NON-LOCAL ELECTRON-POSITRON ENHANCEMENT FACTORS IN SOLIDS

Non-local electron-positron correlation effects in solids are studied. The weighted density approximation is applied to calculations of the non-local electron-positron correlation functions. The calculated weighted density approximation electron-positron enhancement factors for the core electrons are compared with those obtained within the local density approximation. Also, differences in the electron-positron enhancement factors due to the s, p, d and f angular momentum channels of the electron charge density are studied. The formalism is applied to ab initio calculations of positron lifetimes in a variety of metals and silicon. The influence of various approximations to the electron-positron interaction on the positron lifetimes is also presented. The weighted density approximation results are compared to those calculated within the local density approximation, the recent generalized gradient approximation and with experimental data. PACS numbers: 78.70. Bj, The positron lifetime, τ, is an important characteristic of electronic properties of solids. This parameter provides information on the electron density distribution in the host material, thus yielding also useful information on defects in metals and semiconductors. The positron annihilation rate, A = 1/r, is calculated as where nei(re ) is the electron density in the host material, φ+(r p ) is the wave function of a thermalised positron and g(re , rp ) denotes the correlation function of the positron at rp and electrons at re , and r0 and c are the classical electron radius and velocity of light, respectively

Many-body aspects of positron annihilation in the electron gas

We investigate positron annihilation in electron liquid as a case study for many-body theory, in particular the optimized Fermi Hypernetted Chain (FHNC-EL) method. We examine several approximation schemes and show that one has to go up to the most sophisticated implementation of the theory available at the moment in order to get annihilation rates that agree reasonably well with experimental data. Even though there is basically just one number to look at, the electron-positron pair distribution function at zero distance, it is exactly this number that dictates how the full pair distribution behaves: In most cases, it falls off monotonously towards unity as the distance increases. Cases where the electron-positron pair distribution exhibits a dip are precursors to the formation of bound electron--positron pairs. The formation of electron-positron pairs is indicated by a divergence of the FHNC-EL equations, from this we can estimate the density regime where positrons must be localized. This occurs in our calculations in the range 9.4 <= r_s <=10, where r_s is the dimensionless density parameter of the electron liquid.Comment: To appear in Phys. Rev. B (2003

Forecasting: theory and practice

Forecasting has always been at the forefront of decision making and planning. The uncertainty that surrounds the future is both exciting and challenging, with individuals and organisations seeking to minimise risks and maximise utilities. The large number of forecasting applications calls for a diverse set of forecasting methods to tackle real-life challenges. This article provides a non-systematic review of the theory and the practice of forecasting. We provide an overview of a wide range of theoretical, state-of-the-art models, methods, principles, and approaches to prepare, produce, organise, and evaluate forecasts. We then demonstrate how such theoretical concepts are applied in a variety of real-life contexts. We do not claim that this review is an exhaustive list of methods and applications. However, we wish that our encyclopedic presentation will offer a point of reference for the rich work that has been undertaken over the last decades, with some key insights for the future of forecasting theory and practice. Given its encyclopedic nature, the intended mode of reading is non-linear. We offer cross-references to allow the readers to navigate through the various topics. We complement the theoretical concepts and applications covered by large lists of free or open-source software implementations and publicly-available databases

Forecasting: theory and practice

Forecasting has always been in the forefront of decision making and planning. The uncertainty that surrounds the future is both exciting and challenging, with individuals and organisations seeking to minimise risks and maximise utilities. The lack of a free-lunch theorem implies the need for a diverse set of forecasting methods to tackle an array of applications. This unique article provides a non-systematic review of the theory and the practice of forecasting. We offer a wide range of theoretical, state-of-the-art models, methods, principles, and approaches to prepare, produce, organise, and evaluate forecasts. We then demonstrate how such theoretical concepts are applied in a variety of real-life contexts, including operations, economics, finance, energy, environment, and social good. We do not claim that this review is an exhaustive list of methods and applications. The list was compiled based on the expertise and interests of the authors. However, we wish that our encyclopedic presentation will offer a point of reference for the rich work that has been undertaken over the last decades, with some key insights for the future of the forecasting theory and practice

Data for: The role of inventories in the dynamics of the natural gas market

The data used in the article entitled &quot;The role of inventories in the dynamics of the natural gas market&quot; submitted to Energy Economics

Data for: The role of inventories in the dynamics of the natural gas market

The data used in the article entitled &quot;The role of inventories in the dynamics of the natural gas market&quot; submitted to Energy Economics.THIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV

Influence of Light and Shadow on the Topography and Physiognomy of the Landscape

Relacje przestrzenne pomiędzy człowiekiem a krajobrazem są szczególnie ważne w odniesieniu do odbioru wizualnego. Światło i cień to elementy, które budują obraz przestrzeni po zmroku. Analiza struktury topograficznej i fizjonomicznej terenu warunkuje przyjęcie indywidualnego kierunku pracy nad projektem dla konkretnego miejsca. Oświetlenie nie może prowadzić do uniformizacji, dlatego niezbędne jest za każdym razem dokonanie analizy pod względem struktury topograficznej – zdefiniowanie typu krajobrazu oraz fizjonomicznej – określenie roli i funkcji elementów tworzących pokrycie i kompozycję terenu. Dokonanie wymienionego zakresu badań umożliwia przyjęcie właściwego kierunku nocnej narracji krajobrazu. Często umiar, nawiązanie do szlachetniej prostoty i powściągliwość w kształtowaniu przestrzeni światłem mogą być najlepszym sposobem kreacji nocy. Dowodzą tego werdykty sędziów na międzynarodowych konkursach nagradzających przede wszystkim za wrażliwość na środowisko, projektowanie zrównoważone i sztukę zmieniającą wizerunek miejsca

Przestrzenne działania tymczasowe jako narzędzia kreacji miejsca

W artykule podjęto zagadnienie działań tymczasowych jako elementu kreacji miejsca. Koncepcje działań tymczasowych w formie instalacji umiejscowiono na placu Nowy Targ we Wrocławiu, którego przestrzeń, zdaniem autorek, z wielu powodów może być sceną dla efemerycznych form zagospodarowania. Zaproponowane instalacje wpisują się w nurt obserwowany w wielu miejscach świata. Cechują je tymczasowość, zmienność wyrazu, minimalne koszty wykonania, współdziałanie z użytkownikami na etapie powstawania. Jako działania na styku architektury, sztuki i kultury mają na celu wizualną zmianę danego obszaru, przekazanie idei, wzbudzenie zaciekawienia miejscowych i przyjezdnych, wywołanie emocji oraz ułatwienie interakcji społecznych, a tym samym tworzenie i aktywizację miejsca

Electron-Positron Correlations in Paramagnetic Chromium

Several approximations for the electron-positron interaction are applied to study the electron-positron momentum densities of paramagnetic chromium. The non-local and state selective electron-positron correlation functions, evaluated within the weighted density approximation, are used to calculate various positron annihilation characteristics in this system. The weighted density approximation results are compared to the experimental data, and to the corresponding results of the independent particle model, local density approximation, and generalised gradient approximation. Also, the semi-empirical results are used in gaining more physical insight and deriving final conclusions of this work