392 research outputs found
Strain-gradient-induced switching of nanoscale domains in free-standing ultrathin films
We report first-principle atomistic simulations on the effect of local strain
gradients on the nanoscale domain morphology of free-standing PbTiO
ultrathin films. First, the ferroelectric properties of free films at the
atomic level are reviewed. For the explored thicknesses (10 to 23 unit cells),
we find flux-closure domain structures whose morphology is thickness dependent.
A critical value of 20 unit cells is observed: thinner films show structures
with 90 domain loops, whereas thicker ones develop, in addition,
180 domain walls, giving rise to structures of the Landau-Lifshitz
type. When a local and compressive strain gradient at the top surface is
imposed, the gradient is able to switch the polarization of the downward
domains, but not to the opposite ones. The evolution of the domain pattern as a
function of the strain gradient strength consequently depends on the film
thickness. Our simulations indicate that in thinner films, first the 90
domain loops migrate towards the strain-gradient region, and then the
polarization in that zone is gradually switched. In thicker films, instead, the
switching in the strain-gradient region is progressive, not involving
domain-wall motion, which is attributed to less mobile 180 domain
walls. The ferroelectric switching is understood based on the knowledge of the
local atomic properties, and the results confirm that mechanical
flexoelectricity provides a means to control the nanodomain pattern in
ferroelectric systems.Comment: 9 pages, 6 figure
Janus II: a new generation application-driven computer for spin-system simulations
This paper describes the architecture, the development and the implementation
of Janus II, a new generation application-driven number cruncher optimized for
Monte Carlo simulations of spin systems (mainly spin glasses). This domain of
computational physics is a recognized grand challenge of high-performance
computing: the resources necessary to study in detail theoretical models that
can make contact with experimental data are by far beyond those available using
commodity computer systems. On the other hand, several specific features of the
associated algorithms suggest that unconventional computer architectures, which
can be implemented with available electronics technologies, may lead to order
of magnitude increases in performance, reducing to acceptable values on human
scales the time needed to carry out simulation campaigns that would take
centuries on commercially available machines. Janus II is one such machine,
recently developed and commissioned, that builds upon and improves on the
successful JANUS machine, which has been used for physics since 2008 and is
still in operation today. This paper describes in detail the motivations behind
the project, the computational requirements, the architecture and the
implementation of this new machine and compares its expected performances with
those of currently available commercial systems.Comment: 28 pages, 6 figure
Understanding the structure and reactivity of NiCu nanoparticles: An atomistic model
The structure of bimetallic NiCu nanoparticles (NP) is investigated as a function of their composition and size by means of Lattice MonteCarlo (LMC) and molecular dynamics (MD) simulations. According to our results, copper segregation takes place at any composition of the particles. We found that this feature is not size-dependent. In contrast, nickel segregation depends on the NP size. When the size increases, Ni atoms tend to remain in the vicinity of the surface and deeper. For smaller NPs, Ni atoms are located at the surface as well. Our results also showed that most of the metal atoms segregated at the surface area were found to decorate edges and/or form islands. Our findings agree qualitatively with the experimental data found in the literature. In addition, we comment on the reactivity of these nanoparticles.Fil: Quaino, Paola Monica. Universidad Nacional del Litoral. Instituto de QuĂmica Aplicada del Litoral. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Santa Fe. Instituto de QuĂmica Aplicada del Litoral.; Argentina. Universidad Nacional del Litoral. Facultad de IngenierĂa QuĂmica. Programa de ElectroquĂmica Aplicada e IngenierĂa ElectroquĂmica; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Belletti, Gustavo Daniel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad Nacional del Litoral. Instituto de QuĂmica Aplicada del Litoral. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - Santa Fe. Instituto de QuĂmica Aplicada del Litoral.; Argentina. Universidad Nacional del Litoral. Facultad de IngenierĂa QuĂmica. Programa de ElectroquĂmica Aplicada e IngenierĂa ElectroquĂmica; ArgentinaFil: Shermukhamedov, S. A.. Kazan National Research Technological University; RusiaFil: Glukhov, D. V.. Kazan National Research Technological University; RusiaFil: Santos, Elizabeth del Carmen. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico Conicet - CĂłrdoba. Instituto de FĂsica Enrique Gaviola. Universidad Nacional de CĂłrdoba. Instituto de FĂsica Enrique Gaviola; Argentina. Institute of Theoretical Chemistry; Alemania. Universitat Ulm; AlemaniaFil: Schmickler, Wolfgang. Universitat Ulm; Alemania. Institute of Theoretical Chemistry; AlemaniaFil: Nazmutdinov, Renat. Kazan National Research Technological University; Rusi
Effects of biochar addition on long-term behaviour of concrete and mortar
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Simulating spin systems on IANUS, an FPGA-based computer
We describe the hardwired implementation of algorithms for Monte Carlo
simulations of a large class of spin models. We have implemented these
algorithms as VHDL codes and we have mapped them onto a dedicated processor
based on a large FPGA device. The measured performance on one such processor is
comparable to O(100) carefully programmed high-end PCs: it turns out to be even
better for some selected spin models. We describe here codes that we are
currently executing on the IANUS massively parallel FPGA-based system.Comment: 19 pages, 8 figures; submitted to Computer Physics Communication
Biochar-based cement pastes and mortars with enhanced mechanical properties
Nowadays, the environmental impact of cementitious material industry and more generally of building activities is matter of concern, especially in terms of their effects on climate change and consumption of natural resources. Within this context, the aim of this paper is the investigation of the role of biochar, a solid carbonaceous by-product material resulting from biomass pyrolysis/gasification of residual biomass, as a sustainable ingredient for the production of cementitious materials, combining carbon sink properties with enhanced mechanical behavior. Although biochar is mainly investigated as agricultural amendment, there is also evidence that biochar may be an eco-friendly material to improve the sustainable performance of cementitious materials. As outlined in literature, biochar can be used as filler to modify the nanogranular nature of cement matrix, or as substitute of clinker to reduce the emissions of greenhouse gases related to cement production. In this work, biochar is added as micro-nano particles in different cementitious composites, i.e. cement pastes and mortars, as a function of filler or partial substitute of cement. The main mechanical properties of biochar-based materials are then investigated to determine the optimal percentage of biochar addition
Tethered Monte Carlo: computing the effective potential without critical slowing down
We present Tethered Monte Carlo, a simple, general purpose method of
computing the effective potential of the order parameter (Helmholtz free
energy). This formalism is based on a new statistical ensemble, closely related
to the micromagnetic one, but with an extended configuration space (through
Creutz-like demons). Canonical averages for arbitrary values of the external
magnetic field are computed without additional simulations. The method is put
to work in the two dimensional Ising model, where the existence of exact
results enables us to perform high precision checks. A rather peculiar feature
of our implementation, which employs a local Metropolis algorithm, is the total
absence, within errors, of critical slowing down for magnetic observables.
Indeed, high accuracy results are presented for lattices as large as L=1024.Comment: 32 pages, 8 eps figures. Corrected Eq. (36), which is wrong in the
published pape
Ianus: an Adpative FPGA Computer
Dedicated machines designed for specific computational algorithms can
outperform conventional computers by several orders of magnitude. In this note
we describe {\it Ianus}, a new generation FPGA based machine and its basic
features: hardware integration and wide reprogrammability. Our goal is to build
a machine that can fully exploit the performance potential of new generation
FPGA devices. We also plan a software platform which simplifies its
programming, in order to extend its intended range of application to a wide
class of interesting and computationally demanding problems. The decision to
develop a dedicated processor is a complex one, involving careful assessment of
its performance lead, during its expected lifetime, over traditional computers,
taking into account their performance increase, as predicted by Moore's law. We
discuss this point in detail
First language attrition and syntactic subjects: a study of Greek and Italian near-native speakers of English
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