Is Small Perfect? Size Limit to Defect Formation in Pyramidal Pt Nanocontacts

We report high resolution transmission electron microscopy and ab initio calculation results for the defect formation in Pt nanocontacts (NCs). Our results show that there is a size limit to the existence of twins (extended structural defects). Defects are always present but blocked away from the tip axes. The twins may act as scattering plane, influencing contact electron transmission for Pt NC at room temperature and Ag/Au NC at low temperature.Comment: 4 pages, 3 figure

A comprehensive study of electric, thermoelectric and thermal conductivities of Graphene with short range unitary and charged impurities

Motivated by the experimental measurement of electrical and hall conductivity, thermopower and Nernst effect, we calculate the longitudinal and transverse electrical and heat transport in graphene in the presence of unitary scatterers as well as charged impurities. The temperature and carrier density dependence in this system display a number of anomalous features that arise due to the relativistic nature of the low energy fermionic degrees of freedom. We derive the properties in detail including the effect of unitary and charged impurities self-consistently, and present tables giving the analytic expressions for all the transport properties in the limit of small and large temperature compared to the chemical potential and the scattering rates. We compare our results with the available experimental data. While the qualitative variations with temperature and density of carriers or chemical potential of all transport properties can be reproduced, we find that a given set of parameters of the impurities fits the Hall conductivity, Thermopower and the Nernst effect quantitatively but cannot fit the conductivity quantitatively. On the other hand a single set of parameters for scattering from Coulomb impurities fits conductivity, hall resistance and thermopower but not Nernst

Surface Effects on the Mechanical Elongation of AuCu Nanowires: De-alloying and the Formation of Mixed Suspended Atomic Chains

We report here an atomistic study of the mechanical deformation of AuxCu(1-x) atomic-size wires (NWs) by means of high resolution transmission electron microscopy (HRTEM) experiments. Molecular dynamics simulations were also carried out in order to obtain deeper insights on the dynamical properties of stretched NWs. The mechanical properties are significantly dependent on the chemical composition that evolves in time at the junction; some structures exhibit a remarkable de-alloying behavior. Also, our results represent the first experimental realization of mixed linear atomic chains (LACs) among transition and noble metals; in particular, surface energies induce chemical gradients on NW surfaces that can be exploited to control the relative LAC compositions (different number of gold and copper atoms). The implications of these results for nanocatalysis and spin transport of one-atom-thick metal wires are addressed.Comment: Accepted to Journal of Applied Physics (JAP

Low Temperature (ln2) And Uhv Mechanically Controllable Break Junction Setup To Study Quantum Electrical Transport Of Atomic-size Metal Nanowire

Reliable metal nanowire studies requires experimental stringent conditions, as clean samples and environment. In this sense, we have designed and built a dedicated instrument to study electrical transport properties of atomic-size metal contacts based on the mechanically controlled break junction technique, operating at ultra-high-vacuum conditions. Here we describe the chosen setup, its implementation and performance. © 2008 IOP Publishing Ltd.100PART 5Landauer, R., (1957) IBM J. Res. Dev., 1, p. 223Agrait, N., Yeyati, A.L., Van Ruitenbeek, J.M., (2003) Phys. Rep., 377, p. 81Rodrigues, V., Fuhrer, T., Ugarte, D., (2000) Phys. Rev. Lett., 85, p. 4124Hansen, K., Lægsgaard, E., Stensgaard, I., Besenbacher, F., (1997) Phys. Rev. B, 56, p. 2208De Heer, W.A., Frank, S., Ugarte, D., (1997) Z. Phys. B, 104, p. 469Rodrigues, V., Bettini, J., Rocha, A.R., Rego, L.G.C., Ugarte, D., (2002) Phys. Rev. B, 65, p. 153402Rego, L.G.C., Rocha, A.R., Rodrigues, V., Ugarte, D., (2003) Phys. Rev. B, 67, p. 045412Gonzalez, J.C., Rodrigues, V., Bettini, J., Rego, L.G.C., Rocha, A.R., Coura, P.Z., Dantas, S.O., Ugarte, D., (2004) Phys. Rev. Lett., 93, p. 126103Lagos, M., Rodrigues, V., Ugarte, D., (2007) J. Electron Spectroscopy and Related Phenomena, 20, pp. 156-158Rubio, G., Agraït, N., Vieira, S., (1996) Phys. Rev. Lett., 76, p. 2302Smit, R.H.M., Noat, Y., Untiedt, C., Lang, N.D., Van Hemert, M.C., Van Ruitenbeek, J.M., (2002) Nature, 419, p. 906VaLkering A, M.C., Mares, A.I., Untiedt, C., Gavan, K.B., Oosterkamp, T.H., Van Ruitenbeek, J.M., (2005) Review of Scientific Instruments, 76, p. 103903Zhou, C., Muller, C.J., Deshpande, M.R., Sleight, J.W., Reed, M.A., (1995) Appl. Phys. Lett., 67, p. 1160Hansen, K., Nielsen, S.K., Brandbyge, M., Lægsgaard, E., Stensgaard, I., Besenbacher, F., (2000) Appl. Phys. Lett., 77, p. 708Hakkinen, H., Barnett, R.N., Landman, U., (1999) J. Phys. Chem. B, 103, p. 8814Legoas, S.B., Rodrigues, V., Ugarte, D., Galvao, D.S., (2004) Phys. Rev. Lett., 93, p. 216103Novaes, F.D., Da Silva A, J.R., Fazzio, A., Da Silva, E.Z., (2005) J. Appl. Phys. A, 81, p. 1551Barnett, R.N., Hakkinen, H., Scherbakov, A.G., Landman, U., (2004) Nano Lett., p. 41845Hakkinen, H., Barnett, R.N., Scherbakov, A.G., Landman, U., (2000) J. Phys. Chem. B, 104, p. 9063Rodrigues, V., Bettini, J., Silva, P.C., Ugarte, D., (2003) Phys. Rev. Lett., 91, p. 096801Costa-Kramer, J.L., Garcia, N., Olin, H., (1997) Phys. Rev. B, 55, p. 1291

Large-scale unsupervised spatio-temporal semantic analysis of vast regions from satellite images sequences

Temporal sequences of satellite images constitute a highly valuable and abundant resource for analyzing regions of interest. However, the automatic acquisition of knowledge on a large scale is a challenging task due to different factors such as the lack of precise labeled data, the definition and variability of the terrain entities, or the inherent complexity of the images and their fusion. In this context, we present a fully unsupervised and general methodology to conduct spatio-temporal taxonomies of large regions from sequences of satellite images. Our approach relies on a combination of deep embeddings and time series clustering to capture the semantic properties of the ground and its evolution over time, providing a comprehensive understanding of the region of interest. The proposed method is enhanced by a novel procedure specifically devised to refine the embedding and exploit the underlying spatio-temporal patterns. We use this methodology to conduct an in-depth analysis of a 220 km2 region in northern Spain in different settings. The results provide a broad and intuitive perspective of the land where large areas are connected in a compact and well-structured manner, mainly based on climatic, phytological, and hydrological factors

Personalised Trachea Stent Designer: a Knowledge Feature

Modelling of anatomical parts is usually tackled through triangulated models with specialised Bio-CAD applications. If features beyond anatomy are required, geometry is usually translated into NURBS geometry for further modification in parametric feature based design CAD systems. But, they remain quite unmanageable yet. The authors present, validate and implement into a knowledge feature a methodology that generates an anatomically personalised trachea stent based a point cloud data extracted from a Computerised Axial Tomography with an open-source medical data visualisation application

Current sheets at three-dimensional magnetic nulls:effect of compressibility

The nature of current sheet formation in the vicinity of three-dimensional (3D) magnetic null points is investigated. The particular focus is upon the effect of the compressibility of the plasma on the qualitative and quantitative properties of the current sheet. An initially potential 3D null is subjected to shearing perturbations, as in a previous paper [Pontin et al., Phys. Plasmas, in press (2007)]. It is found that as the incompressible limit is approached, the collapse of the null point is suppressed, and an approximately planar current sheet aligned to the fan plane is present instead. This is the case regardless of whether the spine or fan of the null is sheared. Both the peak current and peak reconnection rate are reduced. The results have a bearing on previous analytical solutions for steady-state reconnection in incompressible plasmas, implying that fan current sheet solutions are dynamically accessible, while spine current sheet solutions are not.Comment: to appear in Physics of Plasmas. This version contains updated figures and references, additional discussion, and typos are fixed. This is the second in a series of papers - the first of which (by the same authors) is located at astro-ph/0701462. A version with higher quality figures can be found at http://www.maths.dundee.ac.uk/~dpontin

Evidence of spontaneous spin polarized transport in magnetic nanowires

The exploitation of the spin in charge-based systems is opening revolutionary opportunities for device architecture. Surprisingly, room temperature electrical transport through magnetic nanowires is still an unresolved issue. Here, we show that ferromagnetic (Co) suspended atom chains spontaneously display an electron transport of half a conductance quantum, as expected for a fully polarized conduction channel. Similar behavior has been observed for Pd (a quasi-magnetic 4d metal) and Pt (a non-magnetic 5d metal). These results suggest that the nanowire low dimensionality reinforces or induces magnetic behavior, lifting off spin degeneracy even at room temperature and zero external magnetic field.Comment: 4 pages, 3 eps fig