Crystal Doping Growth And Property Characterization Of Binary FeSe Superconductor

At the beginning of 2008,it was found that when x=0.11 in La(O_1-xF_x)FeAs system,there was the highest superconducting transition temperature of 26k in this system.This discovery attracted great attention,and then a series of iron-based superconductors were found.As the simplest member of the family of iron-based superconductors,binary FeSe is the common layer structure unit of all iron-based superconductors.It is a tetragonal PbO structure.The superconducting phase is?-FeSe,and the superconducting transition temperature is 8-9K.Interestingly,when some Te atoms are used to replace the Featoms in FeSe,the superconducting transition temperature can be increased to 15 K.The superconducting transition temperature of FeSe increases to 36.7k at 8.9GPa.Using the molecular beam epitaxy technology,the superconducting transition temperature of a FeSe monolayer film grown on the substrate of Sr TiO3 substrate can reach 65k.FeSe is the best candidate to study the basic mechanism of superconductivity of iron-based superconductors because of its simple chemical composition,high superconductivity,high Jc and small anisotropy.In this paper,the pure FeSe single crystals and a series of element doped FeSe single crystals with different components have been successfully grown by chemical vapor transport method,and the physical properties of the samples have been studied,mainly including the following aspects:(1)A series of doped S,Cu,Co and pure FeSe single crystals have been successfully grown by chemical vapor transport method.Compared with traditional FeSe single crystal growth methods,such as flux method,descenting crucible method and traveling-solvent floating-zone method,the growth temperature of the chemical vapor transport method is low,and the FeSe single crystal has no any heteropeaks.The crystal quality is high and crystal size is large,the maximum size can reach the centimeter level.All samples get XRD scanning peaks through X-ray diffraction,and the orientation of the peaks is 00l(l=1-4).Through SEM scanning,we can see that the surface of the sample is flat,and the fracture presents layered structure.Through EDX scanning,we can get the characteristic energy spectrum of the elements,and confirm that the elements have been doped successfully.We can get accurate a series of single crystal samples with final composition were obtained by ICP composition analysis.(2)Property characterization of binary FeSe superconductor.The analysis of the magnetic susceptibility and resistivity of the sample shows that the superconducting transition temperature of the FeSe single crystal grown by chemical vapor transport method is 9K.Single crystal has anisotropy and structural phase transitions occurs at88K.The Hall resistance,specific heat and magnetoresistance of FeSe were measured in terms of electric transport.(3)Property characterization of FeSe_1-xS_x(x=0.07,0.13)single crystal.The results of resistance and magnetic susceptibility of the samples show that the superconducting transition temperatures of both are:10.2K and 10K,respectively,and the temperature when the structural phase transition occurs are:77K and 54K,respectively.(4)Property characterization of Fe_0.96Co_0.04Se single crystal.The results of resistance and magnetic susceptibility of the samples show that the superconductivity of FeSe single crystal is completely suppressed.The sample still has anisotropy,and as the temperature decreases,no structural phase transition occurs.In terms of electrical transport,the Hall resistance of the sample was measured.(5)Property characterization of Fe_1-xCu_xSe(x=0.02,0.05,0.10)single crystal.The resistivity results indicate that the superconductivity of the sample doped with2%Cu is completely suppressed.With the decrease of temperature,the resistance shows the behavior from metal to semiconductor when x=0.02,and the resistance shows the behavior of insulator when x=0.05 and 0.1.The magnetic moment analysis of the sample revealed that the sample still had anisotropy and no structural phase transition occurred.The Hall coefficient(R_H)of the FeSe single crystal doped with Cu is not strongly dependent on temperature.