Zoran S. Popović

Electronic structure of the substitutional vacancy in graphene: density-functional and Green's function studies

We study the electronic structure of graphene with a single substitutional vacancy using a combination of the density-functional, tight-binding and impurity Greens function approaches. Density-functional studies are performed with the all-electron spin-polarized linear augmented plane wave (LAPW) method. The three sp2? dangling bonds adjacent to the vacancy introduce localized states (V?) in the mid-gap region, which split due to the crystal field and a Jahn?Teller distortion, while the pz? states introduce a sharp resonance state (V?) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the ? and the ? states, so that these bands are clearly identifiable in the calculated band structure. As to the magnetic moment of the vacancy, the Hunds rule coupling aligns the spins of the four localized V?1??, V?2? and V?? electrons, resulting in an S?=?1 state, with a magnetic moment of 2?B, which is reduced by about 0.3?B due to the anti-ferromagnetic spin polarization of the ? band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of 1.7?B. Using the Lippmann?Schwinger equation, we reproduce the well-known ?1/r decay of the localized V? wave function with distance, and in addition, find an interference term coming from the two Dirac points, previously unnoticed in the literature. The long-range nature of the V? wave function is a unique feature of the graphene vacancy and we suggest that this may be one of the reasons for the widely varying relaxed structures and magnetic moments reported from the supercell band calculations in the literature.

DENSITY-FUNCTIONAL STUDIES OF THE ELECTRONIC STRUCTURE OF THE PEROVSKITE OXIDES : LA1-XCAXMNO3

Using density‐functional methods, we study the electronic structures of the lanthanum‐based ‘‘double‐exchange’’ perovskite magnets. Antiferromagnetic insulating solutions are obtained for both the end members, LaMnO3 and CaMnO3, within the local density approximation (LDA), with the Jahn‐Teller (JT) distortion of the oxygen octrahedron taken into account. The JT distortion splits off the Mn(3d)eg bands producing an energy gap within the LDA, with the bands derived from the (z2−1) orbital, pointed along the long basal‐plane Mn—O bond, occupied and the (x2−y2) bands empty. The on‐site Coulomb repulsion and the intra‐site exchange terms are found to be, respectively, U≂8–10 eV and J≂0.9 eV, from the ‘‘constrained’’ density‐functional theory. The large value of U as compared to the bandwidth indicates that the manganese perovskite oxides are strongly correlated systems.

A comparative study of coulomb-correlated electronic structure of the spin-gapped compound α'-NaV2O5 in Pmmn and P21mn crystal structure

The paramagnetic, spin-polarized, and LDA+ U rotationally invariant, fully self-consistent linear-muffin-tin orbital band structure for the inorganic spin-gapped vanadate α ′ -NaV 2 O 5 has been calculated for both centrosymmetric group P m m n and noncentrosymmetric group P 2 1 m n (earlier assignment which is now denied as a room temperature structure) and twosupposed antiferromagnetic orders. A very small band gap solution and magnetic moments on vanadium atoms are obtained in the framework of local-spin-density functional approximation. The LDA+ U approach has produced the insulating antiferromagnetic solution with more appropriate energy gaps and magnetic moments. We found that the minimal crystal structure changes produce significant changes in the electronic structure and magnetic properties. Starting from an extended four-band tight-binding model, fitted to the paramagnetic P m m n bands, the hopping terms relevant for the calculation of exchange parameters of a spatially anisotropic Heisenberg mo...

Coulomb correlated band structure of one-dimensional SrCuO2

Abstract The spin-restricted, spin-polarized, and LDA + U fully self-consistent linear muffin-tin orbital band structure for the “one-leg ladder” compound SrCuO2 has been studied. An extremely small band gap solution with negligible magnetic moment on Cu sites is obtained in the framework of the LSDA method. On the other hand, the LDA + U method has produced the insulating antiferromagnetic solution with an energy gap of 1.63 eV and a magnetic moment of 0.89 μB. The standard LDA and LSDA SrCuO2 bands around the Fermi level are so similar to the recently discovered spin-Peierls inorganic compound CuGeO3 that one could perhaps expect the appearance of the same type of phase transition in orthorhombic strontium copper oxide SrCuO2.

Effect of adsorbed H atoms on magnetism in monoatomic Fe wires at Ir(100)

By means of ab initio calculations based on density-functional theory we demonstrate that magnetism in monoatomic Fe wires deposited on nanostructured Ir(100) surface can be tuned by their functionalization with hydrogen. The pristine monoatomic Fe wires deposited on nanostructured Ir(100) surface partially covered by H atoms are antiferromagnetic. However, the type of exchange interaction between Fe atoms can be changed by increasing H coverage. At fully hydrogenated Ir surface the Fe wires themselves are decorated with hydrogen, which gives rise to the ferromagnetic coupling between adjacent Fe atoms.