MgB_2类似化合物的超导电性的第一性原理研究与新材料探索
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摘要
自1911年Onnes首先在液态汞中发现超导电性以来，超导一直是人们研究的热点。2001年，日本科学家在二硼化镁(MgB_2)中发现了高达39K的超导电性，这又一次引起了人们对超导尤其是简单金属化合物中的超导电性研究的热潮。
MgB_2中39K的超导电性及其潜在的应用前景提出了3个需要解决的问题：
(1) 其超导本质是什么?
(2) 具有类似结构的化合物中是否也存在超导电性?
(3) 能不能通过掺杂使其超导转变温度或者超导性质得到提高或改善?
目前，根据实验结果和第一性原理计算，MgB_2中超导电性的本质已经得到证实：MgB_2是一种由各向异性的强电子声子相互作用导致的二能隙超导体。但是，对于其类似化合物中超导电性的研究却还存在很多问题：许多简单金属二硼化物中不存在超导电性，关于过渡金属二硼化物中超导电性的实验报道则存在很多的分歧。另外，MgB_2的掺杂实验表明现有的掺杂不仅没有提高其超导转变温度，而且还提出了很多新的问题。为了解释这些实验现象，利用现在已较为成熟的第一性原理计算的方法，我们对MgB_2类似化合物的结构和电子结构进行了研究，并探讨了δ能带和E_(2g)模式声子之间的电声相互作用对类似化合物超导电性的影响。
本论文共分为6个部分，主要内容如下：
第一章首先简要介绍了超导发展的历史和MgB_2中超导电性的发现，然后详细地介绍了MgB_2中的超导电性的本质和目前MgB_2类似化合物中的超导电性的研究现状。MgB_2中的超导电性可以用Eliashberg超导理论得到很好的解释，但是对于MgB_2类似化合物(包括二硼化物和掺杂的MgB_2)，实验结果表明其超导性质较MgB_2发生了很大的变化，而且不同的实验结果之间也存在分歧。
第二章介绍了本文的理论基础和计算方法。首先简要概括了可以解释MgB_2中超导电性的Eliashberg超导理论，然后详细地介绍了密度泛函理论、密度泛函微扰理论以及平面波赝势的计算方法，最后对本文所用的近似方法和软件进行了简要的说明。
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 (MgB_2) by Japanese scientists, which stimulated significant interest in superconductivity, especially in related simple metal compounds.
Superconductivity as high as 39K in MgB_2 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 MgB_2 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 MgB_2-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 MgB_2 are generally introduced, and then the current researches on the superconductivity in MgB_2 were discussed in detail. According to present researches, superconductivity in MgB_2 can be explained by Eliashberg theory, but the physical properties of related materials (including metal diboride and doped MgB_2) have changed very much compared to those of MgB_2. 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.
MgB_2中39K的超导电性及其潜在的应用前景提出了3个需要解决的问题：
(1) 其超导本质是什么?
(2) 具有类似结构的化合物中是否也存在超导电性?
(3) 能不能通过掺杂使其超导转变温度或者超导性质得到提高或改善?
目前，根据实验结果和第一性原理计算，MgB_2中超导电性的本质已经得到证实：MgB_2是一种由各向异性的强电子声子相互作用导致的二能隙超导体。但是，对于其类似化合物中超导电性的研究却还存在很多问题：许多简单金属二硼化物中不存在超导电性，关于过渡金属二硼化物中超导电性的实验报道则存在很多的分歧。另外，MgB_2的掺杂实验表明现有的掺杂不仅没有提高其超导转变温度，而且还提出了很多新的问题。为了解释这些实验现象，利用现在已较为成熟的第一性原理计算的方法，我们对MgB_2类似化合物的结构和电子结构进行了研究，并探讨了δ能带和E_(2g)模式声子之间的电声相互作用对类似化合物超导电性的影响。
本论文共分为6个部分，主要内容如下：
第一章首先简要介绍了超导发展的历史和MgB_2中超导电性的发现，然后详细地介绍了MgB_2中的超导电性的本质和目前MgB_2类似化合物中的超导电性的研究现状。MgB_2中的超导电性可以用Eliashberg超导理论得到很好的解释，但是对于MgB_2类似化合物(包括二硼化物和掺杂的MgB_2)，实验结果表明其超导性质较MgB_2发生了很大的变化，而且不同的实验结果之间也存在分歧。
第二章介绍了本文的理论基础和计算方法。首先简要概括了可以解释MgB_2中超导电性的Eliashberg超导理论，然后详细地介绍了密度泛函理论、密度泛函微扰理论以及平面波赝势的计算方法，最后对本文所用的近似方法和软件进行了简要的说明。
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 (MgB_2) by Japanese scientists, which stimulated significant interest in superconductivity, especially in related simple metal compounds.
Superconductivity as high as 39K in MgB_2 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 MgB_2 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 MgB_2-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 MgB_2 are generally introduced, and then the current researches on the superconductivity in MgB_2 were discussed in detail. According to present researches, superconductivity in MgB_2 can be explained by Eliashberg theory, but the physical properties of related materials (including metal diboride and doped MgB_2) have changed very much compared to those of MgB_2. 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.
引文
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