2,894 research outputs found
Density functional theory for the freezing of soft-core fluids
We present a simple density functional theory for the solid phases of systems
of particles interacting via soft-core potentials. In particular, we apply the
theory to particles interacting via repulsive point Yukawa and Gaussian pair
potentials. We find qualitative agreement with the established phase diagrams
for these systems. The theory is able to account for the bcc-fcc solid
transitions of both systems and the re-entrant melting that the Gaussian system
exhibits.Comment: 7 pages, 4 figure
Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part II: The intrinsic electronic midgap states
We propose a structural model that treats in a unified fashion both the
atomic motions and electronic excitations in quenched melts of pnictide and
chalcogenide semiconductors. In Part I (submitted to J. Chem. Phys.), we argued
these quenched melts represent aperiodic -networks that are highly
stable and, at the same time, structurally degenerate. These networks are
characterized by a continuous range of coordination. Here we present a
systematic way to classify these types of coordination in terms of discrete
coordination defects in a parent structure defined on a simple cubic lattice.
We identify the lowest energy coordination defects with the intrinsic midgap
electronic states in semiconductor glasses, which were argued earlier to cause
many of the unique optoelectronic anomalies in these materials. In addition,
these coordination defects are mobile and correspond to the transition state
configurations during the activated transport above the glass transition. The
presence of the coordination defects may account for the puzzling discrepancy
between the kinetic and thermodynamic fragility in chalcogenides. Finally, the
proposed model recovers as limiting cases several popular types of bonding
patterns proposed earlier, including: valence-alternation pairs, hypervalent
configurations, and homopolar bonds in heteropolar compounds.Comment: 17 pages, 15 figures, revised version, final version to appear in J.
Chem. Phy
Density functional theory of vortex lattice melting in layered superconductors: a mean-field--substrate approach
We study the melting of the pancake vortex lattice in a layered
superconductor in the limit of vanishing Josephson coupling. Our approach
combines the methodology of a recently proposed mean-field substrate model for
such systems with the classical density functional theory of freezing. We
derive a free-energy functional in terms of a scalar order-parameter profile
and use it to derive a simple formula describing the temperature dependence of
the melting field. Our theoretical predictions are in good agreement with
simulation data. The theoretical framework proposed is thermodynamically
consistent and thus capable of describing the negative magnetization jump
obtained in experiments. Such consistency is demonstrated by showing the
equivalence of our expression for the density discontinuity at the transition
with the corresponding Clausius-Clapeyron relation.Comment: 11 pages, 4 figure
Why do ultrasoft repulsive particles cluster and crystallize? Analytical results from density functional theory
We demonstrate the accuracy of the hypernetted chain closure and of the
mean-field approximation for the calculation of the fluid-state properties of
systems interacting by means of bounded and positive-definite pair potentials
with oscillating Fourier transforms. Subsequently, we prove the validity of a
bilinear, random-phase density functional for arbitrary inhomogeneous phases of
the same systems. On the basis of this functional, we calculate analytically
the freezing parameters of the latter. We demonstrate explicitly that the
stable crystals feature a lattice constant that is independent of density and
whose value is dictated by the position of the negative minimum of the Fourier
transform of the pair potential. This property is equivalent with the existence
of clusters, whose population scales proportionally to the density. We
establish that regardless of the form of the interaction potential and of the
location on the freezing line, all cluster crystals have a universal Lindemann
ratio L = 0.189 at freezing. We further make an explicit link between the
aforementioned density functional and the harmonic theory of crystals. This
allows us to establish an equivalence between the emergence of clusters and the
existence of negative Fourier components of the interaction potential. Finally,
we make a connection between the class of models at hand and the system of
infinite-dimensional hard spheres, when the limits of interaction steepness and
space dimension are both taken to infinity in a particularly described fashion.Comment: 19 pages, 5 figures, submitted to J. Chem. Phys; new version: minor
changes in structure of pape
Analysis of Dislocation Mechanism for Melting of Elements: Pressure Dependence
In the framework of melting as a dislocation-mediated phase transition we
derive an equation for the pressure dependence of the melting temperatures of
the elements valid up to pressures of order their ambient bulk moduli. Melting
curves are calculated for Al, Mg, Ni, Pb, the iron group (Fe, Ru, Os), the
chromium group (Cr, Mo, W), the copper group (Cu, Ag, Au), noble gases (Ne, Ar,
Kr, Xe, Rn), and six actinides (Am, Cm, Np, Pa, Th, U). These calculated
melting curves are in good agreement with existing data. We also discuss the
apparent equivalence of our melting relation and the Lindemann criterion, and
the lack of the rigorous proof of their equivalence. We show that the would-be
mathematical equivalence of both formulas must manifest itself in a new
relation between the Gr\"{u}neisen constant, bulk and shear moduli, and the
pressure derivative of the shear modulus.Comment: 19 pages, LaTeX, 9 eps figure
A wavelet based numerical method for nonlinear partial differential equations
The purpose of this paper is to present a wavelet–Galerkin scheme for solving
nonlinear elliptic partial differential equations. We select as trial spaces a nested
sequence of spaces from an appropriate biorthogonal multiscale analysis. This gives
rise to a nonlinear discretized system. To overcome the problems of nonlinearity, we
apply the machinery of interpolating wavelets to obtain knot oriented quadrature
rules. Finally, Newton’s method is applied to approximate the solution in the given
ansatz space. The results of some numerical experiments with different biorthogonal
systems, confirming the applicability of our scheme, are presented.Instituto de Cooperação Científica e Tecnológica Internacional - Acções Integradas Luso-Alemãs (DAAD/ICCTI) - Projecto DAAD/ICCTI nº 01141
Universities and community-based research in developing countries: community voice and educational provision in rural Tanzania
The main focus of recent research on the community engagement role of universities has been in developed countries, generally in towns and cities and usually conducted from the perspectives of universities rather than the communities with which they engage. The purpose of this paper is to investigate the community engagement role of universities in the rural areas of developing countries, and its potential for strengthening the voice of rural communities. The particular focus is on the provision of primary and secondary education. The paper is based on the assumption that in order for community members to have both the capacity and the confidence to engage in political discourse for improving educational capacity and quality, they need the opportunity to become involved and well-versed in the options available, beyond their own experience. Particular attention is given in the paper to community-based research (CBR). CBR is explored from the perspectives of community members and local leaders in the government-community partnerships which have responsibility for the provision of primary and secondary education in rural Tanzania. The historical and policy background of the partnerships, together with findings from two case studies, provide the context for the paper
Chemical bath deposition of textured and compact zinc oxide thin films on vinyl-terminated polystyrene brushes
In this study we investigated the influence of an organic polystyrene brush on the deposition of ZnO thin films under moderate conditions. On a non-modified SiOx surface, island growth is observed, whereas the polymer brush induces homogeneous film growth. A chemical modification of the polystyrene brushes during the mineralization process occurs, which enables stronger interaction between the then polar template and polar ZnO crystallites in solution. This may lead to oriented attachment of the crystallites so that the observed (002) texture arises. Characterization of the templates and the resulting ZnO films were performed with ζ-potential and contact angle measurements as well as scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). Infrared spectroscopy (IR) measurements were used to investigate the polystyrene brushes before and after modification
Theoretical study of the thermal behavior of free and alumina-supported Fe-C nanoparticles
The thermal behavior of free and alumina-supported iron-carbon nanoparticles
is investigated via molecular dynamics simulations, in which the effect of the
substrate is treated with a simple Morse potential fitted to ab initio data. We
observe that the presence of the substrate raises the melting temperature of
medium and large nanoparticles ( = 0-0.16, = 80-1000, non-
magic numbers) by 40-60 K; it also plays an important role in defining the
ground state of smaller Fe nanoparticles ( = 50-80). The main focus of our
study is the investigation of Fe-C phase diagrams as a function of the
nanoparticle size. We find that as the cluster size decreases in the
1.1-1.6-nm-diameter range the eutectic point shifts significantly not only
toward lower temperatures, as expected from the Gibbs-Thomson law, but also
toward lower concentrations of C. The strong dependence of the maximum C
solubility on the Fe-C cluster size may have important implications for the
catalytic growth of carbon nanotubes by chemical vapor deposition.Comment: 13 pages, 11 figures, higher quality figures can be seen in article 9
at http://alpha.mems.duke.edu/wahyu
Status and Plans for the Array Control and Data Acquisition System of the Cherenkov Telescope Array
The Cherenkov Telescope Array (CTA) is the next-generation atmospheric
Cherenkov gamma-ray observatory. CTA will consist of two installations, one in
the northern, and the other in the southern hemisphere, containing tens of
telescopes of different sizes. The CTA performance requirements and the
inherent complexity associated with the operation, control and monitoring of
such a large distributed multi-telescope array leads to new challenges in the
field of the gamma-ray astronomy. The ACTL (array control and data acquisition)
system will consist of the hardware and software that is necessary to control
and monitor the CTA arrays, as well as to time-stamp, read-out, filter and
store -at aggregated rates of few GB/s- the scientific data. The ACTL system
must be flexible enough to permit the simultaneous automatic operation of
multiple sub-arrays of telescopes with a minimum personnel effort on site. One
of the challenges of the system is to provide a reliable integration of the
control of a large and heterogeneous set of devices. Moreover, the system is
required to be ready to adapt the observation schedule, on timescales of a few
tens of seconds, to account for changing environmental conditions or to
prioritize incoming scientific alerts from time-critical transient phenomena
such as gamma ray bursts. This contribution provides a summary of the main
design choices and plans for building the ACTL system.Comment: In Proceedings of the 34th International Cosmic Ray Conference
(ICRC2015), The Hague, The Netherlands. All CTA contributions at
arXiv:1508.0589
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