Gl Theory Of Superconductivity Of Magnesium Diboride,

Since the discovery of superconductivity by Onnes in 1911, researches on superconductivity are always a hot problem. Especially, since the discovery of high critical temperature cooper oxide superconductors in 1986, superconductivity research reached a high level. In China, superconductivity researches were paid more and more attention. In 2001, the superconductivity of magnesium diboride.In this thesis, we study the superconductivity of magnesium diboride based on the two-band Ginzburg-Landau theory. In chapter 1, we review the history of superconductivity research, G-L theory, G-L equation, BCS theory and its main conclusions, and the two-band model. In chapter 2, we present the properties of the system in both the superconductivity state, such as the isotope effect, the Hall effect, the pressure effect, the critical magnetic field, the coherent length and the doping effect. Theory and experiment results show that there are two energy gaps in the superconductor of magnesium diboride, the larger is near 38.5 K, the smaller is by 12 K. Thus, we think that the superconductivity of magnesium system can be described by a two-band model. In chapter 3, the temperature dependence of the coherent length ζ(T), the penetration depth A(T) and the Ginzburg-Landau parameter k(T) = λ(T) / ζ(T) in the vicinity of critical temperature are studied by using two-band G-L model. The calculation results are in better agreement with the experimental results for magnesium diboride than one-band model. In chapter 4, the exact formula of T_c's equation and the isotope effect exponent of two-band s-wave superconductors in weak coupling limit are derived by considering the influence of interband interaction and non-electron interaction are included in our model. The isotope effect exponent of magnesium diboride, α = 0.3 with T_c ≈40K can be found in the weak coupling regine andinterband interaction of electron-phonon interaction show more effect on isotope effect exponent than of non-electron-phonon interaction.