Superconductivity of MgB2 in the BCS framework with emphasis on extrinsic effects on critical temperature

The discovery of superconductivity in MgB2 with an unusually high TC 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 MgB2, but after more than 5 years it is now accepted that MgB2 is a BCS type superconductor with exceptionally high TC. In this review we discuss qualitatively the origin of superconductivity in MgB2 based on both experimental and theoretical results. The variation of TC of MgB2 with various processes, such as irradiation of high energy ions, doping and external pressure, etc., is discussed and reviewed. TC of MgB2 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 TC under these conditions.