Magnesium Diboride Superconductivity Theory

Since the discovery of superconductivity by H. Kamerlingh Onnes in 1911, researches on superconductivity are always a hot problem. Especially, since the discovery of superconductivity of Copper oxide superconductors in 1986, superconductivity studies reached a high level. On January 10th 2001, the superconductivity of MgB₂ system was discovered by J. Akimitsu et. al., the researches on superconductivity reached a new high level.In this thesis, we study the superconductivity of MgB₂ system based on the two-band model.In chapter 1, we review briefly the history of superconductivity study, BCS theory and its main conclusions, and the basic characteristics of superconductors. In chapter 2, we present the properties of the system in both the superconducting state and normal state, such as the isotope effect, the pressure effect and the doping effect. Theory results show that superconductor MgB₂ is a BCS superconductor in substance, but there appear two energy gaps. The relation between the gaps and temperature is close to a BCS like one. Thus, we think the superconductivity of MgB₂ system can be described by a two-band superconductivity model. In chapter 3, by introducing the non electron-phonon interaction on the basis of the two-band model, and using a self-consistent method we calculate the energy gaps at different temperatures, and study the Specific heat of MgB2 superconductor. These results are in agreement with the experimental results. In chapter 4, The temperature dependence of the upper critical field Hc2(T) and the lower critical field Hc1(T) for MgB2, is studied inthe vicinity of Tc, using a two-band Ginzburg-Landau (G-L) theory. And it fits the experiment date very well.In summary, based on the two band model within the framework of BCS theory, we have discussed the Energy gaps and the Specific heat of MgB2 superconductor and the upper critical field Hc2(T ) and the lower critical field Hcl(T). Our results are in agreement with experiments.