Superconductivity of MgB2in the BCS framework with emphasis on extrinsic effects on critical temperature

The discovery of superconductivity in MgB 2 with an unusually high T C of 39 K interested material scientists all over the world. Many of its superconducting properties, including an experimentally observed double energy gap, differ significantly with those of conventional LTS and HTS. Many scientists proposed exotic and unconventional mechanisms to explain the superconductivity in MgB 2 , but after more than 5 years it is now accepted that MgB 2 is a BCS type superconductor with exceptionally high T C . In this review we discuss qualitatively the origin of superconductivity in MgB 2 based on both experimental and theoretical results. The variation of T C of MgB 2 with various processes, such as irradiation of high energy ions, doping and external pressure, etc., is discussed and reviewed. T C of MgB 2 decreases upon irradiation, by applying external pressure and by most of the doping. The reduction of density of states at the Fermi level, increased band scattering and changes in the phonon frequencies are the main factors which reduce the T C under these conditions.