Electron correlation Hubbard

Keyvrords Two-electron reduced density matrix N-representability conditions Strong electron correlation Hubbard models... 

Many electron systems such as molecules and quantum dots show the complex phenomena of electron correlation caused by Coulomb interactions. These phenomena can be described to some extent by the Hubbard model [76]. This is a simple model that captures the main physics of the problem and admits an exact solution in some special cases [77]. To calculate the entanglement for electrons described by this model, we will use Zanardi s measure, which is given in Fock space as the von Neumann entropy [78]. 

The antiferromagnetic state described by the occupation of the lower Hubbard band is stabilized by inclusion of such electron correlation, but the ferromagnetic analog is not. This is a result exactly analogous to the stabilization of the lowest singlet state in cyclobutadiene below the triplet. For the simple density of states used by Hubbard in his treatment he showed in fact that the condition for ferromagnetism was... 

Electron correlation plays an important role in determining the electronic structures of many solids. Hubbard (1963) treated the correlation problem in terms of the parameter, U. Figure 6.2 shows how U varies with the band-width W, resulting in the overlap of the upper and lower Hubbard states (or in the disappearance of the band gap). In NiO, there is a splitting between the upper and lower Hubbard bands since IV relative values of U and W determine the electronic structure of transition-metal compounds. Unfortunately, it is difficult to obtain reliable values of U. The Hubbard model takes into account only the d orbitals of the transition metal (single band model). One has to include the mixing of the oxygen p and metal d orbitals in a more realistic treatment. It would also be necessary to take into account the presence of mixed-valence of a metal (e.g. Cu ", Cu ). 

L.J. Dunne, E.J. Brandas, D-Wave Bipolaronic Condensate with Short Range Repulsive Electronic Correlations in an Extended Hubbard Model of High-Tc Cuprate Superconductors, Adv. Quant. Chem. 40 (2001) 225. 

The aim of this work is to demonstrate that the above-mentioned unusual properties of cuprates can be interpreted in the framework of the t-J model of a Cu-O plane which is a common structure element of these crystals. The model was shown to describe correctly the low-energy part of the spectrum of the realistic extended Hubbard model [4], To take proper account of strong electron correlations inherent in moderately doped cuprate perovskites the description in terms of Hubbard operators and Mori s projection operator technique [5] are used. The self-energy equations for hole and spin Green s functions obtained in this approach are self-consistently solved for the ranges of hole concentrations 0 < x < 0.16 and temperatures 2 K< T <1200 K. Lattices with 20x20 sites and larger are used. 

In addition, comparison to solutions of the Hubbard and PPP models including electron correlation shows the VB wave function to be a more accurate initial approximation than the Hartree Fock solution at the correlation strengths likely to be encountered in realistic semiempirical models. In spite of the qualitative superiority of the VB wave function, systematic computational approaches to more accurate treatment of correlation are still most readily achieved when starting from the independent particle limit, but the correlated wave functions thus built up are likely to be interpretable in valence bond terms. 

In the Hubbard model it was found that phonon-induced charge transfer is spin polarized because of strong electron correlation. The dynamics of spins is strongly modified by the Cu-0 bond-stretching phonons, and the spin excitations are generally greatly softened by phonons10. This must be related... 

To derive the dynamical spin susceptibility in the superconducting state we use the method suggested by Hubbard and Jain[15] that allows to take into account strong electronic correlations. First we add the external magnetic field applied along c-axis into the effective Hamiltonian... 

This is a central result of our paper. Here, xo(q> °>) is the usual BCS-like Lind-hard response function, n(q, co) and Z(q, co) result from the strong electronic correlation effects. In the normal state the expression for n(q, co) has been obtained by Hubbard and Jain [15]. In the superconducting state it is given by... 

Electronic Structure Calculations. We have used first-principles electronic structure calculations as manifest in the (spin) density functional linearized muffin-tin orbital method to examine whether the asymmetry in properties is reflected in a corresponding asymmetry in the one-electron band structure. While in a more complete analysis explicit electron correlation of the Hubbard U type would be intrinsic to the calculation,17 we have taken the view that one-electron bandwidths point to the possible role that correlation might play and that correlation could be a consequence of the one-electron band structure rather than an integral part of the electronic structure. We have chosen the Lai- Ca,Mn03 system for our calculations to avoid complications due to 4f electrons in the corresponding Pr system. 

J. Hubbard, electron correlations in narrow energy bands Proc. Roy. Soc. London A276 238-257 (1963). 

The cause or causes of the opening of a gap in the band structure of trans-PA has been the subject of many theoretical papers and of much debate (see Chapter 11, Section IV.A and reviews and discussions in [17,146,147,181]). It would seem that electron-phonon and electron-election interactions are of comparable importance. If electron correlations are treated by adding a Hubbard on-site interaction term to the SSH Hamiltonian, the available experimental results for tram-PA are best accounted for by taking about equal values for the electron-phonon coupling X and for the Hubbard U. It might be that in other CPs the importance of electron correlations is greater. Note, however, that a U term (on-site interactions) is not enough to treat the correlations correctly, especially if excitons are to be studied (see the discussion of the PDA case above). 

Figure 7.1. The band gap is determined by the d-d electron correlation in the Mott-Hubbard insulator (a), where A > I/. By contrast, the band gap is determined by the charge transfer excitation energy in the charge transfer insulator (b), where U > A.

It is believed that electron correlation plays an important role with the anomalously high resistivity exhibited in marginal metals. Unfortunately, although the Mott-Hubbard model adequately explains behavior on the insulating side of the M-NM transition, on the metallic side, it does so only if the system is far from the transition. Electron dynamics of systems in which U is only slightly less than W (i.e. metallic systems close to the M-NM transition), are not well described by a simple itinerant or localized picture. The study of systems with almost localized electrons is still an area under intense investigation within the condensed matter physics community. A dynamical mean field theory (DMFT) has been developed for the Hubbard model, which enables one to describe both the insulating state and the metallic state, at least for weak correlation. 

Systems exhibiting both strong disorder and electron correlation, so-caUed disordered Mott-Hubbard insulators, are difficult to evaluate. The description of electronic states in the presence of both disorder and correlation is still an unresolved issue in condensed matter physics. Whether disorder or the correlation is the predominant factor in controlling transport properties in a material depends on a complex... 

The theoretical analysis could have started from the premise that the electron-electron interaction (electron correlation) is more important than the electron-phonon interaction. The Hubbard Hamiltonian would then be more appropriate than the SSH one. The Hubbard Hamiltonian has the form ... 

A discussion is given of electron correlations in d- and f-electron systems. In the former case we concentrate on transition metals for which the correlated ground-state wave function can be calculated when a model Hamiltonian is used, i.e. a five-band Hubbard Hamiltonian. Various correlation effects are discussed. In f-electron systems a singlet ground-state forms due to the strong correlations. It is pointed out how quasiparticle excitations can be computed for Ce systems. 

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