The Anisotropic Resistivity Of “122”-Type Iron-Based Superconductors And Transport Properties Of Rare-Earth Trite-llurides System And Magnetic Properties Of Iron-based Arsenides

Sinice the discovery of superconductor at1911,the superconducting materials have been a major topic in condensed matter physics. A large number of scientists engaged in studying the mechanism of superconductivity and obtained many exciting results,such as the BCS theory,Josephson effect and so on.These results provided fundamental basis for further research.The discovery of high-temperature cuprate superconductors was the great breakthrough and caused a craze to explore new su-perconductors.Although people have gained great achievements,the mechanism of high-temperature superconductivity is still not clear,and its application is limited. The discovery of iron-based superconductors once again ignited peoples’ enthusiasm. Comparing the differences and similarities between the two types of high-temperature superconductors,scientists hope to find some commonalities between these two types of high-temperature superconductors in order to explore the mechanism of supercon-ductors,it can provides a direction for the exploration of the higher temperature superconductors.In this dissertation, we systematically studied the anisotropic resistivity of Ca1-xNaxFe2As2and the transport properties of rare-earth tritellurides which was similar to the layered compound.We synthesized the high-quality single crystals of Ca1-xNaxFe2As2superconductors and then studied the reversion behavior of in-plane anisotropy resistivity. We used the ammonothermal method to insert alkali metal in rare-earth tritellurides and got higher superconducting temperature.This dissertation is constructed as following:1. Brief overview of Iron-based High Temperature Superconductors and Some Rare-Earth TritelluridesThis chapter reviewed the crystalline structure of iron-based superconductors, the doping of122iron-based superconductors,its phase diagram and the supercon-ducting properties and so on.We detailedly introduced the measurement method of in-plane resistivity anisotropy of122iron-based superconductors and the different be-haviors in the different materials.We discussed the possible mechanism of the origin of anisotropy.We briefly reviewed the crystalline structure and the transport proper-ties of rare-earth tritellurides,then focused on the charge density wave ordered state in rare-earth tritellurides. This chapter gave a brief introduction of charge density wave and the relationship between charge density wave and superconductivity. From these introduction, we can understand some basic transport properties of iron-based high temperature superconductors and some rare-earth tritellurides.2. Evolution of Anisotropic In-plane Resistivity with doping level in Ca1-xNaxFe2As2Single CrystalsIn this chapter, we measured the in-plane resistivity anisotropy in the underdoped Ca1-xNaxFe2As2single crystals. The anisotropy (indicated by ρb-ρa) appears below a temperature well above magnetic transition temperature TN, being positive (ρb-ρa>0)as x≤0.14. With increasing the doping level to x=0.19, an intersection between ρb and ρa is observed upon cooling, with ρb-ρa<0at low-temperature deep inside magnetically ordered state, while ρb-ρa>0at high temperature. Subsequently, further increase of hole concentration leads to a negative anisotropy ρb-ρpa<0in the whole temperature range. These results manifest that the anisotropic behavior of resistivity in the magnetically ordered state depends strongly on the competition of the contributions from different mechanisms, and the competition between the two contributions results in a complicated evolution of the anisotropy of in-plane resistivity with doping level.3. The Sample Preparation and Transport Properties of KxZrTe3with The Superconducting Temperature7kIn this chapter, we introduced the synthesis of ZrTe3single crystal by using chemi-cal vapor transport method. Then we used the ammonothermal method to insert alkali metal in ZrTe3at room temperature and measured the transport properties by PPMS and SQUID. We observed the superconducting transition at3.5K by the resistivity and magnetic susceptibility measurement. With the increase of K content in KxZrTe3, the superconducting transition temperature is enhanced; when the content of K is1, the superconducting transition temperature reaches the maximum of7K; then we continue to increase the content of K, but the temperature startS to decrease. The maximum in Tc indicates the existence of the optimal content of K. This is the highest superconducting transition temperature in ZrTe3system.