1,531 research outputs found
Self-driven lattice-model Monte Carlo simulations of alloy thermodynamic
Monte Carlo (MC) simulations of lattice models are a widely used way to
compute thermodynamic properties of substitutional alloys. A limitation to
their more widespread use is the difficulty of driving a MC simulation in order
to obtain the desired quantities. To address this problem, we have devised a
variety of high-level algorithms that serve as an interface between the user
and a traditional MC code. The user specifies the goals sought in a high-level
form that our algorithms convert into elementary tasks to be performed by a
standard MC code. For instance, our algorithms permit the determination of the
free energy of an alloy phase over its entire region of stability within a
specified accuracy, without requiring any user intervention during the
calculations. Our algorithms also enable the direct determination of
composition-temperature phase boundaries without requiring the calculation of
the whole free energy surface of the alloy system
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Carbides and Nitrides of Zirconium and Hafnium.
Among transition metal carbides and nitrides, zirconium, and hafnium compounds are the most stable and have the highest melting temperatures. Here we review published data on phases and phase equilibria in Hf-Zr-C-N-O system, from experiment and ab initio computations with focus on rocksalt Zr and Hf carbides and nitrides, their solid solutions and oxygen solubility limits. The systematic experimental studies on phase equilibria and thermodynamics were performed mainly 40-60 years ago, mostly for binary systems of Zr and Hf with C and N. Since then, synthesis of several oxynitrides was reported in the fluorite-derivative type of structures, of orthorhombic and cubic higher nitrides Zr3N4 and Hf3N4. An ever-increasing stream of data is provided by ab initio computations, and one of the testable predictions is that the rocksalt HfC0.75N0.22 phase would have the highest known melting temperature. Experimental data on melting temperatures of hafnium carbonitrides are absent, but minimum in heat capacity and maximum in hardness were reported for Hf(C,N) solid solutions. New methods, such as electrical pulse heating and laser melting, can fill the gaps in experimental data and validate ab initio predictions
What determines whether top public sector executives actually use performance information?
__Abstract__
Performance management has permeated public sector organizations worldwide over the last decades. At its core is the idea of using such information for decision making in a systematic form. Externally, performance information can be used to showcase performance, to give account, or to compare and benchmark. Internally, it can be used to monitor internal developments or to improve operations. A link between performance measurement and the use of this information in decision making is often assumed. Yet, until recently, the actual use of performance information was not very high on the public management research agenda. It is now a common observation that governments have invested substantially in collecting data, yet know relatively little about what drives performance information use
Internal and external use of performance information in public organisations: Results from an international executive survey
Abstract. This paper analyses determinants of public managers´ internal and external use of
performance information. Using a sample of over 3100 top public sector executives in six
European countries, we find evidence for significant country variations, with a more limited
use of performance information in France and Germany. It was also found that the use of
performance information is mainly determined by organizational factors rather than
managers’ individual socio-demographic characteristics. The analysis also found considerable differences in patterns of use between policy fields and a lower use of performance indicators in central government ministries. Finally, the implementation of performance management instruments in an organization has an overall strong effect on the actual use of performance information
Using bond-length dependent transferable force constants to predict vibrational entropies in Au-Cu, Au-Pd, and Cu-Pd alloys
A model is tested to rapidly evaluate the vibrational properties of alloys
with site disorder. It is shown that length-dependent transferable force
constants exist, and can be used to accurately predict the vibrational entropy
of substitutionally ordered and disordered structures in Au-Cu, Au-Pd, and
Cu-Pd. For each relevant force constant, a length- dependent function is
determined and fitted to force constants obtained from first-principles
pseudopotential calculations. We show that these transferable force constants
can accurately predict vibrational entropies of L1-ordered and disordered
phases in CuAu, AuPd, PdAu, CuPd, and PdAu. In
addition, we calculate the vibrational entropy difference between
L1-ordered and disordered phases of AuCu and CuPt.Comment: 9 pages, 6 figures, 3 table
Public management reforms and emerging trends and effects on social cohesion in Europe
__Abstract__
During the past thirty years or so, governments across Europe have grown ever more accustomed to developing initiatives to enhance the performance of key institutions and organizations. At the same time as being subject to the introduction of wide-ranging management reforms, many of those institutions and organizations are now increasingly charged with responding effectively to complex and intractable social problems. Amongst the so-called “wicked issues” public organizations are expected to address is the cohesiveness of the societies that they serve. In fact, European governments have implemented a swathe of initiatives designed to prompt public organizations to devote more energy to addressing the supposed centrifugal tendencies associated with globalisation and the breakdown of traditional social structures. In this context, the main objective of this report is to analyse emerging trends in social cohesion, beginning with socio-economic influences on social solidarity and social order in Europe before evaluating whether public management matters. To do so, the report draws upon primary and secondary research carried out in the COCOPS Work Packages 1-5, and blends this with analysis of Eurobarometer data and interviews with European policy-makers carried out specifically for Work Package 6. The findings of the report suggest first that social cohesion is largely determined by socio-economic disadvantage and that the economic strain associated with the financial crisis has also had a detrimental effect on the cohesiveness of European societies. At the same time, public management also matters, with some reforms and practices found to have beneficial effects for social solidarity and social order – though some others were found to have a negative effect. Since it seems that public management can make a positive difference to the cohesiveness of society, there is good reason for the countries of the EU to continue to invest in developing better policies aimed at promoting social cohesion
First Principles Phase Diagram Calculations for the Octahedral-Interstitial System ZrO,
First principles based phase diagram calculations were performed for the
octahedral-interstitial solid solution system \alpha ZrOX (\alpha Zr[
]_(1-X)OX; [ ]=Vacancy; 0 \leq X \leq 1/2). The cluster expansion method was
used to do a ground state analysis, and to calculate the phase diagram. The
predicted diagram has four ordered ground-states in the range 0 \leq X \leq
1/2, but one of these, at X=5/12, is predicted to disproportionate at T \approx
20K, well below the experimentally investigated range T \approx 420K. Thus, at
T \succeq 420K, the first-principles based calculation predicts three ordered
phases rather than the four that have been reported by experimentalists
Multiplets Matter: The Electronic Structure of Rare-Earth Semiconductors and Semimetals
We demonstrate that a theoretical framework fully incorporating intra-atomic
correlations and multiplet structure of the localized 4f states is required in
order to capture the essential physics of rare-earth semiconductors and
semimetals. We focus in particular on the rare-earth semimetal erbium arsenide
(ErAs), for which effective one-electron approaches fail to provide a
consistent picture of both high and low-energy electronic states. We treat the
many-body states of the Er 4f shell within an atomic approximation in the
framework of dynamical mean-field theory. Our results for the magnetic-field
dependence of the 4f local moment, the influence of multiplets on the
photoemission spectrum, and the exchange splitting of the Fermi surface pockets
as measured from Shubnikov-de Haas oscillations, are found to be in good
agreement with experimental results.Comment: 5 pages, 3 figures, 2 table
Physics and chemistry of hydrogen in the vacancies of semiconductors
Hydrogen is well known to cause electrical passivation of lattice vacancies in semiconductors. This effect follows from the chemical passivation of the dangling bonds. Recently it was found that H in the carbon vacancy of SiC forms a three-center bond with two silicon neighbors in the vacancy, and gives rise to a new electrically active state. In this paper we examine hydrogen in the anion vacancies of BN, AlN, and GaN. We find that three-center bonding of H is quite common and follows clear trends in terms of the second-neighbor distance in the lattice, the typical (two-center) hydrogen-host-atom bond length, the electronegativity difference between host atoms and hydrogen, as well as the charge state of the vacancy. Three-center bonding limits the number of H atoms a nitrogen vacancy can capture to two, and prevents electric passivation in GaAs as well
A first-principles approach to closing the "10-100 eV gap" for charge-carrier thermalization in semiconductors
The present work is concerned with studying accurately the energy-loss
processes that control the thermalization of hot electrons and holes that are
generated by high-energy radiation in wurtzite GaN, using an ab initio
approach. Current physical models of the nuclear/particle physics community
cover thermalization in the high-energy range (kinetic energies exceeding ~100
eV), and the electronic-device community has studied extensively carrier
transport in the low-energy range (below ~10 eV). However, the processes that
control the energy losses and thermalization of electrons and holes in the
intermediate energy range of about 10-100 eV (the "10-100 eV gap") are poorly
known. The aim of this research is to close this gap, by utilizing density
functional theory (DFT) to obtain the band structure and dielectric function of
GaN for energies up to about 100 eV. We also calculate charge-carrier
scattering rates for the major charge-carrier interactions (phonon scattering,
impact ionization, and plasmon emission), using the DFT results and first-order
perturbation theory. With this information, we study the thermalization of
electrons starting at 100 eV using the Monte Carlo method to solve the
semiclassical Boltzmann transport equation. Full thermalization of electrons
and holes is complete within ~1 and 0.5 ps, respectively. Hot electrons
dissipate about 90% of their initial kinetic energy to the electron-hole gas
(90 eV) during the first ~0.1 fs, due to rapid plasmon emission and impact
ionization at high energies. The remaining energy is lost more slowly as phonon
emission dominates at lower energies (below ~10 eV). During the thermalization,
hot electrons generate pairs with an average energy of ~8.9 eV/pair (11-12
pairs per hot electron). Additionally, during the thermalization, the maximum
electron displacement from its original position is found to be on the order of
100 nm.Comment: 23 pages, 20 figures. This LaTex file uses RevTex4.2 from AP
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