Showing papers on "Charge ordering published in 1980"

Magnetic after-effects in Fe3O4and vacancy-doped magnetite

Abstract: The magnetic relaxation spectrum of stoichiometric and vacancy-doped magnetite has been investigated in the temperature range between 4·2 and 130 K. The observed relaxation processes are classified as short- and long-range rearrangements of Fe2+ and Fe3+ ionic states within domain walls. It is proposed that below 35 K a coherent tunnelling process governs the mobility of electrons, but that above 50 K is replaced by a thermally activated hopping process leading to the Verwey transition at 123 K.

Electron correlation theory of Fe3O4

Abstract: Above and below the Verwey transition in Fe3O4 the influence of the short-and long-range order of the extra B-site electrons on the density of states, the d.c conductivity and the optical absorption for infra-red frequencies is investigated. Our theory takes into account the Coulomb interaction between nearest (U1) and next-nearest neighbours (U2), the nearest-neighbour hopping (t), and a bilinear electron–phonon interaction, where U1≫t>U2 is assumed. Good agreement with experimental data is found. The Verwey transition is treated as the condensation of a coupled charge-density phonon mode with Δ5 (1) symmetry. A new charge ordering scheme is obtained: combination of the Δ5 (1) Yamada order (along the a axis) and charge order along the b axis due to U2.

Neutron diffuse scattering in magnetite due to molecular polarons

Abstract: A detailed neutron diffuse scattering study has been carried out in order to verify a model which describes the property of valence fluctuations in magnetite above T/sub V/. This model assumes the existence of a complex which is composed of two excess electrons and a local displacement mode of oxygens within the fcc primitive cell. The complex is called a molecular polaron. It is assumed that at sufficiently high temperatures there is a random distribution of molecular polarons, which are fluctuating independently by making hopping motions through the crystal or by dissociating into smaller polarons. The lifetime of each molecular polaron is assumed to be long enough to induce an instantaneous strain field around it. Based on this model, the neutron diffuse scattering cross section due to randomly distributed dressed molecular polarons has been calculated. A precise measurement of the quasielastic scattering of neutrons has been carried out at 150 K. The observed results definitely show the characteristics which are predicted by the model calculation and, thus, give evidence for the existence of the proposed molecular polarons. From this standpoint, the Verwey transition of magnetite may be viewed as the cooperative ordering process of dressed molecular polarons. Possible extensions ofmore » the model to describe the ordering and the dynamical behavior of the molecular polarons are discussed.« less

Charge ordering in Fe3O4, Ti4O7and bipolarons

Abstract: Charge localization in Ti4O7 and Fe3O4 and the respective insulating states are discussed It is concluded that the ground state in both systems is bipolaronic The concept of such a bipolaron is discussed and the importance of elastic interaction between two localized electrons as the dominant interaction in such systems is emphasized It is concluded that both systems above the long-range ordering temperature are to be considered as bipolaron glasses

The Verwey Transition Revisited

Abstract: Magnetite, the ferrospinel Fe3+[Fe2+Fe3+]04, is ferrimagnetic with atomic moments characteristic of localized, high- spin configurations with orbital-momentum quenched. An unactivated electrical conductivity of low electron mobility u leaves open the question whether the mobile d electrons on the octahedral sites are itinerant (u = ei/m*) or polarons (u = eDo/kT) having no activation energy for a jump between molecular orbitals of Fe-Fe pairs. Below a sharp Verwey transition at about 120K, the mobile charges are ordered; but the structural evidence is still inadequate to clarify whether this is a complex ordering of small polarons or the stabilization of a charge-density wave.

Partial charge ordering in magnetite

Abstract: It is shown that recent models of the charge ordering in magnetite which can explain a- and b-axis doubling are likely to be unstable, even below the Verwey temperature, to the formation of a partially ordered structure which accords with single-crystal neutron results.

Thermopower and electrical conductivity of magnetite near the verwey transition

Abstract: (1980). Thermopower and electrical conductivity of magnetite near the verwey transition. Philosophical Magazine B: Vol. 42, No. 3, pp. 429-430.

Displacive reconstruction phase transitions of clean transition metal and semiconductor surfaces

Abstract: The higher periodicities observed on several clean crystalline surfaces are discussed. Among the models invoked to account for their formation, we focus here on electronic mechanisms, particularly of the charge-density-wave type. A one-dimensional model is introduced to illustrate the continuity between the charge-density wave picture and the chemical bond picture. The reconstruction phase transitions of W and Mo(100) and Si(111) surfaces are discussed in some detail. We also consider their critical behaviour, and the defect structure in the ordered state.

Optical conductivity anisotropy below the Verwey transition in Fe3O4

Abstract: The optical conductivity of magnetite is calculated at zero temperature taking into account the nearest-neighbour Coulomb interaction between the extra B-site electrons. Due to the long-range order the optical conductivity is anisotropic where the anisotropy factor depends on the special long-range order scheme and is calculated for several examples.