Superconductivity Of Magnesium Diboride

After superconductivity was discovered by H.Kamerlingh Onnes in 1911, superconductivity has long been one of the pop subjects in the study of physics. In 2001, the discovery of superconductivity with high TC as high as 39K in MgB2 had generated great excitement in the condensed matter physics. From then on, more and more investigators all over the world began to take part in the tide of the investigation of superconductivity of MgB2.In this paper, we investigate the superconductivity of MgB2, such as pressure effect, isotope effect and transition temperature.In chapter 1, we review the history of superconductivity study in brief. In addition, some properties of superconductor, BCS theory and strong-coupling theory are also introduced.In chapter 2, we discuss the main normal and superconducting state properties of magnesium diboride, including isotope effect, pressure effect, transport properties, Hall effect and superconducting energy gap et al. The results indicate that that MgB2 behaves like a conventional BCS superconductor, and its superconductivity can be discussed within the framework of BCS Theory. At the same time, we take notice that the electronic structure and other normal properties of MgB2 have no much specialties compared with other simple compounds with similar lattice structure, but it holds the transition temperature as high as 39K, which is much higher than that of other intermetallics; in addition, the experiments show that the total isotope effect exponent a is only about 0.32, which deviates from BCS theoretical value 1/2 greatly. All these tell us that MgB2 is not a conventional BCS superconductor purely, and only electron-phonon coupling mechanism cannot explain superconductivity effectively, and some non-electron-phonon interaction should be considered. So in chapter 3 and 4, models with non-electron-phonon interaction are used to investigate the superconductivity of MgB2In chapter 3, Based on McMillan formula of strong-couplingsuperconducting theory, we have a study on the influence of pressure on the superconductivity of MgB2. We obtain the same results as experiments, which further indicates that the superconductivity of MgB2 should fall in the framework of BCS theory. And at the same time, the physical mechanism of the pressure's influence on the superconductivity of MgB2 is discussed.In chaper 4, we use the two-band model with electron-phonon and non-electron-phonon interaction to investigate the transition temperature and isotope effect of MgB2 system by self-consistent method. We obtain the isotope effect exponent a 0.34, which is in agreement with experimental dates. Our investigation shows that the non-electron-phonon interaction plays an important role in the superconductivity of MgB2 system, and it can enhance the transition temperature greatly.