Order parameters in the Verwey phase transition

The Verwey phase transition in magnetite is analyzed on the basis of the Landau theory. The free energy functional is expanded in a series of components belonging to the primary and secondary order parameters. A low-temperature phase with the monoclinic P2/c symmetry is a result of condensation of two order parameters X_3 and \Delta_5 . The temperature dependence of the shear elastic constant C_44 is derived and the mechanism of its softening is discussed.Comment: 4 pages, 1 figur

Comparative study of the electronic structures of Fe3O4 and Fe2SiO4

The electronic properties of two spinels Fe$_3$O$_4$ and Fe$_2$SiO$_4$ are studied by the density functional theory. The local Coulomb repulsion $U$ and the Hund's exchange $J$ between the $3d$ electrons on iron are included. For $U=0$, both spinels are half-metals, with the minority $t_{2g}$ states at the Fermi level. Magnetite remains a metal in a cubic phase even at large values of $U$. The metal-insulator transition is induced by the $X_3$ phonon, which lowers the total energy and stabilizes the charge-orbital ordering. Fe$_2$SiO$_4$ transforms to a Mott insulating state for $U>2$ eV with a gap $\Delta_g\sim U$. The antiferromagnetic interactions induce the tetragonal distortion, which releases the geometrical frustration and stabilizes the long-range order. The differences of electronic structures in the high-symmetry cubic phases and the distorted low-symmetry phases of both spinels are discussed.Comment: 6 pages, 6 figure

Mechanism of the Verwey transition in magnetite

By combining {\it ab initio} results for the electronic structure and phonon spectrum with the group theory, we establish the origin of the Verwey transition in Fe$_3$O$_4$. Two primary order parameters with $X_3$ and $\Delta_5$ symmetries are identified. They induce the phase transformation from the high-temperature cubic to the low-temperature monoclinic structure. The on-site Coulomb interaction $U$ between 3d electrons at Fe ions plays a crucial role in this transition -- it amplifies the coupling of phonons to conduction electrons and thus opens a gap at the Fermi energy. {\it Published in Phys. Rev. Lett. {\bf 97}, 156402 (2006).}Comment: 5 pages, 3 figure

{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.Comment: 7 pages, 8 figure

Nucleation Processes in Incommensurate Phases

The commensurate-incommensurate phase transition in crystals in driven by the nuclei called stripples. The stripple has a structure defined by the symmetries of crystal phases. It is nucleated as a result of thermal fluctuations. Since a macroscopic number of stripples is needed to complete the phase transition a collective nucleation can take place. There are two ideal mechaniscms of collective stripple nucleations : cascade and serial ones. The stripple appearences are illustrated by the results of mocelular-dynamics simulation on the model systems

Vibrational properties of alpha- and sigma-phase Fe-Cr alloy

Experimental investigation as well as theoretical calculations, of the Fe-partial phonon density-of-states (DOS) for nominally Fe_52.5Cr_47.5 alloy having (a) alpha- and (b) sigma-phase structure were carried out. The former at sector 3-ID of the Advanced Photon Source, using the method of nuclear resonant inelastic X-ray scattering, and the latter with the direct method [K. Parlinski et al., Phys. Rev. Lett. {78, 4063 (1997)]. The characteristic features of phonon DOS, which differentiate one phase from the other, were revealed and successfully reproduced by the theory. Various data pertinent to the dynamics such as Lamb-Mossbauer factor, f, kinetic energy per atom, E_k, and the mean force constant, D, were directly derived from the experiment and the theoretical calculations, while vibrational specific heat at constant volume, C_V, and vibrational entropy, S were calculated using the Fe-partial DOS. Using the values of f and C_V, we determined values for Debye temperatures, T_D. An excellent agreement for some quantities derived from experiment and first-principles theory, like C_V and quite good one for others like D and S was obtained.Comment: 4 pages, 3 figure

Structure and elastic properties of Mg(OH)2_2 from density functional theory

The structure, lattice dynamics and mechanical properties of the magnesium hydroxide have been investigated with static density functional theory calculations as well as \it {ab initio} molecular dynamics. The hypothesis of a superstructure existing in the lattice formed by the hydrogen atoms has been tested. The elastic constants of the material have been calculated with static deformations approach and are in fair agreement with the experimental data. The hydrogen subsystem structure exhibits signs of disordered behaviour while maintaining correlations between angular positions of neighbouring atoms. We establish that the essential angular correlations between hydrogen positions are maintained to the temperature of at least 150 K and show that they are well described by a physically motivated probabilistic model. The rotational degree of freedom appears to be decoupled from the lattice directions above 30K

Взаимосвязь инвестиционной политики предприятия с повышением качества и эффективности управления

В современных условиях для эффективной деятельности предприятия особенно актуальной является проблема мобилизации и эффективного использования инвестиций. Инвестиции является неотъемлемой частью деловой активности субъектов хозяйствования, в том числе производственной, инновационной, рыночной, маркетинговой и иной деятельности. Целью данной работы является: изучить теоретические аспекты инвестиций; рассмотреть методологические подходы к оценке эффективности инвестиций; исследовать систему управления инвестиционной деятельностью предприятий; изучить понятие эффективности предприятия; провести анализ инвестиционной деятельности предприятия ООО "Потенциал" провести оценку влияния инвестиций на эффективность деятельности предприятия ООО "Потенциал"In modern conditions, for the efficient operation of an enterprise, the problem of mobilization and efficient use of investments is especially urgent. Investment is an integral part of the business activity of business entities, including production, innovation, market, marketing and other activities. The aim of this work is: to study the theoretical aspects of investment; consider methodological approaches to assessing the effectiveness of investments; explore the system of investment management of enterprises; to study the concept of enterprise efficiency; to analyze the investment activity of the enterprise LLC “Potential” to assess the impact of investments on the efficiency of the enterprise LLC “Potential

Basal-plane Incommensurate Phases in HCP Structures

An Ising model with competing interaction is used to study the appearance of incommensurate phases in the basal plane of an hexagonal closed-packed structure. The calculated mean-field phase diagram reveals various 1q-incommensurate and lock-in phases. The results are applied to explain the basal-plane incommensurate phase in some compounds of the A'A"BX_4 family, like K_2MoO_4, K_2WO_4, Rb_2WO4 and to describe the sequence of high-temperature phase transitions in other compounds of this family.Comment: 8 pages, RevTeX + 4 ps figure