Research On Electronic Transport Characters And Mechanism Of Manganese Perovskite Oxides With Multiple Oxidation States

Research on the manganites has revealed many novel phenomenons,for example,colossal magnetoresistance(CMR), at the same time,it has lad to some new physical concepts,such as,double-exchang and Jahn-Teller polaron and so on.Early research mainly forcused on searching for insulating ferromagnets with a large magnetization for high-frequency applications, and now we has put light on finding out and explaining large negative magnetoresistance effects which appear near and below the Curie temperature. Based on the abundant physical properties, the manganites can be used for solid electrolytes, catalysts, sensors and novel electronic materials. Their rich electronic phase diagrams reflect the fine balance of interactions which determine the electronic ground state. These compounds represent, in microcosm, the interplay of experiment,theory and application which is at the heart of solid state physics.This kind of compouds is the core in the area of microscopic world, experiment , theory and applications of the solid state physics.Mixed-valence manganese oxides (R1-xAx)MnO3 (R rare-earth cation, A alkali or alkaline earth cation), with a structure similar to that of perovskiteCaTiO3, exhibit a rich variety of crystallographic, electronic and magnetic phases.Historically they led to the formulation of new physical concepts such as double exchange and the Jahn-Teller polaron. More recent work on thin flms has revealed new phenomena, including colossal magnetoresistance near the Curie temperature, dense granular magnetoresistance and optically-induced magnetic phase transitions. Issues addressed include the nature of the electronic ground states, the metal-insulator transition as a function of temperature, pressure and applied magnetic field, the electronic transport mechanisms, dielectric and magnetic polaron formation, magnetic localization, the role of cation disorder and the Jahn- Teller effect.Manganese oxides are generally charge-transfer type, but also close to the Mott-Hubbard-type region. Parent-phase LaMnO3 is a charge-transfer-type insulators. When using a small amount of divalent alkaline-earth metal instead of La, the introduction of hole (mainly in the oxygen), the system's chemical potential change, until the ground state from an insulator into a metal, occurs insulator-metal transition (IM). This band fills the control of IM changes greatly affected by the system bandwidth. When using different sizes of the alkaline earth metals do an alternative, system bandwidth will be subject to modulation (A bit larger ionic radius, bandwidth is also larger). IM changes would give rise to the threshold required for carrier concentration is small. On this basis, we will be mixed with alkali metal K which is bound to introduce some sort of doping disorder, which disorder effects on the system is unpredictable.We have synthesised a series of manganese perovskite oxides by hydrothermal methods and annealed the samples in different conditions, that was at different temperatures and different atmosphere.In detail,the samples were dealt at 500℃, 700℃, 900℃and at the atmosphere of oxygen,vacuum (with or without graphite) and air.In the process, oxygen spilled or ran into the sample. And here, I think the second occurred at the the atmosphere of oxygen and air,as a result, La1-x-y CaxKyMnO3+δappeared. And La1-x-y CaxKyMnO3-δappeared in the first condition. As to the value ofδ, it depended on the properties of the compound, and such condition also was ture of YBCO which resulted in superconductivity. In the text, we found out that the conductivity changed after the annealing. What was more, the crystal structure had no change by XRD. In the process of electrical test, we could say that the conductivity grow good for the samples annealed at the atmosphere of oxygen and air,and the opposite condition occured at the atmosphere of vacuum (with or without graphite) which could be explained as a result ofδ.In order to clarify the reason of the changing nature through different anneal-treated samples, we use the X-ray photoelectron spectroscopy for element valence analysis. With the element content analysis of the sample surface, we found that La, Ca, K, Mn element peak intensity is slightly change and the peak is not remove strongly by the external conditions of sample handling process and the sample nature enormous impact did not change . In the contrast ,the oxygen environment has been different, the original oxygen content has changed dramatically, a shoulder structure at the high BE side of O 2p3/2 main peak has been observed which we conclude that derived from the zero valence oxygen.Meanwhile, the corresponding valence-band spectra of sample also has been changed. This phenomenon fully demonstrates that the oxygen in this process plays an important role in the electronic transport properties of the sample. In further studies, we also found room-temperature ferromagnetic phase in our samples.In the process, we also found that supercurrent about 5×104 A/cm2 appeared for the manganite perovskite oxide which could occurred in the superconducting state of the superconductor.We could explain as follows: at first the system formed the long-life exciton in the presence of the applied electrical field and heat, then as a result of the interactions of the exiton and the electron ,two elecrons formed a pair showing out the supercurrent.