【作者】 李萌；

【导师】 吴桂平； 施智祥；

【作者基本信息】 东南大学 ， 凝聚态物理， 2020， 硕士

【摘要】 2014年,研究人员利用高压合成技术制备出了层状超导材料AP_2-xX_x（A=Zr,Hf;X=S,Se）,这类材料与铁基超导体111体系类似,都具有PbFCl型晶体结构,空间群为P4/nmm。随着掺杂含量的增加,T_c相图呈现出与铁基超导体类似的“圆顶”形状,表明该体系中可能存在非常规超导电性。此外,2015年,研究人员在铁基超导体122体系（ThCr₂Si₂结构）中发现了一类新型的超导材料(La_0.5-xNa_0.5+x)Fe₂As₂,该化合物与122体系中其他化合物类似,在低温下发生顺磁到反铁磁转变,同时伴随着从四方相到正交相的结构转变。在(La_0.5-xNa_0.5+x)Fe₂As₂中,可以通过改变La/Na的比值来研究电子和空穴的不对称性,为理解铁基超导体的非常规超导电性提供了新的研究平台。本论文中,我们主要对PbFCl型ZrP_2-xSe_x和ThCr₂Si₂型(La_0.5-xNa_0.5+x)(Fe_1-yCo_y)₂As₂的单晶样品制备及其物理性质进行了研究。本论文内容具体安排如下:第一章,绪论部分简单介绍了超导电性的发现和发展史,并着重介绍了PbFCl型和ThCr₂Si₂型超导材料的研究进展。第二章,介绍了样品的生长方法、测试表征方法主要包括电、磁、比热和高压等测量手段。第三章,我们首次利用化学气相传输法生长出了毫米量级的ZrP_1.49Se_0.51单晶样品（超导转变温度T_c为6.8 K）,并对其本征物性进行了系统的研究。研究结果表明其上临界场μ₀H_c2（T）不能用传统的基于轨道拆对效应的WHH模型描述,同时,类似于多带铁基超导体,上临界场各向异性（38）表现出非单调的温度依赖关系。最后,下临界场和比热的结果表明,ZrP_1.49Se_0.51是一种基于电子-声子耦合机制的第二类BCS超导体。第四章,我们采用自助熔剂法成功生长出了La_0.4Na_0.6Fe₂As₂单晶样品,并对其进行了Co元素化学掺杂和金刚石对顶砧高压物性研究。X射线衍射（XRD）和X射线能谱仪（EDS）测量结果表明Co元素成功的掺杂到Fe位。通过电输运测量,我们发现随着Co元素掺杂含量或者施加压力的提高,La_0.4Na_0.6Fe₂As₂中的反铁磁转变逐渐被抑制,但并没有出现本征的体超导电性,这与铁基超导体中反铁磁转变消失后通常表现出的体超导电性行为非常不符。此外,霍尔效应表明,Co掺杂引入了额外的电子,使得载流子由空穴型占主导变为电子型占主导。最后,对本论文内容进行总结与展望。更多还原

【Abstract】 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 fieldμ₀H_c2（T）deviates from the conventional Werthamer-Helfand-Hohenber（WHH）model with only considering the orbital pair breaking effect.Meanwhile,the anisotropy ofμ₀H_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.更多还原