Magnetic field dependence of conductivity and effective mass of carriers in a model of Mott-Hubbard material

The influence of external magnetic field $h$ on a static conductivity of Mott-Hubbard material which is described by model with correlated hopping of electrons has been investigated. By means of canonical transformation the effective Hamiltonian which takes into account strong intra-site Coulomb repulsion and correlated hopping is obtained. Using a variant of generalized Hartree-Fock approximation the single-electron Green function and quasiparticle energy spectrum of the model have been calculated. The static conductivity $\sigma$ has been calculated as a function of $h$, electron concentration $n$ and temperature $T$. The correlated hopping is shown to cause the electron-hole asymmetry of transport properties of narrow band materials.Comment: 10 pages, 7 figures, submitted to Condensed Matter Physics journa

Phase Diagram of Metal-Insulator Transition in System with Anderson-Hubbard Centers

The model of a strongly correlated system in which periodically spaced Anderson-Hubbard centers are introduced into narrow-band metal is considered. Besides the interactions between localized magnetic moments and strong on-site Coulomb interaction, the model takes into account the hybridization of localized and band states. To study the efect of the lattice deformation on the electrical properties of the system the phonon term and elastic energy have been taken into account. Green functions for band and localized electrons have been found. On this base, the energy spectrum has been investigated as function of model parameters, temperature and external pressure. The criterion of metal-insulator transition for integer value of electron concentration has been derived and the phase diagram of the metal-insulator transition has been built.Comment: presented at 12 International Simposium on Physics of Materials, Prague 4-8.09.201

Ground state ferromagnetism in a doubly orbitally degenerate model

In the present paper the ground state of a double orbitally degenerate model at weak intra-atomic interaction is studied using the Green functions method. Beside the diagonal matrix elements of electron-electron interactions the model includes correlated hopping integrals and inter-atomic exchange interaction. The influence of orbital degeneracy with Hund's rule coupling, correlated hopping and inter-atomic direct exchange on the ferromagnetic ordering is investigated. The expressions for ground state energy and magnetization, the criterion of transition from paramagnetic to ferromagnetic ground state as functions of the model parameters are obtained. The obtained results are compared with some experimental data for magnetic materials.Comment: 13 pages, 11 eps figure

Electron correlations in narrow energy bands: ground state energy and metal-insulator transition

The electron correlations in narrow energy bands are examined within the framework of the Hubbard model. The single-particle Green function and energy spectrum are obtained in a paramagnetic state at half-filling by means of a new two-pole approximation. Analytical expressions for the energy gap, polar states concentration and energy of the system are found in the ground state. Metal-insulator transitions in the model at the change of bandwidth or temperature are investigated. The results obtained are used for interpretation of some experimental data in narrow-band materials.В данiй роботi електроннi кореляцiї у вузьких енергетичних зонах вивчаються в рамках моделi Габбарда. За допомогою нового двополюсного наближення знайдено одночастинкову функцiю Грiна та енергетичний спектр у парамагнiтному станi при половинному заповненнi. В основному станi знайдено аналiтичнi вирази для ширини енергетичної щiлини, концентрацiї полярних станiв та енергiї системи. Дослiджено переходи метал-дiелектрик при змiнi ширини зони та при змiнi температури. Отриманi результати використанi для iнтерпретацiї деяких експериментальних даних для вузькозонних матерiалiв

Energy spectrum of a doubly orbitally degenerate model with non-equivalent subbands

In the present paper we investigate a doubly orbitally degenerate narrowband model with correlated hopping. The model peculiarity takes into account the matrix element of electron-electron interaction which describes intersite hoppings of electrons. In particular, this leads to the concentration dependence of the effective hopping integral. The cases of the strong and weak Hund’s coupling are considered. By means of a generalized mean- field approximation the single-particle Green function and quasiparticle energy spectrum are calculated. Metal-insulator transition is studied in the model at different integer values of the electron concentration. Using the obtained energy spectrum we find criteria of metal-insulator transition.У роботі ми вивчаємо двократно орбітально вироджену вузькозонну модель з корельованим переносом електронів. Особливістю моделі є врахування матричного елемента електрон-електронної взаємодії, який описує міжвузлові переходи електронів. Це приводить, зокрема, до концентраційної залежності ефективного інтеграла пере- носу. Розглянуті випадки сильного та слабкого гундівського зв’язку. За допомогою узагальненого наближення середнього поля розраховані одночастинкова функція Гріна та енергетичний спектр. Перехід метал-діелектрик у моделі досліджений при різних цілих значеннях електронної концентрації. За допомогою отриманого енергетичного спектра знайдено критерії переходу метал-діелектри

Energy spectrum of the organic quasi-1D conductors with NNN and correlated hopping

A model for organic quasi-one-dimensional conductors (TMTTF)₂X and (TMTSF)₂X is considered. The anisotropic character of these compounds is modelled by two different hopping parameters: t between nearest neighbors (NN) in a chain of tetramethyl-tetrathiafulvalene (TMTTF) or tetramethyl-tetraselenfulvalene (TMTSF) molecules and t' between the chains (NNN - between next nearest neighbors). Taking into account the correlated hopping of electrons allows us to describe the effect of site occupancy on hopping processes. In a regime of strong intraatomic correlation, high energy processes are cut off by applying two successive canonical transformations. An effective model is obtained for concentration of electrons n < 1 which contains kinetic exchange terms of antiferromagnetic (AF) nature. Oppositely, NNN hopping and correlated hopping disfavor the AF order. The energy spectrum of the effective model is calculated. Application of the obtained results to quasi-one-dimensional conductors is discussed