Electronic band structure, Fermi surface, structural and elastic properties of two polymorphs of MgFeSeO as possible new superconducting systems

By means of DFT-based first-principles calculations, we examine two polymorphs of the newly synthesized 1111-like MgFeSeO as possible new superconducting systems. We have found that the polymorph with blocks [MgO], where Mg atoms are placed in the centers of O4 tetrahedra, is dynamically unstable - unlike the ZrCuSiAs-type polymorph with oxygen atoms placed in the centers of Mg4 tetrahedra. The characterization of this material covers the structural, elastic properties, electronic band structure, density of electronic states, and Fermi surface. Our calculations suggest that a high critical temperature for MgFeSeO may be achieved as a result of electron or hole doping through ion substitutions or through creation of lattice vacancies.

Electronic properties and Fermi surface for new Fe-free layered superconductor BaTi2Bi2O from first principles

Very recently, as an important step in the development of layered Fe-free pnictide-oxide superconductors, the new phase BaTi2Bi2O was discovered which has the highest TC (about 4.6 K) among all related non-doped systems. In this Letter, we report for the first time the electronic bands, Fermi surface topology, total and partial densities of electronic states for BaTi2Bi2O obtained by means of the first-principles FLAPW-GGA calculations. The inter-atomic bonding picture is described as a high-anisotropic mixture of metallic, covalent, and ionic contributions. Besides, the structural and electronic factors, which can be responsible for the increased transition temperature for BaTi2Bi2O (as compared with related pnictide-oxides BaTi2As2O and BaTi2Sb2O), are discussed.

Electronic band structure, Fermi surface and effect of spin-orbit coupling for tetragonal low-temperature superconductor Bi2Pd from first principles

We have examined theoretically the electronic band structure and Fermi surface of tetragonal low-temperature superconductor Bi2Pd. Our main results are that (i) the Pd 4d and Bi 6p states determine the main peculiarities of the multiple-sheets FS topology, thus for this material the complicated superconducting gap structure with different energy gaps on different FS sheets should be assumed; (ii) the effect of the spin-orbit coupling is of minor importance for the distributions of the near-Fermi electronic states; and (iii) this phase adopts 3D-like type owing to the directional bonds between the adjacent atomic sheets.

Electronic band structure and inter-atomic bonding in layered 1111-like Th-based pnictide oxides ThCuPO, ThCuAsO, ThAgPO, and ThAgAsO from first principles calculations

First-principles FLAPW-GGA band structure calculations were employed to examine the structural, electronic properties and the chemical bonding picture for four ZrCuSiAs-like Th-based quaternary pnictide oxides ThCuPO, ThCuAsO, ThAgPO, and ThAgAsO. These compounds were found to be semimetals and may be viewed as intermediate systems between two main isostructural groups of superconducting and semiconducting 1111 phases. The Th 5f states participate actively in the formation of valence bands and the Th 5f states for ThMPnO phases are itinerant and partially occupied. We found also that the bonding picture in ThMPnO phases can be classified as a high-anisotropic mixture of ionic and covalent contributions: inside [Th2O2] and [M2Pn2] blocks, mixed covalent-ionic bonds take place, whereas between the adjacent [Th2O2]/[M2Pn2] blocks, ionic bonds emerge owing to [Th2O2] to [M2Pn2] charge transfer.

Structural, electronic properties and Fermi surface of ThCr2Si2-type tetragonal KFe2S2, KFe2Se2, and KFe2Te2 phases as parent systems of new ternary iron-chalcogenide superconductors

First principles FLAPW-GGA method was used for the comparative study of the structural and electronic properties of three related tetragonal ThCr2Si2-type phases KFe2Ch2, where Ch are S, Se, and Te. The main trends in electronic bands, densities of states and Fermi surfaces for AFe2Ch2 are analyzed in relation to their structural parameters. We found that at the anion replacements (S<->Se<->Te) any critical changes in electronic structure of KFe2Ch2 phases are absent. On the other hand, our analysis of structural and electronic parameters for hypothetical KFe2Te2 allows to assume that this system may be proposed as perspective parent phase for search of new iron-chalcogenide superconducting materials.

Elastic properties and inter-atomic bonding in new superconductor KFe2Se2 from first principles calculations

Very recently (November, 2010, PRB, 82, 180520R) the first 122-like ternary superconductor KxFe2Se2 with enhanced TC ~ 31K has been discovered. This finding has stimulated much activity in search of related materials and triggered the intense studies of their properties. Indeed already in 2010-2011 the superconductivity (TC ~ 27-33K) was also found in the series of new synthesized 122 phases such as CsxFe2Se2, RbxFe2Se2, (TlK)xFeySe2 etc. which have formed today the new family of superconducting iron-based materials without toxic As. Here, using the ab initio FLAPW-GGA method we have predicted for the first time the elastic properties for KFe2Se2 and discussed their interplay with inter-atomic bonding for this system. Our data reveal that the examined phase is relatively soft material. In addition, this system is mechanically stable, adopts considerable elastic anisotropy, and demonstrates brittleness. These conclusions agree with the bonding picture for KFe2Se2, where the inter-atomic bonding is highly anisotropic and includes ionic, covalent and metallic contributions.

Electronic structure and Fermi surface of new K intercalated iron selenide superconductor KxFe2Se2

Using the ab initio FLAPW-GGA method we examine the electronic band structure, densities of states, and the Fermi surface topology for a very recently synthesized ThCr2Si2-type potassium intercalated iron selenide superconductor KxFe2Se2. We found that the electronic state of the stoichiometric KFe2Se2 is far from that of the isostructural iron pnictide superconductors. Thus the main factor responsible for experimentally observed superconductivity for this material is the deficiency of potassium, i.e. the hole doping effect. On the other hand, based on the results obtained, we conclude that the tuning of the electronic system of the new KxFe2Se2 superconductor in the presence of K vacancies is achieved by joint effect owing to structural relaxations and hole doping, where the structural factor is responsible for the modification of the band topology, whereas the doping level determines their filling.

Magnetic and electronic properties of nitrogen-doped lanthanum sesquioxide La2O3 as predicted from first principles

Using the ab initio FLAPW-GGA method we examine the electronic and magnetic properties of nitrogen-doped non-magnetic sesquioxide La2O3 emphasizing the role of doping sites in the occurrence of d0-magnetism. We predict the magnetization of La2O3 induced by nitrogen impurity in both octahedral and tetrahedral sites of the oxygen sublattice. The most interesting results are that (i) the total magnetic moments (about 1 {mu}B per supercells) are independent of the doping site, whereas (ii) the electronic spectra of these systems differ drastically: La2O3:N with six-fold coordinated nitrogen behaves as a narrow-band-gap magnetic semiconductor, whereas with four-fold coordinated nitrogen is predicted to be a magnetic half-metal. This effect is explained taking into account the differences in N-2pz versus N-2px,y orbital splitting for various doping sites. Thus, the type of the doping site is one of the essential factors for designing of new d0-magnetic materials with promising properties.

First-Principles Prediction of Novel Magnetic Materials Based on ZrCuSiAs-like Semiconducting Pnictide-Oxides

We assumed that significant enlargement of the functional properties of the family of quaternary ZrCuSiAs-like pnictide-oxides, often called also as 1111 phases, which are known now first of all as parent phases for new FeAs superconductors, may be achieved by replacement of nonmagnetic ions by magnetic ions in semiconducting ZrCuSiAs-like phases. We checked this assumption by means of first-principles FLAPW-GGA calculations using a wide-band-gap semiconductor YZnAsO doped with Mn, Fe, and Co as an example. Our main finding is that substitution of Mn, Fe, and Co for Zn leads to drastic transformations of electronic and magnetic properties of the parent material: as distinct from the non-magnetic YZnAsO, the examined doped phases YZn0.89Mn0.11AsO, YZn0.89Fe0.11AsO, and YZn0.89Co0.11AsO behave as a magnetic semiconductor, a magnetic half-metal or as a magnetic gapless semi-metal, respectively.

Stability, structural, elastic and electronic properties of RuN polymorphs from first-principles calculations

First-principles calculations through a FLAPW-GGA method for six possible polymorphs of ruthenium mononitride RuN with various atomic coordination numbers CNs: cubic zinc blende (ZB) and cooperite PtS-like structures with CNs = 4; cubic rock-salt (RS), hexagonal WC-like and NiAs-like structures with CNs = 6 and cubic CsCl-like structure with CN = 8 indicate that the most stable is ZB structure, which is much more preferable for RuN than the recently reported RS structure for synthesized RuN samples. The elastic and electronic properties of ZB-RuN were investigated and discussed in comparison with those for RS-RuN polymorph.