Study Electron Correlation Research Articles

Local Electron Correlations in a Two-Dimensional Hubbard Model on the Penrose Lattice

We study electron correlations in the half-filled Hubbard model on two-dimensional Penrose lattice. Applying the real-space dynamical mean-field theory to large clusters, we discuss how low-temperature properties are affected by the quasiperiodic structure. By calculating the double occupancy and renormalization factor at each site, we clarify the existence of the Mott transition. The spatially-dependent renormalization characteristic of geometrical structure is also addressed.

Retraction: “Study of electronic structure and magnetization correlations in hydrogenated and vacuum annealed Ni doped ZnO” [J. Appl. Phys. 109, 063907 (2011)

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Quantum Effects in Double Ionization of Argon below the Threshold Intensity

So far, nonsequential double ionization (NSDI) of atoms can be well understood within a semiclassical or even classical picture. No quantum effect appears to be required to explain the data observed. We theoretically study electron correlation resulting from NSDI of argon in a low-intensity laser field using a quantum-mechanical S-matrix theory. We show that quantum interference between the contributions of different intermediate excited states of the singly charged argon ion produces a transition from back-to-back to side-by-side emission with increasing laser intensity, which is in close agreement with the experimental data. For higher intensities, this transition is enhanced by the consequences of depletion of the excited states.

NMR study of electronic correlations in Mn-doped Ba(Fe1−xCox)2As2and BaFe2(As1−xPx)2

We probe the real space electronic response to a local magnetic impurity in isovalent and heterovalent doped BaFe2As2 (122) using Nuclear Magnetic Resonance (NMR). The local moments carried by Mn impurities doped into Ba(Fe1-xCox)2As2(Co-122) and BaFe(As1-xPx)2(P-122) at optimal doping induce a spin polarization in the vicinity of the impurity. The amplitude, shape and extension of this polarisation is given by the real part of the susceptibility chi'(r) of FeAs layers, and is consequently related to the nature and strength of the electronic correlations present in the system. We study this polarisation using 75As NMR in Co-122 and both 75As and 31P NMR in P-122. The NMR spectra of Mn-doped materials is made of two essential features. First is a satellite line associated with nuclei located as nearest neighbor of Mn impurities. The analysis of the temperature dependence of the shift of this satellite line shows that Mn local moments behave as isolated Curie moments. The second feature is a temperature dependent broadening of the central line. We show that the broadening of the central line follows the susceptibility of Mn local moments, as expected from typical RKKY-like interactions. This demonstrates that the susceptibility chi'(r) of FeAs layers does not make significant contribution to the temperature dependent broadening of the central line. chi'(r) is consequently only weakly temperature dependent in optimally doped Co-122 and P-122. This behaviour is in contrast with that of strongly correlated materials such as underdoped cuprate high-Tc superconductors where the central line broadens faster than the impurity susceptibility grows, because of the development of strong magnetic correlations when T is lowered. Moreover, the FeAs layer susceptibility is found quantitatively similar in both heterovalent doped and isolvalent doped BaFe2As2.

Comparison study of electron correlation in one-photon and two-photon double ionization of helium

Based on ab initio time-dependent calculations for helium, we study the joint angular distribution (JAD) of the two electrons ionized by absorbing one or two photons. On the JAD plane for the equal energy sharing case, one can identify a common symmetric ejection mode with respect to the laser polarization for three different processes, i.e., one-photon double ionization, two-photon nonsequential ionization, and sequential double ionization. In this symmetric ejection case, the two ejected electrons strongly favor a particular interelectron angle. We show that this angle can mimic the electron correlation strength in these double-ionization processes. The interelectron angle in the three types of double ionization is found to be obviously different.

Probing Angular Correlations in Sequential Double Ionization

We study electron correlation in sequential double ionization of noble gas atoms and HCl in intense, femtosecond laser pulses. We measure the photoelectron angular distributions of Ne+ relative to the first electron in a pump-probe experiment with 8 fs, 800 nm, circularly polarized laser pulses at a peak intensity of a few 10(15) W/cm2. Using a linear-linear pump-probe setup, we further study He, Ar, and HCl. We find a clear angular correlation between the two ionization steps in the sequential double ionization intensity regime.

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.

Study of electronic structure and magnetization correlations in hydrogenated and vacuum annealed Ni doped ZnO

The influence of hydrogen and vacuum annealing on the structural, electronic, and magnetic properties of Ni-doped ZnO pellets viz. Zn0.95Ni0.05O has been investigated using x-ray diffraction (XRD), magnetization, resistance measurements, and x-ray photoelectron spectroscopy (XPS). Rietveld refinement of the XRD patterns confirms that all the polycrystalline samples possess wurtzite type hexagonal structure with no evidence of secondary phases. The superconducting quantum interference device magnetometry measurements exhibit a paramagnetic state for the as-synthesized Zn0.95Ni0.05O. However, the post annealing in H2 as well as in vacuum, drive it to a ferromagnetic state at 300 K. The obtained values of coercivity are 177 and 270 Oe, and the saturation magnetizations are 1.67 and 1.91 emu/g, respectively, for the H2 and vacuum annealed samples. Temperature dependent magnetization results show that the Curie temperatures are nearly the same (∼550 K) for both the annealed samples. Resistance of the annealed samples is three orders of magnitude lower than their as-synthesized counterparts. The XPS results confirm bivalency of Ni ions and creation of O vacancies upon annealing, due to the Ni (3d)–O (2p) hybridization. The ferromagnetism and the consequent electronic property changes are reversible with regard to reheating the samples in air, in agreement with the reported switch “on” and “off” ferromagnetic ordering in the doped ZnO.

Dynamical mean-field theory within an augmented plane-wave framework: Assessing electronic correlations in the iron pnictide LaFeAsO

We present an approach that combines the local-density approximation (LDA) and the dynamical mean-field theory (DMFT) in the framework of the full-potential linear augmented plane-wave method. Wannier-type functions for the correlated shell are constructed by projecting local orbitals onto a set of Bloch eigenstates located within a certain energy window. The screened Coulomb interaction and Hund's coupling are calculated from a first-principles constrained random-phase approximation scheme. We apply this $\text{LDA}+\text{DMFT}$ implementation, in conjunction with a continuous-time quantum Monte Carlo algorithm, to the study of electronic correlations in LaFeAsO. Our findings support the physical picture of a metal with intermediate correlations. The average value of the mass renormalization of the $\text{Fe}\text{ }3d$ bands is about 1.6, in reasonable agreement with the picture inferred from photoemission experiments. The discrepancies between different $\text{LDA}+\text{DMFT}$ calculations (all technically correct) which have been reported in the literature are shown to have two causes: (i) the specific value of the interaction parameters used in these calculations and (ii) the degree of localization of the Wannier orbitals chosen to represent the $\text{Fe}\text{ }3d$ states, to which many-body terms are applied. The latter is a fundamental issue in the application of many-body calculations, such as DMFT, in a realistic setting. We provide strong evidence that the DMFT approximation is more accurate and more straightforward to implement when well-localized orbitals are constructed from a large energy window encompassing $\text{Fe-}3d$, $\text{As-}4p$, and $\text{O-}2p$ and point out several difficulties associated with the use of extended Wannier functions associated with the low-energy iron bands. Some of these issues have important physical consequences regarding, in particular, the sensitivity to the Hund's coupling.

Spectroscopic and electric properties of the LiCs molecule: a coupled cluster study including higher excitations

Aimed at obtaining complete and highly accurate potential energy surfaces for molecules containing heavy elements, we present a new general-order coupled cluster method which can be applied in the framework of the spin-free Dirac formalism. As an initial application we present a systematic study of electron correlation and relativistic effects on the spectroscopic and electric properties of the LiCs molecule in its electronic ground state. In particular, we closely investigate the importance of excitations higher than coupled cluster doubles, spin-free and spin-dependent relativistic effects and the correlation of outer-core electrons on the equilibrium bond length, the harmonic vibrational frequency, the dissociation energy, the dipole moment and the static electric dipole polarizability. We demonstrate that our new implementation allows for highly accurate calculations not only in the bonding region but also along the complete potential curve. The quality of our results is demonstrated by a vibrational analysis where an almost complete set of vibrational levels has been calculated accurately.

Intriguing electron correlation effects in the photoionization of metallic quantum-dot nanorings

We report detailed results on ionization in metallic quantum-dot (QD) nanorings described by the extended Hubbard model at half filling obtained by exact numerical diagonalization. In spite of very strong electron correlations, the ionization spectra are astonishingly scarce. We attribute this scarcity to a hidden quasi-symmetry, generalizing thereby similar results on optical absorption recently reported [Phys. Rev. B 75, 125323 (2007); 77, 165339 (2008)]. Numerical results indicate that this hidden quasi-symmetry of the extended Hubbard model does not evolve into a true (hidden) symmetry but remains a quasi-symmetry in the case of the restricted Hubbard model as well. Based on the observation on the number of significant ionization signals per each spatial symmetry, we claim the existence of a one-to-one map between the relevant ionization signals of the correlated half-filled nanorings and the one-hole and two-hole-one-particle processes possible in the noninteracting case. Similar to the case of optical absorption, numerous avoided crossings (anticrossings) are present in the ionization spectra, which often involve more than two states. The present results demonstrate that ionization could be a useful tool to study electron correlations in metallic QD-nanoarrays, providing information that is complementary to optical absorption.

A local-MP2 approach to the ab initio study of electron correlation in crystals and to the simulation of vibrational spectra: the case of Ice XI

Wave-function-based computational techniques for describing electron correlation effects in periodic systems have a long history. Among early attempts, the method proposed by Colle and Salvetti (Theor Chim Acta 37:329, 1975) more than 30 years ago is noteworthy for its simplicity, power, and far-reaching consequences. The renewed interest in this topic is due to the well-known failure of techniques based on Density Functional Theory when it comes to obtaining very accurate estimates of some important quantities. Here we present the essential features of an ab initio code, (CRYSCOR) recently implemented (Phys Rev B 76: 075101, 075102, 2007), which solves the MP2 equations for crystals by adopting a local-correlation approach and using as a reference the Hartree-Fock solution provided by the CRYSTAL program. As an example of application, we discuss the MP2 corrections to the frequency of some vibrational modes in a proton-ordered structure of water ice (Ice XI).

Charge Order versus Superconductivity in Inhomogeneous Systems

We examine the disordered system with defined attractive centers of carries attraction. By changing concentration of such centers one can model the appearance of a superconducting order. To study electron correlations and to describe disorder effects, the Negative U Hubbard model and the Coherent Potential Approximation (CPA) have been applied. Our approach includes the charge fluctuations on the equal foot.

Ti oxohalides: A playground for electron correlation physics

Abstract Low-dimensional S = 1 2 systems of the form TiO X ( X = Cl , Br ) have been investigated using photoemission spectroscopy. In their pristine form they display unusual spin-Peierls phases and their behavior under pressure suggests a possible metallic phase. While upon electron-doping photoemission spectra reveal the filling of the upper Hubbard band, clear spectroscopic evidence for a metal–insulator transition is difficult due to correlation and coupling effects in these compounds. However, these effects qualify them as almost ideal model systems for the study of strong electron correlations away from half-filling.

Estimates of effective Hubbard model parameters forC20isomers

We report on an effective Hubbard Hamiltonian approach for the study of electronic correlations in ${\text{C}}_{20}$ isomers---cage, bowl, and ring---with quantum Monte Carlo and exact diagonalization methods. The tight-binding hopping parameter, $t$, in the effective Hamiltonian is determined by a fit to density-functional theory calculations, and the on-site Coulomb interaction, $U/t$, is determined by calculating the isomers' affinity energies, which are compared to experimental values. For the ${\text{C}}_{20}$ fullerene cage we estimate ${t}_{\text{cage}}\ensuremath{\simeq}0.68--1.36\text{ }\text{eV}$ and ${(U/t)}_{\text{cage}}\ensuremath{\simeq}7.1--12.2$. The resulting effective Hamiltonian is then used to study the shift of spectral peaks in the density of states of neutral and one-electron-doped ${\text{C}}_{20}$ isomers. Energy gaps are also extracted for possible future comparison with experiments.

Periodic local MP2 method for the study of electronic correlation in crystals: Theory and preliminary applications

A computational technique for solving the MP2 equations for periodic systems using a local-correlation approach and implemented in the CRYSCOR code is presented. The Hartree-Fock solution provided by the CRYSTAL program is used as a reference. The motivations for the implementation of the new code are discussed, and the techniques adopted are briefly recalled. With respect to the original formulation (Pisani et al., J Chem Phys 2005, 122, 094113), many new features have been introduced in CRYSCOR to improve its efficiency and robustness. In particular, an adaptation of the density fitting scheme to translationally periodic systems is described, based on Fourier transformation techniques. Three examples of application are provided, concerning the CO(2) crystal, proton transfer in ice XI, and the adsorption of methane on MgO (001). The results obtained with the periodic LMP2 method for these systems appear more reliable than the ones obtained using density functional theory.

Experimental and theoretical study of electronic correlations in carbon nanotubes and graphite from auger spectroscopy

We give a quantitative estimate of the screened on-site Coulomb repulsion U in carbon nanotubes and graphite. This is obtained by measuring the Auger spectrum of these materials and performing a theoretical study based on the Green's function method proposed by Cini[Solid. St. Comm. 24, 681 (1977)]. The experimental lineshape is very well reproduced by the theory, where only one fit parameter is employed. This allows to extract the value of the screened on-site repulsion between 2p states, which results U = 4.6 eV for nanotubes and U = 2.1 eV for graphite.

Electronic interaction effects in rare‐earth alloys from 2D‐ACAR experiments

AbstractMeasurement of the two dimensional angular correlation of the electron‐positron annihilation radiation (2D‐ACAR) complemented with ab‐initio calculations can provide decisive information about the character of the f‐electrons in rare earth compounds and are the prerequisite to the study of electron correlations. We provide examples of systems where good approximations of the archetype f‐electron localized and f‐electron itinerant behaviours, apply. i) In the case of the antiferromagnetic heavy fermion and superconductor CeIn3 the multisheet Fermi Surface (FS), reconstructed from our measurements in the paramagnetic phase, agrees closely with the predictions of band structure calculations regarding the Ce 4f electrons as fully localized. ii) On the other hand, our studies of the antiferromagnet actinide based UGa3 in the paramagnetic phase, compared with calculations which include the effects due to the non uniform positron density and the electron‐positron correlations, produce a substantial evidence that an unconstrained 5f‐electron itinerant description applies. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ab initio calculations of Stark induced electric dipole transition amplitudes of singly ionized calcium

We have determined the amplitudes for Stark-induced (SI) transitions, of current interest in atomic physics and astrophysics, between mixed states dominated by same parity configurations of some low-lying states of singly ionized calcium. Well established Relativistic Coupled cluster (RCC) method has been used for the spectroscopic study of these transitions. This method has proved to be a very useful tool for rigorous electron correlation studies. We compare our results with experimental data for allowed electric dipole transition amplitudes and energies and also with other theoretical studies.

Dynamical Mean Field within Iterative Perturbation Theory

The problem of strong correlations belongs to the central problem of the condensed matter theory and has intensively been studied by many sophisticated methods. One of them is the dynamical mean field theory (DMFT) [1], which was successfully used to study, e.g. the metal–insulator (Mott) [2] transitions in strongly correlated electron systems. The simplest but nontrivial model that is used to study electron correlations is the Hubbard model, which in the limit of the infinite dimensions, is mapped onto a single impurity Anderson model. However, the Anderson-like models have to be solved together with appropriate self-consistent equations. In the limit of infinite dimension all spatial degrees of freedom are frozen and the self-energy is local (independent of momentum). To solve the mapped single impurity model, one can adopt several computational schemes such as the quantum Monte Carlo method, the non-crossing approximation, the exact diagonalization method or iterative perturbation theory (IPT). The last scheme (i.e. IPT), introduced for the Hubbard model at half-filling [3, 4] and in [5] for arbitrary doping, allows us to solve the single impurity Anderson model in a very efficient and relatively simple way. The objective of this paper is to discuss the numerical implementations of IPT for orbitally degenerated models.