Study Of Electron Correlation Effects Research Articles

Effective interaction for vanadium oxyhydrides Srn+1VnO2n+1Hn ( n=1 and n→∞ ): A constrained-RPA study

Transition metal oxides have been one of the central objects in the studies of electron correlation effects because of their rich variety of physical properties mainly depending on the transition metal element. On the other hand, exploiting the anion degrees of freedom is less popular but can be another promising way to control properties of strongly correlated materials. In particular, oxyhydrides offer a unique playground of strongly correlated low-dimensional electronic structure, where the $s$ orbitals of hydrogen breaks a chemical bond between the cation $t_{2g}$ orbitals. In this study, we evaluate the effective interaction, i.e., the screened Coulomb interaction parameters in low-energy effective models, for vanadium oxyhydrides Sr$_{n+1}$V$_n$O$_{2n+1}$H$_n$ ($n=1,\infty$) using the constrained random-phase approximation (cRPA). We find that the effective interaction in the $t_{2g}$ model, where only the $t_{2g}$ orbitals are explicitly considered, is strongly screened by the $e_g$ bands compared with that for oxides, because the $e_g$ bands are much entangled with the $t_{2g}$ bands in the oxyhydrides. On the other hand, the effective interaction is rather strong in the $d$ model, where all the vanadium $d$ orbitals are explicitly considered, owing to a large energy separation between the V-$d$ bands and the anion bands (O-$p$ and H-$s$), because the O-$p$ states are stabilized by the existence of the hydrogen atoms. These findings suggest that non-trivial and unique correlation effects can take place in vanadium oxyhydrides.

Ab initio studies of electron correlation effects in magnetic dipolar hyperfine interaction of Cs

The hyperfine-structure constant A of the S, P, and D states in Cs atom with the principal quantum number are calculated using the Dirac–Fock approximation, the low-order many-body perturbation theory, and relativistic coupled-cluster method in single and double approximations with and without the inclusion of nonlinear terms. The importance of electron correlation effects, especially the nonlinear corrections of the cluster operators, is demonstrated by comparing the results of various approximations with available experimental values. Moreover, the correlation trends for individual electron correlation effects involving direct, core-polarization, and pair-correlation ones in members of Rydberg series are also investigated in the framework of the coupled-cluster theory. Some interesting features are observed.

Correlated electronic states of SrVO3 revealed by angle-resolved photoemission spectroscopy

In this article, we review recent progress in angle-resolved photoemission (ARPES) studies of the Mott-Hubbard-type correlated electron systems SrVO3. It has the d1 electron configuration and is an ideal model compound to study electron correlation effects in normal metal. ARPES studies of bulk single-crystal SrVO3 and CaVO3 have revealed the difference in the mass renormalization of electrons between them. In-situ ARPES studies of thin films fabricated by the pulsed laser deposition method have clarified not only quasi-particle dispersions, which show a kink like high-Tc cuprates, but also finite dispersions in the incoherent part. Self-energy in a wide energy range has been deduced from the ARPES spectral weight using Kramers–Kronig transformation. The obtained self-energy has several structures which yield the incoherent feature and a pseudogap-like dip similar to the high-Tc cuprates. Quantum-well states in ultrathin films of SrVO3 have revealed sub-bands with correlated electrons. These findings of electron correlation effects outlined in the present article would provide a starting point not only for fundamental condensed-matter physics but also for the development of new devices with correlated electrons.

Dyson orbitals of N2O: Electron momentum spectroscopy and symmetry adapted cluster-configuration interaction calculations

Electron momentum spectroscopy and symmetry adapted cluster-configuration interaction (SAC-CI) theory were combined to study electron correlation effects in nitrous oxide molecule (N(2)O). The SAC-CI General-R method accurately reproduced the experimental ionization spectrum. This bench-marked method was also introduced for calculating the momentum distributions of N(2)O Dyson orbitals. Several calculated momentum distributions with different theoretical methods were compared with the high resolution experimental results. In the outer-valence region, Hartree-Fock (HF), density functional theory (DFT), and SAC-CI theory can well describe the experimental momentum distributions. SAC-CI presented a best performance among them. In the inner-valence region, HF and DFT cannot work well due to the severe breaking of the molecular orbital picture, while SAC-CI still produced an excellent description of experimental momentum profiles because it can accurately take into account electron correlations. Moreover, the thermally averaged calculation showed that the geometrical changes induced by the vibration at room temperature have no noticeable effects on momentum distribution of valence orbitals of N(2)O.

Ab initio studies of electron correlation effects in the atomic parity violating amplitudes in Cs and Fr

We have studied the correlation effects in Cs and Fr arising from the interplay of the residual Coulomb interaction to all orders and the neutral weak interaction which gives rise to the parity violating electric dipole transition to first order, within the framework of the relativistic coupled-cluster theory which circumvents the constrain of explicitly summing over the intermediate states. We observe that the contributions arising from the perturbed doubly excited states are quite significant and hence, any calculation should not be considered accurate unless it includes the perturbed double excitations comprehensively. In this paper, we report a comparative study of various results related to the parity violation in Cs and Fr.

Ab initiowave function-based methods for excited states in solids: Correlation corrections to the band structure of ionic oxides

Ab initio wave function-based methods are applied to the study of electron correlation effects on the band structure of oxide systems. We choose MgO as a prototype closed-shell ionic oxide. Our analysis is based on a local Hamiltonian approach and performed on finite fragments cut from the infinite solid. Localized Wannier functions and embedding potentials are obtained from prior periodic Hartree-Fock (HF) calculations. We investigate the role of various electron correlation effects in reducing the HF band gap and modifying the bandwidths. On-site and nearest-neighbor charge relaxation as well as long-range polarization effects are calculated. Whereas correlation effects are essential for computing accurate band gaps, we found that they produce smaller changes on the HF bandwidths, at least for this material. Surprisingly, a broadening effect is obtained for the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ valence bands. The ab initio data are in good agreement with the energy gap and bandwidth derived from thermoreflectance and x-ray photoemission experiments. The results show that the wave function-based approach applied here allows for well controlled approximations and a transparent identification of the microscopic processes which determine the electronic band structure.

Quantum criticalities induced by Lifshitz transitions

We study electron correlation effects on quantum criticalities of Lifshitz transitions (LTs), using the mean-field theory based on a preexisting broken symmetry order, in two-dimensional systems. In the presence of interactions, LTs may become discontinuous in contrast to the continuous transition in the original proposal by Lifshitz for noninteracting systems. We find that the quantum criticality at the endpoint of discontinuous LTs, which we call the marginal quantum critical point, belongs to a different universality class from that of the mean-field Ising model.

Quantum and Topological Criticalities of Lifshitz Transition in Two-Dimensional Correlated Electron Systems

We study electron correlation effects on quantum criticalities of Lifshitz transitions at zero temperature, using the mean-field theory based on a preexisting symmetry-broken order, in two-dimensional systems. In the presence of interactions, Lifshitz transitions may become discontinuous in contrast to the continuous transition in the original proposal by Lifshitz for noninteracting systems. We focus on the quantum criticality at the endpoint of discontinuous Lifshitz transitions, which we call the marginal quantum critical point. It shows remarkable criticalities arising from its nature as a topological transition. At the point, for the canonical ensemble, the susceptibility of the order parameter chi is found to diverge as ln 1/|delta Delta| when the ``neck'' of the Fermi surface collapses at the van Hove singularity. More remarkably, it diverges as 1/|delta Delta| when the electron/hole pocket of the Fermi surface vanishes. Here delta Delta is the amplitude of the mean field measured from the Lifshitz critical point. On the other hand, for the grand canonical ensemble, the discontinuous transitions appear as the electronic phase separation and the endpoint of the phase separation is the marginal quantum critical point. Especially, when a pocket of the Fermi surface vanishes, the uniform charge compressibility kappa diverges as 1/|delta n|, instead of chi, where delta n is the electron density measured from the critical point. Accordingly, Lifshitz transition induces large fluctuations represented by diverging chi and/or kappa. Such fluctuations must be involved in physics of competing orders and influence diversity of strong correlation effects.

Numerical algorithm for the double-orbital Hubbard model: Hund-coupled pairing symmetry in the doped case

In order to numerically study electron correlation effects in multiorbital systems, we propose a new type of discrete transformation for the exchange (Hund's coupling) and pair-hopping interactions to be used in the dynamical mean field $\text{theory}+\text{quantum}$ Monte Carlo method. The transformation, which is real and exact, turns out to suppress the sign problem in a wide parameter region including non-half-filled bands. This enables us to obtain the dominant pairing symmetry in the double-orbital Hubbard model, which shows that the spin-triplet, orbital-antisymmetric pairing that exploits Hund's coupling is stable in a wide region of the band filling.

Electron correlation effects in band structure of magnetic clusters Mn 12 and Fe 8

We present calculations of electronic structure and local magnetic moment of 3d metals in [Mn 12O 12(CH 3COO) 16(H 2O) 24]·2CH 3COOH·4H 2O and [(C 6H 15N 3) 6Fe 8O 2(OH) 12]Br 7(H 2O)Br·8H 2O, abbreviated Mn 12 and Fe 8Br 8, respectively. The comparison of calculated and experimental soft X-ray spectra shows that p–d mixing is very sensitive to the value of the local Coulomb repulsion parameter U. This result is found to be in agreement with experimental resonant inelastic X-ray scattering spectra (RIXS) and X-ray photoelectron spectra (XPS), which have been used to study electron correlation effects in magnetic clusters. The O 2p–Me 3d separation found in the experiment, is well reproduced by LDA + U band structure calculations, which include on-site Coulomb interactions. The 3d metal L 2,3 RIXS behaviour reflects the presence of the local magnetic moment and confirms the existence of the energy gap in the 3d band of the metal.

Thermodynamic studies of electron correlation effects on organic salts based on BEDT-TTF and DCNQI molecules

A thermodynamic investigation on the electronic properties of several organic salts based on BEDT-TTF and DCNQI molecules is performed by means of high-precision specific-heat measurements in the temperature range between 0.9 and 6 K. The electronic specific-heat coefficient γ for dimeric insulators is almost 0 mJ/mol K 2, which is explained by the picture of Mott–Hubbard gap in the effectively half-filled band. The non-dimeric salt of (DI-DCNQI) 2Ag, on the other hand, shows an unexpectedly large γ value of about 30–40 mJ mol −1 K −2 representing the existence of large low-energy excitations from the charge-ordered insulating ground state. The difference of the low-temperature electronic specific-heat behavior between the dimeric and non-dimeric systems demonstrates that manifestation of electron correlation effects is qualitatively different in these systems.

Analysis of large-scale multi-configuration self-consistent field wave functions by expectation values of local operators

We have developed a method to analyze multi-configuration self-consistent field wave functions for open-shell systems, which may consist of a very large number of Slater determinants, by calculating their expectation values for (local) one- and two-particle operators in a numerically efficient way. Although the proposed formalism is fully general and applies to any operators referring to a subset of arbitrary orthogonal molecular orbitals, the local operators we use here are defined in terms of localized molecular orbitals. This choice permits to perform a chemically intuitive analysis of the local spin, population and charge fluctuation, also allowing to study electron correlation effects in chemical bonding. We present some applications for ionic and covalent systems considering four small molecules, H2, N2, H2O and CH4, as well as a binary transition metal complex, [Fe2S6]6−.

A graphical unitary group approach to study multiplet specific multichannel electron correlation effects in the photoionization of O2

We have implemented an efficient multichannel configuration–interaction complete-active-space (MCCI-CAS) approximation to study electron–correlation effects in molecular photoionization. This approach is based on the graphical unitary group approach (GUGA) for computing matrix elements of the Hamiltonian and includes target relaxation, correlation, and polarization as well as correlation due to coupling between different asymptotic scattering channels. The statistical rule, which partitions the total cross section into multiplets by simple spin statistics, is easily derived in this formalism. The scattering equations are solved using the Schwinger variational method. We present multiplet specific results of a detailed MCCI-CAS Schwinger study of the photoionization of molecular oxygen in the photon energy region of 12.3–20.4 eV, including up to four coupled electronic channels. Our results show the importance of using correlated target states. We have obtained all of the autoionization structure near threshold that has been assigned experimentally. In addition, we predict structure not yet resolved by experiment.

Distributed Gaussian basis sets: description of the Hartree-Fock ground state energies of N2, CO and BF using s- and p-type Gaussian functions

The ground state Hartree-Fock electronic energy for the nitrogen molecule is determined by using basis sets of distributed s- and p-type Gaussian functions and compared with the results of fully numerical (finite difference) Hartree-Fock calculations. Calculations are also reported for the ground states of the CO and BF molecules and a universal distributed Gaussian basis set established. A potential energy curve is calculated for the CO molecule and the calculation of energy derivatives is considered. The use of distributed basis sets of s and p Gaussian functions in the study of electron correlation effects using many-body perturbation theory is demonstrated.

Local-spin-selective X-ray absorption: a new technique to study correlated materials

K edge X-ray absorption spectral shapes can be detected by measuring the Kβ X-ray emission decay channel. Due to the coherence of the excitation and decay processes, the convolution of the spectral shape with the 1s life time broadening disappears. The increased resolution makes possible the study of electron correlation effects in the pre-edge peaks of K edges. Due to the 3p3d exchange interaction, the Kβ decay channel has separate peaks for spin-up and spin-down. This spin-selection is exploited in so-called local-spin-selective X-ray absorption. From its comparison with X-ray magnetic circular dichroism (X-MCD) it is possible to determine the energy dependence of the Fano factor. This is shown for MnP.

Electron correlation effects and induced superconductivity in La xSr 1−xTiO 3

A study of electron correlation effects on the properties of La xSr 1−xTiO 3 is presented using a single band repulsive Hubbard model and employing Gutzwiller approximation. A possibility of inducing superconductivity in this material is suggested.

A comparison of autotasking and macrotasking in a quantum chemical application program on a cray Y-MP computer

The results of some experiments in parallel processing using either autotasking or macrotasking on a dedicated CRAY Y-MP/4 computer under the UNICOS (version 7.0.2) operating system are reported. Two sets of calculations were carried out using the quantum chemical application program ccMBPT ( concurrent computation Many-Body Perturbation Theory) for the study of electron correlation effects in atoms and molecules. In the first set, multiprocessing was achieved by exploiting the autotasking features of the compiler whilst, in the second set, an explicitly macrotasked code was employed. The performance of the system was mentioned as a function of the size of the problem, i.e. the number of virtual orbitals. It is demonstrated that the explicitly macrotasked code leads to uniformly better performance than the autotasked code.

Parallel processing using macro-tasking in a multi-job environment on a CRAY Y-MP computer

The results of some experiments in parallel processing using macro-tasking in a multi-job environment on the CRAY Y-MP computer under the UNICOS operating system are reported. Ten jobs performing identical calculations using the ccMBPT ( concurrent computation Many-Body Perturbation Theory) program for the study of electron correlation effects in atoms and molecules are executed in dedicated sessions. In each session, n ( n = 0, 1,…, 10) of the jobs are macrotasked using four processes and 10 − n jobs are executed as a single process. The performance of the system is monitored as a function of the number of multitasked jobs. It is demonstrated that there is no degradation in the performance of the system as the proportion of multitasked jobs is increased.

Finite cluster studies of electron correlation effects

Abstract A review of the physical properties of small systems described by various model Hamiltonians is given. Their physical properties are found either exactly or by using Monte-Carlo methods, and are compared with properties of bulk systems. It is shown that (i) clusters differ in an essential way from the bulk, having electronic states strongly dependent on the geometry and cluster size, but, at the same time, (ii) important insights into the nature of electron correlation and low-energy excitations in solids may be gained by cluster studies. In particular, the cluster calculations allow one to identify the tendencies to various kinds of symmetry-broken states in the (extended) Hubbard model and give a simple qualitative understanding of the low temperature properties of the Kondo and the heavy-fermion systems. Furthermore, cluster calculations allow one to study such problems as the development of magnetic moment at chemisorbed atoms for strong correlations and the suppression of frustration by quan...

Studies of electron correlation effects in multicharged-ion-atom collisions involving double capture

We review measurements of L-Coster- Kronig and Auger electron production in slow, multicharged collision systems to study electron correlation effects in the process of double electron capture. The n −3 law was confirmed for the production of the Coster-Kronig configurations 1s 22p nl( n ≥ 6) in O 6+ + He collisions. Enhancement of high angular momentum l in specific 1s 22p nl configurations was observed by means of high-resolution measurements of the Coster-Kronig lines. The importance of electron correlation effects in couplings of potential energy curves leading to the 1s 22p nl configurations is verified by means of Landau-Zener model calculations.