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

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

Assessing the Sustainability of Agricultural Systems

This work constructs and tests a comparative technique that can be used to determine the ability of agricultural systems to meet multiple objectives of sustainability. Assessments are based on an agricultural system\u27s ability to maintain five system stocks that represent environmental capital, social capital, human capital, physical capital, and financial capital. The capital stocks are measured as stock objectives, which are decomposed as indicators to calculate agricultural system scores. An examination of the complexity of agro-ecosystems and the multiple equilibria produced by balancing multiple objectives is used to explain the alternative criteria used in this comparative technique to evaluate agricultural systems. The indicators of each of the objectives of the five capitals in this comparative technique are assessed using multi-criteria decision-making. Based upon research and the advice of experts in the field, a survey instrument was developed that uses farming practices to evaluate indicators of the five system stocks. A sample of 23 Maine farms was used to test the survey instrument. Given the sample size of observations and nature of the data, no statistical tests were done for this work. However, the sample allows for preliminary evaluations of the ability of the comparative technique to assess distinctions between farming systems in balancing the multiple objectives of sustainability. From these results, it appears that both organic and conventional farming operations can be adept at balancing the many objectives of sustainability. These results also suggest that due to the scale of operations and marketing channels used by the farms surveyed, some conventional farming operations can meet a Pretty sustainability test better than some organic farming operations. The results from the surveyed farmers demonstrate the intricacies inherent in making the tradeoffs necessary to balance multiple objectives. Farming operations that tended to score well on system stocks such as environmental capital did not always score so well on other system stocks. Future work with a larger sample of farms could demonstrate whether or not the sample results of this pilot project are representative of a broader population

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

Metallic ferromagnetism in a generalized Hubbard model

The possibility of realization of a metallic ferromagnetic state in a generalized Hubbard model with correlated hopping and exchange interaction integrals is investigated. The single-electron energy spectrum recently obtained by means of the mean-field approximation is applied for the description of ground-state and finite-temperature properties of the system. An expression for the Curie temperature is found, and the behavior of the temperature dependences of the magnetization and paramagnetic susceptibility is analyzed. Taking into account the correlated hopping allows to explain certain peculiarities of the ferromagnetic behavior of transition metals and their compounds