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Layered structure and peculiar electronic properties of two-dimensional (2D) materials foster the concept of utilizing them as main components of lithium-ion batteries. Understanding basic physical mechanisms governing the interaction of Li with 2D crystals is of key importance to succeeding in a rational design of cathode and anode materials with superior functionalities. In this study density functional theory was applied to reveal the microscopic picture of Li interaction with 15 2D crystals, including several transition metal oxides and dichalcogenides, carbides of Group XIV elements, functionalized graphene, silicene, and germanene, as well as black phosphorus and Ti2C MXene. We found that the general trend in Li binding can be estimated from positions of conduction band minima of 2D materials, since the energy of the lowest empty electronic states shows a nice correlation with the strength of Li adsorption. At variance to the majority of studied surfaces where the electron transferred from Li is spread across the substrate, in monolayers of carbides of Group XIV elements the interaction with Li and the charge transfer are well localized. This gives rise to their capability to accommodate Li structures with a nearly constant binding energy of alkaline atoms over Li coverages ranging from well-separated adatoms to a full monolayer. © 2018 American Physical Society.

We study the modification of the electronic structure in the strong spin-orbit coupled Sr$_2$IrO$_4$ by epitaxial strain using density functional methods. Structural optimization shows that strain changes the internal structural parameters such as the Ir-O-Ir bond angle, which has an important effect on the band structure. An interesting prediction is the $\Gamma - $X crossover of the valence band maximum with strain, while the conduction minimum at M remains unchanged. This in turn suggests strong strain dependence of the transport properties for the hole doped system, but not when the system is electron-doped. Taking the measured value of the $\Gamma-X$ separation for the unstrained case, we predict the $\Gamma - $X crossover of the valence band maximum to occur for the tensile epitaxial strain $e_{xx} \approx 3\%$. A minimal tight-binding model within the $J_{\rm eff} = 1/2$ subspace is developed to describe the main features of the band structure. The optical absorption spectra under epitaxial strain are computed using density-functional theory, which explains the observed anisotropy in the optical spectra with the polarization of the incident light. We show that the optical transitions between the Ir (d) states, which are dipole forbidden, can be explained in terms of the admixture of Ir (p) orbitals with the Ir (d) bands., Comment: 15 figures

Within density functional theory (DFT) with the generalized gradient approximation (GGA), GGA plus on-site Coulomb repulsion method, and improved version of the modified Becke-Johnson exchange potential suggested recently by Tran and Blaha [F. Tran, P. Blaha, Phys. Rev. Lett. 102 (2009) 2264011 (TB-mBJ), we investigate the electronic structure and optical properties of noncentrosymmetric multiferroic perovskites PbVO3 and BiCoO3. These two compounds, although similar in lattice distortions and population of crystal-field levels, behave quite differently because of the different interplay between the fundamental Kramers degeneracy and the single-ion anisotropy in them. The main characteristic of the calculated TB-mBJ electronic structures is significant rearrangement of the V and Co 3d states near their valence bands tops when compared to the present and earlier GGA and GGA + U calculations of these compounds. This fact causes the different optical responses of the title compounds as well, which are analyzed and interpreted in terms of the calculated electronic structures. A comparison of the calculated properties with available experimental data indicates that the TB-mBJ approach provides a better description of the electronic and optical properties of PbVO3 and BiCoO3 than the standard GGA and GGA + U approaches. (c) 2013 Elsevier B.V. All rights reserved.

We used ab initio calculations based on density functional theory (DFT) to investigate the binding of atomic oxygen on graphene, considering adsorption structures with sizes varying from sub-nanometric clusters to infinite overlayers. From an extensive set of studied configurations of oxygen dimers, trimers and tetramers we demonstrated a strong tendency of adsorbates to form close-packed structures. In the high O coverage regime, corresponding to a C/O ratio of 2:1, we identified two very favorable structures with a distribution of O adatoms which enables both the hybridization of all C-2p(z) states from the graphene pi electron network with the O-2p states, as well as the minimization of the electrostatic repulsion between negatively charged O atoms attached to the same side of the graphene sheet. Using DFT results obtained for small clusters we constructed a simple model able to describe the energetics of the O islands with sizes beyond those that can be directly treated by first principles methods. (C) 2012 Elsevier Ltd. All rights reserved.

Electronic structure and optical properties of copper tungstate CuWO4 were investigated using density functional theory with improved version of exchange potential suggested recently by Tran and Blaha (2009) [7] (TB-mBJ). The calculated band gap value is found to be in excellent agreement with the recent experimental results. The values of the magnetic moments at the Cu sites and O (3) oxygen atomic sites (closest to the Cu2+ ions in antiferromagnetic chains) are in good agreement with experiments also. The principal characteristic of the calculated electronic structure is significant rearrangement of the Cu 3d states near the valence band top as compared to our earlier PBE and PBE + U calculations of the same compound. This fact causes the different optical response of the CuWO4 as well, which is analyzed and interpreted in terms of calculated electronic structure. A comparison of the calculated properties with available experimental data indicates that the TB-mBJ approach provides a better description of the electronic and optical properties of the CuWO4 compound than the standard PBE and PBE + U approaches. (C) 2012 Elsevier B.V. All rights reserved.

Ab initio calculations based on the density-functional theory have been employed to study the electronic and magnetic properties of copper tungstate CuWO4, as well as its optical characteristics in the ultraviolet region, up to 40 eV. The electronic structure around the band gap is dominated by the O p- and the Cu d-states and it is quite different from the recent spin-restricted calculations of the same compound. The most stable antiferromagnetic state and the values of magnetic moments at Cu sites and 0(3) oxygen atomic sites (closest to Cu2+ ions in zigzag antiferromagnetic chains) are in agreement with experiments. The gap is found to be indirect with the acceptable value only after use of the LDA + U rotationally invariant self-consistent full potential linearized augmented plane wave (FP LAPW) approach. The optical spectra are analyzed, compared, and interpreted in terms of calculated band structures. It is shown that absorption process involves significant energy flow from the O ions to the Cu ions. (C) 2010 Elsevier B.V. All rights reserved.

We investigate the electronic structure of the monolayer black phosphorus (BP) using density-functional methods both with and without an applied electric field. We find that a simple one-band tight-binding Hamiltonian based on the ${p}_{z}$ orbitals and nearest-neighbor hopping is sufficient to describe the band structure in the gap region rather well and justification for this is given from symmetry arguments. The anisotropic nature of the band structure leads in turn to an anisotropic Rashba effect, where the magnitude of the spin splitting caused by an applied electric field is not only momentum dependent, but also depends on the direction of $\stackrel{P\vec}{k}$. The Rashba Hamiltonian is generalized for the anisotropic case, which reads: ${H}_{R}={\ensuremath{\alpha}}_{R}(\stackrel{P\vec}{\ensuremath{\sigma}}\ifmmode\times\else\texttimes\fi{}{\stackrel{P\vec}{k}}^{\ensuremath{'}})\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{z}$, where the scaled momentum ${\stackrel{P\vec}{k}}^{\ensuremath{'}}$ contains the anisotropy effect. The Rashba effect is studied quantitatively for BP from ab initio density-functional calculations in the presence of an applied electric field. A byproduct of this work is the demonstration that the strength of the spin-orbit coupling for the outermost electrons in the atoms, which are relevant for the solids, increases only as the Landau-Lifshitz ${Z}^{2}$ scaling with the atomic number $Z$, rather than the higher power ${Z}^{4}$ scaling, as sometimes thought.

We present a systematic study of the sheet carrier density and valence-band offset in the GaN/AlxGa1-xN(0001) heterostructure as a function of x from ab initio density-functional methods. We find that the calculated sheet carrier density increases rapidly with x for xless than or equal to0.3 in good agreement with experiments, but beyond this concentration, it quickly saturates to a value of about 2x10(13) cm(-2). The band offset shows a small asymmetry between the Ga-face and N-face interfaces and changes more or less linearly with x, although a small bowing is found. The layer-projected densities of states indicate the formation of the two-dimensional electron gas at the Ga-face interface and confirm the absence of interface states in the gap. (C) 2004 American Institute of Physics.

We show that the Rashba spin-orbit interaction in $d$ electron solids, which originates from the broken inversion symmetry at surfaces or interfaces, is strongly dependent on the orbital characters of the bands involved. This is studied by developing a tight-binding model in the presence of a uniform perpendicular electric field and spin-orbit coupling. We argue that for valence electrons, the spin-orbit coupling strength scales only as the square of the atomic number. The electric field distorts the $d$ orbitals through the admixture of $p$ and $f$ states and also introduces intersite overlap parameters. Expressions for Rashba coefficients for the bands are obtained in both weak and strong spin-orbit interaction limits and are shown to be orbital dependent. The results are compared with first-principles calculations for model systems, showing good agreement. Our study demonstrates the orbital-dependent gate control of the Rashba effect for the purposes of oxide electronics.

Using density-functional methods, we study the electronic structure of ${\mathrm{Ba}}_{3}\mathrm{Cu}{\mathrm{Sb}}_{2}{\mathrm{O}}_{9}$, a candidate material for the quantum spin liquid behavior. We study both the triangular lattice as well as the recently proposed hexagonal lattice structures with flipped Cu-Sb dumbbells. The band structure near the Fermi energy is described very well by a tight-binding Hamiltonian involving the Cu (${e}_{g}$) orbitals, confirming their central role in the physics of the problem. A minimal tight-binding Hamiltonian for the triangular structure is presented. The Cu (${d}^{9}$) ions (a single ${e}_{g}$ hole in the band structure) present in the compound are expected to be Jahn-Teller centers, while the nature of the Jahn-Teller distortions in this material is still under debate. Solving a simple model by exact diagonalization, we show that electronic correlation effects in general enhance the tendency towards a Jahn-Teller distortion by reducing the kinetic energy due to correlation effects. Our density-functional calculations do indeed show a significant Jahn-Teller distortion of the $\mathrm{Cu}{\mathrm{O}}_{6}$ octahedra when we include the correlation effects within the Coulomb-corrected GGA+U method, so that the Jahn-Teller effect is correlation driven. We argue for the presence of a random static Jahn-Teller distortion in the hexagonal structure rather than a dynamical one because of the broken octahedral symmetry around the $\mathrm{Cu}{\mathrm{O}}_{6}$ octahedra and the potential fluctuations inherently present in the system caused by a significant disorder, which is believed to be present, in particular, due to the flipped Cu-Sb dumbbells.

We report a simple colloidal synthesis of two types of Sb2S 3 nanowires with small band gap and high aspect ratio. Field-emission scanning electron and transmission electron microscopies confirmed formation of high aspect ratio Sb2S3 nanowires, separated in the form of bundles and coalesced with each other in long bars. Diffuse reflectance and absorption spectroscopies revealed that the optical band-gap energies of the synthesized nanowires separated in the form of bundles are 1.56 and 1.59 eV, and coalesced with each other in long bars are 1.36 and 1.28 eV, respectively. The structure refinement showed that Sb2S3 powders belong to the orthorhombic structure with space group Pnma (no. 62). It was found that Sb2S3 nanowires separated in the form of bundles predominantly grow along the [0 1 0] direction being in the needle-like shape. The nanowires coalesced with each other in long bars rise in the form of long bars, are ribbon-like in shape and have expressed {1 0 1} facets which grow along the [0 1 0] direction. No peaks in photoluminescence spectra were observed in the spectral range from 250 to 600 nm. In order to shed more light on the experimental results concerning the band-gap energies and, in the literature generally poorly investigated electronic properties of the synthesized material, we performed theoretical calculations of the electronic structure and optical properties of the Sb2S3 samples synthesized here. This was done on the basis of density functional theory with the generalized gradient approximation, and also with an improved version of the exchange potential suggested recently by Tran and Blaha. The main characteristic is the significant improvement of the band gap value.

The paramagnetic, spin-polarized, and local-density approximation $\mathrm{LDA}+U$ rotationally invariant, fully self-consistent linear muffin-tin orbital band structure for the second inorganic spin-Peierls vanadate ${\ensuremath{\alpha}}^{\ensuremath{'}}{\ensuremath{-}\mathrm{N}\mathrm{a}\mathrm{V}}_{2}{\mathrm{O}}_{5}$ has been calculated. A very small band-gap solution and magnetic moments on both vanadium-type atoms are obtained in the framework of local-spin-density functional approximation. The new rotationally invariant $\mathrm{LDA}+U$ approach has produced the insulating antiferromagnetic solution with an energy gap of 2.7 eV and a magnetic moment of $0.97{\ensuremath{\mu}}_{B}$ only on one type of vanadium atoms. These results are in agreement with experiments to a quite satisfactory degree.

Using the first-principles full-potential linear muffin-tin orbital method in the atomic sphere approximation, we have calculated the electronic structure of the intermetallic compound , and evaluated the electric field gradients (EFGs) at all of the three inequivalent positions (two Hf and one Fe) in its lattice. The main results extracted from the experimental investigations, concerning the different magnitudes and origins of the EFGs at inequivalent Hf sites, are correctly reproduced. The possible mechanisms of formation of the EFGs at these sites are analysed and discussed.

The electronic structure and electric field gradient (EFG) of the intermetallic compound are calculated on the basis of a recently developed first-principles band-structure full-potential (FP) linear-muffin-tin-orbital (LMTO) scheme within the atomic sphere approximation (ASA). The calculated value of the EFG at the Hf sites is in very good agreement with the results obtained by the time-differential perturbed angular correlation (TDPAC) technique used for the experimental study of the EFG at impurity sites in the compound . The asymmetry parameter and the individual contributions to the EFG are discussed. It is shown that various treatments of 4f hafnium electrons have a large influence on the EFG. The formulae for all of the components of the EFG tensor (the contributions from the valence electrons as well as the lattice contribution) are derived and presented also.

The electronic structure of the Hf2Ni intermetallic compound has been obtained using the first-principle full-potential (FP) linear-muffin- tin-orbital (LMTO) method in the atomicsphere approximation (ASA). The electric field gradient (EFG) tensor is calculated for both HI' and Ni sites and compared with the experiment. The theoretical results show that the EFG is determined dominantly by the p electrons. 1. Introd uction The interaction between the nuclear quadrupole moment and the electric field gradient (EFG) at the atomic site with noncubic point symmetry can be determined by various hyperfine interaction measurements, such as: perturbed angular correlation (PAC), perturbed angular distribution, and nuclear magnetic resonance (NMR). These methods were used widely to get information about surfaces, impurities, and vacancy related defects. A lot of experimental investigations of EFG in complex compounds with many atoms per primitive cell have appeared in recent years. Some of them have been devoted to the timedifferential perturbed angular correlation (TDPAC) measurements of nuclear quadrupole interactions in the ordered intermetalic compounds of transition metal Hf. It was shown that EFG reveals interesting properties in these compounds. These compounds represent a challenge for the theory due to their complexity. The explanation of the origin and the magnitude of EFG in one of them, e.g. Hf2Ni, is the subject of the present theoretical investigation.

The electronic structure for the cuprate ${\mathrm{CuGeO}}_{3}$ has been studied by the generalization of the local-density-approximation method for the systems with strong Coulomb correlations. The stable insulating antiferromagnetic solution with an energy gap of $\ensuremath{\sim}3.02\mathrm{eV}$ and a magnetic moment of $0.89{\ensuremath{\mu}}_{B}$ is obtained. According to our results the strong copper on-site Coulomb interaction of $U=9.66\mathrm{eV}$ is the most important quantity for the gap opening in this inorganic spin-Peierls compound.

This work presents the synthesis of F-doped LiFePO4/C composite by the specific modification of the recently suggested synthesis procedure based on an aqueous precipitation of precursor material in molten stearic acid, followed by a high temperature treatment. Besides the lattice parameters and the primitive cell volume reductions, compared to the undoped sample synthesized under the same conditions, the Rietveld refinement also shows that fluorine ions preferably occupy specific oxygen sites. Particularly, the best refinement is accomplished when fluorine ions occupy O(2) sites exclusively. By means of up-to-date electronic structure and total energy calculations this experimental finding is theoretically confirmed. Such fluorine doping also produces closing of the gap in the electronic structure and consequently better conductivity properties of the doped compound. In addition, the morphological and electrochemical performances of the synthesized powder are fully characterized. (C) 2013 Elsevier B.V. All rights reserved.

We report the novel colloidal syntheses in organic media of antimony sesquioxide (Sb2O3) spherical nanoparticles (30-250 nm) and octahedron micro- and nanocrystals (100 nm-4 mu m) depending on the synthetic method conditions. It is observed that small differences in the synthetic procedure cause large differences in the very changeable morphology. The structure of Sb2O3 powders was refined down to the R-factors of 9.57, 7.44, 9.19, 9.78, and 8.30 %. The refinement showed that Sb2O3 powder belongs to the cubic crystal type with space group Fd (3) over barm (No. 227). The values of estimated standard deviations, as well as reliability factors, confirmed that the structure of Sb2O3 was well refined. Ultraviolet and visible (UV-Vis) absorption spectroscopy and diffuse reflectance measurements (DRS) reveal that the optical band gap energies found for the Sb2O3 octahedrons and nanoparticles, micro- and nanocrystals, respectively, are quite independent of the synthetic method conditions and synthesized morphology and is found to be between 4.1-4.4 eV. No peaks in both photoluminescence (PL) emission and excitation spectra have been observed for a broad spectral range, typical for this material. In order to discriminate between conflicting experimental results concerning the band gap energy of this compound, we investigated theoretically the electronic structure and optical properties of one of the cubic sesquioxide Sb2O3 samples synthesized here. This has been done on the basis of density functional theory (DFT) with the generalized gradient approximation (GGA) and improved version of exchange potential suggested recently by Tran and Blaha (TB-mBJ). The main characteristic of the calculated TB-mBJ electronic structure is the significant improvement of the band gap value, which is in perfect agreement with our experimental measurements. The real and imaginary parts of the dielectric tensor are also calculated and interpreted in terms of the obtained electronic structure.

We show that the substitutional vacancy in graphene forms a dynamical Jahn-Teller center. The adiabatic potential surface resulting from the electron-lattice coupling was computed using density-functional methods and subsequently the Schr\"odinger equation was solved for the nuclear motion. Our calculations show a large tunnelling splitting $3 \Gamma$ of about 86 cm$^{-1}$. %, which is large as compared to the typical strain splitting. The effect results in a large delocalization of the carbon nuclear wave functions around the vacancy leading to a significant broadening of the Jahn-Teller active $sp^2\sigma$ electron states. The tunnelling splitting should be observable in electron paramagnetic resonance and two-photon resonance scattering experiments., Comment: 5 pages, 4 figures

We study the electronic structure of graphene with a single substitutional vacancy using a combination of the density-functional, tight-binding, and impurity Green's function approaches. Density functional studies are performed with the all-electron spin-polarized linear augmented plane wave (LAPW) method. The three $sp^2 \sigma$ dangling bonds adjacent to the vacancy introduce localized states (V$\sigma$) in the mid-gap region, which split due to the crystal field and a Jahn-Teller distortion, while the $p_z \pi$ states introduce a sharp resonance state (V$\pi$) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the $\sigma$ and the $\pi$ states, so that these bands are clearly identifiable in the calculated band structure. As for the magnetic moment of the vacancy, the Hund's-rule coupling aligns the spins of the four localized V$\sigma_1 \uparrow \downarrow$, V$\sigma_2 \uparrow $, and the V$\pi \uparrow$ electrons resulting in a S=1 state, with a magnetic moment of $2 \mu_B$, which is reduced by about $0.3 \mu_B$ due to the anti-ferromagnetic spin-polarization of the $\pi$ band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of $1.7 \mu_B$. Using the Lippmann-Schwinger equation, we reproduce the well-known $\sim 1/r$ decay of the localized V$\pi$ 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$\pi$ 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., Comment: 24 pages, 15 figures. Accepted for publication in New Journal of Physics

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.

By means of first-principles calculations using spin-polarized generalized gradient approximation method and ultrasoft pseudopotentials, the electronic and magnetic structure of the recently synthesized orthorhombic phase of bilayer manganite ${\text{Ca}}_{2.5}{\text{Sr}}_{0.5}{\text{GaMn}}_{2}{\text{O}}_{8}$ compound is obtained. Our calculations have shown that the specific antiferromagnetic ordering, in accordance with experimental findings, is the most stable one. The total energies calculated for the several possible magnetic states of the title compound in the orthorhombic phase enable us to estimate spin exchange interactions for nearest ${J}_{nn}=\ensuremath{-}1.60\text{ }\text{meV}$ and next\penalty1000\-\allowhyphens{} nearest neighbors ${J}_{nnn}=\ensuremath{-}0.21\text{ }\text{meV}$. Calculated Curie--Weiss temperature in the mean-field approximation is in excellent agreement with the measured magnetic phase transition temperature.

Transport measurements of the two-dimensional electron gas at the LaAlO3-SrTiO3 interface have found a density of carriers much lower than expected from the "polar catastrophe" arguments. From a detail density-functional study, we suggest how this discrepancy may be reconciled. We find that electrons occupy multiple subbands at the interface leading to a rich array of transport properties. Some electrons are confined to a single interfacial layer and susceptible to localization, while others with small masses and extended over several layers are expected to contribute to transport.

Using density-functional calculations, we study the electronic structure of the purple bronze ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_{6}{\mathrm{O}}_{17}$, which has been proposed to be a paradigm system for the Luttinger liquid behavior. Our results show that the quasi-one-dimensional (1D) electron bands crossing the Fermi energy originate from the Mo atoms on the double zigzag chains with predominant Mo $({d}_{xy})$ character and a Fermi surface that consists of two slightly warped planes, normal to the direction of the zigzag chains. The overall shape and dispersion of the bands as well as the calculated Fermi surface nesting vector are in excellent agreement with recent photoemission measurements. From constrained density-functional calculations of the Coulomb interactions and the calculated Fermi velocity, we estimate the values for the characteristic parameters of the Luttinger liquid, viz., the ratio of the spin-charge velocities to be ${v}_{\ensuremath{\rho}}∕{v}_{s}\ensuremath{\approx}1.8$ and the anomalous dimension characterizing the Fermi surface discontinuity to be $\ensuremath{\alpha}\ensuremath{\approx}0.6$. The general agreement of these values with experiments further strengthens the case for the lithium molybdenum purple bronze as a Luttinger liquid.

Atomic structure and magnetic properties of monatomic cobalt wires self-assembled on a stepped Pt surface are studied using density-functional theory. Nucleation sites and the early stages of wire growth are obtained by calculating the binding energies and diffusion barriers of Co atoms on $\mathrm{Pt}(332)$, and the adsorption geometries of Co dimers and trimers on the same surface. We show that experimentally observed monoatomic Co wires formed at the step edges are kinetically favored over another thermodynamically equally stable structure found in our calculations. The spin magnetic moment of monatomic wires supported by $\mathrm{Pt}(332)$ is $3.74{\ensuremath{\mu}}_{B}$ per Co atom, more than 50% larger than that of free-standing wires. This enhancement is explained in terms of the spin-dependent hybridization between $\mathrm{Pt}\text{\ensuremath{-}}5d$ and $\mathrm{Co}\text{\ensuremath{-}}3d$ states.

The origin of the insulating ground states and the charge disproportionation in the two Fe-based perovskite oxides, viz., $\mathrm{Ca}\mathrm{Fe}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{1∕3}{\mathrm{Sr}}_{2∕3}\mathrm{Fe}{\mathrm{O}}_{3}$, are examined from ab initio electronic structure calculations based on the density-functional methods. We find that correlation effects beyond the local density approximation as well as lattice distortions are necessary to describe the electronic properties of both compounds. It is the intricate interplay between lattice distortion and correlation effect that describes both the charge disproportionation and insulating ground state within the band theory, as is observed in these compounds.

Density-functional studies of the electron states in the dilute magnetic semiconductor GaN:Mn reveal major differences for the case of the Mn impurity at the substitutional site Mn_Ga versus the interstitial site Mn_I. The splitting of the two-fold and the three-fold degenerate Mn(d)states in the gap are reversed between the two cases, which is understood in terms of the symmetry-controlled hybridization with the neighboring atoms. In contrast to Mn_Ga, which acts as a deep acceptor, Mn_I acts as a donor, suggesting the formation of Coulomb-stabilized complexes such as (Mn_Ga Mn_I Mn_Ga), where the acceptor level of Mn_Ga is passivated by the Mn_I donor. Formation of such passivated clusters might be the reason for the observed low carrier-doping efficiency of Mn in GaN. Even though the Mn states are located well inside the gap,the wave functions are spread far away from the impurity center. This is caused by the hybridization with the nitrogen atoms, which acquire small magnetic moments aligned with the Mn moment. Implications of the differences in the electronic structure for the optical properties are discussed.

Using a density functional approach, we study the structural and magnetic properties of the pyrox-ene compound NaTiSi2O6. While all previous workers are taking that NaTiSi2O6 is a quasi-one-dimensional S=1/2 system, our theoretical results indicate that this is a Haldane S=1 chain compound below the phase transition at 210 K. A good agreement is obtained between the calculated and the measured Ti-Ti distances in the dimerized low temperature phase. We present a simple explanation of the flow of the unusual phase transition which is taking place in this compound.

Using a density functional approach, we study the structural and magnetic properties of the pyrox-ene compound NaTiSi2O6. While all previous workers are taking that NaTiSi2O6 is a quasi-one-dimensional S=1/2 system, our theoretical results indicate that this is a Haldane S=1 chain compound below the phase transition at 210 K. A good agreement is obtained between the calculated and the measured Ti-Ti distances in the dimerized low temperature phase. We present a simple explanation of the flow of the unusual phase transition which is taking place in this compound.

The electronic structure of the Ruddlesden-Popper layered perovskite compound Sr 3 Mn 2 O 7 is studied from density-functional calculations using the linear muffin-tin orbitals method. An antiferromagnetic, insulating solution is obtained in agreement with the experiments, with a magnetic moment of about 2.52μ B for each Mn atom. The magnetic interactions between the Mn atoms, both within the bilayer and between the bilayers, are shown to arise from superexchange. The intrabilayer interaction involves the three-site Mn-O-Mn superexchange much like the case of the well-known CaMnO 3 , while the interbilayer exchange, mediated via the longer Mn-O-O-Mn superexchange path, is considerably weaker. Consistent with the layered nature of the compound, we find a strong out-of-plane to in-plane band-mass anisotropy for Sr 3 Mn 2 O 7 (m * z /m * x , y ∼10.9 for electrons and ∼4.2 for holes), while for the related compound LaSr 2 Mn 2 O 7 , which is a ferromagnetic metal, we obtain a strong anisotropy in the resistivity ρ c /ρ a b ∼40 using kinetic transport theory, in qualitative agreement with the experimental value of ∼100.

The microscopic origin of the charge and orbital order in the half-doped manganites is examined from ab initio density-functional calculations and exact diagonalization studies. It is shown that the dominant mechanism responsible for the charge order is the Jahn-Teller coupling, with a lesser but significant contribution from the on-site Coulomb interaction. The band structure shows a sizable interchain coupling between the zigzag chains, leading to a considerable band dispersion normal to the chains, in sharp contrast with the zigzag chain physics.

The paramagnetic, spin-polarized, and LDA+U rotationally invariant, fully self-consistent linear-muffin-tin orbital band structure for the inorganic spin-gapped vanadate alpha-NaV2O5 has been calculated for both centrosymmetric group Pmmn and noncentrosymmetric group P2(1)mn (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 Pmmn bands, the hopping terms relevant for the calculation of exchange parameters of a spatially anisotropic Heisenberg model are estimated. A new frustrating second neighbour coupling, as well as a pronounced diagonal intraladder hopping term, are found.

The cooperative Jahn-Teller coupling between the Mn ${e}_{g}$ electrons and the oxygen octahedral distortions in ${\mathrm{LaMnO}}_{3}$ is studied using ab initio density-functional calculations and tight-binding models. The linear and quadratic vibronic coupling parameters are calculated using density-functional methods. It is shown that the cooperative Jahn-Teller coupling, primarily due to the interoctahedral electron hopping (band structure term), leads to the ordering of the octahedral distortion and simultaneously to orbital ordering. The coupling results in a two-minima adiabatic potential surface in the solid, instead of the three-minima ``Mexican-hat'' surface for the isolated octahedron.

In this paper we suggest a new scheme for obtaining the electronic contribution to the electric field gradient (EFG) at the nucleus. The EFG for all hcp metals from Be to Cd is obtained from band structure calculations using the recently developed scheme of full-potential (FP) linear-muffin-tin-orbital (LMTO) formalism in the atomic-sphere approximation (ASA). A comparison with the most accurate full-potential (FP) linear augmented plane wave (LAPW) calculations and experimental values shows very good agreement. According to the investigations presented in this paper, the FP-LMTO-ASA method is probably the best choice presently for the calculation of the EFG in more complicated compounds.

The spin-restricted, spin-polarized, and LDA-I-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 mu(B). 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. (C) 1998 Elsevier Science Ltd. All rights reserved.

In this paper we suggest a new scheme for obtaining electronic contribution to the electric field gradient (EFG) at the nucleus. The EFG for ali HCP metals from Be to Cd is obtained from band structure calculations using the recently developed scheme of full-potential (FP) linear-muffin-tin-orbital (LMTO) formalism in the atomic-sphere approximation (ASA). This method is computationally very effective. A comparison with the theoretically most accurate full-potential linear augmented plane wave calculations and experimental values shows very good agreement. According to the investigations presented in this paper. FP-LMTO-ASA method is probably the best choice presently for the calculation of the EFG in more complicated compounds. (C) 1998 Elsevier Science B.V. All rights reserved.

The stability of the charge-stacked structure vis-a-vis the charge-alternate structure in the half-doped manganites is studied with a model that includes electronic kinetic energy, onsite and intersite Coulomb interactions, the Jahn-Teller energy, and the antiferromagnetic superexchange between the manganese core spins. It is shown that for a single zigzag chain, the electronic kinetic energy stabilizes the standard chain, with Mn3+ at the bridge site and Mn4+ at the corner site, over the reversed zigzag chain with the two Mn valences interchanged. The electronic kinetic energy and magnetic interactions stabilize the three-dimensional charge-stacked structure, while a large intersite Coulomb interaction Vgreater than or equal toV(c) would stabilize the charge-alternate structure. It is argued that the magnitude of V is small enough that the charge-stacked structure is stabilized in the half-doped manganites such as La1/2Ca1/2MnO3. (C) 2002 American Institute of Physics. 46th Annual Conference on Magnetism and Magnetic Materials, Nov 12-16, 2001, Seattle, Washington

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)e(g) bands producing an energy gap within the LDA, with the bands derived from the a (z(2)-1) orbital, pointed along the long basal-plane Mn-O bond, occupied and the (x(2)-y(2)) bands empty. The on-site Coulomb repulsion and the intra-site exchange terms are found to be, respectively, U similar or equal to 8-10 eV and J similar or equal to 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. (C) 1996 American Institute of Physics. 40th Annual Conference on Magnetism and Magnetic Materials, Nov 06-09, 1995, Philadelphia, Pa

We report on the magnetic properties of HoCo dimers as a model system for the smallest intermetallic compound of a lanthanide and a transition metal atom. The dimers are adsorbed on ultrathin MgO(100) films grown on Ag(100). New for 4f elements, we detect inelastic excitations with scanning tunneling spectroscopy and prove their magnetic origin by applying an external magnetic field. In combination with density functional theory and spin Hamiltonian analysis, we determine the magnetic level distribution, as well as sign and magnitude of the exchange interaction between the two atoms. In contrast to typical 4f-3d bulk compounds, we find ferromagnetic coupling in the dimer. © 2018 American Physical Society.