Study On Iron-based Superconductors And Magnetic Structure Of LiCu₂O₂

Explore for new high-temperature superconductors and pursuit of higher superconductivity temperature have been people's dream, but until early 2008, people know the high-temperature superconductors only exists in copper oxide. In February 2008, the research team led by Professor Hideo Hosono reported that the iron-based material LaO1-xFxFeAs has superconducting transition temperature as high as 26K. This finding sparked interest in this superconducting system, the superconducting temperature soon break through the BCS theory predicted McMillan limit (39K), which is the superconducting temperature limit of traditional superconductors. This is the direct proof of such an iron arsenic superconductors is a new type of high-temperature superconducting materials. This result immediately aroused strong international response, a series of impressive research results in the field soon were got. This new high-temperature superconducting materials provides a new platform for studying the mechanism of high-temperature superconductivity.In this dissertation, we have studied the preparation and physical properties of iron-based superconductors with Sr4Sc2O6M2As2(M = Fe and Co)(426)structure and FeSe(11) structure. We also studied the pressure effect on BaFe2As2 system to see the pressure effect dependence of doping level. For LiCu2O2 system, we studied the complicated magnetic structure and multiferroic behavior at low temperature in this material through specific heat and susceptibility measurement.The whole dissertation is constructed as following:1. Brief overview of research on LiCu2O2 system and iron-based superconductorIn this chapter, the author has reviewed the basic physical properties in LiCu2O2 system and iron-based superconductors. For LiCu2O2 system, we have reviewed the crystalline structure and multiferroic behavior, and also discuss the magnetic structure. For iron-based superconductors, we have introduced the crystalline structure and development history. The pressure effect on iron-based superconductors have been also reviewed.2. Spin orientation in spin frustrated system LiCu2O2In this chapter, we have systematically measured susceptibility, angle dependent magnetization and specific heat on LiCu2O2 single crystal. When magnetic field (H) is rotated within ab plane, a twofold symmetry in magnetization is observed at 50 K and 100 K under H=5T, while a jump in magnetization is observed at certain angle with the twofold symmetry at 10 K. Specific heat shows two successive transitions at Tc1=24.7 K and Tc2=23 K, respectively. The first one at Tc1 shows antiferromagnetic (AFM) nature, while the second one at Tc2 shows ferromagnetic (FM) behavior. These behaviors can be interpreted by the recently reported magnetic structure, and confirm the magnetic structure through another way.3. Structure and physical properties of the new layered oxypnictides Sr4Sc2O6M2As2 (M = Fe and Co)In this chapter, we have successfully prepared the new layered oxypnictides Sr4Sc2O6M2As2 (M =Fe and Co). They adopt the tetragonal structure, being the same as that of Sr4Sc2O6Fe2P2. The lattice constants are a=0.4045nm and c=1.5802nm for M= Fe, and a=0.4045nm and c=1.5695nm for M=Co, respectively. Their transport and magnetic properties have been systematically studied. The temperature dependence of the Hall coefficient and the thermoelectric power for the Sr4Sc2O6Fe2As2 compound show a complicated behavior, similar to that of the ironbased parent compounds LnOFeAs and BaFe2As2. This suggests that Sr4Sc2O6Fe2As2 could be considered as a new parent compound of iron-based superconductors.4. Pressure effect on Superconductivity in Co- and K-doped BaFe2As2 single crystalsIn this chapter, we systematically studied the pressure effect on the superconducting state of Co-doped and K-doped BaFe2As2 single crystals. Similar behavior was observed in these two different systems. In the underdoped region where spin density wave (SDW) transition can be observed in resistivity, pressure tends to have a very large positive effect on TC, and TC increase rapidly with pressure enhanced. In the optimal doped region where SDW transition is suppressed, pressure tends to have no effect on TC, and TC changes a little with pressure applied. In the overdoped region, pressure tends to have negative effect on TC, and TC decrease with pressure enhanced. A detailed phase diagram about the influence of pressure on the superconducting state is given.