| 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. |
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