Role Of Electron Correlation Research Articles

Ab initio electronic and optical spectra of free-base porphyrins: The role of electronic correlation

We present a theoretical investigation of electronic and optical properties of free-base porphyrins based on density functional theory and many-body perturbation theory. The electronic levels of free-base porphine (H(2)P) and its phenyl derivative, free-base tetraphenylporphyrin (H(2)TPP) are calculated using the ab initio GW approximation for the self-energy. The approach is found to yield results that compare favorably with the available photoemission spectra. The excitonic nature of the optical peaks is revealed by solving the Bethe-Salpeter equation, which provides an accurate description of the experimental absorption spectra. The lowest triplet transition energies are in good agreement with the measured values.

Probing collective modes of correlated states of few electrons in semiconductor quantum dots

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons.

Local moments and magnetic order in the two-dimensional Anderson-Mott transition

We study the role of electronic correlation in a disordered two-dimensional model by using a variational wave function that can interpolate between Anderson and Mott insulators. Within this approach, the Anderson-Mott transition can be described both in the paramagnetic sector and in the magnetic sector. In the latter case, we find evidence for the formation of local magnetic moments that order before the Mott transition. The charge gap opening in the Mott insulator is accompanied by the vanishing of the ${\text{lim}}_{q\ensuremath{\rightarrow}0}\overline{⟨{n}_{q}⟩⟨{n}_{\ensuremath{-}q}⟩}$ (the bar denoting the impurity average). The role of a frustrating (second-neighbor) hopping is also discussed, with a particular emphasis to the formation of metastable spin-glass states.

Core-level and valence-band study using angle-integrated photoemission onLaFeAsO0.9F0.1

Using angle-integrated photoemission spectroscopy we have probed the novel ${\text{LaFeAsO}}_{0.9}{\text{F}}_{0.1}$ superconductor over a wide range of photon energies and temperatures. We have provided a full characterization of the orbital character of the valence-band (VB) density of states (DOS) and of the magnitude of the $d\text{\ensuremath{-}}p$ hybridization energy. Finally, we have identified two characteristic temperatures: 90 K where a pseudogap-like feature appears to close and 120 K where a sudden change in the DOS near ${E}_{F}$ occurs. We associate these phenomena with the spin density wave magnetic ordering and the structural transition seen in the parent compound, respectively. These results suggest the important role of electron correlation, spin physics, and structural distortion in the physics of Fe-based superconductors.

Role of electron correlation and long range magnetic order in the electronic structure of [formula omitted

The room temperature photoemission spectra collected at different surface sensitivities reveal qualitatively different surface and bulk electronic structures in CaRuO 3 and SrRuO 3 . The extracted bulk spectra are dominated by the coherent feature intensity with a weak correlation induced feature at higher binding energies. The First principle calculations provide a wonderful representation of the bulk spectra for the effective electron correlation strength, U / W ∼ 0.2 as expected for highly extended 4d systems. This resolves a long-standing issue that arose due to the prediction of large U / W similar to 3d systems. Photoemission spectra across the magnetic phase transition reveal unusual evolution exhibiting a large reduction in the coherent feature intensity in the bulk spectrum of SrRuO 3 , while the bulk spectrum in CaRuO 3 remains almost the same down to the lowest temperature studied.

The role of electron correlations in the double ionization of helium by fast protons

We consider the double ionization (DI) of a helium atom by fast protons and study the role of electron correlations in this process. We develop a quantum-mechanical approach that takes into account the interaction of the emitted electrons in continuum and the dynamical charge screening of the ejected particles, which depends on their ejection kinematics. The interaction of the emitted electrons between themselves and with the core is described in the model of approximate 3C functions, while the dynamical charge screening is described by introducing effective charges of the emitted electrons and the ion core, which are determined by the particle momenta. The derived closed analytical expressions for the differential ionization cross sections have been applied to the case of a coplanar particle ejection geometry at various momenta q transferred to the atom. Analysis of our calculations has shown that the developed model describes adequately the available experimental data. Including the dynamical charge screening has a significant effect on the DI cross section and improves considerably the agreement between theory and experiment.

Structural and electronic properties of Mg, Zn, and Cd from Hartree-Fock and density functional calculations including hybrid functionals

The crystal structures of Zn and Cd deviate from an ideal hexagonal close packing by a significantly increased $c∕a$ ratio. In order to investigate the electronic reason for this deviation, especially with regard to the nearly ideal hcp element Mg, Hartree-Fock and density functional theory calculations were performed, employing various functionals within the local density or the generalized gradient approximations as well as hybrid functionals. The cohesive energy, lattice constants optimized with respect to the energy and elastic constants were computed. The role of electronic correlation in consideration of the filled $d$-shell is emphasized, postulating different intra- and inter-layer interactions, both in Zn and Cd. On the potential energy surface in the space of varying lattice constants, a path is explored that corresponds to a uniaxial compression along the $c$ axis. In contrast to Mg, the potential energy surface of Zn and Cd is very flat along this path, and an electronic topological transition occurs, leading to a Mg-like band structure.

Fast electron and X-ray scattering as a tool to study target's structure

We concentrate on several relatively new aspects of the study of fast electron and X-ray scattering by atoms and atom-like objects, namely endohedral atoms and fullerenes. However, main attention is given to fast charge particle scattering. We show that the corresponding cross-sections, being expressed via so-called generalized oscillator strengths (GOS), give information on the electronic structure of the target and on the role of electron correlations in it. We consider what sort of information became available when analyzing the dependence of GOS upon their multipolarity, transferred momentum q and energy ω. To obtain theoretical results, we employ both the one-electron Hartree–Fock approximation and account for the multi-electron correlation in the target, using the random phase approximation with exchange. We demonstrate the role of non-dipole corrections in the small-angle fast-electron inelastic scattering. There dipole contribution dominates while non-dipole corrections can be considerably and controllably enhanced as compared to the case of low and medium energy photoionization. We show also that analyses of GOS for discrete level excitations permit to clarify their multipolarity. The results of calculations of Compton excitation and ionization cross-sections are presented. Attention is given to cooperative effects in inelastic fast electron-atom scattering that results in directed motion of the secondary electrons, a phenomenon that is similar to “drag currents” in photoionization. We demonstrate how one should derive GOS for endohedral atoms, e.g. A@ C 60 and what is the additional information that can be obtained from corresponding GOS. Most of discussions are illustrated by the results of concrete calculations.

Role of electronic correlations in the transport characteristics of molecular junctions

We study the effects of electronic correlations in the transport characteristics of this molecular junction made of the two-atom molecule connected to the noninteracting leads. We compare the results of two different approaches: (1) the spin unrestricted Hartree–Fock (UHF) method and (2) equation-of-motion (EOM) technique, allowing us to treat accurately all the intramolecular electron interactions.

Electron correlations in a C20 fullerene cluster

The ground-state properties of C$_{20}$ fullerene clusters are determined in the framework of the Hubbard model by using lattice density-functional theory (LDFT) and scaling approximations to the interaction-energy functional. Results are given for the ground-state energy, kinetic and Coulomb energies, local magnetic moments, and charge-excitation gap, as a function of the Coulomb repulsion $U/t$ and for electron or hole doping $\delta$ close half-band filling ($|\delta| \le 1$). The role of electron correlations is analyzed by comparing the LDFT results with fully unrestricted Hartree-Fock (UHF) calculations which take into account possible noncollinear arrangements of the local spin-polarizations. The consequences of the spin-density-wave symmetry breaking, often found in UHF, and the implications of this study for more complex fullerene structures are discussed.

Angular distribution in two-photon double ionization of helium by intense attosecond soft-x-ray pulses

We investigate two-photon double ionization of helium by intense ({approx_equal}10{sup 15} W/cm{sup 2}) ultrashort ({approx_equal}300 as) soft-x-ray pulses (E=91.6 eV). The time-dependent two-electron Schroedinger equation is solved using a coupled channel method. We show that for ultrashort pulses the angular distribution of ejected electrons depends on the pulse duration and provides insight into the role of electron correlations in the two-electron photoemission process. The angular distribution at energies near the 'independent-electron' peaks is close to dipolar while it acquires in the 'valley' of correlated emission a significant quadrupolar component within a few hundred attoseconds.

Electric dipole transition amplitudes for 207Pb+

Electric dipole transition amplitudes of certain low-lying states of Pb+ have been calculated using the relativistic coupled-cluster theory and compared with previous calculations. The role of electron correlation is found to be important. Some of the results we have obtained would be useful in estimating the size of the parity non-conserving amplitude for the 6p2P1/2 → 6p2P3/2 transition.

Electron Correlation in Metals

Since the discovery of high Tc superconductivity, the role of electron correlation in superconductivity has been an important new issue in condensed matter physics. Here the role of electron correlation in metals is explained in detail on the basis of the Fermi liquid theory. The book discusses the following issues: enhancements of electronic specific heat and magnetic susceptibility; effects of electron correlation on transport phenomena such as electric resistivity andHall coefficient; magnetism; Mott transition and unconventional superconductivity. These originate commonly from the coulomb repulsion between electrons. In particular, superconductivity in strongly correlated electron systems is discussed with a unified point of view. This book is written to explain interesting physics in metals for undergraduate and graduate students and researchers in condensed matter physics.

Electron correlation effects in hyperfine interactions in 45Sc and 89Y

Relativistic coupled-cluster theory has been employed to calculate the magnetic dipole and electric quadrupole hyperfine structure constants for the stable isotopes 45Sc and 89Y. The results are in good agreement with available experimental data. The role of electron correlation is found to be important. The trend exhibited by these effects in the systems under consideration with d-valence electrons is rather different from previously studied s and p single valence atomic systems. The contributions of certain higher order correlation effects are about 10–20%. Electric quadrupole hyperfine structure constants for 4d2D3/2 and 4d2D5/2 states are presented for the first time.

Temperature Dependence of the Optical Spectral Weight in the Cuprates: Role of Electron Correlations

We compare calculations based on the dynamical mean-field theory of the Hubbard model with the infrared spectral weight W(Omega,T) of La(2-x)SrxCuO4 and other cuprates. Without using fitting parameters we show that most of the anomalies found in W(Omega,T) with respect to normal metals, including the existence of two different energy scales for the doping and the T dependence of W(Omega,T), can be ascribed to strong correlation effects.

Theoretical study of polarons and self-trapped excited states in one-dimensional C 60 crystal

The role of electron correlation, conjugation of π-electrons on formation of polarons and excitons in C 60 solids has been investigated. For description of the polaron and exciton we use a generalized Su–Shrieffer–Heeger (SSH) model for the intermolecular and intramolecular degrees of freedom including a Hubbard-type on-site interaction. It is shown that in the case of a linear chain with one extra electron (hole), for weak π-conjugation a polaron is generated. In the case of the electron excitation from the highest occupied state in the upper valence band to the lowest unoccupied state in the lower conductance band, a Frenkel exciton is formed on the central C 60 molecule in the supercell at small conjugation parameter. The formation of the charge-transfer exciton is observed in the case of the electron excitation from the highest occupied state in the upper valence band to the third lowest unoccupied state in the lower conduction band.

Role of electronic correlations on the ground-state properties and on the pressure-induced metal-insulator transition inBaVS3

We investigated the structural, magnetic and electronic properties of the hexagonal perovskite $\mathrm{Ba}\mathrm{V}{\mathrm{S}}_{3}$ by means of first-principles calculations within the density functional theory in the local spin density approximation (LSDA) that includes the Hubbard repulsion term $U\phantom{\rule{0.3em}{0ex}}(\mathrm{LSDA}+U)$ to take into account electronic correlations. We find that the $\mathrm{LSDA}+U$ scheme greatly improves on the LSDA results previously reported, and quantitatively accounts for all ground state properties found experimentally. First, the $\mathrm{LSDA}+U$ predicts an orthorhombic structure and a quasi-metallic ground state with a long-range antiferromagnetic (AFM) order in the quasi-hexagonal $ab$-plane, ferromagnetically (FM) coupled along the $c$-axis. Second, we studied the stability of competing crystal structures and competing magnetic orderings in terms of exchange integrals. The results account well for the experimental pressure-dependence of the metal-insulator transition and for the chemical-pressure induced AFM-FM transition reported recently in Sr-substituted samples. In particular, at the experimental value of the volume, $V$, we obtain an energy gap $\ensuremath{\Delta}=47\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, which falls in the range of experimental values $(43\char21{}59\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$, while, at smaller $V$ values, we find an AFM-FM transition, in agreement with the above effects of Sr-induced chemical pressure. Finally, in the metallic phase, we find a nearly isotropic electrical conductivity, in agreement with experiments, despite the presence of quasi-one-dimensional chains of $\mathrm{V}{\mathrm{S}}_{6}$ along the $c$-axis. We account for such three-dimensional conductivity in terms of the strong interaction between $V$ and $S$ orbitals within each $\mathrm{V}{\mathrm{S}}_{3}$ chain and between adjacent chains.

High-harmonic generation by quantum-dot nanorings

Exact numerical results are obtained within the extended Hubbard Hamiltonian for nanorings consisting of Ag quantum dots (QD’s ) with C6v symmetry which interact with a circularly polarized light. The results show that the high-harmonic generation (HHG) spectra obtained from such artificial “molecules” are more pronounced than the HHG spectra obtained from a real molecule such as benzene. Our studies show that the HHG spectra obtained from the QD nanorings consist of two plateaus while only one plateau appears for benzene. The role of electron correlations in the generation of the high-order harmonics is studied, and it is shown that it can increase the intensity of the high-order harmonics. Mainly affected are the harmonics which are located in the second plateau. Selection rules for the produced high harmonics and a new “synergetic” selection rule for the symmetry of the states contributing to the HHG spectrum, a combined effect of spatial and charge conjugation symmetries, are discussed.

Equilibrium geometries of low-lying isomers of some Li clusters, within Hartree-Fock theory plus bond order or MP2 correlation corrections.

In a recent study by Kornath et al. [J. Chem. Phys. 118, 6957 (2003)], the Li(n) clusters with n=2, 4, and 8 have been isolated in argon matrices at 15 K and characterized by Raman spectroscopy. This has prompted us to carry out a theoretical study on such clusters up to n=10, using Hartree-Fock theory, plus low-order Møller-Plesset perturbation corrections. To check against the above study of Kornath et al., as a by-product we have made the same approximations for n=6 and 8 as we have for n=10. This has led us to emphasize trends with n through the Li(n) clusters for (i) ground-state energy, (ii) HOMO-LUMO energy gap, (iii) dissociation energy, and (iv) Hartree-Fock eigenvalue sum. The role of electron correlation in distinguishing between low-lying isomers is plainly crucial, and will need a combination of experiment and theory to obtain decisive results such as that of Kornath et al. for Li(8). In particular, it is shown that Hartree-Fock theory plus bond order correlations does account for the experimentally observed symmetry T(d) symmetry for Li(8).

Electronic structure and states in two-dimensional triangular cobalt oxides: The role of electronic correlation

We obtain the electronic states and structures of two-dimensional cobalt oxides, Na$_{x}$CoO$_{2}$ (x=0, 0.35, 0.5 and 0.75) by utilizing the full-potential linear muffin-tin orbitals (FP-LMTO) methods, from which some essential electronic interaction parameters are estimated: the bare on-site Coulomb interaction of cobalt U$_{dd}$=7.5 eV renormalizes to 5 eV for x=0.35, the $pd$ hybridizations t$_{pd\sigma}$ and t$_{pd\pi}$ are -1.40 and 0.70 eV, respectively. The density of states at E$_{F}$ decreases from 6-7 states/eV in the local density approximation (LDA) to about 1.0 states/eV in the LDA+U scheme. The role of the intercalation of water molecules and the microscopic mechanism of the superconductivity in Na$_{0.35}$CoO$_{2}$$\cdot$mH$_{2}$O is discussed.