Showing papers on "Charge ordering published in 1988"
TL;DR: In this paper, the atomic parameters used in the extended Hubbard (EHB) models for transition metal oxides were determined so as to reproduce ab initio results and experimental results.
Abstract: The atomic parameters used in the extended Hubbard (EHB) models for transition metal oxides were determined so as to reproduce ab initio results and experimental results. The EHB model was investigated in relation to classification of transition metal oxides into the Mott-Hubbard and charge transfer (CT) semiconductors. The model calculations indicate that both spin and charge fluctuations are of importance to the high-Tc superconductivity.
31 citations
TL;DR: In this paper, it was shown that in such systems, depending on intramolecular parameters of electron transfer and exchange interaction, one or two phase transitions are possible, and an anomalous temperature behavior of the magnetic moment of charge-ordered crystals was predicted.
28 citations
TL;DR: In this article, it was confirmed by new magnetization measurements that a magnetic transition occurs sharply at 30 K in the monoclinic pyrrhotite Fe7S8.
Abstract: It is confirmed by new magnetization measurements that a magnetic transition occurs sharply at 30 K in the monoclinic pyrrhotite Fe7S8. Though its nature is not yet fully understood, this transition shows some similarities with the Verwey transition and can provide a powerful tool for the identification of pyrrhotite in rocks.
26 citations
TL;DR: In this article, a new optical effect in the reflectivity spectrum of synthetic single crystals of magnetite (Fe 3 O 4 ) is presented, which consists of many absorption lines in the far-infrared that will be interpreted as an evidence of tunnel modes between different valencies of iron.
16 citations
01 Jan 1988
TL;DR: The metal-insulator transition in magnetite is also known as the Verwey transition as discussed by the authors, where the electronic charge associated with Fe2+ and Fe3+ ions in the octahedral B sites is completely delocalized.
Abstract: A number of mixed-valence transition metal oxides show metal-insulator transitions characterized by a large conductivity decrease and a specific heat anomaly. A typical example is magnetite, Fe3O4, which has one conducting electron for two cationic sites (Fe2+ and Fe3+). Instead of being metallic at T = 0K, it is an insulator at low temperatures due to charge localization. The metal-insulator transition in magnetite is also known as the Verwey transition, because Verwey (1939) first pointed out that above the transition (T v = 119 K) the electronic charge associated with Fe2+ and Fe3+ ions in the octahedral B sites is completely delocalized, whereas below the transition Fe2+ and Fe3+ occur in their distinct ionic states (Verwey and Haymaan, 1941).
12 citations
TL;DR: In this article, electrical resistivity and thermoelectric power measurements have been made on single crystals of manganese ferrites, Mn/sub x/Fe/sub 3-x/O/sub 4/ (x=0, 0.5, 0 6, 0 7, 0 8, 0 9, and 0.95) in the temperature range 10 K to 300 K.
Abstract: Electrical resistivity and thermoelectric power measurements have been made on single crystals of manganese ferrites, Mn/sub x/Fe/sub 3-x/O/sub 4/ (x=0, 0.5, 0.7, 0.8, 0.9, and 0.95) in the temperature range 10 K to 300 K. Below the Verwey transition T/sub v/ of magnetite, the thermoelectric power is strongly influenced by the oxygen nonstoichiometry of the samples whereas the resistivity exhibits hardly any dependence on the changes of the oxygen content. Starting from the lowest temperatures, the electrical properties are explained in terms of the impurity band, variable range hopping, small polaron band, and small polaron hopping conduction mechanisms where the long-range and the short-range orderings have to be taken into account. >
2 citations
TL;DR: In this article, the interplay between charge disproportionation and superconductivity resulting from localization and Bose condensation of local hole pairs in systems with strong electron-phonon interaction is analyzed on the basis of the negative U Hubbard model.
Abstract: The interplay between charge disproportionation and superconductivity resulting from localization and Bose condensation of local hole pairs in systems with strong electron-phonon interaction is analysed on the basis of the “negative U” Hubbard model. The Landau free energy functional with two competing order parameters is derived from microscopic consideration. An example of the resulting T -x phase diagram is presented.
2 citations
TL;DR: In this article, the interplay between charge density waves (CDW) and Bose condensation of local hole pairs in systems with strong electron-phonon interaction was analyzed on the basis of the negative U Hubbard Hamiltonian and local pair superconductor model.
Abstract: The interplay between charge density waves (CDW) and Bose condensation of local hole pairs in systems with strong electron-phonon interaction [1] is analysed on the basis of the negative U Hubbard Hamiltonian and local pair superconductor model The Landau free energy functional with two competing order parameters is derived from microscopic considerations and the temperature-concentration (T - x) phase diagrams relevant for La2-xMxCuO4 (M = Ba, Ca, Sr) compounds are calculated