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Atomic Size

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Daniel C Fredrickson – 1st expert on this subject based on the ideXlab platform

  • progress in visualizing Atomic Size effects with dft chemical pressure analysis from isolated atoms to trends in ab5 intermetallics
    Journal of Chemical Theory and Computation, 2014
    Co-Authors: Veronica M Berns, Joshua Engelkemier, Brandon J Kilduff, Daniel C Fredrickson

    Abstract:

    The notion of Atomic Size poses an important challenge to chemical theory: empirical evidence has long established that atoms have spatial requirements, which are summarized in tables of covalent, ionic, metallic, and van der Waals radii. Considerations based on these radii play a central role in the design and interpretation of experiments, but few methods are available to directly support arguments based on Atomic Size using electronic structure methods. Recently, we described an approach to elucidating Atomic Size effects using theoretical calculations: the DFT-Chemical Pressure analysis, which visualizes the local pressures arising in crystal structures from the interactions of Atomic Size and electronic effects. Using this approach, a variety of structural phenomena in intermetallic phases have already been understood in terms that provide guidance to new synthetic experiments. However, the applicability of the DFT-CP method to the broad range of the structures encountered in the solid state is limit…

  • first principles elucidation of Atomic Size effects using dft chemical pressure analysis origins of ca36sn23 s long period superstructure
    Journal of Chemical Theory and Computation, 2013
    Co-Authors: Joshua Engelkemier, Veronica M Berns, Daniel C Fredrickson

    Abstract:

    The space requirements of atoms are empirically known to play key roles in determining structure and reactivity across compounds ranging from simple molecules to extended solid state phases. Despite the importance of this concept, the effects of Atomic Size on stability remain difficult to extract from quantum mechanical calculations. Recently, we outlined a quantitative yet visual and intuitive approach to the theoretical analysis of Atomic Size in periodic structures: the DFT-Chemical Pressure (DFT-CP) analysis. In this Article, we describe the methodological details of this DFT-CP procedure, with a particular emphasis on refinements of the method to make it useful for a wider variety of systems. A central improvement is a new integration scheme with broader applicability than our earlier Voronoi cell method: contact volume space-partitioning. In this approach, we make explicit our assumption that the pressure at each voxel is most strongly influenced by its two closest atoms. The unit cell is divided i…

  • dft chemical pressure analysis visualizing the role of Atomic Size in shaping the structures of inorganic materials
    Journal of the American Chemical Society, 2012
    Co-Authors: Daniel C Fredrickson

    Abstract:

    Atomic Size effects have long played a role in our empirical understanding of inorganic crystal structures. At the level of electronic structure calculations, however, the contribution of Atomic Size remains difficult to analyze, both alone and relative to other influences. In this paper, we extend the concepts outlined in a recent communication to develop a theoretical method for revealing the impact of the space requirements of atoms: the density functional theory-chemical pressure (DFT-CP) analysis. The influence of Atomic Size is most pronounced when the optimization of bonding contacts is impeded by steric repulsion at other contacts, resulting in nonideal interAtomic distances. Such contacts are associated with chemical pressures (CPs) acting upon the atoms involved. The DFT-CP analysis allows for the calculation and interpretation of the CP distributions within crystal structures using DFT results. The method is demonstrated using the stability of the Ca2Ag7 structure over the simpler CaCu5-type al…

J. Cuevas – 2nd expert on this subject based on the ideXlab platform

  • thermal conductance of metallic Atomic Size contacts phonon transport and wiedemann franz law
    Physical Review B, 2017
    Co-Authors: Jan C Klockner, Manuel Matt, Peter Nielaba, Fabian Pauly, J. Cuevas

    Abstract:

    Motivated by recent experiments [Science 355, 6330 (2017); Nat. Nanotechnol. 12, 430 (2017)], we present here an extensive theoretical analysis of the thermal conductance of AtomicSize contacts made of three different metals, namely gold (Au), platinum (Pt) and aluminum (Al).

  • orbital origin of the electrical conduction in ferromagnetic Atomic Size contacts insights from shot noise measurements and theoretical simulations
    Physical Review B, 2016
    Co-Authors: Ran Vardimon, Peter Nielaba, Manuel Matt, J. Cuevas

    Abstract:

    With the goal to elucidate the nature of spin-dependent electronic transport in ferromagnetic Atomic contacts, we present here a combined experimental and theoretical study of the conductance and shot noise of metallic Atomic contacts made of the 3d ferromagnetic materials Fe, Co, and Ni. For comparison, we also present the corresponding results for the noble metal Cu. Conductance and shot noise measurements, performed using a low-temperature break junction setup, show that in these ferromagnetic nanowires: (i) there is no conductance quantization of any kind, (ii) transport is dominated by several partially-open conduction channels, even in the case of single-atom contacts, and (iii) the Fano factor of large contacts saturates to values that clearly differs from those of monovalent (nonmagnetic) metals. We rationalize these observations with the help of a theoretical approach that combines molecular dynamics simulations to describe the junction formation with nonequilibrium Green’s function techniques to compute the transport properties within the Landauer-Buettiker framework. Our theoretical approach successfully reproduces all the basic experimental results and it shows that all the observations can be traced back to the fact that the d bands of the minority-spin electrons play a fundamental role in the transport through ferromagnetic AtomicSize contacts. These d bands give rise to partially open conduction channels for any contact Size, which in turn lead naturally to the different observations described above. Thus, the transport picture for these nanoscale ferromagnetic wires that emerges from the ensemble of our results is clearly at variance with the well established conduction mechanism that governs the transport in macroscopic ferromagnetic wires, where the d bands are responsible for the magnetism but do not take part in the charge flow.

  • quantum thermopower of metallic Atomic Size contacts at room temperature
    Nano Letters, 2015
    Co-Authors: Charalambos Evangeli, J. Cuevas, Manuel Matt, Peter Nielaba, Fabian Pauly, Laura Rincongarcia, Gabino Rubiobollinger, Nicolás Agraït

    Abstract:

    We report conductance and thermopower measurements of metallic AtomicSize contacts, namely gold and platinum, using a scanning tunneling microscope (STM) at room temperature. We find that few-atom gold contacts have an average negative thermopower, whereas platinum contacts present a positive thermopower, showing that for both metals, the sign of the thermopower in the nanoscale differs from that of bulk wires. We also find that the magnitude of the thermopower exhibits minima at the maxima of the conductance histogram in the case of gold nanocontacts while for platinum it presents large fluctuations. Tight-binding calculations and Green’s function techniques, together with molecular dynamics simulations, show that these observations can be understood in the context of the Landauer–Buttiker picture of coherent transport in Atomic-scale wires. In particular, we show that the differences in the thermopower between these two metals are due to the fact that the elastic transport is dominated by the 6s orbita…

J. M. Van Ruitenbeek – 3rd expert on this subject based on the ideXlab platform

  • Atomic Size oscillations in conductance histograms for gold nanowires and the influence of work hardening
    Physical Review Letters, 2005
    Co-Authors: I K Yanson, O I Shklyarevskii, Sz Csonka, H Van Kempen, S Speller, A I Yanson, J. M. Van Ruitenbeek

    Abstract:

    Nanowires of different nature have been shown to self-assemble as a function of stress at the contact between two macroscopic metallic leads. Here we demonstrate for gold wires that the balance between various metastable nanowire configurations is influenced by the microstructure of the starting materials and we discover a new set of periodic structures, which we interpret as due to the Atomic discreteness of the contact Size for the three principal crystal orientations.

  • conductance fluctuations as a tool for investigating the quantum modes in Atomic Size metallic contacts
    Physical Review B, 2000
    Co-Authors: B. Ludoph, J. M. Van Ruitenbeek

    Abstract:

    Recently it has been observed that the conductance fluctuations of AtomicSize gold contacts are suppressed when the conductance is equal to an integer multiple of the conductance quantum. The fact that these contacts tend to consist exclusively of fully open or closed modes has been argued to be the origin for this suppression. Here the experiments have been extended to a wide range of metallic elements with different chemical valences, and they provide information about the relation between the mode composition and statistically preferred conductance values observed in conductance histograms.

  • THERMOPOWER OF AtomicSize METALLIC CONTACTS
    Physical Review B, 1999
    Co-Authors: B. Ludoph, J. M. Van Ruitenbeek

    Abstract:

    The thermopower and conductance of AtomicSize metallic contacts have been simultaneously measured using a mechanically controllable break junction. For contacts approaching Atomic dimensions, abrupt steps in the thermopower are observed which coincide with jumps in the conductance. The measured thermopower for a large number of AtomicSize contacts is randomly distributed around the value for large contacts and can be either positive or negative in sign. However, it is suppressed at the quantum value of the conductance G_0 = 2e^2/h. We derive an expression that describes these results in terms of quantum interference of electrons backscattered in the banks.