Magnetic Transition And Substitution Effect In Superconducting Sr₂VO₃FeAs

The recently discovered iron pnictide compounds have become a new family of high temperature superconductors. It has broken cuprate "monopoly" in the physics of high-temperature superconductivity (HTSC) compounds and revived hopes on more deep theoretical understanding of mechanisms of HTSC. Empirically, it is found that the superconducting transition temperature rises when the inter-conducting-layer spacing distance is enlarged in the FeAs-based superconductors. Therefore fabricating a compound with larger spacing distance between the FeAs layers becomes an important way to look for materials with higher Tc. Sr2VO3FeAs with a rather large spacing distance between the neighboring FeAs layers was found to show superconductivity with Tc=37.2 K without intensive carrier doping; Moreover it has special perovskite oxide layer; and special physical properties have been found in it, different from other iron pnictide superconductorsFirstly, the magnetic properties of Sr2VO3FeAs will be investigated systematically. It is found that Sr2VO3FeAs undergoes two magnetic phase transitions, at 55 K and 150 K respectively. At different temperatures, a singlet spectrum with no magnetic splitting is observed as a main component of each Mossbauer spectrum, suggesting that the magnetism doesn't derive from the Fe atoms. X-ray diffraction (XRD) at different temperatures is applied, but no structural phase transition is found. The two magnetic anomalies are both suppressed by the Mg substitution for V in Sr2VO3FeAs, suggesting that they derive from the V atoms. We conclude that superconductivity is confined to the FeAs planes whereas magnetisms due to the V perovskite layer, and. The magnetic anomaly at about 55 K, is likely to be derived from the itinerant electrons of V3+ or canting of the AFM aligned V3+ spins.Secondly, Sr2VO3-δFeAs with different oxygen deficiencies has been successfully fabricated and studied. We find that a small amount of oxygen deficiencies seems beneficial to form purer phase. At the same time, with the increase of oxygen deficiency, the magnetic anomalies at 55 K and 150 K were suppressed, the superconducting transition temperature drops down monotonically and the sample becomes semiconductor little by little.Besides, a mixed occupancy of vanadium and iron will be studied by changing the content of the Fe and V atoms in the sample. The physical properties become different when the content of the Fe and V atoms are changed. With the increase of Fe-content, the Tc is increased to 37 K suddenly. However the two magnetic anomalies are both suppressed and the sample becomes semiconductor while the V-content is increased. Considering the results of the research on oxygen deficiencies, we conclude that the oxygen deficiencies result in more serious mixed occupancy in this sample. Thus, the V doping introduces disorder in the iron arsenide layer, and changes both crystal and electronic structures, which is responsible for the complex properties. Therefore, clues may be provided to understand the complex properties by the research on the mixed occupancy and oxygen deficiencies, which are worthy of further exploration.