456 research outputs found
Electrical conductivity measured in atomic carbon chains
The first electrical conductivity measurements of monoatomic carbon chains
are reported in this study. The chains were obtained by unraveling carbon atoms
from graphene ribbons while an electrical current flowed through the ribbon
and, successively, through the chain. The formation of the chains was
accompanied by a characteristic drop in the electrical conductivity. The
conductivity of carbon chains was much lower than previously predicted for
ideal chains. First-principles calculations using both density functional and
many-body perturbation theory show that strain in the chains determines the
conductivity in a decisive way. Indeed, carbon chains are always under varying
non-zero strain that transforms its atomic structure from cumulene to polyyne
configuration, thus inducing a tunable band gap. The modified electronic
structure and the characteristics of the contact to the graphitic periphery
explain the low conductivity of the locally constrained carbon chain.Comment: 21 pages, 9 figure
Energetics and stability of nanostructured amorphous carbon
Monte Carlo simulations, supplemented by ab initio calculations, shed light
into the energetics and thermodynamic stability of nanostructured amorphous
carbon. The interaction of the embedded nanocrystals with the host amorphous
matrix is shown to determine in a large degree the stability and the relative
energy differences among carbon phases. Diamonds are stable structures in
matrices with sp^3 fraction over 60%. Schwarzites are stable in low-coordinated
networks. Other sp^2-bonded structures are metastable.Comment: 11 pages, 7 figure
In situ radiographic investigation of de lithiation mechanisms in a tin electrode lithium ion battery.
The lithiation and delithiation mechanisms of multiple Sn particles in a customized flat radiography cell were investigated by in amp; 8197;situ synchrotron radiography. For the first time, four de lithiation phenomena in a Sn electrode battery system are highlighted 1 amp; 8197;the de lithiation behavior varies between different Sn particles, 2 amp; 8197;the time required to lithiate individual Sn particles is markedly different from the time needed to discharge the complete battery, 3 amp; 8197;electrochemical deactivation of originally electrochemically active particles is reported, and 4 amp; 8197;a change of electrochemical behavior of individual particles during cycling is found and explained by dynamic changes of de lithiation pathways amongst particles within the electrode. These unexpected findings fundamentaly expand the understanding of the underlying de lithiation mechanisms inside commercial lithium ion batteries LIBs and would open new design principles for high performance next generation LIB
Morphological evolution of electrochemically plated stripped lithium microstructures by synchrotron X ray phase contrast tomography
Due to its low redox potential and high theoretical specific capacity, Li metal has drawn worldwide research attention because of its potential use in next generation battery technologies such as Li S and Li O2. Unfortunately, uncontrollable growth of Li microstructures LmSs, e.g., dendrites, fibers during electrochemical Li stripping plating has prevented their practical commercialization. Despite various strategies proposed to mitigate LmS nucleation and or block its growth, a fundamental understanding of the underlying evolution mechanisms remains elusive. Herein, synchrotron in line phase contrast X ray tomography was employed to investigate the morphological evolution of electrochemically deposited dissolved LmSs nondestructively. We present a 3D characterization of electrochemically stripped Li electrodes with regard to electrochemically plated LmSs. We clarify fundamentally the origin of the porous lithium interface growing into Li electrodes. Moreover, cleavage of the separator caused by growing LmS was experimentally observed and visualized in 3D. Our systematic investigation provides fundamental insights into LmS evolution and enables us to understand the evolution mechanisms in Li electrodes more profoundl
Theory of the anomalous Hall effect from the Kubo formula and the Dirac equation
A model to treat the anomalous Hall effect is developed. Based on the Kubo
formalism and on the Dirac equation, this model allows the simultaneous
calculation of the skew-scattering and side-jump contributions to the anomalous
Hall conductivity. The continuity and the consistency with the
weak-relativistic limit described by the Pauli Hamiltonian is shown. For both
approaches, Dirac and Pauli, the Feynman diagrams, which lead to the
skew-scattering and the side-jump contributions, are underlined. In order to
illustrate this method, we apply it to a particular case: a ferromagnetic bulk
compound in the limit of weak-scattering and free-electrons approximation.
Explicit expressions for the anomalous Hall conductivity for both
skew-scattering and side-jump mechanisms are obtained. Within this model, the
recently predicted ''spin Hall effect'' appears naturally
Electronic transport through ordered and disordered graphene grain boundaries
The evolution of electronic wave packets (WPs) through grain boundaries (GBs) of various structures in graphene was investigated by the numerical solution of the time-dependent Schrödinger equation. WPs were injected from a simulated STM tip placed above one of the grains. Electronic structure of the GBs was calculated by ab-initio and tight-binding methods. Two main factors governing the energy dependence of the transport have been identified: the misorientation angle of the two adjacent graphene grains and the atomic structure of the GB. In case of an ordered GB made of a periodic repetition of pentagon-heptagon pairs, it was found that the transport at high and low energies is mainly determined by the misorientation angle, but the transport around the Fermi energy is correlated with the electronic structure of the GB. A particular line defect with zero misorientation angle Lahiri et al., behaves as a metallic nanowire and shows electron-hole asymmetry for hot electrons or holes. To generate disordered GBs, found experimentally in CVD graphene samples, a Monte-Carlo-like procedure has been developed. Results show a reduced transport for the disordered GBs, primarily attributed to electronic localized states caused by C atoms with only two covalent bonds. © 2013 Elsevier Ltd. All rights reserved
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