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$_{60}$ 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 (C$_8$H$_8$) 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 sourc

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