Full-potential Density-functional Calculations Research Articles

Investigation on structural, electronic and magnetic properties of Co2FeGe Heusler alloy: Experiment and theory

Experimental and computational studies are performed on Co2FeGe Heusler alloy. Significantly large saturation magnetization (Ms), Curie temperature (Tc) and spin wave stiffness constant (D) of 6.1 µB/f.u., 1073 K and 10.4 nm2-meV respectively were observed experimentally. The experimental values of Tc and D are reported first time and are among the highest reported values of the same in Heusler alloy domain. The Co2FeGe Heusler alloy strictly follows Slater-Pauling (SP) rule, however, the minor experimental deviation from its SP value is justified by doing full-potential density functional calculations, which gives more accurate result when electron–electron correlation parameter (U) is taken into account with conventional generalized gradient approximation (GGA) scheme. Effect of lattice strain and U on the total and atom-specific magnetic moments and spin-polarization is studied in detail from which we conclude that they have a key role in the deviation of the experimental results from the expected theoretical values. Dynamical stability of the alloy is studied from the phonon dispersion curve and the effect of U is found to be distinct on the phonon density of states. Role of different types of atomic site disorders on the saturation magnetization is also explored computationally and related to the experimental results. The computational results provide in-depth and detailed study followed by experimental validations.

Magneto-optical Kerr effect in ZnTMO2 (TM=Cr, Mn, Fe, Co and Ni)

First principles generalized gradient full potential density-functional calculations were performed to predict the optical and magneto-optical (MO) properties of the chalcopyrite compounds ZnTMO2, TM=Cr, Mn, Fe, Co and Ni. Detailed investigation of the electronic band structure and density of states is reported. The optical properties in the 0–8eV energy range are analyzed in terms of band structure transitions. As for the magneto–optical properties, our results show that the studied compounds have peaks in the Kerr rotation ranging from infrared to ultraviolet radiation, with ZnFeO2 having the highest Kerr rotation angle of 10° and −8.46° at 0.35eV and 4.59eV, respectively. The peaks in the Kerr spectra were assigned to the optical and magneto–optic contributions. Our calculated function of merit for these compounds indicates that these compounds might be useful for technological application in high density storage.

Assessment of Functionals for First-Principle Studies of the Structural and Electronic Properties ofδ-Bi2O3

Fully relativistic full-potential density functional calculations with an all-electron linearized augmented plane waves plus local orbitals method were carried out to perform a comparative study on the structural and electronic properties of the cubic oxideδ-Bi2O3phase, which is considered as one of the most promising materials in a variety of applications including fuel cells, sensors, and catalysts. Three different density functionals were used in our calculations, LDA, the GGA scheme in the parametrization of Perdew, Burke, and Ernzerhof (PBE96), and the hybrid scheme of Perdew-Wang B3PW91. The examined properties include lattice parameter, band structure and density of states, and charge density profiles. For this modification the three functionals reveal the characteristics of a metal and the existence of minigaps at high symmetry points of the band structure when spin-orbit coupling is taken into account. Density of states exhibits hybridization of Bi 6s and O 2p orbitals and the calculated charge density profiles exhibit the ionic character in the chemical bonding of this compound. The B3PW91 hybrid functional provided a better agreement with the experimental result for the lattice parameter, revealing the importance of Hartree-Fock exchange in this compound.

AB INITIO INVESTIGATIONS OF THE MAGNETISM IN DILUTED MAGNETIC SEMICONDUCTOR Fe-DOPED GaN

In this paper, we present a first principle investigation on Fe -doped GaN with wurtzite and zinc-blend structure using full potential density functional calculations. Data point out that the magnetic behavior of Fe -doped GaN system is strongly dependent on Fe doping configurations. In agreement with the experimental reports, and independently by doping, antiferromagnetism occurs in the zinc-blend structure, while in the wurtzite structure ferromagnetism depends on the Fe doping configurations. Detailed analyses combined with density of state calculations support the assignment that the ferromagnetism is closely related to the impurity band at the origin of the hybridization of Fe 3d and N 2p states in the Fe -doped GaN of wurtzite phase.

Recent Advances in the Korringa-Kohn-Rostoker Green Function Method

The Korringa-Kohn-Rostoker (KKR) Green function (GF) method is a technique for all-electron full-potential density-functional calculations. Similar to the historical Wigner-Seitz cellular method, the KKR-GF method uses a partitioning of space into atomic Wigner-Seitz cells. However, the numerically demanding wave-function matching at the cell boundaries is avoided by use of an integral equation formalism based on the concept of reference Green functions. The advantage of this formalism will be illustrated by the recent progress made for very large systems with thousands of inequivalent atoms and for very accurate calculations of atomic forces and total energies.

Graphene and some of its structural analogues: full-potential density functional theory calculations

Using full-potential density functional calculations we have investigated the structural and electronic properties of graphene and some of its structural analogues, viz., monolayer (ML) of SiC, GeC, BN, AlN, GaN, ZnO, ZnS and ZnSe. While our calculations corroborate some of the reported results based on different methods, our results on ZnSe, the two dimensional bulk modulus of ML-GeC, ML-AlN, ML-GaN, ML-ZnO and ML-ZnS and the effective masses of the charge carriers in these binary mono-layers are something new. With the current progress in synthesis techniques, some of these new materials may be synthesized in near future for applications in nano-devices.

Strain-tunable band gap in graphene/h-BN hetero-bilayer

Using full-potential density functional calculations within local density approximation (LDA), we predict that mechanically tunable band-gap and quasi-particle-effective-mass are realizable in graphene/hexagonal-BN hetero-bilayer (C/h-BN HBL) by application of in-plane homogeneous biaxial strain. While providing one of the possible reasons for the experimentally observed gap-less pristine-graphene-like electronic properties of C/h-BN HBL, which theoretically has a narrow band-gap, we suggest a schematic experiment for verification of our results which may find applications in nano-electromechanical systems (NEMS), nano opto-mechanical systems (NOMS) and other nano-devices based on C/h-BN HBL.

Detection of stacking faults breaking the [110]/[11¯0] symmetry in ferromagnetic semiconductors (Ga,Mn)As and (Ga,Mn)(As,P)

We report high resolution x-ray diffraction measurements of (Ga,Mn)As and (Ga,Mn)(As,P) epilayers. We observe a structural anisotropy in the form of stacking faults which are present in the (111) and (11-1) planes and absent in the (-111) and (1-11) planes. The stacking faults produce no macroscopic strain. They occupy 0.01 - 0.1 per cent of the epilayer volume. Full-potential density functional calculations evidence an attraction of Mn_Ga impurities to the stacking faults. We argue that the enhanced Mn density along the common [1-10] direction of the stacking fault planes produces sufficiently strong [110]/[1-10] symmetry breaking mechanism to account for the in-plane uniaxial magnetocrystalline anisotropy of these ferromagnetic semiconductors.

Anisotropic Multiband Many-Body Interactions inLuNi2B2C

We present a comprehensive de Haas-van Alphen study on the nonmagnetic borocarbide superconductor LuNi2B2C. The analysis of the angular-dependent effective masses for different bands in combination with full-potential density functional calculations allowed us to determine the mass-enhancement factors, lambda, for the different electronic bands and their wave-vector dependences. Our data clearly show the anisotropic multiband character of the superconductivity in LuNi2B2C.

Coarse-grained density functional theories for metallic alloys: Generalized coherent-potential approximations and charge-excess functional theory

The class of the generalized coherent-potential approximations (GCPAs) to the density functional theory (DFT) is introduced within the multiple scattering theory formalism with the aim of dealing with ordered or disordered metallic alloys. All GCPA theories are based on a common ansatz for the kinetic part of the Hohenberg\char21{}Kohn functional and each theory of the class is specified by an external model concerning the potential reconstruction. Most existing DFT implementations of CPA-based theories belong to the GCPA class. The analysis of the formal properties of the density functional defined by GCPA theories shows that it consists of marginally coupled local contributions. Furthermore, it is shown that the GCPA functional does not depend on the details of the charge density and that it can be exactly rewritten as a function of the appropriate charge multipole moments to be associated with each lattice site. A general procedure based on the integration of the $qV$ laws is described that allows for the explicit construction of the same function. The coarse-grained nature of the GCPA density functional implies a great deal of computational advantages and is connected with the $O(N)$ scalability of GCPA algorithms. Moreover, it is shown that a convenient truncated series expansion of the GCPA functional leads to the charge-excess functional (CEF) theory [E. Bruno et al., Phys. Rev. Lett. 91, 166401 (2003)], which here is offered in a generalized version that includes multipolar interactions. CEF and the GCPA numerical results are compared with status of art linearized augmented plane wave (LAPW) full-potential density functional calculations for 62 bcc- and fcc-based ordered CuZn alloys, in all the range of concentrations. Two facts clearly emerge from these extensive tests. In the first place, the discrepancies between GCPA and CEF results are always within the numerical accuracy of the calculations, both for the site charges and the total energies. In the second place, the GCPA (or the CEF) is able to very carefully reproduce the LAPW site charges and a good agreement is obtained also about the total energies.

Magnetic and elastic properties ofYCo5andLaCo5under pressure

High hydrostatic pressure destroys the strong ferromagnetism in $\mathrm{Y}{\mathrm{Co}}_{5}$. The transformation proceeds in a stepwise fashion, as a first-order phase transition $({P}_{\mathrm{crit}}=18\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, and is accompanied by an isomorphic (without change of symmetry) lattice collapse and a topological change of the Fermi surface. Accurate full-potential density-functional calculations enable us to ascribe these phenomena to a quasi-one-dimensional $3d$-like band, whose top under ambient pressure is situated $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi level. Lattice compression drives the top of the nondispersive band---and the associated peak in the majority density of states---upward in energy, until they reach the Fermi level and the system becomes unstable. A similar transition at a somewhat higher pressure $(23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ is predicted for the isomorphous compound $\mathrm{La}{\mathrm{Co}}_{5}$.

A full-potential linearized augmented plane wave (FP-LAPW) study of atomic carbon, nitrogen, and oxygen chemisorption on the (1 0 0) surface of δ-Pu

Fully relativistic full-potential density functional calculations with an all-electron linearized augmented plane wave plus local orbitals method have been performed to investigate the electronic and geometric structures of atomic carbon, nitrogen, and oxygen chemisorption on the (1 0 0) surface of δ-Pu. For all chemisorption processes, the center adsorption site is found to be the most preferred site with chemisorption energies of 7.964, 7.665, and 8.335 eV for the C, N, and O adatoms, respectively. The respective optimized distances of the C, N, and O adatoms from the surface were found to be 0.26, 0.35, and 0.48 Å. The work functions and the net magnet moments, respectively, increased and decreased in all cases compared with the bare δ-Pu (1 0 0) surface. In particular, the work function shift is largest for the least preferred top site and lowest for the most preferred center site. A detailed analysis of partial charges inside the atomic spheres, charge density distributions, and the local density of states have been performed to investigate the nature of the interaction between the surface Pu atoms and the adatoms.

Magneto-optical Kerr effects of half-metallic ferromagnetic transition metal chalcogenides in zinc-blende and wurtzite structures

The electronic structure and polar magneto-optical Kerr effect (MOKE) of the transition metal chalcogenides, such as CrSe, CrTe, and VTe, in zinc-blende and wurtzite structures are studied by full-potential density-functional calculations. The $p\text{\ensuremath{-}}d$ exchange interaction in these half-metallic ferromagnets is estimated and found to be ferromagnetic. The MOKE is quite large in all of these compounds. It is found that the peaks in Kerr rotation spectra lower than $3.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are enhanced by the plasma resonance due to the metallic feature in the majority spin channel, while those higher than $3.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ originate from the quite large off-diagonal optical conductivity element. The CrTe exhibits a larger Kerr effect than the CrSe since the spin-orbit coupling of $p$ orbitals in Te is stronger than that in Se, while the larger Kerr effect in VTe than in CrTe is quite unexpected because the magnetic moment in VTe is smaller than in CrTe. This strange result is understood in terms of transitions from the underlying band structure. Our calculations can give hints to the understanding of MOKE and the design of the new materials with technologically desirable magneto-optical properties through tailoring their electronic band structures.

The role of covalency in the orbital-order of 3d 1 perovskites

We investigate the interplay between lattice distortion, bond formation and orbital ordering in A TiO 3 ( A = La , Y) perovskites using full-potential density-functional calculations and ELF analysis. It is shown that the lattice distortion implies a dramatic reorganization of the bonds between A cations and oxygen on the one hand, and titanium and oxygen on the other hand. Along the series cubic LaTiO 3, orthorhombic LaTiO 3, orthorhombic YTiO 3 an increasing asymmetrization of the environment of Ti is taking place. We show that changes in chemical bonding are relevant for lifting the degeneracy of Ti 3d orbitals across the metal–insulator transition and we sketch the use of the ELF to visualize orbital ordering.

Ab initio correlation approach to a ferric wheel-like molecular cluster

We present an ab initio study of electronic correlation effects in a molecular cluster derived from the hexanuclear ferric wheel [LiFe6(OCH3)12-(dbm)6]PF6. The electronic and magnetic properties of this cluster have been studied with all-electron Hartree-Fock, full-potential density functional calculations and multi-reference second-order perturbation theory. For different levels of correlation, a detailed study of the impact of the electronic correlation on the exchange parameter was feasible. As the main result, we found that the influence of the bridge oxygen atoms on the exchange parameter is less intense than the influence of the apical ligand groups, which is due to the geometry of the cluster. With respect to the cluster model approach, the experimental value of the exchange parameter was affirmed.

First principles study of ferromagnetism inTi0.0625Zn0.9375O

We present ab initio full-potential density functional calculations of the electronic structure of ${\mathrm{Ti}}_{0.0625}{\mathrm{Zn}}_{0.9375}\mathrm{O}$. We show that it is likely for Ti to order ferromagnetically in $\mathrm{ZnO}$, forming a dilute magnetic semiconductor, in agreement with experiment. The ferromagnetic ordering occurs despite Ti being nonmagnetic in its natural phase. ${\mathrm{Ti}}_{0.0625}{\mathrm{Zn}}_{0.9375}\mathrm{O}$ is shown to possess a magnetic moment of $0.63{\ensuremath{\mu}}_{B}$ per supercell, with a spin-polarized impurity band at the bottom of the $\mathrm{ZnO}$ conduction band and the Fermi level lying within the former band. The impurity band is found to primarily arise from the hybridization of the Ti $3d$ level with O $2p$ states.

Electronic and magnetic properties of a hexanuclear ferric wheel

The electronic and magnetic properties of the hexanuclear ferric wheel [LiFe6(OCH3)12-(dbm)6]PF6 have been studied with all-electron Hartree-Fock and full-potential density functional calculations. The best agreement for the magnetic exchange coupling is at the level of the B3LYP hybrid functional. Surprisingly, the Hartree-Fock approximation gives the wrong sign for the exchange coupling. The local density approximation and the gradient corrected functional PBE strongly overestimate the exchange coupling due to the too large delocalization of the d-orbitals. These findings are supported by results from the Mulliken population analysis for the magnetic moments and the charge on the individual atoms.

Huge magneto-optical effects in half-metallic double perovskites

Using generalized-gradient-corrected full-potential density-functional calculations we have studied the magneto-optical properties of double perovskites A2BB8O6, A= Ca, Sr, and Ba; B= Fe; and B8 = Mo, W, and Re. Sr2FeWO6 has the maximum polar Kerr rotation of 3.87° and specific Faraday rotation of 4.5 3 10 5 deg cm ˛1 . All other compounds have Kerr rotation more than 1°, except Ca 2FeMoO6 which has maximum Kerr rotation of 0.5°. Our electronic structure studies show that all these compounds are half-metallic in the ferromagnetic configuration. The large Kerr rotation is found to be due to the combined effects of relatively large exchange splitting from Fe, large spin-orbit coupling due to 4 d or 5d elements, large offdiagonal conductivity, and half-metallicity. Materials with large magneto-optical (MO) effects have attracted a lot of attention in both basic and applied research. The motivation stems from desire to develop erasable MO memories and high density disks with MO readout. However, it is still a challenging problem to find a material with large MO rotation and also possessing a collection of necessary magnetic properties. The interaction of electromagnetic radiation with magnetized matter manifests itself as MO effects. The MO effect due to reflection is called MO Kerr effect (MOKE) and that due to transmission is called MO Faraday effect. Planepolarized light, when reflected from a metal surface or transmitted through a thin film that has nonzero magnetization will become elliptically polarized with ellipticity (e K in reflection mode and eF in transmission mode). The major axis is rotated by the angle u (uK in reflection mode and uF in transmission mode) relative to the polarization axis of the incident beam. The MOKE is of three major types, polar, longitudinal, and equatorial. The polar Kerr effect for which the direction of the macroscopic magnetization of the ferromagnetic material and the propagation direction of plane polarized incident beam are perpendicular to the plane of the surface, is the most interesting case and used in practical

Effect of oxygen stoichiometry on spin, charge, and orbital ordering in manganites

Using full-potential density-functional calculations we show that oxygen stoichiometry plays an important role on spin, charge, and orbital ordering in manganites. The electronic structure and magnetic properties of ${\mathrm{LaBaMn}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}}$ have been studied for $\ensuremath{\delta}=0,0.5,$ and 1; for $\ensuremath{\delta}=0$ and 0.5 the system exhibits charge, orbital, and antiferromagnetic spin ordering, whereas at $\ensuremath{\delta}=1$ the charge and orbital orderings disappear but the spin ordering remains. We also bring out an insulator-to-metal transition upon going from $\ensuremath{\delta}=0$ to 1. The study suggests that one can manipulate the charge and orbital orderings in certain perovskite-like oxides by merely varying the oxygen stoichiometry and hence design oxides with desired electrical and magnetic properties.

Spin, charge, and orbital orderings in oxides with dual-valent transition metal ions

Using generalized-gradient-corrected full-potential density-functional calculations, we have studied the electronic structure and magnetic properties of YBaMn 2O 5, Sr 4Fe 4O 11, and Ca 3Co 2O 6. In these phases, the 3 d transition metal ions have dual valence. We have studied the electronic structure using site-, angular momentum-, and orbital-projected density of states. The charge and orbital ordering are analyzed in terms of the calculated electron-density distribution, charge density, and electron localization function. The oxygen vacancy, cation radii, and crystal-field effects are found to play an important role for the various ordering phenomena in these compounds.