Combined Configuration Interaction Research Articles

Ferromagnetic Insulating Ground-State Resolved in Mixed Protons and Oxygen Vacancies-Doped La0.67Sr0.33CoO3 Thin Films via Ionic Liquid Gating.

The realization of ferromagnetic insulating ground state is a critical prerequisite for spintronic applications. By applying electric field-controlled ionic liquid gating (ILG) to stoichiometry La0.67Sr0.33CoO3 thin films, the doping of protons (H+) has been achieved for the first time. Furthermore, a hitherto-unreported ferromagnetic insulating phase with a remarkably high Tc up to 180 K has been observed which can be attributed to the doping of H+ and the formation of oxygen vacancies (VO). The chemical formula of the dual-ion migrated film has been identified as La2/3Sr1/3CoO8/3H2/3 based on combined Co L23-edge absorption spectra and configuration interaction cluster calculations, from which we are able to explain the ferromagnetic ground state in terms of the distinct magnetic moment contributions from Co ions with octahedral (Oh) and tetrahedral (Td) symmetries following antiparallel spin alignments. Further density functional theory calculations have been performed to verify the functionality of H+ as the transfer ion and the origin of the novel ferromagnetic insulating ground state. Our results provide a fundamental understanding of the ILG regulation mechanism and shed light on the manipulating of more functionalities in other correlated compounds through dual-ion manipulation.

A theoretical study of ferrocene based on combined configuration interaction singles (CIS) and time-dependent density functional theory (TDDFT) approach

The state-of-the art density functional theory (DFT) is used to clearly resolve the two parallel cyclopentadienyl rings of ferrocene, which are either staggered (D5d symmetry) or eclipsed (D5h symmetry), in their ground-state conformation. Present result revealed that the eclipsed conformer with D5h point group represents the true minimum ground state structure of ferrocene. Natural population analysis is used to determine how atomic charge is distributed across different atoms of ferrocene D5h conformer and also the distribution of electrons in the core, valence, and Rydberg sub-shells. It is further investigated in potential energy scan that the rotation of the dihedral angle δ from 0° to 3π/5 will reproduce three times D5h or D5d conformers periodically as the period of 2π/5 due to the pentagonal structure of the CP ring. Further to examine optical spectra in the ultraviolet-visible (UV–vis) range, configuration interaction single (CIS) and time-dependent density functional theory (TDDFT) have conducted which help in locating the significant electronic shifts between different energy levels. Absorption spectra for high spin states were also generated in order to comprehend the characteristics of low-lying spin excitation. According to our estimates, the greatest absorption intensity is restricted to an energy range of 4–6 eV. Knowledge of ferrocene conformers will improve the research on other metallocenes and their derivatives, which have applications in biotechnology, nanotechnology, and solar technology.

Calculation of energies and hyperfine-structure constants of U+233 and U233

We carried out calculations of the energies and magnetic dipole hyperfine-structure constants of the low-lying states of $^{233}\mathrm{U}^{+}$ and $^{233}\mathrm{U}$ using two different approaches. With six valence electrons and a very heavy core, uranium represents a major challenge for precision atomic theory even using large-scale computational resources. The first approach combines configuration interaction (CI) with a method allowing us to include core-valence correlations to all orders of the perturbation theory over residual Coulomb interaction. The second approach is a pure CI method which allows the use of different initial approximations. We present a detailed analysis of all calculated properties and discuss the advantages and disadvantages of each of these methods. We report a preliminary value of the U nuclear magnetic moment and outline the need for further experiments.

Laser spectroscopy of the yPJo7 states of Cr i

Here we report measured and calculated values of decay rates of the 3d$^4$($^5$D)4s4p($^3$P$^{\rm{o}}$)\ y$^7$P$^{\rm{o}}_{2,3,4}$ states of Cr I. The decay rates are measured using time-correlated single photon counting with roughly 1% total uncertainty. In addition, the isotope shifts for these transitions are measured by laser induced fluorescence to roughly 0.5% uncertainty. The decay rate calculations are carried out by a hybrid approach that combines configuration interaction and the linearized coupled cluster method (CI+all-order method). The measurements provide a much needed precision benchmark for testing the accuracy of the CI+all-order approach for such complicated systems with six valence electrons, allowing to significantly expand its applicability. These measurements also demonstrate operation of a cryogenic buffer gas beam source for future experiments with MgF molecules toward quantum blackbody thermometry.

Quantitative investigation of the 4f occupation in the quasikagome Kondo lattice CeRh1−xPdxSn

CeRhSn with the Ce atoms forming a quasikagome lattice in the hexagonal plane has recently been discussed in the context of quantum criticality driven by magnetic frustration. Furthermore, it has been reported that the successive substitution of Rh by Pd leads to magnetic order. Here we have investigated the change of the 4$f$ occupation in the substitution series CeRh$_{1-x}$Pd$_x$Sn for for $x$ = 0, 0.1, 0.3, 0.5, 0.75 by means of photoelectron spectroscopy with hard x-rays (HAXPES). The quantitative analysis of the core level spectra with a combined full multiplet and configuration interaction analysis shows a smooth decrease of the 4$f^0$ contribution with rising $x$ due to an increase of the effective 4$f$ binding energy $\varepsilon_{4f}$ and the reduction of the effective hybridization $V_\text{eff}$. We further compare valence band data with the calculated partial density of states and find that the Pd 4$d$ states are about 1eV further away from the Ce 4$f$ states at the Fermi energy than the Rh 4$d$ states. In fact, the effective binding energy $\varepsilon_{4f}$ of the 4$f$ states in the configuration interaction analysis of the core level spectra decreases by the same amount.

Detection of the Lowest-Lying Odd-Parity Atomic Levels in Actinium.

Two lowest-energy odd-parity atomic levels of actinium, 7s^{2}7p^{2}P_{1/2}^{o}, 7s^{2}7p^{2}P_{3/2}^{o}, were observed via two-step resonant laser-ionization spectroscopy and their respective energies were measured to be 7477.36(4) and 12 276.59(2) cm^{-1}. The lifetimes of these states were determined as 668(11) and 255(7)ns, respectively. In addition, we observed the effect of the hyperfine structure on the line for the transition to ^{2}P_{3/2}^{o}. These properties were calculated using a hybrid approach that combines configuration interaction and coupled-cluster methods, in good agreement with the experiment. The data are of relevance for understanding the complex atomic spectra of actinides and for developing efficient laser cooling and ionization schemes for actinium, with possible applications for high-purity medical-isotope production and future fundamental physics experiments.

First-principles many-body theory for charged and neutral excitations: Trion fine structure splitting in transition metal dichalcogenides

A hybrid theory which combines configuration interaction and Green's function approaches is proposed here to treat charged and neutral excitations. The theory is parameter free and reduces to the well-known Bethe-Salpeter equation (BSE) in the case of excitons. However, unlike the BSE, the theory can be applied to calculate any excitation beyond the exciton. As this type of computation is generally time consuming, we show that in the case of Wannier-type excitations, the localization in reciprocal space can be used to reduce the required computation load. We apply our approach to excitons and trions in ${\mathrm{WS}}_{2}$ and ${\mathrm{MoS}}_{2}$ transition metal dichalcogenides monolayers and obtain optical spectra, binding energies, and dark-bright exciton splitting in good agreement with experimental measurements. Moreover, in the case of ${\mathrm{WS}}_{2}$, we have found that the negative trion peak shows a fine structure splitting (FSS) of 15 meV which corresponds to the FSS for an isolated monolayer.

Calculation of atomic spectra and transition amplitudes for the superheavy element Db ( Z =105)

Atomic spectra and other properties of superheavy element dubnium (Db, Z=105) are calculated using recently developed method combining configuration interaction with perturbation theory (the CIPT method, Dzuba et al, Phys. Rev. A, {\bf 95}, 012503 (2017)). These include energy levels for low-lying states of Db and Db~II, electric dipole transition amplitudes from the ground state of Db, isotope shift for these transitions and ionisation potential of Db. Similar calculations for Ta, which is lighter analog of Db, are performed to control the accuracy of the calculations.

Clock-related properties of Lu+

The singly-ionized lutetium has a number of fortuitous properties well suited for a design of an optical clock and corresponding applications. In this work, we study Lu+ properties relevant to a development of the clock using the relativistic high-precision method combining configuration interaction and the linearized coupled-cluster approaches. The systematic effects due to interaction of an external electric-field gradient with the quadrupole moment and the dynamic correction to the blackbody radiation shift are studied and uncertainties are estimated. The value of the 5d6s 1D_2 polarizability is predicted. We also demonstrate that Lu+ is a good candidate to search for variation of the fine-structure constant.

Determination of static dipole polarizabilities of Yb atom**Project supported by the National Natural Science Foundation of China (Grant Nos. 91536106 and U1332206), the Strategic Priority Research Program (Category B) of the Chinese Academy of Sciences (Grant No. 21030300), and the National Key Research and Development Program of China (Grant No. 2016YFA0302104).

We determine the static values of the scalar and tensor dipole polarizabilities of the ground, 6s6p , and 6s6p states of the Yb atom. These results can be useful in many experiments undertaken using this atom. We employed a combined configuration interaction (CI) method and a second-order many-body perturbation theory (MBPT) to evaluate energies and electric dipole (E1) matrix elements of many low-lying excited states of the above atom. These values are compared with the other available theoretical calculations and experimental values. By combining these E1 matrix elements with the experimental excitation energies, we estimate the dominant valence correlation contributions to the dipole polarizabilities of the above states. The core contribution is obtained from the finite field approach. We also compare these values with the other theoretical results as there are no precise experimental values that are available for these properties.

Atomic properties of actinide ions with particle-hole configurations

We study the effects of higher-order electronic correlations in the systems with particle-hole excited states using a relativistic hybrid method that combines configuration interaction and linearized coupled-cluster approaches. We find the configuration interaction part of the calculation sufficiently complete for eight electrons while maintaining good quality of the effective coupled-cluster potential for the core. Excellent agreement with experiment was demonstrated for a test case of La$^{3+}$. We apply our method for homologue actinide ions Th$^{4+}$ and U$^{6+}$ which are of experimental interest due to a puzzle associated with the resonant excitation Stark ionization spectroscopy (RESIS) method. These ions are also of interest to actinide chemistry and this is the first precision calculation of their atomic properties.

Relativistic all-order many-body calculation of energies, wavelengths, and M1 and E2 transition rates for the 3d n configurations in tungsten ions.

Energy levels, wavelengths, magnetic-dipole and electric-quadrupole transition rates between the low-lying states are evaluated for W51+ to W54+ ions with 3d n (n = 2 to 5) electronic configurations by using an approach combining configuration interaction with the linearized coupled-cluster single-double method. The QED corrections are directly incorporated into the calculations and their effect is studied in detail. Uncertainties of the calculations are discussed. This study of such highly charged ions with the present method opens the way for future applications allowing an accurate prediction of properties for a very wide range of highly charged ions aimed at providing precision benchmarks for various applications.

Combining configuration interaction with perturbation theory for atoms with a large number of valence electrons

A version of the configuration interaction (CI) method is developed which treats highly excited many-electron basis states perturbatively, so that their inclusion does not affect the size of the CI matrix. This removes, at least in principle, the main limitation of the CI method in dealing with many-electron atoms or ions. We perform calculations of the spectra of iodine and its ions, tungsten, and ytterbium as examples of atoms with open $s$, $p$, $d$ and $f$-shells. Good agreement of the calculated data with experiment illustrates the power of the method. Its advantages and limitations are discussed.

Ionization potentials of superheavy elements No, Lr, and Rf and their ions.

We predict ionization potentials of superheavy elements No, Lr, and Rf and their ions using a relativistic hybrid method that combines configuration interaction (CI) with the linearized coupled-cluster approach. Extensive study of the completeness of the four-electron CI calculations for Hf and Rf was carried out. As a test of theoretical accuracy, we also calculated ionization potential of Yb, Lu, Hf, and their ions, which are homologues of the superheavy elements of this study.

Ground state spin and excitation energies in half-filled Lieb lattices

We present detailed spectral calculations for small Lieb lattices having up to $N=4$ number of cells, in the regime of half-filling, an instance of particular relevance for the nano-magnetism of discrete systems such as quantum dot arrays, due to the degenerate levels at mid-spectrum. While for the Hubbard interaction model -and even number of sites- the ground state spin is given by the Lieb theorem, the inclusion of long range interaction -or odd number of sites- make the spin state not a priori known, which justifies our approach. We calculate also the excitation energies, which are of experimental importance, and find significant variation induced by the interaction potential. One obtains insights on the mechanisms involved that impose as ground state the Lieb state with lower spin rather than the Hund one with maximum spin for the degenerate levels, showing this in the first and second order of the interaction potential for the smaller lattices. The analytical results concorde with the numerical ones, which are performed by exact diagonalization calculations or by a combined mean-field and configuration interaction method. While the Lieb state is always lower in energy than the Hund state, for strong long-range interaction, when possible, another minimal spin state is imposed as ground state.

EXTENDED RELATIVISTIC CONFIGURATION INTERACTION AND MANY-BODY PERTURBATION CALCULATIONS OF SPECTROSCOPIC DATA FOR THE N ≤ 6 CONFIGURATIONS IN Ne-LIKE IONS BETWEEN Cr xv AND Kr xxvii

ABSTRACT Level energies, wavelengths, electric dipole, magnetic dipole, electric quadrupole, and magnetic quadrupole transition rates, oscillator strengths, and line strengths from combined relativistic configuration interaction and many-body perturbation calculations are reported for the 201 fine-structure states of the , , , , , , and configurations in all Ne-like ions between Cr xv and Kr xxvii. Calculated level energies and transition data are compared with experiments from the National Institute of Standards and Technology (NIST) and CHIANTI databases, and other recent benchmark calculations. The mean energy difference with the NIST experiments is only 0.05%. The present calculations significantly increase the amount of accurate spectroscopic data for the n > 3 states in a number of Ne-like ions of astrophysical interest. A complete data set should be helpful for analyzing new observations from solar and other astrophysical sources, and is also likely to be useful for modeling and diagnosing a variety of plasmas, including astronomical and fusion plasma.

Linear scaling approach for atomistic calculation of excitonic properties of 10-million-atom nanostructures

Numerical calculations of excitonic properties of novel nanostructures, such as nanowire and crystal phase quantum dots, must combine atomistic accuracy with an approachable computational complexity. The key difficulty comes from the fact that excitonic spectra details arise from atomicscale contributions that must be integrated over a large spatial domain containing a million and more of atoms. In this work we present a step-by-step solution to this problem: combined empirical tight-binding and configuration interaction scheme that unites linearly scaling computational time with the essentials of the atomistic modeling. We benchmark our method on the example of wellstudied self-assembled InAs/GaAs quantum dot. Next, we apply our atomistic approach to crystal phase quantum dots containing more than 10 million atoms.

SYSTEMATIC CALCULATIONS OF ENERGY LEVELS AND TRANSITION RATES OF BE-LIKE IONS WITH Z = 10–30 USING A COMBINED CONFIGURATION INTERACTION AND MANY-BODY PERTURBATION THEORY APPROACH

We report calculations of energy levels and radiative rates for transitions among the lowest 116 fine-structure levels arising from the configurations in Be-like ions with Z = 10–30. The wavelengths, oscillator strengths, line strengths, and radiative rates for all possible electric dipole, magnetic dipole, electric quadrupole, and magnetic quadrupole transitions among the 116 levels have been calculated using the combined configuration interaction and many-body perturbation method. The accuracy of the results is determined through extensive comparisons with existing laboratory measurements and theoretical results. The present complete set of results should be of great help in line identification and the interpretation of spectra, as well as in the modeling and diagnostics of astrophysical and fusion plasmas.

CI-MBPT: A package of programs for relativistic atomic calculations based on a method combining configuration interaction and many-body perturbation theory

This package of programs allows us to carry out relativistic calculations for many-electron atoms and ions. One can find energy levels and a number of atomic properties: g-factors, magnetic-dipole and electric-quadrupole hyperfine structure constants, electric- and magnetic-multipole transition amplitudes, and matrix elements of the parity nonconserving interactions. Method of calculation is based on a combination of conventional configuration interaction (CI) method and many-body perturbation theory (MBPT). The former explicitly accounts for the interaction between valence electrons, while the latter includes core–core and core–valence correlations. These two methods are combined to acquire benefits from both approaches and attain better accuracy. Program summaryProgram title: CI-MBPTCatalogue identifier: AEWV_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEWV_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 102640No. of bytes in distributed program, including test data, etc.: 583539Distribution format: tar.gzProgramming language:Fortran 90.Computer: Intel Core i5 CPU, 3.2 GHz.Operating system: Linux (CentOS 5, Ubuntu 12.04 LTS, SUSE 13.2).RAM: 8 GbClassification: 2.1.Nature of problem: Prediction of atomic or ionic energy levels and different observables in the framework of relativistic approach.Solution method: The package of programs determines energy levels and associated many-electron wave functions for states of atoms and ions in the pure CI or CI+MBPT approximations. Using the wave functions, different atomic properties can be obtained, including g-factors, magnetic-dipole and electric-quadrupole hyperfine structure constants, electric- and magnetic-multipole transition amplitudes, P-odd and P,T-odd amplitudes.Restrictions: The package is not designed for calculations of high Rydberg and autoionizing states. It becomes inefficient for the number of valence electrons exceeding four or five. It has not been tested for superheavy elements. The maximal number of Hartree–Fock–Dirac (HFD) orbitals when HFD equations are solved is 32.Unusual features: One-electron orbitals outside the nucleus are defined on radial grid points. Inside the nucleus they are described in a form of the Taylor expansion over r/R, where R is the nuclear radius.Additional comments: All programs have been compiled, linked, and tested with both “ifort” and freely available “gfortran”.Running time: Changes from tens of minutes for atoms with two valence electrons to tens of hours for more complex systems.

SYSTEMATIC CALCULATIONS OF ENERGY LEVELS AND TRANSITION RATES OF C-LIKE IONS WITH Z = 13-36

Based on systematic calculations using a combined relativistic configuration interaction and a many-body perturbation theory (MBPT) approach, we provide a complete and consistent data set for 46 levels belonging to the 2s 22p 2, 2s2p 3, 2p 4, 2s 22p3s, 2s 22p3p, and 2s 22p3d configurations in C-like ions with 13 ≤ Z ≤ 36. The data set includes energy levels as well as electric dipole, magnetic dipole, electric quadrupole, and magnetic quadrupole transition properties. Extensive comparisons with available observed and calculated results are made and indicate that the present MBPT calculations are highly accurate. The present data set can be used reliably for many purposes, such as the line identification of observed spectra, and modeling and diagnostics of astrophysical and fusion plasmas.