We present a new scheme to extract numerically optimal interatomic potentials from large amounts of data produced by first-principles calculations. The method is based on fitting the potential to ab initio atomic forces of many atomic configurations, including surfaces, clusters, liquids and crystals at finite temperature. The extensive data set overcomes the difficulties encountered by traditional fitting approaches when using rich and complex analytic forms, allowing to construct potentials with a degree of accuracy comparable to that obtained by ab initio methods. A glue potential for aluminium obtained with this method is presented and discussed.

https://iopscience.iop.org/article/10.1209/0295-5075/26/8/005/pdf

Interatomic Potentials from First-Principles Calculations: The Force-Matching Method

Quantum impurity models describe an atom or molecule embedded in a host material with which it can exchange electrons. They are basic to nanoscience as representations of quantum dots and molecular conductors and play an increasingly important role in the theory of "correlated electron" materials as auxiliary problems whose solution gives the "dynamical mean field" approximation to the self energy and local correlation functions. These applications require a method of solution which provides access to both high and low energy scales and is effective for wide classes of physically realistic models. The continuous-time quantum Monte Carlo algorithms reviewed in this article meet this challenge. We present derivations and descriptions of the algorithms in enough detail to allow other workers to write their own implementations, discuss the strengths and weaknesses of the methods, summarize the problems to which the new methods have been successfully applied and outline prospects for future applications.

Continuous-time Monte Carlo methods for quantum impurity models

A new, versatile method of calculating positron states in solids is described. The main purpose is to develop a practical scheme accurate enough to reliably predict annihilation characteristics for positrons trapped at defect clusters of varying size, geometry and impurity environment, in order to help analyse experimental data. The positron potential is constructed from the full lattice electrostatic potential, and a local-density approximation for the correlation potential. The three-dimensional positron Schrodinger equation is solved by finite-element techniques. Enhancement effects in the annihilation rate are parametrised in a simple way, both for simple and transition metals. The applications discussed include vacancies, vacancy clusters and impurity-vacancy complexes in a number of metals. Good agreement with experiment in cases where comparison is possible suggests a good predictive power for the scheme.

https://iopscience.iop.org/article/10.1088/0305-4608/13/2/009/pdf

Defect spectroscopy with positrons: a general calculational method

We implemented the charge self-consistent combination of density-functional theory and dynamical mean-field theory (DMFT) in two full-potential methods, the augmented plane-wave and the linear muffin-tin orbital methods. We categorize the commonly used projection methods in terms of the causality of the resulting DMFT equations and the amount of partial spectral weight retained. The detailed flow of the dynamical mean-field algorithm is described, including the computation of response functions such as transport coefficients. We discuss the implementation of the impurity solvers based on hybridization expansion and an analytic continuation method for self-energy. We also derive the formalism for the bold continuous time quantum Monte Carlo method. We test our method on a classic problem in strongly correlated physics, the isostructural transition in Ce metal. We apply our method to the class of heavy-fermion materials ${\text{CeIrIn}}_{5}$, ${\text{CeCoIn}}_{5}$, and ${\text{CeRhIn}}_{5}$ and show that the $\text{Ce}\text{ }4f$ electrons are more localized in ${\text{CeRhIn}}_{5}$ than in the other two, a result corroborated by experiment. We show that ${\text{CeIrIn}}_{5}$ is the most itinerant and has a very anisotropic hybridization, pointing mostly toward the out-of-plane In atoms. In ${\text{CeRhIn}}_{5}$ we stabilized the antiferromagnetic DMFT solution below 3 K, in close agreement with the experimental Neel temperature.

/pdf/dynamical-mean-field-theory-within-the-full-potential-3uj9yzaame.pdf

Dynamical mean-field theory within the full-potential methods: Electronic structure of CeIrIn 5 , CeCoIn 5 , and CeRhIn 5

Actinide elements produce a plethora of interesting physical behaviors due to the $5f$ states. This review compiles and analyzes progress in the understanding of the electronic and magnetic structure of the $5f$ states in actinide metals. Particular interest is given to electron energy-loss spectroscopy and many-electron atomic spectral calculations, since there is now an appreciable library of core $d\ensuremath{\rightarrow}\text{valence}$ $f$ transitions for Th, U, Np, Pu, Am, and Cm. These results are interwoven and discussed against published experimental data, such as x-ray photoemission and absorption spectroscopy, transport measurements, and electron, x-ray, and neutron diffraction, as well as theoretical results, such as density-functional theory and dynamical mean-field theory.

Nature of the 5f states in actinide metals

Structures of nanoparticles in $\text{Fe-}16\text{Cr-}4.5\text{Al-}0.3\text{Ti-}2\text{W-}0.37{\text{Y}}_{2}{\text{O}}_{3}$ (K3) and $\text{Fe-}20\text{Cr-}4.5\text{Al-}0.34\text{Ti-}0.5{\text{Y}}_{2}{\text{O}}_{3}$ (MA956) oxide dispersion strengthened (ODS) ferritic steels produced by mechanical alloying (MA) and followed by hot extrusion have been studied using high-resolution transmission electron microscopy (HRTEM) techniques to understand the formation mechanism of nanoparticles in MA/ODS steels. The observations of Y-Al-O complex-oxide nanoparticles in both ODS steels imply that decomposition of ${\text{Y}}_{2}{\text{O}}_{3}$ in association with internal oxidation of Al occurred during mechanical alloying. While the majority of oxide nanoparticles formed in both steels is ${\text{Y}}_{4}{\text{Al}}_{2}{\text{O}}_{9}$, a few oxide particles of ${\text{YAlO}}_{3}$ are also observed occasionally. These results reveal that Ti $(0.3\text{ }\text{wt}\text{ }\mathrm{%})$ plays an insignificant role in forming oxide nanoparticles in the presence of Al $(4.5\text{ }\text{wt}\text{ }\mathrm{%})$. HRTEM observations of crystalline nanoparticles larger than $\ensuremath{\sim}2\text{ }\text{nm}$ and amorphous or disordered cluster domains smaller than $\ensuremath{\sim}2\text{ }\text{nm}$ provide an insight into the formation mechanism of oxide nanoparticle in MA/ODS steels, which we believe from our observations involves solid-state amorphization and recrystallization. The role of nanoparticles in suppressing radiation-induced swelling is revealed through TEM examinations of cavity distributions in ion-irradiated Fe-14Cr and K3-ODS ferritic steels. HRTEM observations of helium-filled cavities (helium bubbles) preferably trapped at nanoscale oxide particles and clusters in ion-irradiated K3-ODS are presented.

Formation mechanism and the role of nanoparticles in Fe-Cr ODS steels developed for radiation tolerance

A calibration procedure for the response of silicon surface-barrier detectors to heavy ions

Pulse-height defects for heavy ions in a silicon surface-barrier detector

The interaction between positrons and dislocations is considered from a new point of view, which leads to a new interpretation of positron results obtained for metal samples containing dislocations. The positron binding energy in a perfect dislocation line is assumed to be small (<or approximately=0.1 eV). The observed changes in the positron annihilation parameters due to the presence of dislocations then originate from annihilation in point-like defects (e.g. jogs) associated with the dislocation line. The positron-dislocation line trapping rate due to phonon emission is calculated for various binding energies and temperatures and is found to be unexpectedly large. It is shown that this interpretation predicts strong temperature dependences for the positron trapping rate in annealed samples at temperatures below 300K due to positron detrapping.

http://iopscience.iop.org/article/10.1088/0305-4608/10/10/019/pdf

An alternative interpretation of positron annihilation in dislocations

The temperature dependence of positron annihilation in aluminum has been investigated over the range 20-435\ifmmode^\circ\else\textdegree\fi{}C by simultaneous measurements of positron lifetime and Doppler broadening of the annihilation spectrum. The analyses of lifetime data are critically examined, especially with respect to the effects of instrumental resolution-function uncertainties. The lifetime and Doppler-broadening data have been analyzed in terms of the two-state trapping model. Application of this model to the measurement of the vacancy-formation enthalpy ${E}_{\mathrm{v}}^{F}$ in aluminum is discussed in light of the observation of a strong temperature dependence of the apparent positron lifetime in the bulk or lattice state above 370\ifmmode^\circ\else\textdegree\fi{}C. The monovacancy formation enthalpy in aluminum, ${E}_{1\mathrm{v}}^{F}=0.66\ifmmode\pm\else\textpm\fi{}0.02$ eV, was determined for temperatures below 372\ifmmode^\circ\else\textdegree\fi{}C. This is compared with values previously reported.

Michael J. Fluss

Papers

Formation mechanism and the role of nanoparticles in Fe-Cr ODS steels developed for radiation tolerance

A calibration procedure for the response of silicon surface-barrier detectors to heavy ions

Pulse-height defects for heavy ions in a silicon surface-barrier detector

An alternative interpretation of positron annihilation in dislocations

Measurements of the vacancy formation enthalpy in aluminum using positron annihilation spectroscopy