87 research outputs found
Diffusion rates of Cu adatoms on Cu(111) in the presence of an adisland nucleated at FCC or HCP sites
The surface diffusion of Cu adatoms in the presence of an adisland at FCC or
HCP sites on Cu(111) is studied using the EAM potential derived by Mishin {\it
et al.} [Phys. Rev. B {\bf 63} 224106 (2001)]. The diffusion rates along
straight (with close-packed edges) steps with (100) and (111)-type microfacets
(resp. step A and step B) are first investigated using the transition state
theory in the harmonic approximation. It is found that the classical limit
beyond which the diffusion rates follow an Arrhenius law is reached above the
Debye temperature. The Vineyard attempt frequencies and the (static) energy
barriers are reported. Then a comparison is made with the results of more
realistic classical molecular dynamic simulations which also exhibit an
Arrhenius-like behavior. It is concluded that the corresponding energy barriers
are completely consistent with the static ones within the statistical errors
and that the diffusion barrier along step B is significantly larger than along
step A. In contrast the prefactors are very different from the Vineyard
frequencies. They increase with the static energy barrier in agreement with the
Meyer-Neldel compensation rule and this increase is well approximated by the
law proposed by Boisvert {\it et al.} [Phys. Rev. Lett. {\bf 75} 469 (1995)].
As a consequence, the remaining part of this work is devoted to the
determination of static energy barriers for a large number of diffusion events
that can occur in the presence of an adisland. In particular, it is found that
the corner crossing diffusion process for triangular adislands is markedly
different for the two types of borders (A or B). From this set of results the
diffusion rates of the most important atomic displacements can be predicted and
used as input in Kinetic Monte-Carlo simulations
Efficient and accurate three dimensional Poisson solver for surface problems
We present a method that gives highly accurate electrostatic potentials for
systems where we have periodic boundary conditions in two spatial directions
but free boundary conditions in the third direction. These boundary conditions
are needed for all kind of surface problems. Our method has an O(N log N)
computational cost, where N is the number of grid points, with a very small
prefactor. This Poisson solver is primarily intended for real space methods
where the charge density and the potential are given on a uniform grid.Comment: 6 pages, 2 figure
Pond acoustic sampling scheme: a draft protocol for rapid acoustic data collection in small waterbodies
1. Freshwater conservation is vital to the maintenance of global biodiversity. Ponds are a critical, yet often under-recognized, part of this, contributing to overall ecosystem functioning and diversity. They provide habitats for a range of aquatic, terrestrial, and amphibious life, often including rare and declining species.
2. Effective, rapid, and accessible survey methods are needed to enable evidence-based conservation action, but freshwater taxa are often viewed as “difficult”—and few specialist surveyors are available. Datasets on ponds are therefore limited in their spatiotemporal coverage.
3. With the advent of new recording technologies, acoustic survey methods are becoming increasingly available to researchers, citizen scientists, and conservation practitioners. They can be an effective and noninvasive approach for gathering data on target species, assemblages, and environmental variables. However, freshwater applications are lagging behind those in terrestrial and marine spheres, and as an emergent method, research studies have employed a multitude of different sampling protocols.
4. We propose the Pond Acoustic Sampling Scheme (PASS), a simple protocol to allow a standardized minimal sample to be collected rapidly from small waterbodies, alongside environmental and methodological metadata. This sampling scheme can be incorporated into a variety of survey designs and is intended to allow access to a wide range of participants, without requiring complicated or prohibitively expensive equipment.
5. Adoption of this sampling protocol would enable consistent sound recordings to be gathered by researchers and conservation organizations, and allow the development of landscape-scale surveys, data sharing, and collaboration within an expanding freshwater ecoacoustic community—rather than individual approaches that produce incompatible datasets. The compilation of standardized data would improve the prospects for effective research into the soundscapes of small waterbodies and aid freshwater conservation efforts
Orbital contribution to the magnetic properties of nanowires: Is the orbital polarization ansatz justified?
We show that considerable orbital magnetic moments and magneto-crystalline
anisotropy energies are obtained for a Fe monatomic wire described in a
tight-binding method with intra-atomic electronic interactions treated in a
full Hartree Fock (HF) decoupling scheme. Even-though the use of the orbital
polarization ansatz with simplified Hamiltonians leads to fairly good results
when the spin magnetization is saturated this is not the case of unsaturated
systems. We conclude that the full HF scheme is necessary to investigate low
dimensional systems
Analysis of Clean Transition Metal Surfaces by Core Level Spectroscopy
The shifts in the binding energy of core electrons detected by high resolution X-ray photoelectron spectroscopy are a very sensitive probe of the chemical bonding of the excited atom. Since the surface atoms have their geometrical environment perturbed, their core levels are shifted from their bulk positions. A very large number of experiments have been performed on the 4f core level positions of tantalum and tungsten for various orientations of the surface plane. Systematic trends have been put forward and explained by theoretical models. Furthermore, the analysis of the angular variation of the core level line intensities gives structural information when compared with theoretical calculations. In the case of W(100) a single scattering theory is sufficient to reproduce experimental data. Finally we show that, in some particular cases, the core level lineshapes may differ strongly from a Doniach Sunjic model. The temperature dependence of their widths due to core hole-phonon coupling can be reproduced within the independent boson theory
Temperature dependence of the magnetic Casimir-Polder interaction
We analyze the magnetic dipole contribution to atom-surface dispersion
forces. Unlike its electrical counterpart, it involves small transition
frequencies that are comparable to thermal energy scales. A significant
temperature dependence is found near surfaces with a nonzero DC conductivity,
leading to a strong suppression of the dispersion force at T > 0. We use
thermal response theory for the surface material and discuss both normal metals
and superconductors. The asymptotes of the free energy of interaction and of
the entropy are calculated analytically over a large range of distances. Near a
superconductor, the onset of dissipation at the phase transition strongly
changes the interaction, including a discontinuous entropy. We discuss the
similarities with the Casimir interaction beween two surfaces and suggest that
precision measurements of the atom-surface interaction may shed new light upon
open questions around the temperature dependence of dispersion forces between
lossy media.Comment: 11 figure
Lifetime of the first and second collective excitations in metallic nanoparticles
We determine the lifetime of the surface plasmon in metallic nanoparticles
under various conditions, concentrating on the Landau damping, which is the
dominant mechanism for intermediate-size particles. Besides the main
contribution to the lifetime, which smoothly increases with the size of the
particle, our semiclassical evaluation yields an additional oscillating
component. For the case of noble metal particles embedded in a dielectric
medium, it is crucial to consider the details of the electronic confinement; we
show that in this case the lifetime is determined by the shape of the
self-consistent potential near the surface. Strong enough perturbations may
lead to the second collective excitation of the electronic system. We study its
lifetime, which is limited by two decay channels: Landau damping and
ionization. We determine the size dependence of both contributions and show
that the second collective excitation remains as a well defined resonance.Comment: 18 pages, 5 figures; few minor change
Broken-Bond Rule for the Surface Energies of Noble Metals
Using two different full-potential ab-initio techniques we introduce a
simple, universal rule based on the number of broken first-neighbor bonds to
determine the surface energies of the three noble metals Cu, Ag and Au. When a
bond is broken, the rearrangement of the electronic charge for these metals
does not lead to a change of the remaining bonds. Thus the energy needed to
break a bond is independent of the surface orientation. This novel finding can
lead to the development of simple models to describe the energetics of a
surface like step and kink formation, crystal growth, alloy formation,
equilibrium shape of mesoscopic crystallites and surface faceting.Comment: 4 pages, 2 figure
Giant orbital moments are responsible for the anisotropic magnetoresistance of atomic contacts
We study here, both experimentally and theoretically, the anisotropy of
magnetoresistance in atomic contacts. Our measurements on iron break junctions
reveal an abrupt and hysteretic switch between two conductance levels when a
large applied field is continuously rotated. We show that this behaviour stems
from the coexistence of two metastable electronic states which result from the
anisotropy of electronic interactions responsible for the enhancement of
orbital magnetization. In both states giant orbital moments appear on the low
coordinated central atom in a realistic contact geometry. However they differ
by their orientation, parallel or perpendicular, with respect to the axis of
the contact. Our explanation is totally at variance with the usual model based
on the band structure of a monatomic linear chain, which we argue cannot be
applied to 3d ferromagnetic metals
- …