First Principle Investigation On Superconductivity And Magnetic Property Of Iron-based Compounds And MnCr Compound

High-temperature superconductors (abbreviated high-Tc or HTS) are materials that have a superconducting transition temperature (Tc) above 30 K (-243.2℃). From 1960 to 1980,30 K was thought to be the highest theoretically possible Tc. The first high-Tc superconductor was discovered in 1986 by IBM Researchers Karl Muller and Johannes Bednorz, for which they were awarded the Nobel Prize in Physics in 1987. Fe-based superconductors were discovered in 2008. Physicist believe that it is a "significant progress" in high-temperature superconductivity.Possible mechanisms for superconductivity in the cuprates are still the subject of considerable debate and further research. Certain aspects common to all materials have been identified. Similarities between the antiferromagnetic low-temperature state of the undoped materials and the superconducting state that emerges upon doping, primarily the dx2-y2 orbital state of the Cu2+ions, suggest that electron-electron interactions are more significant than electron-phonon interactions in cuprates-making the superconductivity unconventional. Recent work on the Fermi surface has shown that nesting occurs at four points in the antiferromagnetic Brillouin zone where spin waves exist and that the superconducting energy gap is larger at these points. The weak isotope effects observed for most cuprates contrast with conventional superconductors that are well described by BCS theory. Most undoped iron-based superconductors show a tetragonal-orthorhombic structural phase transition followed at lower temperature by magnetic ordering, similar to the cuprate superconductors. However, they are poor metals rather than Mott insulators. Strong evidence that the Tc value varies with the As-Fe-As bond angles has already emerged and shows that the optimal Tc value is obtained with undistorted FeAs4 tetrahedra. The symmetry of the pairing wavefunction is still widely debated, but an extended s-wave scenario is currently favoured.Doping by electron or hole, the critical temperature of iron-based superconductors has been increased to 55K. The parent compounds show antiferromagnetic orderings in low temperature and more complex noncollinear magnetic structures according to neutron diffraction experiments. The superconducting realized by chemical doping or high-pressure suppress magnetic ordering; therefore, understanding the origin of magnetism of these compounds is key to reveal the mechanisms for superconductivity. In this paper, using WIEN2k package based on full-potential linearized augmented plane wave method and its noncollinear magnetic version WIENNCM package, we calculated the noncollinear magnetic structure and electronic structure of the Fe-based parent superconductors. We report the magnetic structure, magnetism and superconductivity of PrFeAsO, CeFeAsO and NdFeAsO. Moreover, the paper investigates the magnetism and electronic structure of two mixed iron-sulfur compounds. Details as follows:1. The noncollinear magnetic ground state in CeFeAsO has been investigated by density-functional theory. Many groups discussed the electronic structure of LaFeAsO, but a few calculations were performed on the others. One of the reasons is that LaFeAsO has the simplest magetic structure and it is not necessary to consider the La spin. However, the superconducting critical temperature Tc in these compounds increase with the increasement of the rare-earth ionic size and LaFeAsO1-xFx has the lowest transition temperature. Giving the electronic structure of LnFeAsO including the rare-earth spin is significant and essential in this domain. It is found that the magnitude of Ce magnetic moment, which is independent of its spin direction, is 0.87μB, in accord with experimental value 0.83(2)μB. Whereas the spin-orbit coupling is considered, the Ce orbital moments change with the different internal magnetic field and affect its total magnetic moment. One type of Ce ions has the magnetic moment 0.909μB, which is very close to the experimental value 0.83(2)μB. Another type of Ce ions has the magnetic moment 0.488μB, which has not been reported yet in experiments. It is also found that the rare earth magnetic moments in NdFeAsO and PrFeAsO are two times over than experimental observations. The different rare earth errors between theory and experiment imply that magnetism is related to the onset of superconducting critical temperature. However, the calculated Fe magnetic moment is over 2.0μB in all solutions. From the bandstructure and density of states one can find that Ce-4f and Fe-3d orbits are the major contribution to the Fermi level. In CeFeAsO, there are four bands crossing the Fermi level atΓ(0,0,0) point, wherein four hole-like pockets are formed. Furthermore, we find the Fermi surface shape is varies with the Ce spin direction, indicating the electroconductibility affected by the Ce spin direction directly. In particular, if the Ce spin is perpendicular to the FeAs plane, the electronic field gradient will change from the negative value into positive one.2. Noncollinear magnetic investigations of the ground state in PrFeAsO have been performed by density-functional theory. We calculated the total energy and made structure optimization, and the electronic density of states of PrFeAsO was analyzed. There are three different magnetic structures in PrFeAsO defined by experiments. Based on these magnetic structures we studied four collinear and four noncollinear cases. The ground state is found to take the ordering proposed by Zhao, in which FeAs plane is of stripe antiferromagnetism and Pr spins are perpendicular to Fe spins. The electronic density of states indicates that for PrFeAsO the increase of the electron Coulomb interaction leads to the decrease of the conductivity.3. The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation GGA approach were applied to study the new mixed valence compound Ba2F2Fe1.5S3. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the Felll and Fell ions arising from the bridging S atoms, which validate the experimental assumptions that there is low dimensional antiferromagnetic interaction in Ba2F2Fe1.5S3. The spin magnetic moment mainly comes from the FeⅢand Fell ions with smaller contribution from S anion. By analysis of the band structure, we find that the compound has semiconductor property.4. The first principles within the full potential linearized augmented plane wave (FP-LAPW) method with the generalized gradient approximation (GGA) approach were applied to study the new mixed valence compound Ba2F2Fe1.5S3. The density of states, the electronic band structure and the spin magnetic moment are calculated. The calculations reveal that the compound has an antiferromagnetic interaction between the FeⅢand Fell ions arising from the bridging S atoms, which validate the experimental assumptions that there is a low-dimensional antiferromagnetic interaction in Ba2F2Fe1.5S3. The spin magnetic moment mainly comes from the FeⅢand Fell ions with smaller contribution from S anion. By analysis of the bandstructure, we find that the compound has half-metallic property.5. First-principles calculations have been performed to study the electronic structure and the magnetic properties for the cyanide-bridged MnⅡCrⅢcoordination polymer crystal. The calculations were based on density-functional theory and the full potential linearized augmented plane wave. From the calculated energy and the density of states, it is found that [Mn(NNdmenH)(H2O)][Cr(CN)6]H2O and dehydrated [Mn(NNdmenH)][Cr(CN)6] have metallic ferromagnetic ground state. Experiment result exhibited these two compounds have ferrimagnetic ordering at 35.2 and 60.4 K, respectively. We believe they take phase transition below 12 K. The spin magnetic moments of these two compounds are mainly assembled at the Mn", CrⅢatom, with a few contributions from the oxygen, nitrogen, carbon atoms.