284 research outputs found
Density-density functionals and effective potentials in many-body electronic structure calculations
We demonstrate the existence of different density-density functionals
designed to retain selected properties of the many-body ground state in a
non-interacting solution starting from the standard density functional theory
ground state. We focus on diffusion quantum Monte Carlo applications that
require trial wave functions with optimal Fermion nodes. The theory is
extensible and can be used to understand current practices in several
electronic structure methods within a generalized density functional framework.
The theory justifies and stimulates the search of optimal empirical density
functionals and effective potentials for accurate calculations of the
properties of real materials, but also cautions on the limits of their
applicability. The concepts are tested and validated with a near-analytic
model.Comment: five figure
Novel properties of the Kohn-Sham exchange potential for open systems: application to the two-dimensional electron gas
The properties of the Kohn-Sham (KS) exchange potential for open systems in
thermodynamical equilibrium, where the number of particles is non-conserved,
are analyzed with the Optimized Effective Potential (OEP) method of Density
Functional Theory (DFT) at zero temperature. The quasi two-dimensional electron
gas (2DEG) is used as an illustrative example. The main findings are that the
KS exchange potential builds a significant barrier-like structure under slight
population of the second subband, and that both the asymptotic value of the KS
exchange potential and the inter-subband energy jump discontinuously at the
one-subband (1S) -> two-subband (2S) transition. The results obtained in this
system offer new insights on open problems of semiconductors, such as the
band-gap underestimation and the band-gap renormalization by photo-excited
carriers.Comment: 7 pages, 3 figures, uses epl.cls(included), accepted for publication
in Europhysics Letter
Exact-exchange density-functional theory for quasi-two-dimensional electron gases
A simple exact-exchange density-functional method for a quasi-two-dimensional
electron gas with variable density is presented. An analytical expression for
the exact-exchange potential with only one occupied subband is provided,
without approximations. When more subbands are occupied the exact-exchange
potential is obtained numerically. The theory shows that, in contradiction with
LDA, the exact-exchange potential exhibits discontinuities and the system
suffers a zero-temperature first-order transition each time a subband is
occupied. Results suggesting that the translational symmetry might be
spontaneously broken at zero temperature are presented. An extension of the
theory to finite temperatures allows to describe a drop in the intersubband
spacing in good quantitative agreement with recent experiments.Comment: 14 pages, 3 figure
Estudo da dimensão das partículas de café torrado moído encapsulado através da difração a laser: Influência nos parâmetros físico-químicos da bebida
A indústria de café em cápsulas tem registado em Portugal um crescimento
exponencial de vendas representando atualmente 60% do mercado. Este
crescimento traduziu-se na necessidade das empresas inovarem em técnicas de
otimização e reprodutibilidade dos parâmetros físico-químicos e organoléticos dos
produtos. A distribuição de dimensão das partículas do café torrado moído
influencia vários destes parâmetros de qualidade e constância dos produtos finais.
A análise granulométrica, efetuada por torres de crivos, revela-se insuficiente para
a caraterização eficaz dos produtos e análise do tamanho das partículas de café
torrado moído. A avaliação da distribuição através da técnica de difração a laser, é
uma técnica inovadora, só muito recentemente aplicada ao café, que foi aplicada
na Kaffa, Galvão & Noronha Lda. para otimizar a fase de moagem e a padronização
das caraterísticas físico-químicas e qualidade dos produtos finais. Foi comparada a
capacidade para analisar diferentes blends, diferentes linhas de produção e
moagens e a variabilidade dos produtos ao longo do tempo utilizando dois
equipamentos, uma torre de crivos e um analisador de dimensão de partículas, por
difração a laser Fritsch, com unidade de dispersão a álcool isopropílico. Foi
efetuado o estudo da reprodutibilidade dos resultados e a otimização dos
parâmetros de trabalho do analisador de partículas para caraterização e controlo de
qualidade do produto café moído conduzindo assim a melhorias no processo
produtivo e analítico.
A técnica de difração a laser revelou-se particularmente eficaz, sobretudo nas
moagens mais finas, permitindo separar as partículas de dimensão inferior a 250
μm, que ficam compactadas nos crivos, e obter curvas de distribuição de partículas
com grande reprodutibilidade, para uma definição mais rigorosa da especificação
técnica dos produtos. Neste trabalho verificou-se também a influência da moagem
na extração dos compostos do café. Paralelamente avaliaram-se as características
organoléticas e físico-químicas da bebida como o pH, o extrato aquoso e a
concentração de fenóis. Pretende-se relacionar este conhecimento ao estudo da
atividade antioxidante em curso, nos vários blends e estimar os benefícios para a
saúde.info:eu-repo/semantics/publishedVersio
Controlling the gap of fullerene microcrystals by applying pressure: the role of many-body effects
We studied theoretically the optical properties of C fullerene
microcrystals as a function of hydrostatic pressure with first-principles
many-body theories. Calculations of the electronic properties were done in the
GW approximation. We computed electronic excited states in the crystal by
diagonalizing the Bethe-Salpeter equation (BSE). Our results confirmed the
existence of bound excitons in the crystal. Both the electronic gap and optical
gap decrease continuously and non-linearly as pressure of up to 6 GPa is
applied. As a result, the absorption spectrum shows strong redshift. We also
obtained that "negative" pressure shows the opposite behavior: the gaps
increase and the optical spectrum shifts toward the blue end of the spectrum.
Negative pressure can be realized by adding cubane (CH) or other
molecules with similar size to the interstitials of the microcrystal. For the
moderate lattice distortions studied here, we found that the optical properties
of fullerene microcrystals with intercalated cubane are similar to the ones of
an expanded undoped microcrystal. Based on these findings, we propose doped C60
as active element in piezo-optical devices.Comment: Final version accepted by PRB. The review history is included in the
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Quantum Monte Carlo Assessment of the Relevance of Electronic Correlations in Defects and EOS in Metals
We have developed a highly accurate computational capability to calculate the equation of state (EOS) and defect formation energies of metallic systems. We are using a newly developed algorithm that enables the study of metallic systems with quantum Monte Carlo (QMC) methods. To date, technical limitations have restricted the application of QMC methods to semiconductors, insulators and the homogeneous electron gas. Using this new 'QMC for metals' we can determine, for the first time, the significance of correlation effects in the EOS and in the formation energies of point defects, impurities, surfaces and interfaces in metallic systems. These calculations go beyond the state-of-the-art accuracy which is currently obtained with Density Functional Theory approaches. Such benchmark calculations can provide more accurate predictions for the EOS and the formation energies of vacancies and interstitials in simple metals. These are important parameters in determining the mechanical properties as well as the micro-structural evolution of metals in irradiated materials or under extreme conditions. We describe the development of our 'QMC for metals' code, which has been adapted to run efficiently on a variety of computer architectures including BG/L. We present results of the first accurate quantum Monte Carlo calculation of an EOS of a realistic metallic system that goes beyond the homogeneous electron gas
Laser-wakefield accelerators for medical phase contrast imaging : Monte Carlo simulations and experimental studies
X-ray phase contrast imaging (X-PCi) is a very promising method of dramatically enhancing the contrast of X-ray images of microscopic weakly absorbing objects and soft tissue, which may lead to significant advancement in medical imaging with high-resolution and low-dose. The interest in X-PCi is giving rise to a demand for effective simulation methods. Monte Carlo codes have been proved a valuable tool for studying X-PCi including coherent effects. The laser-plasma wakefield accelerators (LWFA) is a very compact particle accelerator that uses plasma as an accelerating medium. Accelerating gradient in excess of 1 GV/cm can be obtained, which makes them over a thousand times more compact than conventional accelerators. LWFA are also sources of brilliant betatron radiation, which are promising for applications including medical imaging. We present a study that explores the potential of LWFA-based betatron sources for medical X-PCi and investigate its resolution limit using numerical simulations based on the FLUKA Monte Carlo code, and present preliminary experimental result
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