Single Crystal Growth And Transport Properties Of PbFCl-type And ThCr₂Si₂-type Superconductors

In 2014,the layered superconductors AP_2-xX_x(A=Zr,Hf;X=S,Se)were synthesized by using high-pressure technology.Such materials are similar to the 111-type iron-based superconductors(IBSs),which crystallize in a PbFCl-type crystal structure with P4/nmm space group.As x increases,the superconducting phase diagram exhibits a dome-like shape,which resembles to that of IBSs.Moreover,a novel kind of 122-type(ThCr₂Si₂-type)IBSs,(La_0.5-xNa_0.5+x)Fe₂As₂,were discovered in2015.This material exhibits a magnetic transition from paramagnetic to antiferromagnetic order at low temperatures that is concomitant with a structural transition from tetragonal to orthorhombic phase,in analogy to other compounds of 122-type IBSs.In(La_0.5-xNa_0.5+x)Fe₂As₂,electron-hole asymmetry could be studied by simply varying the La/Na ratio,which provides a new platform for studying the unconventional superconductivity in IBSs.In this thesis,we mainly studied the single crystal growth and transport properties of PbFCl-type ZrP_2-xSe_x and ThCr₂Si₂-type(La_0.5-xNa_0.5+x)(Fe_1-yCo_y)₂As₂.The details of this thesis are as follows:In chapter 1,we briefly introduce the discovery and development of superconductivity,and then mainly review the current research progress of both PbFCl-type and ThCr₂Si₂-type superconductors.In chapter 2,we describe the single crystal growth method and physical properties characterization employed by electricity resistivity,magnetism,specific heat,and high pressure.In chapter 3,for the first time we report the single crystal growth of millimeter-size ZrP_1.49Se_0.51by using a chemical vapor transport method,which shows a superconducting transition temperature T_c with 6.8 K.It is found that the upper critical field?₀H_c2(T)deviates from the conventional Werthamer-Helfand-Hohenber(WHH)model with only considering the orbital pair breaking effect.Meanwhile,the anisotropy of?₀H_c2(T),(38),shows a nonmonotonic temperature dependence,which is similar to the multiband IBSs.Finally,the results of the lower critical field and specific heat show that ZrP_1.49Se_0.51 is a weakly electron–phonon coupled type-II BCS superconductor.In chapter 4,we successfully grow the La_0.4Na_0.6Fe₂As₂ single crystals by using self-flux method and study the effects of Co doping and high pressures.The results of X-ray Diffraction(XRD)and Energy Dispersive X-ray Spectroscopy(EDS)show that Co dopant is successfully doped into the crystal lattice.By electrical resistivity measurement,we find that the antiferromagnetic transition in La_0.4Na_0.6Fe₂As₂ is gradually suppressed with the increase of Co doping or applied pressures.However,no intrinsic bulk superconductivity is observed,in sharply contrast with other IBSs where bulk superconductivity is generally emerged after the suppression of the antiferromagnetic order.In addition,Hall effect shows that extra electrons are successfully introduced via Co doping,which changes the dominant carriers from hole-type to electron-type.In the end,all the work of the thesis is summarized,and I put forward my own points of view and give suggestion for future research works.