Intra-atomic Coulomb Repulsion Research Articles

Interface structure and bias dependence of VN/AlN/VN tunnel junction: A semi-empirical calculation

Vanadium Nitride based magnetic heterostructures have opened up unprecedented opportunities in the field of spintronics owing to their half metallicity and a high value of spin polarization. In this paper, a tight-binding model of epitaxially grown VN/AlN/VN heterostructure is developed with different types of bonding at the left and right interface. The Slater–Koster tight-binding scheme is used to obtain the on-site and hopping energies of the model by fitting them to bulk DFT band structures. Electron transport across the VN/AlN/VN heterostructure is then studied using the non-equilibrium Green’s function method, which works well within the tight-binding framework. An asymmetry in the interface bonding of the structure was found to produce a resonance peak in the transmission spectra at around 0.5 V. Beyond 0.5 V, the tunnel junction registers a large antiparallel current resulting in a negative tunnel magnetoresistance. Moreover, the presence of a magnetic impurity in the insulating barrier enhances tunneling across the junction resulting in a large current that decreases with the value of intra-atomic Coulomb repulsion, U, at the impurity site.

Magnetic Compton profiles of Ni beyond the one-particle picture: Numerically exact and perturbative solvers of dynamical mean-field theory

We calculated the magnetic Compton profiles (MCPs) of Ni using density functional theory supplemented by electronic correlations treated within dynamical mean-field theory (DMFT). We present comparisons between the theoretical and experimental MCPs. The theoretical MCPs were calculated using the KKR method with the perturbative spin-polarized T-matrix fluctuation exchange approximation DMFT solver, as well as with the full potential linear augmented planewave method with the numerically exact continuous-time quantum Monte Carlo DMFT solver. We show that the total magnetic moment decreases with the intra-atomic Coulomb repulsion $U$, which is also reflected in the corresponding MCPs. The total magnetic moment obtained in experimental measurements can be reproduced by intermediate values of $U$. The spectral function reveals that the minority X$_2$ Fermi surface pocket shrinks and gets shallower with respect to the density functional theory calculations.

Phase transitions in a model of BEDT-TTF compound electron subsystem

Model of electronic subsystem of (BEDT-TTF)2X family is studied theoretically within Green function equation of motion approach. The configurational representation of the model Hamiltonian which allows analytical calculation of the energy spectrum is developed. Effective Hamiltonian of the model takes into account the strong intra-site Coulomb, intersite exchange interaction of electrons and correlated hopping of electrons both within the conducting plane and in transversal direction. With rise of intraatomic Coulomb repulsion in the energy spectrum a gap opens and the transition to antiferromagnetic state is realized. At the external pressure and decreasing temperature the transition from insulator to metallic phase occurs. The experimental phase diagrams for quasi-twodimensional organic conductors with strong electron correlations and correlated hopping are discussed.

The structural, electronic, magnetic, and mechanical properties of perovskite oxides PbM1/2Nb1/2O3(M = Fe, Co and Ni)

In this study, the structural, electronic, magnetic, and mechanical properties of perovskite oxides PbM[Formula: see text]Nb[Formula: see text]O3(M = Fe, Co and Ni) are investigated. The systems are treated in ferromagnetic order. The calculations are carried out in the framework of density functional theory (DFT) within the plane-wave pseudopotential method. The exchange-correlation potential is approximated by generalized-gradient spin approach (GGA). The intra-atomic Coulomb repulsion is also taken into account in calculations (GGA + U). We have considered two generalized-gradient spin approximation functionals, which are Perdew–Burke–Ernzerhof (PBE) and PBE for solids (PBEsol) for structural parameter calculations when it included Hubbard potential. Although the spin-polarized electronic band structures of PbCo[Formula: see text]Nb[Formula: see text]O3and PbNi[Formula: see text]Nb[Formula: see text]O3systems exhibit metallic property in ferromagnetic phase, a bandgap is observed in spin-down states of PbFe[Formula: see text]Nb[Formula: see text]O3resulting in half-metallic behavior. The main reason for this behavior is attributed to the hybridization between [Formula: see text]-states of transition metal atoms and [Formula: see text]-states of oxygen atoms. The stability mechanically and the calculated mechanical properties by using elastic constants show that these compounds are mechanically stable in tetragonal phase and have anisotropic character mechanically.

On the Extended Holstein–Hubbard Model for Epitaxial Graphene on Metal

A model that combines the extended Hubbard model involving intra-atomic and interatomic Coulomb repulsion and the Holstein model describing the interaction of a band electron with an Einstein phonon is proposed. Three regions of the phase diagram are considered. The regions correspond to the states of spin- and charge-density waves and the state uniform in spin and charge. Numerical estimations for the Rh, Ir, and Pt substrates show that Coulomb interaction plays a leading part, making possible transitions from the uniform state to the states of spin- and charge-density waves.

Расширенная модель Холстейна-Хаббарда для эпитаксиального графена на металле

AbstractA model that combines the extended Hubbard model involving intra-atomic and interatomic Coulomb repulsion and the Holstein model describing the interaction of a band electron with an Einstein phonon is proposed. Three regions of the phase diagram are considered. The regions correspond to the states of spin- and charge-density waves and the state uniform in spin and charge. Numerical estimations for the Rh, Ir, and Pt substrates show that Coulomb interaction plays a leading part, making possible transitions from the uniform state to the states of spin- and charge-density waves.

Tunable magnetic interaction in hydrogenated epitaxial graphene modulated by the SiC substrate

We show that the ${d}^{0}$ ferromagnetism with high Curie temperature $({T}_{c})$ can be achieved in the electron-doped hydrogenated epitaxial graphene on certain SiC substrates through first-principles calculations. The pristine systems are found to be Mott insulators regardless of SiC polytype $(2H,4H$, or $6H)$ which, however, plays a significant role in the modulation of magnetic interaction. Carrier doping enhances the ferromagnetic coupling due to the double-exchange mechanism and thus realizes the phase transition from antiferromagnetism to ferromagnetism. A ${T}_{c}$ of around 400 K is predicted for graphene on the $2H$-SiC substrate. We employ a nondegenerate Hubbard model to demonstrate how the SiC affects the interfacial magnetism in intra-atomic Coulomb repulsion and intersite hopping interactions.

Time–Energy Uncertainty and Electronic Correlation in H+–Graphite Collisions

The formation of negative ions in the scattering of protons by a highly oriented pyrolytic graphite (HOPG) surface is theoretically and experimentally analyzed for a large scattering angle and compared with previous results obtained in the same system but for a forward scattering geometry. These experiments were motivated by the fact that the interaction of a hydrogen atom with a surface is the prototype system for studying the intra-atomic Coulomb repulsion in an s-like valence orbital localized in the atom. We tried to answer the open questions related to the electronic correlation effects and the influence of the detailed surface band structure by using appropriate theoretical models. The comparison with the experiment of theoretical results obtained by using different limit approximations of the electronic repulsion in the atomic state shows the expected validity ranges according to the ion velocity. However, the most remarkable conclusion obtained from this comparison is the nonvalidity of an adiabatic picture of the energy levels shifted by the interactions with the surface atoms, when the energy uncertainty introduced by the ion velocity becomes of the order of the electronic repulsion in the hydrogen ground state.

Oxygen vacancies at titanate interfaces: Two-dimensional magnetism and orbital reconstruction

We show that oxygen vacancies at titanate interfaces induce a complex multiorbital reconstruction which involves a lowering of the local symmetry and an inversion of t2g and eg orbitals resulting in the occupation of the eg orbitals of Ti atoms neighboring the O vacancy. The orbital reconstruction depends strongly on the clustering of O vacancies and can be accompanied by a magnetic splitting between the local eg orbitals with lobes directed towards the vacancy and interface dxy orbitals. The reconstruction generates a two-dimensional interface magnetic state not observed in bulk SrTiO3. Using generalized gradient approximation (LSDA) with intra-atomic Coulomb repulsion (GGA+U), we find that this magnetic state is common for titanate surfaces and interfaces.

Structural relaxation and metal-insulator transition at the interface between SrTiO 3 and LaAlO 3

The electronic structure of interfaces between LaAlO 3 and SrTiO 3 is studied using local spin density approximation (LSDA) with intra-atomic Coulomb repulsion (LSDA + U). We find that the nature of the interface metallic states is strongly affected by the type of the structure (sandwich or bilayer) and by the termination surface of LaAlO 3. In all structures the atomic relaxation plays a crucial role in the electronic properties of the system. While in sandwiches the structural relaxation produces a significant polarization in SrTiO 3 and a splitting of Ti 3 d xy orbitals, in AlO 2-terminated bilayers the relaxation occurs primarily in LaAlO 3 and results in an insulator-metal transition which has been observed experimentally with increasing thickness of the LaAlO 3 layer.

Mechanism of orbital reconstruction at the interfaces of transition metal oxides

Orbital reconstruction at interfaces between ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6}$ and SrO-terminated ${\text{SrTiO}}_{3}$ is studied using local spin-density approximation (LSDA) with intra-atomic Coulomb repulsion $(\text{LSDA}+U)$. The change in population of interfacial $\text{Cu}\text{ }3d$ orbitals results in stabilization of a new oxidation state $3{d}^{8}$, which involves an additional modification of orbital occupancies in the nearest SrO and ${\text{TiO}}_{2}$ layers. We find that an increase in electron charge in $\text{Cu}\text{ }3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states counterbalances a depopulation of $3{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals which induces, on account of the on-site Coulomb repulsion $U$, a splitting of $3{d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ states at ${\text{CuO}}_{2}\text{-SrO}$ interfaces.

Interface hole doping in cuprate-titanate superlattices

The electronic structure of interfaces between YBa$_2$Cu$_3$O$_6$ and SrTiO$_3$ is studied using local spin density approximation (LSDA) with intra-atomic Coulomb repulsion (LSDA+U). We find a metallic state in cuprate/titanate heterostructures with the hole carriers concentrated substantially in the CuO$_2$-layers and in the first interface TiO$_2$ and SrO planes. This effective interface doping appears due to the polarity of interfaces, caused by the first incomplete copper oxide unit cell. Interface-induced high pre-doping of CuO$_2$-layers is a key mechanism controlling the superconducting properties in engineered field-effect devices realized on the basis of cuprate/titanate superlattices.

Metallic ferromagnetism in an orbitally degenerate system of strongly correlated electrons

Ferromagnetic ordering in a generalization of the Hubbard model with correlated hopping to the case of twofold orbital degeneracy has been studied. An effective Hamiltonian of the model has been derived for the case of strong intra-atomic Coulomb repulsion and strong Hund's rule coupling. The energy of the system and the magnetization in the ground state have been calculated for various densities of states. The magnetization at non-zero temperature and Curie temperature of system have been obtained numerically. The important peculiarity of the concentration dependence of Curie temperature is its decrease in some region of electron concentration with increase of magnetic moment. This allows us to interpret the peculiar behavior of Curie temperature in the compound Fe 1 - x Co x S 2 with the change of conduction band filling.

Localized nature of valence-hole states in Ca2+xY2−xCu5O10 shown by O 1s XAS

The hole doping effects on the O 1s x-ray absorption spectrum (XAS) of a quasi-one-dimensional charge-transfer insulator, Ca2+xY2−xCu5O10, are discussed on the basis of numerically-exact diagonalization calculations applied to finite-size cluster models. The angle-resolved photoemission and inverse one are also calculated to understand the characteristic features in the XAS. The role of “edge-share”-type crystal structure, as well as that of electron correlation, is emphasized to understand the localized nature of the electronic states. The dp model Hamiltonian used in the O 1s XAS calculation can be mapped onto a single-band Hubbard model with the intra-atomic Coulomb repulsion, U, and the first and second neighbor hoppings, t1 and t2 (t1 ≈ t2).

Quantum magnetic excitations from stripes in copper oxide superconductors.

In the copper oxide parent compounds of the high-transition-temperature superconductors the valence electrons are localized--one per copper site--by strong intra-atomic Coulomb repulsion. A symptom of this localization is antiferromagnetism, where the spins of localized electrons alternate between up and down. Superconductivity appears when mobile 'holes' are doped into this insulating state, and it coexists with antiferromagnetic fluctuations. In one approach to describing the coexistence, the holes are believed to self-organize into 'stripes' that alternate with antiferromagnetic (insulating) regions within copper oxide planes, which would necessitate an unconventional mechanism of superconductivity. There is an apparent problem with this picture, however: measurements of magnetic excitations in superconducting YBa2Cu3O6+x near optimum doping are incompatible with the naive expectations for a material with stripes. Here we report neutron scattering measurements on stripe-ordered La1.875Ba0.125CuO4. We show that the measured excitations are, surprisingly, quite similar to those in YBa2Cu3O6+x (refs 9, 10) (that is, the predicted spectrum of magnetic excitations is wrong). We find instead that the observed spectrum can be understood within a stripe model by taking account of quantum excitations. Our results support the concept that stripe correlations are essential to high-transition-temperature superconductivity.

Metal-insulator transition in the double perovskites

We construct an effective Hamiltonian for the motion of electrons among the transition-metal ions of ordered double perovskites like ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6},$ in which strong intra-atomic Coulomb repulsion U is present in only one of the inequivalent transition metal sites. Using a slave-boson formalism, we construct a phase diagram that describes a charge-transfer transition between insulating and metallic behavior as the parameters of the model are changed. The parameters for ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ are estimated from first-principles calculations and a transition to the insulating state with negative pressure is obtained.

Theory of the spin dynamics of bulk Fe and ultrathin Fe(1 0 0) films

We present theoretical studies of the spin dynamics of bulk Fe and of ultrathin Fe(1 0 0) films, within an empirical tight binding description of the one electron energies. We use a Slater–Koster parameterization of the bulk paramagnetic energy bands, and apply these to the film. Ferromagnetic ground states are generated by introducing intra-atomic Coulomb repulsion within the d shell of each atom. The intra-atomic Coulomb matrix elements are expressed in terms of three Racah parameters, whose values are chosen to describe the ferromagnetism of the bulk. We calculate the transverse dynamic spin susceptibility of both the bulk and a film with a (1 0 0) surface, within the random phase approximation (RPA). Such an approach describes the spin wave excitations of these systems, and their Stoner spectrum as well.

Stoner criterion of ferromagnetism and moment saturation in the Hubbard model with an applied magnetic field

The intraatomic Coulomb repulsion of itinerant electrons provides another Stoner mechanism of band splitting leading to ferromagnetism. The behavior of the magnetization versus external magnetic field is derived by an approximate treatment of the local correlations in a simple mean-field approximation for 1D, 2D, and 3D cubic lattices. The criteria for first-order phase transition and full spin alignment in an applied magnetic field are found. These attempts should be regarded as a way to obtain an upper bound for the magnetic moments. The appearance of ferromagnetism is seen for larger values of the intraatomic interaction U and lattice parameter a. This study provides a better understanding of the mechanism of strong ferromagnetic saturation and may help explain the behavior of magnetic moments of certain Fe sites in FeN compounds.

Memory loss and Auger processes in a many-body theory of charge transfer.

Charge transfer between hyperthermal alkali atoms and metallic scattering surfaces is an experimental and theoretical arena for many-body interactions. To model new facets, we use a generalized time-dependent Newns-Anderson Hamiltonian that includes electron spin, multiple atomic orbitals with image shifted levels, intra-atomic Coulomb repulsion, and resonant exchange. A variational electronic many-body wave function solves the dynamical problem. The wave function consists of sectors with zero and one particle-hole pair and goes beyond earlier work with the inclusion of amplitudes for a neutral atom plus an electron-hole pair. Higher-order sectors with more than one particle-hole pair are suppressed by powers of 1/N; hence the wave-function ansatz is equivalent to a 1/N expansion. The equations of motion are integrated numerically without further approximation. This solution shows improved loss of memory --- the final charge state is independent of the initial one --- in agreement with theoretical and experimental expectations. Understanding of this phenomenon is deepened through an analysis of entropy production. By studying the independent-particle approximation, and by examining the role played by different sectors of the Hilbert space in entropy production, we arrive at necessary and sufficient conditions for loss of memory to occur in the many-body solution. As further tests of the theory, we reproduce the experimentally observed peak in the excited neutral Li(2p) occupancy at intermediate work functions starting from different initial conditions. Next, we include Auger processes by adding two-body interaction terms to the many-body Hamiltonian. Several types of Auger processes are considered, and these are shown to affect the final-state occupancies at low work functions because phase space enlarges rapidly as the work function is lowered. Preliminary experimental evidence for an upturn in the Li(2p) occupancy at the lowest work functions thus may be explained by Auger transitions. Finally, we comment on the plausibility of observing a signature of the Kondo resonance in charge transfer experiments. \textcopyright{} 1996 The American Physical Society.

1/N-Expansion Study of aU-Finite Degenerate Anderson Model

We propose a model convenient for studying properties of the degenerate Anderson models including finite intra-atomic Coulomb repulsion U by using the perturbation expansion from the large limit of the spin-orbital degeneracy N (1/ N -expansion). This is to extend a series of 1/ N -expansion studies on the U -infinite degenerate impurity and lattice Anderson models carried out by Nagoya group for including effects of finite U . As a first step, we study a case with a single Anderson impurity to investigate crossover behavior between magnetic state and non-magnetic state as a function of U by calculating average number of the localized electron, specific heat, charge susceptibility and spin susceptibility in the leading order of (1/ N ) and also the Kondo temperature and scattering rate of the conduction electrons up to the next leading order of (1/ N ).