Transport And Magnetic Properties And Phase Diagram Of "122"-typed Iron-Based High Temperature Superconductors

Superconducting materials possess two important features:zero resistance and Messiner effects. These unique features make it important for practical applications in various fields, and the superconducting physics develops gradually in-depth along with the discovery of variety of different types of superconducting materials. The super-conducting transition temperature of conventional superconductors is lower than40K, greatly limiting the practical application of the superconducting materials. Cuprate superconductors were found in1986, which is a great breakthrough of high temper-ature superconductivity, causing a craze to explore new superconductors with higher critical temperature and superconducting mechanism around the world. When com-pared with the conventional BCS superconductors, cuprate high-temperature super-conductors have lots of novel physical properties. However, no well-recognized unified picture of the theoretical descriptions of these novel physical properties as well as the superconducting mechanism of cuprate superconductors was achieved. In early2008, Japanese scientists found the iron-based superconductors, which were subsequently confirmed to be the second class of high-temperature superconductors. The struc-ture of iron-based superconductors is similar with that of cuprate superconductors, superconductivity occurs on the iron-based plane, leading to a two-dimensional su-perconducting structure. Considering the similarities and differences between these two types of high-temperature superconductors, scientists hope to explore a clearer picture of the superconducting mechanism through the comparative study of these two types of high-temperature superconductors.In this dissertation, we systematically studied the anomalous transport proper-ties, thermoelectric properties and Hall angles of122-typed iron-based superconduc-tors Ba1-xKxFe2As2and Ba(Fe1-xCox)2As2, giving the phase diagrams of the super-conducting temperature, thermoelectric power at room temperature, and power-law temperature dependence of Hall angle evolving with the electron or hole doping con-tent. We also measured the transport and magnetic properties of a series of Cu-doped SrFe2As2single crystals, and determined the valence of Cu ions by x-ray photoelectron spectroscopy. In addition, we studied the structural properties, magnetic properties and specific heat of series of Co-and Zn-doped In3Cu2VO9polycrystals, and discussed the changes of ground states for various samples with different types of doping ions. This dissertation is divided into the following six chapters:1. Brief Overview of High Temperature Superconductors and Spin-frustrated systemsThis chapter briefly reviewed the structure, phase diagram of high-temperature cuprate superconductors, as well as the anomalous electronic and magnetic properties of the normal state, such as linear temperature dependence of resistivity, anomalous temperature dependence of Hall angle, pseudogap and other characteristics. As the focus of this chapter, we described in detail several types of structure, electronic and magnetic phase diagram and abnormal transport properties of iron-based high-temperature superconductors. Two low-energy ordered states-spin density wave and charge density wave, were often observed in these two types of high-temperature superconductors. This chapter gave a brief introduction of these two density waves as well as the experimental evidence, and also discussed the relationship between these two density wave and superconductivity. Finally, we briefly reviewed some properties of geometry frustrated and spin frustrated systems. Through these presentations, we can understand some basic properties of high-temperature superconductors and frustrated systems.2. Thermoelectric Properties of Electron-and Hole-doped BaFe2As2In this chapter, we investigate the resistivity and thermoelectric power properties for Co-or K-doped BaFe2As2systems. Resistivity and thermoelectric power of these two systems show systematic trend with continuous changes of K-and Co-doping content. TC as a function of Co-or K-doping content exhibits a domelike behavior in both of the systems. The thermoelectric power at room temperature, S300k, shows a domelike behavior as the function of doping level in Ba1-xKxFe2As2system, while monotonously decreases in BaFe2-xCoxAs2system. It is intriguing that the supercon-ducting samples simultaneously show large absolute values of thermoelectric power and good conductivity. The power factor shows a peak at low temperature, suggest-ing possible applications in thermoelectric cooling modules around the liquid-nitrogen temperature range.3. Power-law Temperature Dependent Hall Angle in the Normal State and its Correlation with Superconductivity in iron-pnictidesIn this chapter, we report Hall measurements of the normal states in K-and Co-doped BaFe2As2and NaFe1-xCoxAs. We found that a power-law temperature de-pendence of Hall angle, cotθH∝Tβ, prevails in the normal state within a temperature range well above the structural, spin-density-wave and superconducting transitions for all samples with various doping levels. The power β is～4for the parent and heavily underdoped compounds, and becomes～3for the superconducting samples. The sudden change of β from4to3occurs at a doping level near the emergence of superconductivity. Our data show that β～3is clearly tied to the superconduc-tivity, and suggest a connection between the physics in the normal state and the superconductivity. These findings shed light on the mechanism of high-temperature superconductivity in iron-pnictides.4. Physical Properties in Hole-doped SrFe2-xCuxAs2Single CrystalsWe report the structural, magnetic, and electronic transport properties of SrFe2-xCuxAs2single crystals grown by self-flux technique. Both SrCu2As2and SrFe2As2crystallize in ThCr2Si2-type (122-type) structure at room temperature, but exhibit distinct magnetic and electronic transport properties. According to the x-ray photoelectron spectroscopy(XPS) Cu-2p core line position, resistivity, susceptibil-ity, and positive Hall coefficient, SrCu2As2is an sp-band metal with Cu in the3d10electronic configuration corresponding to the valence state Cu1+. Compared with SrCu2As2, the almost unchanged Cu-2p core line position in SrFe2-xCuxAs2indicates that partial Cu substitutions for Fe in SrFe2As2may result in hole doping rather than the expected electron doping. No superconductivity is induced by Cu substitution on Fe sites, even though the structural/spin density wave(SDW) transition is gradually suppressed with increasing Cu doping.5. Electronic and Magnetic Phase Diagram in KxFe2-ySe2Superconduc-torsIn this chapter, we report electronic and magnetic phase diagram of KxFe2-ySe2system as a function of Fe valence. A series of KxFe2-ySe2single crystals used in this study were grown by using Bridgman method.The compositions of crystals were determined using an energy-dispersive x-ray spectrometer(EDS). Combining with transport properties, magnetic properties, thermoelectric power and TEM images, we find a superconducting phase(Ⅱ) sandwiched between two AFM insulating phases(Ⅰ and Ⅲ). The two insulating phases are characterized by two distinct superstructures caused by Fe vacancy orders with modulation wave vectors of q1=(1/5,3/5,0) and q2=(1/4,3/4,0), respectively. Therefore, the insulating phase in the region Ⅰ and Ⅲ could arise from the Fe vacancy order. Transport properties show that the K and Fe contents have great influence on the structural and physical properties of AxFe2-ySe2system. The superconducting phase Ⅱ has a higher Fe content of about1.63to1.67with a lower K content of about0.74, while the two insulating phases have a lower Fe content of about1.61. Negative Seebeck coefficient at300K was observed in phase Ⅲ with higher K content, while positive value in phase Ⅰ with lower K content. Antiferromagnet (AFM) and superconducting orders are phase-separated in the samples of region Ⅱ. Our findings cast new insight on the origin and mechanism of superconductivity in iron-based superconductors.6. Magnetic properties of the doped spin-1/2honeycomb-lattice com-pound In3Cu2VO9In this chapter, we report magnetic properties in the Co-and Zn-doped spin-1/2honeycomb-lattice compound InaCuaVO9. The static susceptibility x(T) of In3CU2VO9shows a broad maximum at T0～180K characteristic of a low-dimensional antiferromagnet, passes through a kink at T1～38K followed by a peak at T2～28K. Though specific heat experiments show no long-range ordering down to2K in In3Cu2VO9, analyses of electronic structures and states suggest that the ground state of undoped In3Cu2VO9is probably a Neel antiferromagnet. When Cu2+ions are par-tially substituted by Co2+ions, both the impurity potential scattering and magnetic scattering induced by the magnetic Co2+ions enhance the three-dimensional charac-ter of the magnetic coupling and lead to an AFM long-range order. Replacement of Cu2+with nonmagnetic Zn2+ions weakens the AFM correlation between Cu2+ions, leading to suppression of the AFM state.