412 research outputs found
Structure maps for hcp metals from first principles calculations
The ability to predict the existence and crystal type of ordered structures
of materials from their components is a major challenge of current materials
research. Empirical methods use experimental data to construct structure maps
and make predictions based on clustering of simple physical parameters. Their
usefulness depends on the availability of reliable data over the entire
parameter space. Recent development of high throughput methods opens the
possibility to enhance these empirical structure maps by {\it ab initio}
calculations in regions of the parameter space where the experimental evidence
is lacking or not well characterized. In this paper we construct enhanced maps
for the binary alloys of hcp metals, where the experimental data leaves large
regions of poorly characterized systems believed to be phase-separating. In
these enhanced maps, the clusters of non-compound forming systems are much
smaller than indicated by the empirical results alone.Comment: 7 pages, 4 figures, 1 tabl
Electronic transport properties of quasicrystals: a Review
We present a review of some results concerning electronic transport
properties of quasicrystals. After a short introduction to the basic concepts
of quasiperiodicity, we consider the experimental transport properties of
electrical conductivity with particular focus on the effect of temperature,
magnetic field and defects. Then, we present some heuristic approaches that
tend to give a coherent view of different, and to some extent complementary,
transport mechanisms in quasicrystals. Numerical results are also presented and
in particular the evaluation of the linear response Kubo-Greenwood formula of
conductivity in quasiperiodic systems in presence of disorder.Comment: Latex, 28 pages, Journ. of Math. Phys., Vol38 April 199
Thermodynamic properties of binary HCP solution phases from special quasirandom structures
Three different special quasirandom structures (SQS) of the substitutional
hcp binary random solutions (, 0.5, and 0.75) are
presented. These structures are able to mimic the most important pair and
multi-site correlation functions corresponding to perfectly random hcp
solutions at those compositions. Due to the relatively small size of the
generated structures, they can be used to calculate the properties of random
hcp alloys via first-principles methods. The structures are relaxed in order to
find their lowest energy configurations at each composition. In some cases, it
was found that full relaxation resulted in complete loss of their parental
symmetry as hcp so geometry optimizations in which no local relaxations are
allowed were also performed. In general, the first-principles results for the
seven binary systems (Cd-Mg, Mg-Zr, Al-Mg, Mo-Ru, Hf-Ti, Hf-Zr, and Ti-Zr) show
good agreement with both formation enthalpy and lattice parameters measurements
from experiments. It is concluded that the SQS's presented in this work can be
widely used to study the behavior of random hcp solutions.Comment: 15 pages, 8 figure
An empirical test for cellular automaton models of traffic flow
Based on a detailed microscopic test scenario motivated by recent empirical
studies of single-vehicle data, several cellular automaton models for traffic
flow are compared. We find three levels of agreement with the empirical data:
1) models that do not reproduce even qualitatively the most important empirical
observations,
2) models that are on a macroscopic level in reasonable agreement with the
empirics, and 3) models that reproduce the empirical data on a microscopic
level as well.
Our results are not only relevant for applications, but also shed new light
on the relevant interactions in traffic flow.Comment: 28 pages, 36 figures, accepted for publication in PR
Structure stability in the simple element sodium under pressure
The simple alkali metal Na, that crystallizes in a body-centred cubic
structure at ambient pressure, exhibits a wealth of complex phases at extreme
conditions as found by experimental studies. The analysis of the mechanism of
stabilization of some of these phases, namely, the low-temperature Sm-type
phase and the high-pressure cI16 and oP8 phases, shows that they satisfy the
criteria for the Hume-Rothery mechanism. These phases appear to be stabilized
due to a formation of numerous planes in a Brillouin-Jones zone in the vicinity
of the Fermi sphere of Na, which leads to the reduction of the overall
electronic energy. For the oP8 phase, this mechanism seems to be working if one
assumes that Na becomes divalent metal at this density. The oP8 phase of Na is
analysed in comparison with the MnP-type oP8 phases known in binary compounds,
as well as in relation to the hP4 structure of the NiAs-type
Predicting Crystal Structures with Data Mining of Quantum Calculations
Predicting and characterizing the crystal structure of materials is a key
problem in materials research and development. It is typically addressed with
highly accurate quantum mechanical computations on a small set of candidate
structures, or with empirical rules that have been extracted from a large
amount of experimental information, but have limited predictive power. In this
letter, we transfer the concept of heuristic rule extraction to a large library
of ab-initio calculated information, and demonstrate that this can be developed
into a tool for crystal structure prediction.Comment: 4 pages, 3 pic
The Structure of Barium in the hcp Phase Under High Pressure
Recent experimental results on two hcp phases of barium under high pressure
show interesting variation of the lattice parameters. They are here interpreted
in terms of electronic structure calculation by using the LMTO method and
generalized pseudopotential theory (GPT) with a NFE-TBB approach. In phase II
the dramatic drop in c/a is an instability analogous to that in the group II
metals but with the transfer of s to d electrons playing a crucial role in Ba.
Meanwhile in phase V, the instability decrease a lot due to the core repulsion
at very high pressure. PACS numbers: 62.50+p, 61.66Bi, 71.15.Ap, 71.15Hx,
71.15LaComment: 29 pages, 8 figure
Precision Study of Positronium: Testing Bound State QED Theory
As an unstable light pure leptonic system, positronium is a very specific
probe atom to test bound state QED. In contrast to ordinary QED for free
leptons, the bound state QED theory is not so well understood and bound state
approaches deserve highly accurate tests. We present a brief overview of
precision studies of positronium paying special attention to uncertainties of
theory as well as comparison of theory and experiment. We also consider in
detail advantages and disadvantages of positronium tests compared to other QED
experiments.Comment: A talk presented at Workshop on Positronium Physics (ETH Zurich, May
30-31, 2003
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