Of Mgb <sub> 2 </ Sub> Doping First Principle Of The Electronic Structure And Optical Properties

The discovery of superconductivity of MgB₂ system has attracted much attention on the duality compound.Different kinds of methods have been used to study the physics property of pure MgB₂ and doped MgB₂ systems.In this thesis,we use the first principle method based on the density functional theory to study the electronic structure,superconductivity and optical property of the pure MgB₂ and the doped MgB₂ systems.The main content is as follows:(1) The electronic structure,superconductivity and optical property of pure MgB₂ and the doped MgB₂ systems are introduced.(2) The density functional theory and CASTEP calculation process are discussed.(3) The bond population,the band structure,the density of states of the Al and Li doped MgB₂ systems are studied respectively,further,the critical temperatures of different systems are calculated by the method based on BCS theory.The obtained results show that B-B bond of the Mg_6/8Al_2/8B₂ system is weaker than that of the Mg_6/8Al_2/8B₂ system,while the B-Mg bond of the Mg_6/8Al_2/8B₂ system is stronger than that of the Mg_6/8Al_2/8B₂ system,and the B-Al bond is further stronger than B-Li bond.Compared with the MgB₂ system,Al doping makes the energy band move to the lower energy direction,Li doping makes the energy band move to the higher energy direction;the total DOS on Fermi level N(E_F) and the critical temperature of the Mg_6/8Al_2/8B₂ system decrease,and the total DOS on Fermi level N(E_F) and the critical temperature of the Mg_6/8Al_2/8B₂ system increase.(4) The bond population,the band structure,the density of states and the optical property of the Be-C co-doped MgB₂ systems are studied,further,the critical temperatures of different doped systems are calculated by the method based on BCS theory.The obtained results show that in Mg(B_7/8Be_1/16C_1/16)₂ system,Be-B covalent bond is weaker than B-B covalent bond, and C-B covalent is also weaker than B-B covalent bond,and some B-B bonds are ionic bonds.The total DOS on Fermi level N(E_F) of the Be-C co-doped system is bigger than that of C doped system,but smaller than that of Be doped system.When the incidence direction and wavenumber of the incidence light are identical,in the region of 0～10000cm^-1,the real part of optical conductivityσ₁ of the Be-C co-doped system is bigger than that of C doped system,smaller than that of Be doped system,and in the region of 0～60000cm^-1,the number of effective carriers N_eff(ω) of Be-C co-doped system is bigger than that of C doped system. and smaller than that of Be doped system.The critical temperature of the Be-C co-doped system is higher than that of C doped system,and lower than that of Be doped system.These results are related to the compensation effect between the two kinds of doping. (5) The bond population,the band structure,the density of states and the optical property of the Ag doped MgB₂ system are studied,respectively.The obtained results show that B-B bond decreases,and the B-Mg bond increases a little compared with the pure MgB₂ system. B-Ag bond is covalent bond.The total DOS on Fermi level N(E_F) of Ag doped system is bigger than that of the pure MgB₂ system.In the low energy region,the optical conductivity of the Ag doped system increases.After doping,the number of the effective carriers increases.In conclusion,compared with the MgB₂ system,Al doping makes the energy band move to the lower energy direction,Li doping makes the energy band move to the higher energy direction;the total DOS on Fermi level N(E_F) and the critical temperature of the Mg_6/8Al_2/8B₂ system decrease,and the total DOS on Fermi level N(E_F) and the critical temperature of the Mg_6/8Li_2/8B₂ system increase.The compensation effect happens in the Be-C co-doped case, this can be illustrated in the electronic structure,the critical temperature and the optical property of the system.After Ag doping,the number of the effective carriers increases.In the low energy region,the optical conductivity of the Ag doped system increases.