【作者】 汪洋；

【导师】 石磊；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2010， 博士

【摘要】 铜氧化物高温超导体REBa₂Cu₃O_7-δ（RE是稀土元素）和巨磁阻锰氧化物A_1-xA’_xMnO₃（A是三价稀土离子,A’是二价离子）都是具有钙钛矿或者类钙钛矿的电子强关联体系。在这两种材料中,人们都对可变价的稀土元素在RE位或A位掺杂对其物理性质的影响十分感兴趣。在Pr掺杂的REBa₂Cu₃O_7-δ材料中,Pr的化合价及其对高温超导电性的抑制机理方面一直就存在着争议;在A_1-xA’_xMnO₃材料中,Ce的化合价及其对晶体结构、电输运性质和磁性质的不同影响也引起了人们的争论。在本论文中,我们对这两种材料中变价稀土元素的掺杂效应做了研究,在其化合价的变化规律以及引起的晶体结构变化、输运行为和磁性质方面,取得了一些有意义的结果。论文分为以下四个部分:第一章首先简要介绍了REBa₂Cu₃O_7-δ铜氧化物高温超导体的晶体结构、相关物理性质、掺杂效应,特别介绍了Pr掺杂REBa₂Cu₃O_7-δ的物理性质、Pr掺杂抑制超导电性的机理之争和Pr的化合价之争;然后简要介绍了A_1-xA’_xMnO₃锰氧化物巨磁阻材料的晶体结构、物理机制、相关物理性质以及掺杂效应,以及Ce在A位的掺杂的造成的不同的物理性质和化合价之争。最后,比较了两种材料中可变价元素掺杂的异同。第二章通过对Pr和Ca等量双掺的Y_1-2xPr_xCa_xBa₂Cu₃O_7-δ（x=0⁰.14）系列样品的电阻率和温度关系的详细研究和分析,我们发现其超导转变温度TC随掺杂量x的变化行为在x<0.10时偏离了Pr⁴⁺和Ca²⁺等量双掺造成的线性关系,而在x>0.10时回到了原本期望的线性关系中。剩余电阻率、常温电阻率和常温电阻率温度系数的数据,也都出现了在x=0.10附近的平台以及平台前后存在的不同物理规律的表现。通过对其物理内涵的分析以及和前人工作的比较研究,我们认为,以上的物理现象都是Y_1-2xPr_xCa_xBa₂Cu₃O_7-δ体系中Pr的化合价的变化以及相应的对高温超导抑制机制的改变造成的。当x<0.10时,Pr的化合价相对接近+3,其抑制超导的主要机制为空穴局域;当x>0.10时,Pr的化合价增高并接近+4,其抑制超导的主要机制为空穴填充。X射线光电光谱（XPS）的测量和分析也证明了我们的观点。第三章从前人的工作中我们看到,Ce掺杂的锰氧化物从宽带系到窄带系（指Mn的eg的单电子导带带宽）都表现出了丰富的物理性质和不同的作用机理。在窄带系中,其反常的晶体结构变化以及磁性质的研究都支持Ce的化合价为+4。在宽带系中,类Kondo效应和高温Ce磁序的出现都支持Ce的化合价为+3。我们通过Ca²⁺代替Sr²⁺,减小了Ce掺杂的宽带系锰氧化物Pr_0.3Ce_0.2Ca_xSr_0.5-xMnO₃（x≤0.125）的A位平均离子半径和单电子带宽,发现Pr_0.3Ce_0.2Ca_xSr_0.5-xMnO₃中即有和Ce⁴⁺掺杂窄带系锰氧化物类似的晶体结构反常,又有Ce³⁺显著的类Kondo效应和反常磁序的出现;另外随着单电子带宽的变小（x的增大）,Ce³⁺的类Kondo效应逐渐减弱,Ce³⁺出现磁序的温度Tmax和居里温度TC之间的差距也逐渐接近。这些现象都支持Ce的化合价随着单电子带宽的减少从宽带系时的+3价渐渐向+4价转化。样品的饱和磁矩也随着A位离子半径的减少或者x的增加而增大,支持了Ce³⁺的减少和Mn³⁺的增多。我们认为,在半掺杂锰氧化物中,随着单电子带宽的减少,Ce的化合价会从+3渐渐变向+4,其对材料物理性质的影响机制也会做相应的改变。另外,样品的Ce磁序的特征温度并未在磁化强度的测量中反映出来,但我们发现ESR共振谱强度-温度曲线能够反映Ce磁序的出现。第四章Pr_0.3Ce_0.2Sr_0.5MnO₃在不同的气氛中退火会具有不同的氧含量,虽然没有影响其居里温度,但仍然具有很多不同的物理性质。X射线衍射（XRD）数据和Raman光谱数据表明无论是氧含量的增加还是减少,都会增大锰氧八面体的倾角和畸变,从而影响到其双交换作用。根据实验结果我们认为,氮气退火（减小氧含量）的Pr_0.3Ce_0.2Sr_0.5MnO₃由于只改变Mn离子的价态,只影响到Mn的铁磁相和反铁磁相的竞争。氧气退火（增加氧含量）的Pr_0.3Ce_0.2Sr_0.5MnO₃会同时提升Mn离子和Ce离子的价态,不但会影响到Mn的铁磁相和反铁磁相的竞争,还会影响到Ce的类Kondo效应和磁序,从而使其具有更加复杂的电输运性质和磁性质。虽然Ce掺杂会在系统中引入新的绝缘-金属相变,但由于Ce的磁序是由Mn的磁序诱导产生,在这个体系中,对电输运性质和磁性质决定性作用的还是Mn离子的铁磁相和反铁磁相的竞争。更多还原

【Abstract】 Both the high-Tc cuprate superconductor （HTCS） REBa₂Cu₃O_7-δ （RE is rare-earth elements） and the colossal magnetoresistance （CMR） manganites A_1-xA’_xMnO₃ （A is trivalent rare-earth ions, A’is divalent ions） are the strongly correlated electronic systems which have the perovskite or perovskite-like structures. The physical properties affected by doping to the RE or A sites with the valence-variable rare-earth elements in the two materials are interesting. In the Pr doping REBa₂Cu₃O_7-δ materials, there always have been some debates about the the valence of Pr and the physical principles of the suppression of superconductivity by Pr doping. In the A_1-xA’_xMnO₃ materials, there also have been debates about the valence of Ce and its different effects to the crystal structures, the electronic transport properties, and the magnetic properties. In this dissertation, we studied the doping-effects of the valence-variable rare-earth elements both in the two materials. Some important results about the regularitys of the variations of valence, and the variations of crystal structure, electronic transport property, and magnetic property caused by doping were obtained. The dissertation contains four parts:In Chapter 1, firstly, the crystal sturctures, physical properties and doping effects of the HTCS REBa₂Cu₃O_7-δ were simply reviewed, and the physical properties of Pr doping REBa₂Cu₃O_7-δ, debates of the physical principles of the suppression of superconductivity by Pr doping,debates of the valence of Pr were especially reviewed. Secondly, the crystal sturctures, physical principles, physical properties, and the doping effects of the CMR manganites A_1-xA’_xMnO₃ were simply reviewed, and the different physcial properties caused by Ce substituting for A, the debates of the valence of Ce were also reviewed,. At last,the similarity of and the difference between those two doping effects of valence-variable elements in the two materials were also discussed.In Chapter 2, by investigating the variation of resistivity with temperature of the Pr and Ca double doping Y_1-2xPr_xCa_xBa₂Cu₃O_7-δ with 0≤x≤0.14, a deviation of the linearly decreasing of critical temperature of superconductivity Tc with increasing x which caused by the equal amount double doping of Pr⁴⁺ and Ca²⁺ was observed when x<0.10. when x>0.10, the relation between Tc decreasing and x increasing are linear. The residual resistivity, 300K resistivity and the temperature coefficient of resistivity at 300K also exhibit abnormal behaviors around x=0.10. We suggested that it is the transition of Pr valence from +3 to +4 and the variation of the principles of the depression of TC caused by the valence variation which result in those physical phenomenons. When x<0.10, the valence of Pr is close to +3 and the Tc depression is mainly caused by hole location. When x>0.10, the valence of Pr is close to +4 and the Tc depression is mainly caused by hole filling. The X-ray photoelectron spectroscopy （XPS） measurements also support our conclusion.In Chapter 3, by reviewing the the doping effects of Ce in A_1-xA’_xMnO₃, we found that there are so many physical properties and different physical principles appearing from the wide-band system to narrow-band system in the Ce doping A_1-xA’_xMnO₃ （the band means the one electron conduction band of eg of Mn）. In narrow-band systems, the appearance of abnormal variations of crystal structure and the investigation of magnetic properties support the Ce⁴⁺. In wide-band systems, the appearance of Kondo-like interaction and the abnormal magnetic order of Ce moments support the Ce³⁺. By Ca²⁺ substituting for Sr²⁺, which decreasing the average ion radii of A site and the width of one electron conduction band of Pr_0.3Ce_0.2Ca_xSr_0.5-xMnO₃ （x≤0.125）, we found that, the abnormal variation of crystal structure appearing in Ce⁴⁺ doping narrow-band manganites and the Kondo-like interaction and abnormal magnetic order of Ce³⁺ appearing in Ce³⁺ doping wide-band manganties coexist in our samples. The Kondo-like interaction caused by Ce³⁺ weakens and the difference between the temperature of the onset of magnetic ordering of Ce³⁺ moments Tmax and the Curie Temperature TC decreases with the width of band decreasing（or the x increaing）. All the phenomenons mentioned above support the transition from Ce³⁺ in wide-band system to Ce⁴⁺ when the width of band decreases. The saturation magnetic moment of our samples increases with the average ion radii decreasing or x increasing, which supports the Ce³⁺ decreasing and Mn³⁺ increasing. We suggested that in manganites A0.5A’0.5MnO3, there are transition of the valence of Ce ion from +3 to +4 and the variation of physical principles of the materials when the width of one electron conduction band decreases. In addition, although the temperatures of the onset of magnetic ordering of Ce³⁺ moments disappear in the measurements of magnetization,they appear in the curves of the intensity of electron spin resonance （ESR） vs Temperture.In Chapter 4, we found that annealing Pr_0.3Ce_0.2Sr_0.5MnO₃ at different atmosphere, which causes different content of oxygen in the meterials, affects the TC hardly, but causes lots of difference of the physical properties. The datas of X-ray diffraction （XRD） and the Raman spectrum suggest that the tilting and distortion of the octahedron increase with the content of oxygen either increasing or decreasing, which affect the double exchange interaction. By investigating the results of our experiments, we suggested that, the Pr_0.3Ce_0.2Sr_0.5MnO₃ annealed in nitrogen change the valence of Mn ions only, and affect the competition between ferromagnetic （FM） and antiferromagnetic （AFM） phase of Mn only; the Pr_0.3Ce_0.2Sr_0.5MnO₃ annealed in oxygen can change both the valence of Mn ions and the one of Ce ions, and affect not only the competition between FM and AFM phase of Mn, but also the Kondo-like interaction and magnetic order of Ce, which cause more complex electronic transport properties and magnetic properties. Although the Ce doping induces a new insulator-metal（IM） transtion in manganites, because of the magnetic order of Ce inducing by the one of Mn, the ultimate effects to the electronic transport properties and magnetic properties come from the competition between FM and AFM phase of Mn ions.更多还原