鉄系超伝導体Ba(Fe1-xCox)2As2と関連物質の弾性的性質に関する研究

The discovery of superconductivity in LaFeAsO1_xFx with the transition temperature of Tsc = 26 K, puts the iron pnictide superconductors in the first stage of the experimental and theoretical studies in superconducting research. The discovery of iron-based superconductors with high phase transition temperature was unexpected, because it contains a magnetic element Fe, for this reason it has opened a new avenue of research so far. Main discussion in this system now is focused on the mechanism of the superconductivity. In this thesis we have studied the elastic constant properties of Ba(Fe1_xCox)2As2. The parent compound of this material shows a stripe magnetic order with a structural change from tetragonal to orthorhombic. This work mainly discuss the roles and relations of the structural and magnetic fluctuation of this system. The thesis is organized as follows. We will first provide the necessary background knowledge of the iron-based superconductors in Chapter 1. Chapter 2 serves as various experimental methods and their apparatus for elastic constants, electrical resistivity, heat capacity measurements. In chapter 3, we will show the experimental results of all elastic constants, electrical resistivity, heat capacity. In chapter 4, we discussed the large elastic softening in C66 towards to the structural phase transition temperature. We analyzed the origin of the large elastic softening in C66 based on localized picture and based on band picture and find some parameters and summarized the phase diagram of Ba(Fe1_xCox)2As2. At last we discussed the correlation between elastic anomaly and superconductivity in iron based superconductor Ba(Fe1_xCox)2As2, and revealed the role of orbital in the emergence of superconductivity in iron-based superconductors. In chapter 5, we focused our attention on the three-dimensional character of this system. In our experimental tool, ultrasonic measurement, the corresponding elastic strain modulates physical quantities along c-axis through the elongation and contraction of the inter-layer spacing. So by examining the corresponding elastic constant C33, we can obtain the three-dimensional character of this system. Our studies show that the correlation of C33 with Tsc as well as the in-plane fluctuation appearing in C66. In chapter 6 we will demonstrate the large elastic anomalies in heavy fermion superconductor Rh17S15. In this chapter we summarized the introduction, experimental method, results and discussion in individual. In this chapter we will show the elastic constants of C11, C44, CL = 1/2(C11 + Cl2 + 2C44), CE = 1/2 (C11 - Cl2) and discuss the similarities with the iron-based superconductors. is the main contents of the thesis. In this chapter we will demonstrate the existence of the interlayer fluctuation and the highlight the roles of the magnetic fluctuation. In chapter 7, we will conclude our results of ultrasonic measurements of Ba(Fe1-xCox)2As2. Chapter 8 is Appendix for the analyzing of the elastic constants.甲第237