Of Mgb <sub> Similar Compounds 2 </ Sub> First Principles Study Of Superconductivity And New Materials To Explore

Superconductivity is one of the most interesting areas since it was first discovered in Mercury by Onnes in 1911. In 2001, superconductivity as high as 39K was revealed in Magnesium Diboride (MgB₂) by Japanese scientists, which stimulated significant interest in superconductivity, especially in related simple metal compounds.Superconductivity as high as 39K in MgB₂ and its potential applications raised three basic questions:(1) What's the origin of its relatively high superconductivity?(2) Is there superconductivity or even higher transition temperature in related materials?(3) Can its transition temperature and superconducting properties be improved by doping?At present, the origin of superconductivity in MgB₂ has been confirmed according to detailed experiments and first principles calculation: it is a kind of two gap superconductor where anisotropic electron-phonon interaction (EPI) dominates. However, there are lots of problems in researches on related materials: no superconductivity was found in other simple metal diboride and experimental reports on superconductivity in transition metal diboride usually don't agree; studies on the doping effects non only didn't verify its superconducting origin, but also bring about lots of new questions. In order to answer these questions, structure and electronic structure of MgB₂-related materials are investigated using first principles calculations, and further the effects of the electron-phonon interaction between σ band and E_2g phonon mode on superconductivity are discussed in related compounds. My thesis is divided into 6 parts, their main contents are summarized below: In chapter 1 the history of superconductivity and the discovery of superconductivity in MgB₂ are generally introduced, and then the current researches on the superconductivity in MgB₂ were discussed in detail. According to present researches, superconductivity in MgB₂ can be explained by Eliashberg theory, but the physical properties of related materials (including metal diboride and doped MgB₂) have changed very much compared to those of MgB₂. There are disagreements between different experimental results on the superconducting transition temperature of related materials, and there are no good theoretical explanations for those experimental results now.