173 research outputs found
Conductance oscillations in metallic nanocontacts
We examine the conductance properties of a chain of Na atoms between two metallic leads in the limit of low bias. Resonant states corresponding to the conductance channel and the local charge neutrality condition cause conductance oscillations as a function of the number of atoms in the chain. Moreover, the geometrical shape of the contact leads influences the conductivity by giving rise to additional oscillations as a function of the lead opening angle.Peer reviewe
Ecological and genetic basis of metapopulation persistence of the Glanville fritillary butterfly in fragmented landscapes
Ecologists are challenged to construct models of the biological consequences of habitat loss and fragmentation. Here, we use a metapopulation model to predict the distribution of the Glanville fritillary butterfly during 22 years across a large heterogeneous landscape with 4,415 small dry meadows. The majority (74%) of the 125 networks into which the meadows were clustered are below the extinction threshold for long-term persistence. Among the 33 networks above the threshold, spatial configuration and habitat quality rather than the pooled habitat area predict metapopulation size and persistence, but additionally allelic variation in a SNP in the gene Phosphoglucose isomerase (Pgi) explains 30% of variation in metapopulation size. The Pgi genotypes are associated with dispersal rate and hence with colonizations and extinctions. Associations between Pgi genotypes, population turnover and metapopulation size reflect eco-evolutionary dynamics, which may be a common feature in species inhabiting patch networks with unstable local dynamics.Peer reviewe
Electronic resonance states in metallic nanowires during the breaking process simulated with the ultimate jellium model
We investigate the elongation and breaking process of metallic nanowires
using the ultimate jellium model in self-consistent density-functional
calculations of the electron structure. In this model the positive background
charge deforms to follow the electron density and the energy minimization
determines the shape of the system. However, we restrict the shape of the wires
by assuming rotational invariance about the wire axis. First we study the
stability of infinite wires and show that the quantum mechanical
shell-structure stabilizes the uniform cylindrical geometry at given magic
radii. Next, we focus on finite nanowires supported by leads modeled by
freezing the shape of a uniform wire outside the constriction volume. We
calculate the conductance during the elongation process using the adiabatic
approximation and the WKB transmission formula. We also observe the correlated
oscillations of the elongation force. In different stages of the elongation
process two kinds of electronic structures appear: one with extended states
throughout the wire and one with an atom-cluster like unit in the constriction
and with well localized states. We discuss the origin of these structures.Comment: 11 pages, 8 figure
Interplanetary magnetic structures guiding solar relativistic particles
Relating in situ measurements of relativistic solar particles to their parent
activity in the corona requires understanding the magnetic structures that
guide them from their acceleration site to the Earth. Relativistic particle
events are observed at times of high solar activity, when transient magnetic
structures such as Interplanetary Coronal Mass Ejections (ICMEs) often shape
the interplanetary magnetic field (IMF). They may introduce interplanetary
paths that are longer than nominal, and magnetic connections rooted far from
the nominal Parker spiral. We present a detailed study of the IMF
configurations during ten relativistic solar particle events of the 23rd
activity cycle to elucidate the actual IMF configuration guiding the particles
to Earth, where they are measured by neutron monitors. We use magnetic field
(MAG) and plasma parameter measurements (SWEPAM) from ACE, and determine
interplanetary path lengths of energetic particles through a modified version
of the velocity dispersion analysis based on energetic particle measurements
with SoHO/ERNE. We find that the majority (7/10) of the events is detected in
the vicinity of an ICME. Their interplanetary path lengths are found to be
longer (1.5-2.6 AU) than those of the two events propagating in the slow solar
wind (1.3 AU). The largest apparent path length is found in an event within the
fast solar wind, probably due to enhanced pitch angle scattering. The derived
path lengths imply that the first energetic and relativistic protons are
released at the Sun at the same time as electron beams emitting type III radio
bursts. The timing of the first high-energy particle arrival at Earth is
dominantly determined by the type of IMF in which the particles propagate.
Initial arrival times are as expected from Parker's model in the slow solar
wind, and significantly larger in or near transient structures such as ICMEs
A novel multigrid method for electronic structure calculations
A general real-space multigrid algorithm for the self-consistent solution of
the Kohn-Sham equations appearing in the state-of-the-art electronic-structure
calculations is described. The most important part of the method is the
multigrid solver for the Schroedinger equation. Our choice is the Rayleigh
quotient multigrid method (RQMG), which applies directly to the minimization of
the Rayleigh quotient on the finest level. Very coarse correction grids can be
used, because there is no need to be able to represent the states on the coarse
levels. The RQMG method is generalized for the simultaneous solution of all the
states of the system using a penalty functional to keep the states orthogonal.
The performance of the scheme is demonstrated by applying it in a few molecular
and solid-state systems described by non-local norm-conserving
pseudopotentials.Comment: 9 pages, 3 figure
Model study of adsorbed metallic quantum dots: Na on Cu(111)
We model electronic properties of the second monolayer Na adatom islands
(quantum dots) on the Cu(111) surface covered homogeneously by the first Na
monolayer. An axially-symmetric three-dimensional jellium model, taking into
account the effects due to the first Na monolayer and the Cu substrate, has
been developed. The electronic structure is solved within the local-density
approximation of the density-functional theory using a real-space multigrid
method. The model enables the study of systems consisting of thousands of
Na-atoms. The results for the local density of states are compared with
differential conductance () spectra and constant current topographs from
Scanning Tunneling Microscopy.Comment: 10 pages, 8 figures. For better quality figures, download
http://www.fyslab.hut.fi/~tto/cylart1.pd
Constructing identity through symbols by groups demanding self-determination: Bosnian Serbs and Iraqi Kurds
This contribution revisits the question over which much ink has been spilled in the study of national self-determination; who are the people? More specifically, the authors ask how national identity in self-determination claims is constructed. Drawing on observations from two case studies, they submit that cultural/ethnic definitions of national identity continue to underwrite self-determination claims. The authors argue that these practices have been central to the process of defining and reproducing the group identity on behalf of which the claim to political autonomy is made. The use of symbols and practices referring to territorially bound distinct nations with different linguistic and cultural features compared with other groups inhabiting the state reinforces the assertiveness of self-determination claims. Despite their differences, Bosnian Serbs and Iraqi Kurds typically follow similar trajectories in their use of ethnic, cultural and territorial symbols to reinstate the validity of their demands
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