Quantum Monte Carlo study of a positron in an electron gas

Quantum Monte Carlo calculations of the relaxation energy, pair-correlation function, and annihilating-pair momentum density are presented for a positron immersed in a homogeneous electron gas. We find smaller relaxation energies and contact pair-correlation functions in the important low-density regime than predicted by earlier studies. Our annihilating-pair momentum densities have almost zero weight above the Fermi momentum due to the cancellation of electron-electron and electron-positron correlation effects

An efficient k.p method for calculation of total energy and electronic density of states

An efficient method for calculating the electronic structure in large systems with a fully converged BZ sampling is presented. The method is based on a k.p-like approximation developed in the framework of the density functional perturbation theory. The reliability and efficiency of the method are demostrated in test calculations on Ar and Si supercells

Ab initio Random Structure Searching

It is essential to know the arrangement of the atoms in a material in order to compute and understand its properties. Searching for stable structures of materials using first-principles electronic structure methods, such as density functional theory (DFT), is a rapidly growing field. Here we describe our simple, elegant and powerful approach to searching for structures with DFT which we call ab initio random structure searching (AIRSS). Applications to discovering structures of solids, point defects, surfaces, and clusters are reviewed. New results for iron clusters on graphene, silicon clusters, polymeric nitrogen, hydrogen-rich lithium hydrides, and boron are presented.Comment: 44 pages, 23 figure

Electronic structure of superconducting graphite intercalate compounds: The role of the interlayer state

Although not an intrinsic superconductor, it has been long--known that, when intercalated with certain dopants, graphite is capable of exhibiting superconductivity. Of the family of graphite--based materials which are known to superconduct, perhaps the most well--studied are the alkali metal--graphite intercalation compounds (GIC) and, of these, the most easily fabricated is the C${}_8$K system which exhibits a transition temperature $\bm{T_c\simeq 0.14}$K. By increasing the alkali metal concentration (through high pressure fabrication techniques), the transition temperature has been shown to increase to as much as $\bm 5$K in C${}_2$Na. Lately, in an important recent development, Weller \emph{et al.} have shown that, at ambient conditions, the intercalated compounds \cyb and \cca exhibit superconductivity with transition temperatures $\bm{T_c\simeq 6.5}$K and $\bm{11.5}$K respectively, in excess of that presently reported for other graphite--based compounds. We explore the architecture of the states near the Fermi level and identify characteristics of the electronic band structure generic to GICs. As expected, we find that charge transfer from the intercalant atoms to the graphene sheets results in the occupation of the $\bm\pi$--bands. Yet, remarkably, in all those -- and only those -- compounds that superconduct, we find that an interlayer state, which is well separated from the carbon sheets, also becomes occupied. We show that the energy of the interlayer band is controlled by a combination of its occupancy and the separation between the carbon layers.Comment: 4 Figures. Please see accompanying experimental manuscript "Superconductivity in the Intercalated Graphite Compounds C6Yb and C6Ca" by Weller et a

The Rise and Fall of American Queensware 1807-1822

. This article examines the history of several manufacturers of American queensware in Philadelphia, Pennsylvania, and beyond. Our research reveals that efforts to produce queensware were more extensive and widespread than previously thought. This survey expanded as we discovered references to contemporary queensware potteries in other parts of the United States during the first two decades of the 19th century. In all, 14 queensware-manufacturing ventures are identified and described from Pennsylvania, New Jersey, Ohio, what is now West Virginia, Vermont, and New Hampshire. Much of this research is drawn from period newspaper notices, advertisements, and surviving personal correspondence. The period sources provide a view of the experimental nature of this industry, document the search for raw materials, and describe various aspects of the manufacturing process

Domestic Queensware in Kensington-Fishtown: Excavating Philadelphia\u27s Waterfront Neighborhoods

Ongoing archaeological excavation undertaken by URS/AECOM along the I-95 corridor in Kensington-Fishtown in Philadelphia have brought to light 18th and 19th century domestic and industrial life along a three-mile section of the Delaware River waterfront. Excavation has revealed over 400 shaft features, yard deposits, and industrial foundations yielding over one million artifacts from a three mile section of the Delaware River waterfront. A small quantity of domestic queensware has been recovered from barrel and wood-lined box privies and from an early 19th century drain feature. The recovery of domestic queensware in Kensington-Fishtown has revealed that this ware had become part of the domestic fabric of early 19th century consumers in this part of the city

Electrostatic force driven helium insertion into ammonia and water crystals under pressure

AbstractHelium, ammonia and ice are among the major components of giant gas planets, and predictions of their chemical structures are therefore crucial in predicting planetary dynamics. Here we demonstrate a strong driving force originating from the alternation of the electrostatic interactions for helium to react with crystals of polar molecules such as ammonia and ice. We show that ammonia and helium can form thermodynamically stable compounds above 45 GPa, while ice and helium can form thermodynamically stable compounds above 300 GPa. The changes in the electrostatic interactions provide the driving force for helium insertion under high pressure, but the mechanism is very different to those that occur in ammonia and ice. This work extends the reactivity of helium into new types of compounds and demonstrates the richness of the chemistry of this most stable element in the periodic table.</jats:p