The Electronic Structural Calculation Of BaX (X=S,Se,Te)

The density functional theory with local density approximation (DFT-LDA) as a ground state theory,i.e. the solutions based on the Kohn-Sham equation is a powerful tool for studying the groud state properties in many particle systems just like semiconductor materials,but it is bad to describe the excited state properties。A key problem is that the exchange-correlation interaction in excited states is differ from the ground states。In recent years,nevertheless,several electronic-excitation theories have been developed。The most important theoretical and computational method include the many-body perturbation theory which is based on the quasi-particle concept and the Green function equation。Among them the central ingredient is an electron's self-energy operatorΣthat describes the exchange correlation effect beyond Hartree approximation。The implementations of the above theories are unavoidable to introduce some of approximations。A good approximation is the GW approach by Hedin。But it is shown from computation simulations for many real condensed matter systems that the GW approach is a successful method for electronic-excitation problems。As we know,by way of a kind of photoelectron materials,IIA-VIB compound have got more and more attention in photoelectron technology field because of their outstanding optoelectronic characteristic,so it is very important to study their correlative character。So far,people have made lots of systemic analysis and research for compounds which are made of Be,Mg,Ca and VI element,at the first principle and GW approximation there have been systemic reports,but Concerning the electronic structure of barium's VIB compound ,people don't understand it fully,moreover,there are only computational research at the density functional theory with local density approximation。The existent main problem is that not only band gap(～2eV)calculated by using LDA(local density approximation) has a large discrepancy compared with experimental results(～3.6eV),but also theoretic band gap type(direct of indirect)is inconsistent for different investigation groups。What is more...