Electron-positron Correlation Research Articles

Many-body theory calculations of positron binding to hydrogen cyanide

Positron bound state properties in hydrogen cyanide are studied via many-body theory calculations that account for strong positron-electron correlations including positron-induced polarization, screening of the electron–positron Coulomb interaction, virtual-positronium formation and positron–hole repulsion. Specifically, the Dyson equation is solved using a Gaussian basis, with the positron self-energy in the field of the molecule calculated using the Bethe–Salpeter equations for the two-particle and particle–hole propagators. The present results suggest near cancellation of screening corrections to the bare polarization, and the non-negligible role of the positron–hole interaction. There are no existing measurements to compare to for HCN. Previous configuration interaction (CI) and fixed-node diffusion Monte Carlo (FN-DMC) calculations give positron binding energies in the range 35–44 meV, most of which used a single even-tempered basis centred near the nitrogen atom. Using a similar single-centre positron basis we calculate a positron binding energy of 41 meV, in good agreement. However, we find that including additional basis centres gives an improved description of the positron wave function near the nuclei, and results in a converged binding energy in the range 63–73 meV (depending on geometry and approximation to the positron–molecule correlation potential used).Graphical

Production of positronium chloride: A study of the charge exchange reaction between Ps and Cl.

We present cross sections for the formation of positronium chloride (PsCl) in its ground state from the charge exchange between positronium (Ps) and chloride (Cl-) in the range of 10 meV-100eV Ps energy. We have used theoretical models based on the first Born approximation in its three-body formulation. We simulated the collisions between Ps and Cl- using abinitio binding energies and positronic wave functions at both the mean-field and correlated levels extrapolated to the complete basis set limit. The accuracy of these abinitio data was benchmarked on the PsF system with the existing highly accurate results, including the very recent quantum Monte Carlo results. We have investigated Ps excited states up to n = 4. The results suggest that the channel Ps(n = 2) is of particular interest for the production of PsCl in the ground state and shows that an accurate treatment of correlation effects (i.e., electron-electron and electron-positron correlations) leads to a significant change in the magnitude of the PsCl production cross section with respect to the mean-field level.

Many-body theory of positronium scattering and pickoff annihilation in noble-gas atoms

The many-body-theory approach to positronium-atom interactions developed in [Phys. Rev. Lett. \textbf{120}, 183402 (2018)] is applied to the sequence of noble-gas atoms He-Xe. The Dyson equation is solved separately for an electron and positron moving in the field of the atom, with the entire system enclosed in a hard-wall spherical cavity. The two-particle Dyson equation is solved to give the energies and wave functions of the Ps eigenstates in the cavity. From these, we determine the scattering phase shifts and cross sections, and values of the pickoff annihilation parameter $^1Z_\text{eff}$ including short-range electron-positron correlations via vertex enhancement factors. Comparisons are made with available experimental data for elastic and momentum-transfer cross sections and $^1Z_\text{eff}$. Values of $^1Z_\text{eff}$ for He and Ne, previously reported in [Phys. Rev. Lett. \textbf{120}, 183402 (2018)], are found to be in near-perfect agreement with experiment, and for Ar, Kr, and Xe within a factor of 1.2.

Enhancement of positron binding energy in molecules containing π bonds

Observation of vibrational Feshbach resonances in the annihilation spectra of positrons on molecules provides a direct measurement of the positron-molecule binding energy ${\ensuremath{\varepsilon}}_{B}$. Annihilation measurements are presented for ring hydrocarbons with different numbers of $\ensuremath{\pi}$ bonds, for which it is observed that the presence of $\ensuremath{\pi}$ bonds generally increases the positron binding energies. These molecules were chosen because other global molecular parameters (e.g., polarizability, dipole moment, and geometry) are approximately constant, so the observed differences in ${\ensuremath{\varepsilon}}_{B}$ can be related to changes in the nature of the bonds. The molecular ionization potential ${E}_{\mathrm{i}}$ is an exception: for these molecules, the inclusion of $\ensuremath{\pi}$ bonds tends to decrease ${E}_{\mathrm{i}}$, and the number of $\ensuremath{\pi}$ bonds also exhibits a correlation with ${\ensuremath{\varepsilon}}_{B}$. Comparison with other molecules with $\ensuremath{\pi}$ bonds indicates that the changes in ${\ensuremath{\varepsilon}}_{B}$ are better correlated with the changing electronic structure of the bonds rather than with a direct dependence of ${\ensuremath{\varepsilon}}_{B}$ on ${E}_{\mathrm{i}}$. The relationship between the dependence of ${\ensuremath{\varepsilon}}_{B}$ on the number of $\ensuremath{\pi}$ bonds and electron-positron correlation effects (such as virtual positronium formation) is discussed.

Calculation of Low-Energy Positron-Atom Scattering with Square-Integrable Wavefunctions

The variational method is applied to the low-energy positron scattering and annihilation problem. The ultimate aim of the investigation is to find a computationally economical way of accounting for strong electron–positron correlations, including the effect of virtual positronium formation. The method is applied to the study of elastic s-wave positron scattering from a hydrogen atom. A generalized eigenvalue problem is set up and solved to obtain s-wave positron–hydrogen scattering phase shifts within 8×10−3 rad of accepted values. This is achieved using a small number of terms in the variational wavefunction; in particular, only nine terms that depend on the electron–positron distance are included. The annihilation parameter Zeff is also calculated and is found to be in good agreement with benchmark calculations.

Correlated Wave Functions for Electron-Positron Interactions in Atoms and Molecules.

The positron, as the antiparticle of the electron, can form metastable states with atoms and molecules before its annihilation with an electron. Such metastable matter–positron complexes are stabilized by a variety of mechanisms, which can have both covalent and noncovalent character. Specifically, electron–positron binding often involves strong many-body correlation effects, posing a substantial challenge for quantum-chemical methods based on atomic orbitals. Here we propose an accurate, efficient, and transferable variational ansatz based on a combination of electron–positron geminal orbitals and a Jastrow factor that explicitly includes the electron–positron correlations in the field of the nuclei, which are optimized at the level of variational Monte Carlo (VMC). We apply this approach in combination with diffusion Monte Carlo (DMC) to calculate binding energies for a positron e+ and a positronium Ps (the pseudoatomic electron–positron pair), bound to a set of atomic systems (H–, Li+, Li, Li–, Be+, Be, B–, C–, O– and F–). For PsB, PsC, PsO, and PsF, our VMC and DMC total energies are lower than that from previous calculations; hence, we redefine the state of the art for these systems. To assess our approach for molecules, we study the potential-energy surfaces (PES) of two hydrogen anions H– mediated by a positron (e+H22–), for which we calculate accurate spectroscopic properties by using a dense interpolation of the PES. We demonstrate the reliability and transferability of our correlated wave functions for electron–positron interactions with respect to state-of-the-art calculations reported in the literature.

Anomalous internal pair creation

In recent electron-positron angular correlation measurements the observed significant enhancements relative to the internal pair creation at large angles was interpreted as indication of the creation of J^{pi }=1^{+} boson called X17 particle. In this paper it is brought up that such enhancements can be generated by higher order processes. It is found that nuclear transitions, the transition energy of which is significantly lower than the whole transition energy, can cause peaked angle dependence in electron-positron angular correlation.

Zr vacancies and their complexes with hydrogen in monoclinic zirconia: formation energies and positron lifetimes

Cation vacancies in metal oxides have high formation energies and, thus, are not as abundant as their oxygen-related counterparts. Nonetheless, they can be readily created during non-equilibrium processes. Positron-annihilation spectroscopy (PAS) is a well suited technique of probing such defects since negatively-charged lattice vacancies are deep potential wells which can act as positron traps. The present study reports first-principles calculations of positron trapping at Zr monovacancies in monoclinic zirconia. The binding of positrons and associated lifetimes for these defects were obtained within two-component density-functional theory under different approximations for the electron–positron correlation. The role of hydrogen was also explored. Low-energy vacancy–hydrogen defect complexes were determined for one and two hydrogen atoms bound to a single Zr monovacancy. Hydrogen decoration of the vacancy affected the localization of the positron leading to an appreciable decrease of the lifetime. The study was supplemented with PAS measurements on samples of monoclinic zirconia. From the PAS data two distinct, high-intensity components were resolved originating from different defect states. The corresponding lifetimes of 187 and 225 ps were very close to the theoretical predictions.

First-principles study of the formation energies and positron lifetimes of vacancies in the Yttrium-Aluminum Garnet Y3Al5O12

Lattice vacancies are a major concern for the use of the Y3Al5O12 garnet (YAG) in optical applications. They are known to trap charge carriers preventing them from reaching luminescence centers. This reduces useful photon emission and deteriorates performance. Recent efforts to characterize such defects include experimental works by positron-annihilation spectroscopy (PAS) where extensive positron trapping was reported and attributed to defects made up of both cation and oxygen vacancies. The present study reports first-principles calculations for monovacancy and divacancy defects in YAG by means of conventional and two-component density-functional theory. The defect formation energies and corresponding charge-transition levels in the gap were initially determined. The ability of the lower-energy defects to act as positron-trapping centers was then examined. Corresponding positron lifetimes and binding energies to defects were calculated and compared to the experimental PAS data. The lifetimes of aluminum monovacancies agreed well with experiment if gradient corrections are included in the electron-positron correlation. Association of oxygen with aluminum vacancies was found to lead to stable negatively-charged divacancy complexes which also trap positrons. These defects are characterized by longer positron lifetimes, in agreement with the experimental observations.

Positron binding to hydrocarbon molecules: calculation using the positron–electron correlation polarization potential

Positron binding energies of naphthalene and alkanes (CnH2n+2 with n = 3 − 16) are calculated using the correlation polarization potential approach, where the short-range positron–electron correlation potential is modeled by the density-functional expression taken from a homogenous electron gas model. In the case of naphthalene, the calculated positron binding energy is found to reasonably agree with the experimental measurement. In the case of CnH2n+2 with the linear all-trans conformation we found positive positron binding energies for n ≥ 8 while the positron is not bound for n ≤ 7. This result cannot reproduce the previous experimental study, where the positron was bound for all alkanes with 3 ≤n ≤ 16. In addition, our calculated positron binding energies for n ≥ 9 are much larger than the experimental values although the generalized gradient approximation could improve the calculated values. We also investigated the conformer dependence of the positron binding energy for C16H34 and found that the positron binding energies significantly depend on the conformational structure; hairpin-like and crown-like structures generally have large positron binding energies, while single and multiple gauche structures have smaller binding energies.

Positron lifetimes of bare and hydrogenated zirconium vacancies in cubic yttria-stabilized zirconia: an ab initio study

Recent studies by positron-annihilation spectroscopy (PAS) in single-crystal and nanostructured yttria-stabilized zirconia (YSZ) revealed extensive positron trapping at vacancy-type point defects and other structural imperfections. The present work reports first principles calculations of formation energies and positron lifetimes of Zr vacancies in 10.3 mol% cubic YSZ. The lifetime calculations are based on two-component density-functional theory within the local-density approximation for the electron-positron correlation energy. Gradient-correction effects were also examined. Zr monovacancies were found to be potent trapping sites for positrons with large binding energies and lifetimes in excess of 220 ps. A strong dependency of the lifetimes on the Zr site and local coordination number was observed, a consequence of the structural disorder of the stabilized lattice. In contrast, oxygen vacancies did not act as attractive centers for positrons, either as single defects or as nearest neighbors to seven-fold coordinated Zr vacancies. Defect association of the Zr vacancies with hydrogen led to stable (-H) complexes with hydrogen bound to oxygen ions which are nearest neighbors to the vacancies. The negatively-charged (-H) defects were also found to trap positrons with corresponding lifetimes strongly dependent upon the hydrogen content within the complex.

Time-Dependent Multicomponent Density Functional Theory for Coupled Electron-Positron Dynamics.

Electron-positron interactions have been utilized in various fields of science. Here we develop time-dependent multicomponent density functional theory to study the coupled electron-positron dynamics from first principles. We prove that there are coupled time-dependent single-particle equations that can provide the electron and positron density dynamics, and derive the formally exact expression for their effective potentials. Introducing the adiabatic local density approximation to time-dependent electron-positron correlation, we apply the theory to the dynamics of a positronic lithium hydride molecule under a laser field. We demonstrate the significance of the coupling between electronic and positronic motion by revealing the complex positron detachment mechanism and the suppression of electronic resonant excitation by the screening effect of the positron.

Application of the weighted-density approximation to the accurate description of electron-positron correlation effects in materials

We discuss positron-annihilation lifetimes for a set of illustrative bulk materials within the framework of the weighted-density approximation (WDA). The WDA can correctly describe electron-positron correlations in strongly inhomogeneous systems, such as surfaces, where the applicability of (semi-)local approximations is limited. We analyze the WDA in detail and show that the electrons which cannot screen external charges efficiently, such as the core electrons, cannot be treated accurately via the pair correlation of the homogeneous electron gas. We discuss how this problem can be addressed by reducing the screening in the homogeneous electron gas by adding terms depending on the gradient of the electron density. Further improvements are obtained when core electrons are treated within the LDA and the valence electron using the WDA. Finally, we discuss a semiempirical WDA-based approach in which a sum rule is imposed to reproduce the experimental lifetimes.

First-Principles Study on Positron States at Li-Adsorbed Al(100) Surfaces: Effect of Electron-Positron Correlation

We investigated positron states at Li-adsorbed Al(100) surfaces using \textit{ab-initio} two-component density functional theory. The results thus obtained by using semi-local electron-positron (e-p) correlation functionals with the generalized gradient approximation well reproduce the experimental results, showing that reliable e-p correlation functionals are needed to properly describe the various properties of positron surface state.

Parameter-Free Gradient Correction for Positron States in Oxides

Recently, the theory of gradient-corrected electron-positron correlations in solids was improved and extended in order to avoid the adjustable, empirical parameter α which is now part of theory and a smooth function of the electron density.This new, parameter-free approach is applied to selected oxides in order to discuss their interstitial space morphology reflected by the positron charge distribution. In addition, the positron lifetime and affinity are calculated using a highly precise computational method. An attempt is made to correlate these quantities with the volume of the reduced formula unit.The results for some oxides - such as Li-ion battery cathode materials and high temperature superconductors - are discussed in detail and prospects for future experiments are given.

Calculation of Positron Binding Energies and Electron-Positron Annihilation Rates for Atomic Systems with the Reduced Explicitly Correlated Hartree-Fock Method in the Nuclear-Electronic Orbital Framework.

Although the binding of a positron to a neutral atom has not been directly observed experimentally, high-level theoretical methods have predicted that a positron will bind to a neutral atom. In the present study, the binding energies of a positron to lithium, sodium, beryllium, and magnesium, as well as the electron-positron annihilation rates for these systems, are calculated using the reduced explicitly correlated Hartree-Fock (RXCHF) method within the nuclear-electronic orbital (NEO) framework. Due to the lack of explicit electron-positron correlation, NEO Hartree-Fock and full configuration interaction calculations with reasonable electronic and positronic basis sets do not predict positron binding to any of these atoms. In contrast, the RXCHF calculations predict positron binding energies and electron-positron annihilation rates in qualitative agreement with previous highly accurate but computationally expensive stochastic variational method calculations. These results illustrate that the RXCHF method can successfully describe the binding of a positron to a neutral species with no dipole moment. Moreover, the RXCHF method will be computationally tractable for calculating positron binding to molecular systems. The RXCHF approach offers a balance of accuracy and computational tractability for studying these types of positronic systems.

Advanced characterization of lithium battery materials with positrons

Cathode materials are crucial to improved battery performance, in part because there are not yet materials that can maintain high power and stable cycling with a capacity comparable to that of anode materials. Our parameter-free, gradient-corrected model for electron-positron correlations predicts that spectroscopies based on positron annihilation can be deployed to study the effect of lithium intercalation in the oxide matrix of the cathode. The positron characteristics in oxides can be reliably computed using methods based on first-principles. Thus, we can enable a fundamental characterization of lithium battery materials involving positron annihilation spectroscopy and first-principles calculations. The detailed information one can extract from positron experiments could be useful for understanding and optimizing both battery materials and bi-functional catalysts for oxygen reduction and evolution.

Insight into the electron–positron correlations in metals through the looking glass

A semi-empirical analysis of the positron annihilation experimental spectra indicates for a strong sensitivity of the two-particle electron–positron (e–p) enhancement factor to the l=s, p, d, f character of the initial electronic state [1,2]. The essential discrepancy between the models consists in the dependence of the relevant correlation functions on the energy of the annihilating electron. The present contribution contains a theoretical study of the e–p enhancement factors for s, p, d and f states as a function of the electron energy. The slope of the resulting characteristics is directly related to the degree of localisation of the s, p, d and f electrons in the electron density of states. This effect occurs especially for d electrons in transition metals, in favour to the approach of Ref. [1].The energy dependence of the two-particle correlation functions is also a source of controversy between various theoretical approaches. The energy dependent enhancement factors describe properly the positron interaction with delocalised s and p electrons, but this approach overestimates the high momentum components of the e–p momentum densities, dominated by the localised d and f states. On the contrary, the calculations that employ the energy averaged enhancement factors match better with experiment for localised d and f electrons, but they hardly reproduce experimental spectra for nearly-free electron populations. An attempt to visit two sides of the looking glass is made in the theory of the present work. The model combines the properties of both approaches. The resulting e–p momentum densities and enhancement factors are in good agreement with the experimental data for simple, noble and transition metals, both in the low and high momentum region.

Effect of dipole polarizability on positron binding by strongly polar molecules

A model for positron binding to polar molecules is considered by combining the dipole potential outside the molecule with a strongly repulsive core of a given radius. Using existing experimental data on binding energies leads to unphysically small core radii for all of the molecules studied. This suggests that electron–positron correlations neglected in the simple model play a large role in determining the binding energy. We account for these by including the polarization potential via perturbation theory and non-perturbatively. The perturbative model makes reliable predictions of binding energies for a range of polar organic molecules and hydrogen cyanide. The model also agrees with the linear dependence of the binding energies on the polarizability inferred from the experimental data (Danielson et al 2009 J. Phys. B: At. Mol. Opt. Phys. 42 235203). The effective core radii, however, remain unphysically small for most molecules. Treating molecular polarization non-perturbatively leads to physically meaningful core radii for all of the molecules studied and enables even more accurate predictions of binding energies to be made for nearly all of the molecules considered.

Exploring positron characteristics utilizing two new positron–electron correlation schemes based on multiple electronic structure calculation methods**Project supported by the National Natural Science Foundation of China (Grant Nos. 11175171 and 11105139).

We make a gradient correction to a new local density approximation form of positron–electron correlation. The positron lifetimes and affinities are then probed by using these two approximation forms based on three electronic-structure calculation methods, including the full-potential linearized augmented plane wave (FLAPW) plus local orbitals approach, the atomic superposition (ATSUP) approach, and the projector augmented wave (PAW) approach. The differences between calculated lifetimes using the FLAPW and ATSUP methods are clearly interpreted in the view of positron and electron transfers. We further find that a well-implemented PAW method can give near-perfect agreement on both the positron lifetimes and affinities with the FLAPW method, and the competitiveness of the ATSUP method against the FLAPW/PAW method is reduced within the best calculations. By comparing with the experimental data, the new introduced gradient corrected correlation form is proved to be competitive for positron lifetime and affinity calculations.