Coulomb Correlation Effects Research Articles

Valence and origin of metal–insulator transition in Mn doped SrRuO 3 studied by electrical transport, X-ray photoelectron spectroscopy and LSDA+ U calculation

We have studied the valence and electronic properties of Mn doped SrRuO 3 using electrical transport measurement, X-ray photoelectron spectroscopy (XPS) and local (spin) density approximation plus Coulomb interaction strength calculation (LSDA+ U). The resistivity data revealed that the system undergoes transition from metal to insulator at the critical Mn doping level, x∼0.2, which is accompanied by the structural transition from orthorhombic to tetragonal crystal symmetry. Besides, the significant reduction of the spectral weight at the coherent zone (0.8 eV) of the valence band is observed for x>0.2. The core XPS spectra suggest that both the transition elements exist in the mixed ionic pair, Ru +4/Ru +5↔Mn +3/Mn +4. The detail analysis of the results suggests that the Coulomb correlation effect in conjugation with localization of the charge carriers predominate over the mixed ionic pair effect and responsible for the metal–insulator transition in the series.

A Theoretical Analysis of Instantaneous Coulomb Renormalizations in a Single Quantum Dot Pump-Probe Experiment

We present a theoretical analysis of Coulomb-correlation effects on the optical spectra of a single negatively charged quantum dot. The considered initial and final many particle states are adapted and motivated by a recent two-color pump-probe experiment. We demonstrate that the lowest energy transition is noticeably affected by exchange interactions if the initial state consists of a hot trion. It turns out that for initial states as obtained after spin-conserving carrier relaxation new absorption and emission lines emerge.

Effect of Coulomb correlation on electron transport through a concentric quantum ring–quantum dot structure

We study transfer of a single-electron through a quantum ring capacitively coupled to the charged quantum dot placed in its center. For this purpose we solve the time-dependent Schr\"odinger equation for the pair of particles: the electron traveling through the ring and the other carrier confined within the quantum dot. The correlation effects due to the interaction between the charge carriers are described in a numerically exact manner. We find that the amplitude of Aharonov-Bohm oscillations of the transfer probability is significantly affected by the presence of the dot-confined carrier. In particular the Coulomb correlation leads to inelastic scattering. When the inelastic scattering is strong the transmission of electron through the ring is not completely blocked for $(n+1/2)$ magnetic flux quanta.

Metal-insulator transitions and magnetism in correlated band insulators: FeSi andFe1−xCoxSi

The $\text{LDA}+\text{DMFT}$ (local density approximation combined with dynamical mean-field theory) computation scheme has been used to study spectral and magnetic properties of FeSi and ${\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{Si}$. Having compared different models, we conclude that a correlated band insulator scenario in contrast to Kondo insulator model agrees well with FeSi band structure and experimental data. Coulomb correlation effects lead to band narrowing of the states near the Fermi level with mass renormalization parameter ${m}^{\ensuremath{\ast}}\ensuremath{\approx}2$ in agreement with the results of angle-resolved photoemission spectroscopy. Temperature dependence of spectral functions and magnetic susceptibility calculated in DMFT reproduces transition from nonmagnetic semiconductor to metal with local magnetic moments observed experimentally. Cobalt doping leads to ferromagnetism that has itinerant nature and can be successfully described by the $\text{LDA}+\text{DMFT}$ method.

Electronic structures and magnetic properties of Zn-doped colossal magnetoresistance materials [formula omitted

The electronic structures and magnetic properties of Fe 1 − x Zn x Cr 2S 4 ( x = 0.0 , 0.5 , 0.75 , 1.0 ) have been studied within the generalized gradient approximation (GGA) and GGA + U schemes. The calculated results reveal that with increasing Zn content the cell volume shrinks slightly; the system changes from ferrimagnetism to antiferromagnetism. The materials were found to be semiconductor in the compositional range of 0 ⩽ x ⩽ 0.5 and insulator in 0.75 ⩽ x ⩽ 1.0 . Our calculations reveal that the semiconducting ground state for FeCr 2S 4 is both the Jahn–Teller effect and the Coulomb correlation effect. The calculated results for the magnetic phase transition are in a good agreement with experiments.

Intricate coupling between ion–ion and ion–surface correlations in double layers asillustrated by charge inversion—combined effects of strong Coulomb correlations andexcluded volume

Many-body correlations in electrolyte systems are important when the electrostaticcoupling and/or the volume fraction of ions are not low. Such correlations areignored in the traditional theories of electrolytes based on the Poisson–Boltzmannapproximation. In the general case, the ion density profiles (ion–surface correlationfunctions) and the ion–ion correlation functions in diffuse electric double layersare strongly interdependent. Both have to be included in the treatment of thesystem to capture many essential properties. In this work the coupling between theion–ion and ion–surface correlations and effects of this coupling are illustratedexplicitly and graphically (visually). The average forces that act on the ions inthe double layer are analysed. This leads to an understanding of mechanisms inaction in the inhomogeneous electrolyte near a surface. Charge separation in anelectrolyte outside an uncharged surface and charge inversion of highly chargedsurfaces are thereby used as examples of what insights can be gained by this kindof approach. Some links to mechanisms behind like-charge attraction are alsodiscussed.

Ferromagnetism in two band metals: Combined effect of Coulomb correlation, hybridization and band widths

We study the possibility of ferromagnetism in metals. The metal is described by two hybridized bands one of which includes Hubbard correlation whereas the other is uncorrelated. We parametrize the ratio of the band widths and their centers as well. The original Hamiltonian is transformed into an effective and simpler one. Only one site retains the full correlation (U) while in the others acts as an internal field, the self-energy, in the framework of an alloy analogy approximation. This field, in turn, is self-consistently determined by imposing the translational invariance of the problem. For several total electronic occupation numbers ( n total ) we compare the spin dependent free energies with the corresponding paramagnetic ones. We present several results pointing out the mechanism by which the self-consistency introduces a sort of constraints, for given values of band width and band shift.

Cross effect of Coulomb correlation and hybridization in the occurrence of ferromagnetism in two shifted band transition metals

In this work we discuss the occurrence of ferromagnetism in transition-like metals. The metal is represented by two hybridized($V$) and shifted $(\epsilon_s$) bands one of which includes Hubbard correlation whereas the other is uncorrelated. The starting point is to transform the original Hamiltonian into an effective one. Only one site retains the full correlation (U) while in the others the correlations are represented by an effective field, the self-energy(single-site approximation). This field is self-consistently determined by imposing the translational invariance of the problem. Thereby one gets an exchange split quasi-particle density of states and then an electron-spin polarization for some values of the parameters $(U,V, \alpha, \epsilon_s)$, $\alpha$ being the ratio of the effective masses of the two bands and of the occupation number $n$.

Coulomb correlation effects in LaFeAsO: An LDA + DMFT(QMC) study

Effects of Coulomb correlation on the LaFeAsO electronic structure are investigated by the LDA + DMFT(QMC) method (combination of the local density approximation with the dynamic mean-field theory; impurity solver is a quantum Monte Carlo algorithm). The calculation results show that LaFeAsO is in the regime of intermediate correlation strength with a significant part of the spectral density moved from the Fermi energy to the Hubbard bands and far from the edge of the metal-insulator transition. Correlations affect iron d-orbitals differently. The t 2g states (xz, yz and x 2 − y 2 orbitals) have a higher energy due to crystal field splitting and are nearly half-filled. Their spectral functions have a pseudogap with the Fermi level position on the higher subband slope. The lower energy e g set (xy and 3z 2 − r 2 orbitals) have occupancies significantly larger than 1/2 with typically metallic spectral functions.

Effective bandstructure in the insulating phase versus strong dynamical correlations in metallicVO2

Using a general analytical continuation scheme for cluster dynamical mean-field calculations, we analyze real-frequency self-energies, momentum-resolved spectral functions, and one-particle excitations of the metallic and insulating phases of ${\text{VO}}_{2}$. While for the former dynamical correlations and lifetime effects prevent a description in terms of quasiparticles, the excitations of the latter allow for an effective bandstructure. We construct an orbital dependent, but static one-particle potential that reproduces the essentials of the full many-body spectrum. Yet, the ground state is well beyond a static one-particle description. The emerging picture gives a nontrivial answer to the decade-old question of the nature of the insulator, which we characterize as a ``many-body Peierls'' state, stressing the joint effect of lattice symmetry breaking and Coulomb correlations.

Electron correlations in narrow energy bands: modified polar model approach

The electron correlations in narrow energy bands are examined within the framework of the modied form of polar model. This model permits to analyze the effect of strong Coulomb correlation, inter-atomic exchange and correlated hopping of electrons and explain some peculiarities of the properties of narrow-band materials, namely the metal-insulator transition with an increase of temperature, nonlinear concentration dependence of Curie temperature and peculiarities of transport properties of electronic subsystem. Using a variant of generalized Hartree-Fock approximation, the single-electron Green’s function and quasi-particle energy spectrum of the model are calculated. Metal-insulator transition with the change of temperature is investigated in a system with correlated hopping. Processes of ferromagnetic ordering stabilization in the system with various forms of electronic DOS are studied. The static conductivity and effective spin-dependent masses of current carriers are calculated as a function of electron concentration at various DOS forms. The correlated hopping is shown to cause the electron-hole asymmetry of transport and ferromagnetic properties of narrow band materials.

Density Functional Theory Studies of Spin, Charge, and Orbital Ordering in YBaT2O5(T = Mn, Fe, Co)

Spin, charge, and orbital orderings are influenced by electron/hole doping, cation radii, oxygen stoichiometry, temperature, magnetic field, and so on. In order to understand the role of electron/hole doping, we have studied variations in spin, charge, and orbital ordering in terms of d-band filling for YBaT 2O 5 (T = Mn, Fe, Co). The calculations were performed using density functional theory as implemented in the full-potential linearized augmented-plane-wave method. We have carried out calculations for nonmagnetic, ferromagnetic, and antiferromagnetic configurations. A ferrimagnetic ground state was established for YBaMn 2O 5, whereas YBaFe 2O 5 and YBaCo 2O 5 have antiferromagnetic ground states; all of these results are in agreement with experimental findings. The effects of spin-orbit coupling, the Hubbard U parameter, and orbital polarization on the magnetic properties were also analyzed. The electronic band characteristics were analyzed using total as well as site- and orbital-projected densities of states. Inclusion of spin-orbit coupling and Coulomb correlation effects in the calculations was found to be important in order to reproduce the experimentally established semiconducting behaviors of YBaFe 2O 5 and YBaCo 2O 5. In order to quantify the charges at each atomic site, we made use of the Bader "atom-in-molecule" concept and Born effective-charge (BEC) analyses. The structural optimizations and BEC tensor calculations were performed using the VASP-PAW method. The different types of charge and orbital orderings in these compounds were visualized using the energy-projected density matrices of the d electrons. Substantial differences in ordering patterns with respect to d-band filling emerged. Ordering of the d z (2) orbital of Mn in YBaMn 2O 5 gave rise to G-type ferrimagnetic spin ordering along the c direction and checkerboard-type charge ordering, whereas ordering of the d x (2) - y (2) orbital of Fe in YBaFe 2O 5 caused Wollan-Koehler G-type antiferromagnetic spin ordering along the b direction and stripe-type charge ordering. Similarly, a complex pattern of orbital ordering in YBaCo 2O 5 activated spin and charge orderings similar to those in YBaFe 2O 5.

Coulomb blockade in graphene nanodisks

Graphene nanodisk is a graphene derivative with a closed edge. The trigonal zigzag nanodisk with size $N$ contains a zero-energy sector with the $\text{SU}(N)$ symmetry. We investigate electron-electron interaction effects within the zero-energy sector. We explicitly derive the direct and exchange interactions, which explicitly break the $\text{SU}(N)$ symmetry. Then, by regarding a nanodisk as a quantum dot with an internal degree of freedom, we analyze the nanodisk-lead system consisting of a nanodisk and two leads. By employing the standard Green's function method, we reveal Coulomb blockade effects in this system. As the chemical potential increases, a peculiar series of plateaux and dips develops in the occupation number and a peculiar series of Coulomb blockade peaks develops in the conductance, which reflect the energy spectrum of a nanodisk with a broken $\text{SU}(N)$ symmetry. It is argued that dips emerge in the occupation number due to a Coulomb correlation effect and that tiny Coulomb blockade peaks emerge due to a tunneling effect between the nanodisk and leads.

Theory of shot noise in high-current space-charge-limited field emission

This paper studies the electron shot noise of high current space-charge limited (SCL) field emission, including the effects of both quantum partitioning and Coulomb correlation of the emitted tunneling electrons inside a gap. The Fano factor $(\ensuremath{\gamma})$ is calculated over a wide range of applied voltages, gap spacings, and electron pulse lengths in classical, quantum and relativistic regimes. It is found that the effect of space charge smoothing (observed in thermionic emission) is no longer valid for high current field emission, as the space charge field will increase the potential barrier and thus decrease the degree of quantum partitioning (larger $\ensuremath{\gamma}$). However, the shot noise of SCL field emission in a nanogap may be suppressed due to the combination of Coulomb repulsion and exchange-correlation potential among the tunneling electrons, where the dynamics of the electrons inside the gap are treated by using a quantum mean field model. Coulomb correlation can be ignored when the pulse length of the electron beam is much smaller than the gap transit time. Possible experimental settings are discussed in order to observe the shot noise suppression of field emission with and without the space charge effects.

Quantum interference and Coulomb correlation effects in spin-polarized transport through two coupled quantum dots

Spin-dependent transport through two coupled single-level quantum dots attached to ferromagnetic leads with collinear (parallel and antiparallel) magnetizations is analyzed theoretically by the nonequilibrium Green function technique. Transport characteristics, in particular, linear and nonlinear differential conductance and tunnel magnetoresistance associated with the magnetization rotation from antiparallel to parallel alignment, are calculated numerically with intradot Coulomb interaction taken into account. The relevant Green functions are derived by the equation of motion method within the Hartree-Fock decoupling scheme. The dot occupations and Green functions are calculated self-consistently, and the numerical analysis is focused on the interference (Fano antiresonance) and Coulomb interaction effects. It is shown that the presence of Fano antiresonance depends on the sign of the nondiagonal coupling elements.

Influence of symmetry and Coulomb correlation effects on the optical properties of nitride quantum dots

The electronic and optical properties of self-assembled InN/GaN quantum dots (QDs) are investigated by means of a tight-binding model combined with configuration interaction calculations. Tight-binding single particle wave functions are used as a basis for computing Coulomb and dipole matrix elements. Within this framework, we analyze multi-exciton emission spectra for two different sizes of a lens-shaped InN/GaN QD with wurtzite crystal structure. The impact of the symmetry of the involved electron and hole one-particle states on the optical spectra is discussed in detail. Furthermore we show how the characteristic features of the spectra can be interpreted using a simplified Hamiltonian which provides analytical results for the interacting multi-exciton complexes. We predict a vanishing exciton and biexciton ground state emission for small lens-shaped InN/GaN QDs. For larger systems we report a bright ground state emission but with drastically reduced oscillator strengths caused by the quantum confined Stark effect.

Correlation Effects in EOM-CCSD for the Excited States: Evaluated by AIM Localization Index (LI) and Delocalization Index (DI)

We have extended the evaluation and interpretation of QTAIM (quantum theory of atoms in molecules) localization and delocalization indices lambda (LI) and delta (DI) to electronic excited states by studying ground states (at HF and CCSD levels) and excited states (at CIS and EOM-CCSD) of H2C=CH2, HCCH, H2C=O, H2C=S, CO2, CS2, and SO2. These molecules undergo extensive geometrical changes upon the excitation to the valence adiabatic excited singlet state. The importance of Coulomb correlation effects was demonstrated by comparing the LIs and DIs at none-correlated levels (HF and CIS) and those at correlated levels (CCSD and EOM-CCSD). In interpreting the changes in the magnitudes of the LIs and DIs, we made use of simple molecular orbital and Walsh-diagram analyses. Coulomb correlation is important in determining the magnitude of the LIs and DIs and obtaining geometries that are close to experiment.

Correlation effects in partially ionized mass asymmetric electron-hole plasmas

The effects of strong Coulomb correlations in dense three-dimensional electron-hole plasmas are studied by means of unbiased direct path integral Monte Carlo simulations. The formation and dissociation of bound states, such as excitons and biexcitons, is analyzed and the density-temperature region of their appearance is identified. At high density, the Mott transition to the fully ionized metallic state (electron-hole liquid) is detected. Particular attention is paid to the influence of the hole to electron mass ratio M on the properties of the plasma. Above a critical value of about M=80 formation of a hole Coulomb crystal was recently verified [Bonitz, Phys. Rev. Lett. 95, 235006 (2005)] which is supported by additional results. Results are related to the excitonic phase diagram of intermediate valent Tm[Se,Te], where large values of M have been observed experimentally.

Electron Transport through Double Quantum Dot System with Inter-Dot Coulomb Interaction

A theoretical approach to a problem of electron transport through double quantum dot systems based on non-equilibrium Green function formalism using equation of motion method is presented. I−V characteristics and differential conductance are calculated and discussed in detail in the intermediate regime with tunneling rate between the quantum dots comparable to coupling constants with external electrodes. Effects of inter-dot Coulomb correlations are studied for various values of interaction parameter U . It is shown that the interaction influences transport properties in a pronounced way and apart from the simple Coulomb blockade additional effects can be obtained. When energy levels of two quantum dots are not aligned, the asymmetry in conductance characteristics is closely related to a voltage dependence of population numbers in both quantum dots. For a one bias polarization electrons are well localized in quantum dots in a low voltage region, whereas for the opposite one they are partly delocalized.

Dielectric mismatch effects in two-electron zero-dimensional nanosystems

Two-electron Coulomb interaction and correlation effects in several spherically symmetric zero-dimensional semiconductor heterostructures are investigated in the large dielectric mismatch regime. Specifically, a semiconductor quantum dot (QD) embedded in air or a vacuum, an air-filled nanocavity in a semiconductor matrix, and a weakly confined ${D}^{\ensuremath{-}}$ center (two electrons bounded to a hydrogenic donor impurity in a QD embedded in air). A strong self-polarization-induced radial localization of the electronic density at the heterojunction interface yields surface states. In these states, the polarization of the electron-electron interaction strongly affects dynamics. For a low dielectric constant of the semiconductor building material, Wigner-like localization of the electronic density occurs. As we increase the dielectric constant, it is gradually suppressed. We prove that this gradual suppression is originated by the enhanced strength of the polarization potential accompanying the increase in permittivity. Additionally, in the presence of a weakly confined ${D}^{\ensuremath{-}}$ center in a QD, a transition phase from volume to surface states takes place. It is characterized, in a wide range of quantum dot radii, by a strong ground state reconstruction and a zero ${D}^{\ensuremath{-}}$ binding energy.