Ab-initio Studies On The Iron-Arsenic-based Superconductors And Dilute Magnetic Aemiconductor Sn_1-xZn_xO₂

First principle calculation is an effective means to explore superconducting mechanism and new materials. Using the first principle, people have been successfully studied some new iron-based superconductor materials with high TC. Based on the first principle to study iron-based superconductors is usefull way to complete iron base superconducting mechanism and provides important information and theoretical guidance for experimental study.In this project, the density functional theory (DFT) is employed by using VASP package to investigate the stability of the system structure, electronic structure, and the magnetic characteristics of (Ca4Al2O6-x)(Fe2As2),(Sr4Al2O6)(Fe2As2),(Ca3Al2O5-y)(Fe2As2) and Sn1-xZnxO2systems. The essential results and conclusions we have obtained in this work could be outlined as follows:(1) Density-functional theory calculations are performed on (Ca4Al2O6-x)(Fe2As2) to address the structural stability and electronic properties. The total-energy calculations show that the ground state of parent compound (Ca4Al2O6)(Fe2As2) is striped anti-ferromagnetic order with anti-ferromagnetic coupling between Fe layers. The oxygen vacancy formation energy at4f site (-6.02eV) is smaller than that at2c site (-6.33eV) for (Ca4Al2O6-x)(Fe2As2)(x=1) phase, which means oxygen vacancies can be easily formed at4f site. The further studies show that the ground state of (Ca4Al2O6-x)(Fe2As2)(x=0.25) is identical to the case of (Ca4Al2O6)(Fe2As2), which implies that the spin density wave (SDW) may coexist with superconductivity in (Ca4Al2O5.75)(Fe2As2) phase. The densities of states (DOS) of (Ca4Al2O6-x)(Fe2As2)(x=0,0.25) show that both spin up and spin down DOS at the Fermi energy level become larger obviously due to oxygen vacancies introduced, which means introducing oxygen vacancies in (Ca4Al2O6)(Fe2As2) are benefit for improving its conductivity. These results will be helpful for synthesizing new42622-type structured iron-based superconductors with oxygen vacancies. (2) Inspired by the experiments of (Sr2VO3)2Fe2As2and (Ca4Al2O6-x)(Fe2As2), the density-functional theory calculations are carried out to predict a possible parent compound (Sr4Al2O6)(Fe2As2) for iron-based superconductors. The magnetic orders of Fe atoms are easy to adopt striped anti-ferromagnetic (S-AFM) order for its ground state. The longest dFe-As and the angle (Fe-As-Fe) most close to109.47°are forecasted compared with (Sr2VO3)2Fe2As2and (Ca4Al2O6-x)(Fe2As2) for the nonmagnetic (NM) configuration, which may imply that (Sr4Al2O6)(Fe2As2) is a potential parent compound of high-TC superconductors. The densities of states around the Fermi energy (EF) mainly come from the Fe-3d orbital for both NM and S-AFM configurations. The band structure computation shows that this compound is anisotropic and the Fermi surface sheets of (Sr4Al2O6)(Fe2As2) are similar to (Ca4Al2O6)(Fe2As2) for the NM configuration. The result will be helpful for looking for new type of iron-based superconductors.(3) In this work, the density-functional theory calculations are preformed on the newly discovered superconductor (Ca3Al2O5-y)(Fe2As2)(y=0,0.5) to address its magnetic ordering and electronic properties. Within the LSDA approach, the total-energy calculations show that the ground state of parent compound (Ca3Al2O5)(Fe2As2) is of striped anti-ferromagnetic (S-AFM) order, and the oxygen vacancy can be easily formed at8g site and the magnetic order in this compound tend to be striped anti-ferromagnetic ordering. The electronic densities of states around the Fermi energy (EF) mainly come from the Fe-3d orbitals for both NM and S-AFM configurations of (Ca3Al2O5-y)(Fe2As2)(y=0) system. The band structure calculations show that the parent compound is anisotropic in electronic properties and the Fermi surface sheets (five) for the NM phase which is somewhat different from (Ca4Al2O6)(Fe2As2). The total DOSs for both spins around the EF become larger obviously, when oxygen vacancies are introduced at8g site for the stable S-AFM configuration. Within the LSDA+U framework, a strong correlation insulating gap develops with the increase of Hubbard U is found for the parent compound (Ca3Al2O5-y)(Fe2As2), and the transition from insulator to metal is also forecasted when the oxygen vacancies are introduced into8g site for the stable configuration. The ground state of this compound could be ascribed as a strongly correlated insulator and the oxygen vacancy yields the transition from insulate to metal.(4) Density-functional calculations are carried out to study the magnetic properties and electronic structures of Zinc-doped SnO2systems with and without oxygen (tin) vacancies. The magnetic moment almost increases linearly with the increase of impurity concentration (from4%to25%) in the cases without oxygen and tin vacancies. The induced magnetic moments mainly come from the first shell oxygen atoms surrounding the doped Zn atom. The magnetic moments become smaller (larger) when oxygen (tin) vacancy is introduced, which is not as Co and Ni doped SnO2systems. Oxygen (tin) vacancy defects are easily formed at the nearest distance from the doped Zn atom and the possible largest magnetic moments will appear at the concentration of Zn atom between6.25%and12.5%. The calculated results also show that Zn-Zn are ferromagnetic coupling, and how the magnetic moments vary with Zn-Zn distance when only two Zn atoms were doped into a SnO2supercell. However, Zn-Zn are anti-ferromagnetic coupling when three Zn atoms are doped in the same SnO2supercell. The results may be helpful for further study on TMs doped SnO2systems.