Transport Properties And Electronic Phase Diagram Of Directly And Indirectly Electron-Doped CaHFeAs System

Due to the broad application prospects and potential commercial value, search-ing for new superconductor with higher superconducting transition temperature keep scientists motivated. Although BCS theory explain the behavior of conventional su-perconductor successfully, but the failure of BCS theory work in cuprate has led physicists taken a new look at the mechanism of unconventional superconductivity. Physicists need to study a new class of high-Tc superconductor and find the under-lying relevant physics. The discovery of iron-based superconductors in2008offer a good opportunity for physicists to study the similarities and differences between iron-based superconductor and cuprate, it may pave the way to understand unconventional superconductivity.In this dissertation, we have a deeply and thoroughly research in direct and in-direct CaHFeAs doping system. We perform the electrical and magnetic transport measurements, and finally obtained the electronic phase diagram of CaHFe1-xCoxAs and Ca1-xNdxHFeAs system. By comparing the similarities and differences between the directly or indirectly electron-doped CaHFeAs system and previous iron-based superconductors, we deem that CaHFeAs has a special physical properties compare with other iron-based superconductors.This dissertation is divided into the following three chapters:1. Brief Overview of iron-based SuperconductorIn this chapter, we briefly reviewed the structure and its phase diagram of iron based superconductors, analyzed the similarities and differences among in these struc-tures, and giving several important relationship between the structure and supercon-ductivity. Besides, we also summarized the application of high pressure synthesis technique in iron based superconductor research. Finally, we introduce the concept of density wave, such as charge density wave and spin density wave. The relationship between these ground states of density wave with superconductivity are also intro-duced.2. Transport properties and electronic phase diagram of directly electron-doped1111-type iron arsenide hydrideIn this chapter, we report the successful synthesized CaHFe1-xCoxAs (0≤ x≤0.45) by using a high-pressure and high-temperature solid-state reaction method. By measuring electrical and magnetic properties, we mapped out the phase diagram of the system. The anomaly observed in both resistivity and magnetic susceptibility was continuously suppressed by increasing Co concentration. As doping level increases to0.07, superconductivity is induced. The maximum superconducting transition tem-perature Tc=23.8K and the full superconductive shielding fraction are observed at the optimal doping level x=0.15. The superconducting dome in the phase diagram shows a relatively wide and flat shape. The zero-temperature upper critical field Hc2for an optimally doped sample with x=0.15and an overdoped sample with x=0.3are69.3and37.7T, respectively. The temperature dependences of the upper critical fields show upward curvatures, consistent with the multi-band nature of the system.3. Transport properties and electronic phase diagram of indirectly electron-doped1111-type iron arsenide hydrideIn this chapter, we investigate the Nd doped CaHFeAs, which were synthesized by high pressure technique. Unlike Co doping, which is induce electron in FeAs layer directly, Nd doping is a case of indirect electron doping. As doping level increases to0.05, superconductivity is induced. The maximum superconducting transition tem-perature Tc=46K at the optimal doping level x=0.2, which is higher than that direct electron doping in CaHFeAs via Co substitute for Fe site. We confirm the bulk superconductivity by magnetic and heat capacity measurements.