Study On The Cu Substitution Effect On The Fe Sites In（Tl,Rb）Fe_2-xCu_xSe₂ System

Since the discovery of superconductivity with Tc～26K in LaOFFeAs by H. Hosono et al in 2008, much attention has been paid on these iron-based superconductors. The superconductivity has been induced by the substitution of rare earth elements (Sm, Nd, Pr, Ce, Gd and Y) for La, Sr for La (hole doped), or the partially substitution of F for O (electron doped), or applying high pressure in the synthesis or measurement process. In these superconductors [1111-type], the highest transition temperature up to now is as high as 56K, which breaks the dominance of the copper oxides in the high temperature superconductors (HTSC). The iron-based superconductors can be divided into four families according to their structural feature, which contain the similar Fe-based tetragonal layers:(Ba,K)Fe2As2 [122-type], LiFeAs [111-type] and Fe(Te, Se, S) [11-type]. At the end of 2010, a new system of iron-based superconductors ((Tl,K,Rb)FexSe2 with Tc～30K was reported by our group and Chen group, independently. In our paper, we were the first group that pointed out that the super-lattice of the Fe-vacancy exists in this system and suggested that the superconductivity is near an insulating antiferromagetic (AFM) phase, which triggered another wave of remarkable interest on the iron-based superconductors again. After many efforts over the past year, three conclusions have been reached for this new system:the existence of the V5xV5 Fe-vacancy super-lattice, block anti-ferromagnetic ground state and the absence of Hole Pocket in the Fermi surface. These results challenged the current viewpoints of the S±symmetry in Cooper pair and the anti-ferromagnetic order induced by nesting effect.In this thesis, a series of Cu doped (Tl,Rb)Fe2-xCuxSe2 (x<1.2) crystals were grown successfully. The studies on the Cu doping effect in (Tl,Rb)Fe2-xCuxSe2 system were carried out by chemical analysis, structure, electrical transport and magnetism measurements. The main results are presented as follows, (1) the copper substitution effect on iron site resulted in a phase separation along the c-axis direction; (2) For the copper concentration x< 0.26, the copper substitution suppresses Tc and superconducting volume fraction; (3) In the region of 0.26≤x≤1.2 the (T1,Rb)Fe2-xCuxSe2 crystals exhibit semiconductor behavior and spin glass behavior at low temperatures. A metal-insulator transition occurs near x=0.26. At the end of this study, several questions need to be explored further, such as:whether the phase separation along c-axis direction is resulted from the copper substitution or it is arrived from the intrinsic behavior of the system? For x> 0.26 crystals, what is the physical origin of the spin glass state?My master’s thesis is divided into three chapters:the first chapter presents a review on the research history of superconductivity; the second chapter introduces the experiment methods, preparation of samples and measuring method applied:the last one shows the main experimental results. At the end of this thesis, we give a summary of this paper.