【作者】 林海；

【导师】 闻海虎； 祝熙宇；

【作者基本信息】 南京大学 ， 物理学， 2018， 博士

【摘要】 1911年,Onnes首先发现了超导电性,由此开辟了超导研究领域。1986年,Bednorz和Muller在铜氧化物发现了高温超导电性,开启了高温超导研究的热潮。2008年,Hosono则发现了高温超导的另一个大家族-铁基超导体。铜氧化物和铁基高温超导体各自以CuO2和FeAs/FeSe层为超导层,并通过替换超导层之间的层状结构,获得了大量的新结构超导体。在FeSe基超导体中,由于FeSe层的电中性和弱结合力,一般高温烧结的固相合成手段不太适用于FeSe基超导体。在这样的背景下,以相对低温的液相反应为主导的插层法/去插层法成为了探索FeSe基超导体的主要手段。这种手段还常被用于研究其他有着奇异性质的过渡金属层状化合物。本文将介绍由该方法探索合成出来的三种新铁基超导材料和三种新的过渡金属层状材料,以及对它们的物性测量结果。本文第一章首先介绍了超导的基本概念,然后从材料和理论两个方面介绍了超导研究的发展历史,着重介绍了铁基超导体的研究进展。另外,我们还简要地从玻尔兹曼公式出发推导了多带模型的霍尔电阻和磁阻公式。第二章主要介绍了在实验室中合成样品所采用的固相反应法、熔剂法和水热反应法等方法,详细记录了各种铁基超导体晶体的合成过程,简单介绍了结构分析、成分分析、磁化、电阻等测量方法和测量仪器。在第三章中,我们利用插层法/去插层法合成了高质量的超导体Li1-xFexOHFeSe单晶,并研究了其磁化性质和不同条件下的电阻行为。我们发现Li1-xFexOHFeSe的临界电流密度比相分离的KxFe2-ySe2高一个数量级,有着显著的体超导性质。同时Li1-xFexOHFeSe的超导性十分稳定,其Tc随电流增加而变化不大,随磁场增加而略有压制,说明Li1-xFexOHFeSe的超导性质很稳定。在第四章中,我们利用去插层法成功地将KxFe2-yS2中的K离子完全抽出,首次得到了大面积的FeS单晶,并对FeS单晶样品进行了物性测量。不同磁场方向下的电阻数据显示,FeS单晶的各向异性度约为5.8,且有一个很强的磁阻效应,磁阻最高可到290%(H = 9 T,T= 10 K)。同时,我们观察到一个非线性的霍尔效应,体现出多带效应。利用近似的两带模型,我们计算得到电子能带和空穴能带的基本参数,发现两个能带对输运性质贡献相仿。在第五章中,通过将KxFe2-yS2中的K去插层再将LiOH插层的两个步骤,我们成功制备出高质量的超导体LiOHFeS单晶。XRD衍射谱显示出我们的超导LiOHFeS同时存在两个具有不同c轴晶格常数的相,其中一个相的c=8.91A,接近于非超导LiOHFeS的c=8.96A,而另一个相的c=8.71A更小些,我们认为这个相则可能是超导性的来源,这一结果暗示晶格常数c和超导可能有着直接的关系。对超导体LiOHFeS的磁化测量和输运测量得到LiOHFeS的Tc约为2.8 K,且临界场非常小。插层法/去插层法在铁硫族超导材料的合成与探索中被广泛使用,我们尝试以此法探索、合成和研究其他新型超导材料。但作为结果,我们得到了两种不超导的新化合物。在第六章中,我们首先用固相反应法合成出新的过渡金属层状化合物CsV2Se2-xO(x = 0,0.5),然后用碘的四氢呋喃溶液将结构中的Cs完全抽出,得到了另一种新的过渡金属层状化合物V2Se20。我们的电阻和磁化测量结果显示:CsV2Se20呈现出类半导体行为,CsV2Se1.50呈现出金属性,V2Se2O则呈现出绝缘性。同时在CsV2Se2O和CsV2Se1.5O中发现了分别发生在168K和150K的疑似CDW/SDW相变的反常点。更奇特的是,V2Se20的电阻率在2 K～300 K范围内都正比于log(1/T),这种对数型的电阻-温度依赖关系也出现在电子关联很强的铜氧化物母体。另外,用居里-外斯定律拟合磁化率曲线我们发现,V2Se2O中的平均V原子磁矩大于其母体CsV2Se2-xO的,进一步说明了 V2Se20体系中的电子存在着很强的局域性和关联性。最后在第七章中,我们对全文进行了总结。更多还原

【Abstract】 In 1911,Onnes as the first person observed the superconductivity and opened up the new era of research on superconductivity.In 1986,Bednorz and Muller found high-temperature superconductivity in cuprates,which triggered a wave of reseach on high-temperature superconductors.In 2008,Hosono found high-temperature super-conductivity in iron pnictides,which give rise to a new family of high temperature superconductors,iron-based superconductors.Both the cuprates and iron-based su-perconductors are lay erred compounds,with the superconducting CuO2 planes and FeAs/FeSe layers,respectively.By replacing the layered structures between their su-perconducting layers,numerous new superconductors have been obtained.For the FeSe-based superconductors,due to the electrical neutrality and weak Van der Waals interaction of FeSe layers,the usually-used synthesizing method of solid-phase sin-tering at high temperature is not quite suitable for the FeSe-based superconductors.Therefore,the intercalation/de-intercalation method,dominated by liquid-phase reac-tions at relatively lower temperature,has become an important way to explore FeSe-based superconductors or other transition-metal layered compounds with unexpected properties.This thesis will introduce several new iron-based superconductors and two new vandium-based layered compounds which are synthesized by this method,as well as their physical properties.Among them some were discovered by us.In Chapter 1,the basic concepts of superconductivity are introduced at first.Sec-ondly,the research history of superconductivity is introduced from two aspects:mate-rials and theories.And the research progress of iron-based superconductors specially emphasized.Thirdly,we give the derivation of the Hall resistance and magnetoresis-tance under the multi-band model from the Boltzmann formula.In Chapter 2,we mainly introduce some synthesizing methods ever used in the synthesis of our samples,such as solid-state method,flux method and hydrothermal method.And we record the detailed process of synthesizing some iron-based super-conductor crystals.In addition,we give a brief introduction to some methods and devices/instruments that are used for structure analysis,component analysis,magneti-zation measurement,resistivity measurement and so on.In Chapter 3,by using the intercalation/deintercalation method,we successfully synthesized high-quality single crystals of the superconductor Li1-xFexOHFeSe and study the magnetic properties and the transport properties at different currents and magnetic fields.It is found that the critical current density of Li1-xFexOHFeSe is one order of magnitude higher than that of the phase separated KxFe2-ySe2,which clearly indicates bulk superconductivity in it.Furthermore,Tc of Li1-xFexOHFeSe changes barely with increasing current but is slightly suppressed with increasing magnetic field,which indicates that its superconductivity is very robust.In Chapter 4,by using the deintercalation method,we successfully extracted the K ions from KxFe2-yS2 and got the large FeS single crystals for the first time.We measured the physical properties of FeS single crystal samples.The resistivity under different magnetic field directions shows that FeS single crystal has a large anisotropy,which is about 5.8,and a strong magnetoresistance effect.The maximum magnetore-sistance can reach 290%(H=9T,T=10K).In addition,we observed a nonlinear Hall effect,indicating the multi-band effect.Using the two-band model,we calculated the basic parameters of the hole band and electron band.We found that these two bands make balanced contributions to electric conduction.In Chapter 5,by using the intercalation/deintercalation method,we successful-ly synthesized high quality LiOHFeS superconducting single crystals,by extracting K from KxFe2-yS2and inserting LiOH back into it.The XRD data shows that our su-perconducting LiOHFeS has two phases with different c-axis lattice constants.One of the two phases has c=8.91A,which is close to c=8.96A of the non superconducting LiOHFeS.While the other one has a smaller c=8.71A,which is possibly responsible for superconductivity.This result implies that the lattice constant c may be directly related to the superconductivity.From the magnetization measurement and transport measurement of superconducting LiOHFeS,we understand that LiOHFeS has a sharp superconducting transition at about 2.8K,and a very small critical field.In Chapter 6,by using the similar intercalation/de-intercalation method which is widely used in the synthesis of iron chalcogenide superconductors,we try to explore other new superconductors.As a result,we got two new layered materials which show no superconductivity.At first,we synthesized a new transition metal layered com-pound CsV2Se2-xO(x = 0,0.5)by solid state reaction.Then,we extracted Cs from the CsV2Se20 in iodine tetrahydrofuran solution,and get another brand-new layered com-pound V2Se2O.The resistivity and magnetization measurements show that CsV2Se20 behaves like a semiconductor,CsV2Se1.5O is a metal,while V2Se2O is a insulator.And a CDW/SDW-like anomaly was found in CsV2Se20 and CsV2Se1.5O at 168 K and 150 K,respectively.Interestingly,the resistivity of V2Se20 is roughly proportional to log(1/T)in the temperature range from 2 K to 300 K.This logarithmic resistivity tem-perature dependence also appears in some parent phases of cuprate high-temperature superconductors,which is related to strong electron correlation.Using Curie’s law,we calculated the magnetic moments of these three systems.We found that the magnetic moment per V atom in V2Se2O is larger than that in its parent CsV2Se2-xO,indicating that electrons in V2Se20 system have a stronger localization and correlation effect.We make a summary in the end.更多还原