【作者】 储松南；

【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】 2001年,Cava组发现MgCNi3具有超导电性(Tc-8K),结构为反钙钛矿结构(P m-3m空间群),Mg位于立方结构的顶点位置,面心的Ni原子构成一个Ni6八面体,C位于体心。由于富含元素Ni的材料一般具有磁性,但MgCNi3却具有超导电性而未表现出磁性。因此该材料引起了广泛的关注,大量的实验和理论计算工作相继展开,揭示MgCNi3是一个以电子为载流子的单能隙的超导体,但在超导机制上仍未有统一的观念。上临界场,比热,NMR,13C的同位素效应,单晶的磁场穿透深度以及后期测量的隧穿谱给出的是传统s波超导体,但早期的隧道谱给出了非传统的配对方式,多晶的磁场穿透深度测量给出的结论也是一个非s波的超导体。除了MgCNi3外,研究的热点还扩展到其他反钙钛矿结构材料,此后又发现了有超导趋势的NbBxRh,CaBxPd3,和YBRh3,超导转变温度跟MgCNi3接近的CdCNi3(Tc=2.5-3.2K)和ZnNNi3(Tc～3K).除了超导电性外,反钙钛矿结构的材料由于含有大量的磁性金属元素(Mn,Co,Fe等),所以还有丰富的磁性行为,另外还有最近倍受关注的负热膨胀,及零膨胀特性。第一章我们较详细的介绍了MgCNi3的制备方法,晶体结构,物理性质,电子结构,声子结构,及元素掺杂,给出了MgCNi3比较清晰的物理图像,并对研究中的问题进行了归纳总结。第二章我们的研究从MgCNi3入手,通过B掺杂,以及C空位数改变的MgCNi3的XPS研究,确定了C-Ni共价键间的电荷转移,揭示了在C位掺杂时会造成电荷重新分配,进而影响到超导电性,并揭示出C位原子在MgCNi3超导中扮演的重要角色。第三章,我们先按照传统的观念,将MgCNi3当作一个合金材料,来研究它的电阻率曲线上的两个特殊温度点(150K,50K),发现了该模型的局限性。随后我们在B掺杂的MgCNi3中发现了空穴导电的超导行为,使我们认识到在MgCNi3中存在电子型和空穴型两种载流子,由此我们引入了MgCNi3的双带模型。在该模型下,认为MgCNi3中同时存在电子型和空穴型两种载流子,但有效质量(或者费米速率)不同。它可以很好的解释MgCNi3正常态的电阻行为,也提出了MgCNi3以空穴作为超导的载流子的可能性。这与我们实验结果是一致的。第四章我们将双带模型在反钙钛矿体系中进行推广,在反钙钛矿体系中寻找具有负的电阻率-温度曲线斜率的材料,结果在ZnCNi3-xMnx中发现了这种现象。在霍尔系数的测量中,我们发现Mg0.25Zn0.75CNi3在降温到～205K时载流子类型由空穴型转为电子型。结合霍尔系数的测量我们发现,在反钙钛矿材料中的负的电阻率-温度曲线斜率是在空穴导电占主导的情况下才会出现的结果,通常的金属导电行为可以是电子型导电,也可以是空穴型导电,是两种载流子共同作用的结果。在对ZnCNi3-xMnx的磁化率的进行测量后,我们发现了50K温度处的磁信号异常,提供了电阻率-温度曲线上的50K特殊温度点产生的一个可能性解释。综合我们研究工作,在MgCNi3中,由于体心非金属元素C的引入,它与面心的过渡元素Ni的d电子有共价作用,同时材料的能带结构发生改变,使材料中同时存在电子型和空穴型两种载流子,实际的输运行为是这两种载流子共同作用的结果,另外MgCNi3的超导电性与C位原子,以及空穴载流子都是密切相关的。此结果或许可以类比到其他的反钙钛矿结构材料,为他们的研究提供帮助,因为他们都可以看作是非金属元素(迄今报道的只有B,C,N三种元素)插入到AuCu3结构金属的体心位置。更多还原

【Abstract】 In the year2001, the superconductivity (Tc～8K) in MgCNi3was discovered by Cava et al. MgCNi3has an antiperovskite structure (P m-3m) with Mg occupies the vertex of the cubic structure, Ni occupies the face center to form a Ni6octahedron, and C occupies the body center respectively. Surprisingly, superconductivity was discovered in MgCNi3, in which the high proportion of Ni metal suggests that magnetic interactions may result in ferromagnetism rather than superconductivity. Many experiments and theoretical calculations have revealed that MgCNi3is a singlegap superconductor with electric carriers, but there still are questions of the superconductive mechanism. The measurements of upper critical field, specific heat, NMR,13C isotope effect, penetration depth in single crystal, and later tunneling spectra support s-wave pairing in MgCNi3, whereas earlier tunneling spectra and penetration depth measurements in polycrystal have been interpreted in terms of an unconventional pairing state. Except MgCNi3, many other materials with antiperovskite structure have also been studied. Later, NbBxRh3, CaBxPd3, and YBRh3were found to have a trend to be superconductive; CdCNi3(Tc=2.5-3.2K) and ZnNNi3(Tc～3K) have the similar superconductive transition temperature with MgCNi3. Because the materials with antiperovskite structures have a high proportion of magnetic metal (Mn, Co, Fe, et al), the magnetic properties are important in these compounds. Recently negative thermal expansion and zero thermal expansion are also broadly studied in these compounds.In chapter one:we introduced the preparation, crystal structure, physical properties, electronic structure, phonon structure, element doping in MgCNi3. Our research is based on the figure given here.In chapter two:we prepared B doping and C vacancy MgCNi3, The covalent bond properties of C-Ni were studied by the XPS, and the distribution of electron charge under the C-site doping of MgCNi3was revealed, that will affect the superconductivity.In chapter three:first, the conductivities of MgCNi3with two special temperature points150K and50K were studies in conventional metallic property, and the limitation was revealed. After the hole-like carriers were found in B doing MgCNi3by Hall measurement, two-bands model was applied in these compounds. Under this model, there are two types of carriers (hole-like carriers and electron-like carriers), and it is conceivable that superconductivity in MgCNi3was due to hole-like carriers. This was consistent with our experimental results.In chapter four:the antiperovskite materials (ZnCNi3-xMnx) with negative slope in the resistivity-temperature curve were discovered thereunder two-bands model, and it was found that the carriers changed from hole-like to electron-like near205K in the Hall measurement of Mg0.2sZn0.75CNi3. In the antiperovskite materials, the appearance of negative slope in the resistivity-temperature curve was must under the condition that hole-like carriers were the dominant carriers, but the carriers under metallic behavior could be hole-like or electron-like. The abnormity near50K in the magnetization curve of ZnCNi3-xMnx may have a relation with the special temperature point50K in resistivity curve.In MgCNi3, the body center atom C forms covalent bond with Ni. There are two types of carriers (hole-like carriers and electron-like carriers) in this compound, and the conductivity origins from the two types of carriers. In addition, the superconductivity in this material is in contact with the C-site atom and the hole-like carriers. These may be helpful in the research of other antiperovskite structure materials which can be regarded as that the nonmetallic atoms are placed in the body center of AuCu3alloy.更多还原