Intra-atomic Correlation Effects Research Articles

AbInitio Multiplet-Plus-Cumulant Approach for Correlation Effects in X-Ray Photoelectron Spectroscopy.

The treatment of electronic correlations in open-shell systems is among the most challenging problems of condensed matter theory. Current approximations are only partly successful. Ligand-field multiplet theory has been widely successful in describing intra-atomic correlation effects in x-ray spectra, but typically ignores itinerant states. The cumulant expansion for the one-electron Green's function has been successful in describing shake-up effects but ignores atomic multiplets. More complete methods, such as dynamic mean-field theory can be computationally demanding. Here, we show that separating the dynamic Coulomb interactions into local and longer-range parts with abinitio parameters yields a combined multiplet-plus-cumulant approach that accounts for both local atomic multiplets and satellite excitations. The approach is illustrated in transition metal oxides and explains the multiplet peaks, charge-transfer satellites, and distributed background features observed in XPS experiment.

Ni 3s-hole states in NiO by non-orthogonal configuration interaction

The origin of the features in the Ni 3s X-ray photoelectron spectrum of NiO is investigated using a non-orthogonal configuration interaction approach for an embedded [NiO 6] cluster. We study the interplay of inter-atomic screening with the metal core hole and intra-atomic exchange and electron correlation effects. We show that the spectrum can be described in terms of only few key configurations, provided that orbital relaxation effects are explicitly taken into account for the excited charge transfer configurations. The strength of this approach has been demonstrated earlier for those final states that have a high-spin coupling. In the present contribution the analysis is extended to include low-spin coupled 3s-hole states. The effects of enlarging the embedded cluster and of an improved representation of the nearest cluster surroundings were studied for the high-spin final states. We found only minor effects on the computed peak separations.

Intra-atomic correlation effects and the electronic and magnetic properties in nanotubes

The Gutzwiller method has been applied to investigate the intra-atomic correlation in the \ensuremath{\pi}-band of tubule $B(0,1)n$ versus the number of hexagons on the circumference $n.$ The consequences of the correlation on the electronic (gap) and magnetic (susceptibility) properties have been examined. The critical on-site correlation term $U$ to obtain ferromagnetism or antiferromagnetism has been derived with respect to $n.$ The transition from a paramagnetic to a diamagnetic magnetization obtained by Davids, Wang, Saxena, and Bishop has been studied within the present framework.

Influence of intra-atomic correlation effects on 3dstates in transition-metal compounds

The influence of intra-atomic correlation effects on 3d states was investigated in connection with the problem of explaining the metallic properties of TiO and VO. It has been shown that correlation effects tend to decrease the value of Hubbard parameter U and to increase the bandwidth W of the upper Hubbard band, i.e., to decrease the ratio U/W. This tendency is considerably more pronounced for early transition metal atoms. \textcopyright{} 1996 The American Physical Society.

Accurate Heitler-London interaction energy for He2

A 75-term Gaussian geminal wave function for the helium atom that has a variational energy within 0.42 μhartree of the exact one is constructed. It predicts an electron density that agrees to better than 0.4% with the predictions of energetically superior Hylleraas wave functions to electron-nucleus distances as large as 6 a 0 . The first-order Heitler-London interaction energy E (1) between a pair of helium atoms was computed using an antisymmetrized product of this Gaussian geminal wave function for each of the atoms. This interaction energy is an essential component in the exchange-Coulomb model for the He 2 potential. Our E (1) is probably converged to within 0.03 μhartree for interatomic distances between 3.0 a 0 and 7.5 a 0 . The Coulombic part of the interaction energy was checked by computations using even more accurate Hylleraas wave functions for the monomers. Comparison with an E (1) value computed from self-consistent-field atomic wave functions shows that intra-atomic correlation effects range between 4% and 9%.

Accurate Heitler-London interaction energy for He 2

A 75-term Gaussian geminal wave function for the helium atom that has a variational energy within 0.42 μhartree of the exact one is constructed. It predicts an electron density that agrees to better than 0.4% with the predictions of energetically superior Hylleraas wave functions to electron-nucleus distances as large as 6 a 0. The first-order Heitler-London interaction energy E (1) between a pair of helium atoms was computed using an antisymmetrized product of this Gaussian geminal wave function for each of the atoms. This interaction energy is an essential component in the exchange-Coulomb model for the He 2 potential. Our E (1) is probably converged to within 0.03 μhartree for interatomic distances between 3.0 a 0 and 7.5 a 0. The Coulombic part of the interaction energy was checked by computations using even more accurate Hylleraas wave functions for the monomers. Comparison with an E (1) value computed from self-consistent-field atomic wave functions shows that intra-atomic correlation effects range between 4% and 9%.

Nonadiabatic effects in charge transfer in atom-surface scattering.

Using a recently developd many-body theory, we investigate the velocity dependence of charge transfer in atom-surface scattering. We show that under certain conditions, intra-atomic correlation effects can introduce nontrivial sources of nonadiabaticity. For situations where a sharp Kondo resonance is formed near the Fermi energy, the system can exhibit strong nonadiabatic effects even at low velocities. This nonadiabatic behavior is caused by the relatively long-time scale for the formation or survival of the Kondo peak.

Non-adiabatic effects in charge transfer caused by electron correlation

Using a recently developed many-electron theory, we investigate the velocity dependence of charge transfer probabilities in atom-surface scattering. By analysis of the instantaneous spectral function of the atomic levels, the microscopic origins of non-adiabaticity can be determined. We show that, under certain conditions, intra-atomic correlation effects can introduce novel and non-trivial sources of non-adiabaticity, due to the formation of the Kondo resonance near the Fermi level of the substrate. The narrow width of the Kondo resonance introduces an additional long time scale, which persists even at low velocities.

Theoretical description of quasiresonant charge exchange in atom-surface collisions.

Many recent ion-surface scattering experiments, e.g., ${\mathrm{He}}^{+}$\ensuremath{\rightarrow}Pb, show dramatic oscillations of the neutralization probability as a function of the inverse ion velocity. These oscillations are qualitatively similar to the Stueckelberg oscillations that occur during quasiresonant charge exchange in atom-atom collisions. We present here a general many-body approach for the description of such charge-exchange processes near surfaces. We show that for the proper understanding of the oscillatory behavior it is crucial to include intra-atomic correlation effects. The interference between different tunneling channels influences both the amplitude and the frequency of these oscillations.

Intra-atomic correlation effects in charge transfer

In this paper, we discuss the effects of intra-atomic correlation on charge-transfer processes in atom-surface scattering. Intra-atomic correlation effects can conveniently be described using projection operators and a non-equilibrium Green's function technique. In the low velocity regime, it can be proved that the populations of the atomic states can be calculated directly from the solution of a system of coupled master equations. We apply this formalism to some typical atom-surface scattering situations and show that proper account of intra-atomic correlation effects must be taken for an accurate description of the charge-transfer process.

Anisotropic charge transfer between hydrogenic states and a metallic substrate.

By measuring the angular distribution of photon emission from desorbed excited atoms it is possible to measure the anisotropy in charge-transfer rates between metallic states and the oriented excited states outside of the surface. The measured anisotropy of 2.5 shows excellent agreement with the calculated value of 2.54 and clearly demonstrates that intra-atomic correlation effects must be taken into account when describing charge transfer.

Derivation of a master equation for charge-transfer processes in atom-surface collisions.

The time-dependent Anderson model for multiple levels interacting with a continuum is studied using the slave-boson Green's-function technique, with an eye to understanding charge-transfer phenomena for an atomic species moving outside a metallic surface. It is shown that in the finite-temperature and low-velocity limit, the equations for the occupation numbers satisfy a master equation. Application to charge-exchange processes in atom-surface-scattering experiments shows that the presence of strong intra-atomic correlation effects can drastically change the charge-transfer dynamics.

Correlation Effects of Third-order Perturbation in the Extended Hubbard Model

Using the local approach, we have performed a third�order perturbation calculation to investigate the effects of intra-atomic electron correlation and electron and spin correlation between nearest neighbour sites in the extended Hubbard model. We found that significant correction of the third order over the second order results and, in comparison with the results of the third-order perturbation where only the intra-atomic electron correlation is included, the influence of the electron and spin correlation between nearest neighbour sites on the correlation energy is non-negligible.

Intraatomic correlation effects for the He–He dispersion and exchange–dispersion energies using explicitly correlated Gaussian geminals

The coupling of the intermolecular interaction with the intramolecular correlation effects is considered using the coupled cluster (CC) formalism. The CC equations for the dispersion energy are presented and their relation to the double perturbation theory is analyzed. An approximate scheme based on partial decoupling of the CC equations is applied for the He–He interaction. Numerical results are obtained using explicitly correlated Gaussian geminal basis set. They confirm the importance of the intraatomic (apparent) correlation effects and agree very well with the experimentally derived potential.

Interaction potential between a helium atom and metal surfaces.

By employing an S-matrix theory for evanescent waves, the repulsive potential between a helium atom and corrugated metal surfaces has been calculated. P-wave interactions and intra-atomic correlation effects were found to be very important. The corrugation part of the interaction potential is much weaker than predicted by the effective-medium theory. Application to Cu, Ni, and Ag (110) surfaces gives good agreement with experiment without any adjustable parameters.

Exchange-perturbation calculations of the interaction energy between Be atoms including intra-atomic correlation effects

The interaction energy between Be atoms in the region of small overlap is calculated by means of exchange-perturbation theory. Intra-atomic correlation corrections to the first-order energy and to the dispersion energy are allowed for. The potential energy curve reveals no minimum for R ⩾ 6.0 a o and the results are fairly close to those of Liu and McLean. It is argued that Be 2 is neither a van der Waals complex nor a typical molecule.

Charge states for protons moving in an electron gas: intra-atomic correlation and surface effects

Intra-atomic correlation and surface effects on the charge states of protons moving in an electron gas have been analysed. The main conclusion is that surface effects are of primary importance to the understanding of the experimental charge fractions of protons moving at low velocities. By means of a simple model, the authors have shown that proton velocities can be correlated with those effective electron densities determining the charge states of protons.

Theory of the electronic and atomic structure of Si(111): Surface spin-polarization effects

The electronic structure 1×1 and 2×1 reconstructed Si(111) surfaces are examined using a simple theoretical model which incorporates intra-atomic correlation effects. Electron spin polarization is shown to lead to a low-energy nonmetallic ground state for the nonbuckled Si(111) surface which is energetically more favorable than the metallic, half-filled band state. Spin- polarization and spin-ordering effects are shown to be important for the buckled surface. The incorporation of electronic correlations and spin in the calculations yields results consistent with photoemission, optical, and low energy electron diffraction data. The calculated oscillator strength for optical transitions between dangling-bond bands and its anisotropy are found to be in satisfactory agreement with experimental data. The buckling is estimated to be 0.3 Å.

Analysis of intra-atomic correlation effects on the first order interaction energy between He atoms

The first order interaction energy between two He atoms is calculated with a large CI function (reproducing 97·2 per cent of the correlation energy of the He atom) for interatomic distances R within the range 3·0–5·6 a 0. It is shown that the intra-atomic correlation effect gives a repulsive contribution which increases with R up to 7 per cent of the uncorrelated result in the region of the van der Waals minimum (R = 5·6 a 0). Basis set effects and approximate methods of estimation of the intramolecular correlation effects are also discussed.

A simple semi-empirical approach to the intermolecular potential of van der Waals systems

A simple potential model for the evaluation of isotropic pairwise interaction energies is proposed and used here to predict the lowest triplet state curve of all alkali-alkali interactions. The model, which for the alkali dimers contains no adjustable parameters, expresses the interaction potential in terms of the Hartree-Fock interaction energy, V HF(R), and the interatomic correlation energy as approximated semi-empirically from the second order dispersion energy calculated including the effect of charge overlap between the electron clouds of the two interacting species, V inter/disp(R). Lack of repulsive near Hartree-Fock ab initio results for the triplet state of most alkali dimers has been circumvented by proposing a generalized screened Coulomb type repulsion for these systems. The sum of the additional contributions to the interaction energy [V intra(R) + V intra-inter(R)], which arise from intra-atomic correlation effects and intra-inter coupling terms, is also shown to be well approximated by int...