alignment induces a substantial device conductance of $\sim 2.2 \times (2e^2/h)$. A gate potential can inhibit charge transfer and introduce a conductance gap near $E_F$, changing the current-voltage characteristics from metallic to semi-conducting, thereby producing a field effect molecular current switch

Perturbation Study of the Conductance through an Interacting Region Connected to Multi-Mode Leads

We study the effects of electron correlation on transport through an interacting region connected to multi-mode leads based on the perturbation expansion with respect to the inter-electron interaction. At zero temperature the conductance defined in the Kubo formalism can be written in terms of a single-particle Green's function at the Fermi energy, and it can be mapped onto a transmission coefficient of the free quasiparticles described by an effective Hamiltonian. We apply this formulation to a two-dimensional Hubbard model of finite size connected to two noninteracting leads. We calculate the conductance in the electron-hole symmetric case using the order $U^2$ self-energy. The conductance shows several maximums in the $U$ dependence in some parameter regions of $t_y/t_x$, where $t_x$ ($t_y$) is the hopping matrix element in the $x$- ($y$-) directions. This is caused by the resonance occurring in some of the subbands, and is related with the $U$ dependence of the eigenvalues of the effective Hamiltonian.Comment: 17 pages, 12 figures, to be published in J.Phys.Soc.Jpn. 71(2002)No.