Full-potential LMTO Research Articles

Magnetic-field-induced effects in the electronic structure of itinerant d- and f-metal systems

A paramagnetic response of transition metals and itinerant d- and f-metal compounds in an external magnetic field is studied by employing ab initio full-potential LMTO method in the framework of the local spin density approximation. Within this method the anisotropy of the magnetic susceptibility in hexagonal close-packed transition metals is evaluated for the first time. This anisotropy is owing to the orbital Van Vleck-like paramagnetic susceptibility, which is revealed to be substantial in transition-metal systems due to hybridization effects in the electronic structure. It is demonstrated that compounds TiCo, Ni3Al, YCo2, CeCo2, YNi5, LaNi5, and CeNi5 are strong paramagnets close to the quantum critical point. For these systems the Stoner approximation underestimates the spin susceptibility, whereas the calculated field-induced spin moments provide a good description of the large paramagnetic susceptibilities and magnetovolume effects. It is revealed that an itinerant description of hybridized f electrons produces magnetic properties of the compounds CeCo2, CeNi5, UAl3, UGa3, USi3, and UGe3 in close agreement with experiment. In the uranium compounds UX3 the strong spin-orbit coupling together with hybridization effects give rise to peculiar magnetic states in which the field-induced spin moments are antiparallel to the external field, and the magnetic response is dominated by the orbital contribution.

A study of transition metal K-edge x-ray absorption spectra ofLaBO3 (B = Mn, Fe,Co, Ni), La2CuO4, and SrMnO3 using partial density of states

Transition metal K-edge x-ray absorption near edge structure (XANES) studies have been carried outon LaBO3 (B = Mn, Fe,Co, Ni), La2CuO4, and SrMnO3 compounds. The theoretical spectra have been calculated using the transition metal (TM)4p density of states (DOS) obtained from full-potential LMTO density functional theory.The exchange–correlation functional used in this calculation is taken under the localdensity approximation (LDA). The comparison of experimental spectra with the calculatedones indicates that single-particle transitions under the LDA are sufficient to generateall the observed XANES, including that which has earlier been attributed tomany-body shake-up transitions and core–hole potentials. The present studyreveals that all the experimentally observed features are mainly due to distributionin TM 4p DOS influenced by hybridization with other orbitals. Specifically, forLaMnO3, the feature earlier attributed to the shake-up process is seen toarise from hybridization of Mn 4p with La 6p and O 2p orbitals; inLa2CuO4 the features attributed to the core hole potential correspond to hybridization ofCu 4p with La 6p, La 5d, and O 2p orbitals. To see the effect of inhomogeneouselectronic charge distribution and on-site Coulomb and exchange interaction(U′) on the XANES of these compounds the generalized-gradient approximation andU′ corrections are incorporated in the calculations. These corrections do not generate any newfeatures in the spectra but affect the detailed intensity and positions of some of thefeatures.

Calculated electronic structure and measured X-ray photoemission spectrum of UAuSb 2

The electronic band structure for the tetragonal ferromagnetic compound UAuSb 2 was calculated by the full potential LMTO method. The XPS was measured on single crystals freshly cleaved under high vacuum. The calculated spectrum for the valence band compares well with the measured one. The calculated magnetic moment is significantly smaller than estimated from measurements of magnetization. The cylindrical shape of the Fermi surface is an indication of quasi-two dimensionality of the properties of UAuSb 2 related to its crystal structure having the unit cell elongated along the tetrahedral direction.

Electronic structure and magnetic properties of the α and γ phases of iron, their solutions with carbon, and cementite

The full-potential LMTO method was used to comparatively analyze the band structure, magnetic and cohesive characteristics, energies of formation of α and γ iron, their carbon solid solutions, and cementite. It has been found that the solid solution γ-Fe + C, similarly to γ-Fe, is characterized by an antiinvar transition and that cementite demonstrates invar behavior. The similarity of curves of the density of states, cohesive energies, and magnetic moments for the supersaturated solid solution of carbon in the fcc iron and cementite suggests the preferable formation of cementite in regions of concentration inhomogeneity of carbon distribution in the solid solution of γ iron.

Electron–phonon superconductivity in hole-doped diamond: A first-principles study

Abstract In this work we propose that the observed superconductivity in hole-doped diamond is due to an electron–phonon mechanism and substantiate the idea by first-principles full potential LMTO linear-response calculations. Our calculations, based on the virtual crystal approximation (VCA), show that the holes at the top of the zone-centered, degenerate σ bonding valence band couple strongly to the optical bond-stretching modes, in a very similar way as found in MgB 2 . We discuss the main similarities and differences between the two systems, as well as the doping dependence of T c in diamond and the possibility of observing superconductivity in Si and Ge.

Pressure Effect on Magnetic Properties of UX 3 (X=Al, Ga, In, Si, Ge) Compounds

Magnetic susceptibility χ of UX3 compounds (X=Al, Ga, In, Si, Ge) is studied under pressure up to 2 kbar at some fixed temperatures in the range 78-300 K by a modified Faraday method. A pronounced pressure effect on χ has been revealed, and the magnetovolume derivatives, dlnχ/dlnV, are found to be temperature independent, except for UAl3 and UIn3. A pressure effect on the Neel temperature TN was also detected in antiferromagnetic UGa3 and UIn3. The electronic structures of the UX3 compounds were calculated by employing a relativistic full-potential LMTO method. The evaluated χ and magnetic moments in UX3 and their volume derivatives are in agreement with experimental data.

All-electron self-consistent GW approximation: application to Si, MnO, and NiO.

We present a new kind of self-consistent GW approximation based on the all-electron, full-potential linear muffin-tin orbital method. By iterating the eigenfunctions of the GW Hamiltonian, self-consistency in both the charge density and the quasiparticle spectrum is achieved. We explain why this form of self-consistency should be preferred to the conventional one. Some results for Si (a representative semiconductor) are presented. Finally we consider many details in the electronic structure of the antiferromagnetic insulators MnO and NiO. Excellent agreement with experiment is shown for many properties, suggesting that a Landau quasiparticle (energy band) picture provides a reasonable description of electronic structure even in these correlated materials.

Ab-initio Based Diffusive Studies of Plutonium with Relevance to Nuclear Waste Management

AbstractAn approach to study the diffusion of plutonium (Pu) waste into the material of the storage containers is presented. The delta phase of Pu has been examined, in this article, using a spin and orbitally polarized ab-initio (full potential LMTO) approach [1, 2]. These computations are relatively inexpensive, and it is the intention to see if the results obtained would be satisfactory (compared to the accurate SIC-LDA computations [1-2]) for diffusive studies of Pu into the material of storage containers (for example, into FCC iron as a surrogate for steel [2]). The combination of LDA and spin and orbitally polarized LMTO calculations that we use successfully predict a delocalized 5f, (i.e. bonding) monoclinic ground state and a localized FCC excited state with a density about 8% greater than experiment (compared to the 30% error from pure LDA calculations by various groups [3-6]). The density of states is computed and compared to experimental photoemission studies; there is qualitative agreement. The spin, orbital and total magnetic moment for the FCC phase is computed; the results are qualitatively in agreement with those of other studies. An analytical method for studying the time-dependent diffusion of Pu across an interface with another atomic species is presented.

Lifetimes ofdholes in Cu and Au: Full-potential LMTO approach

We report on theoretical studies of hole lifetimes in Cu and Au by means of the ab initio all electron full potential linear method of ``muffin-tin'' orbitals approach. We find that the momentum-averaged d-hole lifetimes in Cu, Au deviate much less from the lifetimes of holes in free-electron-like $s,p$ bands than it was found in previous ab initio studies. However, at symmetry points L and X and along the $X\ensuremath{-}W$ direction of the Brillouin zone the differences between the lifetimes of d holes and $s,p$ holes are big. The lifetime of a hole in the top ${X}_{5}$ d state of Cu is in good agreement with the experimental data. The lifetimes for lower d states deviate from experimental results, which we refer to neglecting spin-orbit coupling and interactions described by higher order terms of many-body perturbation theory. We discuss the calculational details responsible for the precision of the lifetime evaluations.

Dielectric functions and quasi-particle lifetimes in Ag: full-potential LMTO and LAPW GW approaches

We report theoretical studies of dielectric functions and quasi-particle lifetimes in Ag by means of ab initio full-potential linear ‘muffin-tin’-orbitals LMTO GW and LAPW approaches. We show that the methods provide better description of the dielectric functions then the previous studies based on local density approximation of the density functional theory. In particular, the calculated electron energy loss function contains plasmon absorption peak which was absent in earlier calculations. The computed lifetimes of holes in the highest d-states are in good agreement with the experimental data. At lower energy large discrepancy between calculated and experimental data is observed which we refer to neglecting the vertex corrections.

Electronic structure of Ag 2S, band calculation and photoelectron spectroscopy

The electronic structure of the silver chalcogenide compound Ag 2S has been investigated, experimentally using photoelectron spectroscopy, and theoretically using the full-potential LMTO calculation. The photoemission data taken using a synchrotron photon source is compared with the calculated valence density of states (DOS). The band structure is also calculated for the high-temperature cubic disorder phase. For that sake, a hypothetical ordered structure model is utilized instead of the disorder structure. From the hitherto reported crystallographic data and the calculated total energies, microscopic mechanisms of the phase transition and superionic conduction are discussed.

Effect of metal vacancies on the band structure of Nb, Zr, and Y diborides

Energy band structures of metal-deficient hexagonal diborides M$_{0.75}$B$_2$ (M = Nb, Zr and Y) were calculated using the full-potential LMTO method. The metal vacancies change the density of states near the Fermi level and this effect is quite different for III-V group transition metal diborides. Contradictory data on superconductivity in diborides may be supposed to be connected with nonstoichiometry of samples. Vacancy formation energies are estimated and analyzed.

Band structure of ZrB2, VB2, NbB2, and TaB2 hexagonal diborides: Comparison with superconducting MgB2

The band structure and Fermi surface parameters of ZrB2, VB2, NbB2, and TaB2 hexagonal diborides are considered in the framework of the self-consistent full-potential LMTO method in comparison with the relevant parameters of a MgB2 isostructural superconductor. The factors responsible for the superconducting properties of AlB2-like diborides are analyzed, and the results obtained are compared with previous calculations and available experimental data.

Electronic structure and magnetic properties of lithium manganese spinels

Electronic and magnetic structures of the spinel-type lithium–manganese oxides Li x Mn 2O 4, x=0,0.5,1, are studied ab initio by employing a full-potential LMTO method. The effect of the orthorhombic distortion on electronic structure and magnetism of LiMn 2O 4 was investigated, and our calculations do not show a substantial charge ordering at the structural transition from the cubic spinel to the orthorhombic structure.

Electronic properties of new Ca(AlxSi1−x )2 and Sr(GaxSi1−x )2 superconductors in crystalline and nanotubular states

The band structure of new layered (AlB2 type) Ca(AlxSi1−x)2 and Sr(GaxSi1−x)2 superconductors is studied by the first-principle full-potential LMTO method. It has been shown that the superconducting properties of ternary silicides are due to the high density of (Ca,Sr)d states at the Fermi level, whereas the growth of TC in going from Sr(GaxSi1−x)2 to Ca(AlxSi1−x)2 is associated with the increase in phonon frequencies due to the decrease in atomic masses. Simulations are performed for the electronic properties of hypothetical (11,11) and (20,0) CaAlSi and SrGaSi nanotubes. In going from the crystalline to the nanotubular state, the silicide systems retain metal-like properties. The template and “film rolling” techniques can be used for obtaining silicide nanotubes.

Full-potential LMTO study on the electronic structure of heavy-fermion compound LiV2O4

The electronic structure of heavy fermion compound LiV2O4 has been calculated using a self-consistent full-potential LMTO method. The results show that the conduction bands in this compound are formed from V 3 d states with a bandwidth of 2.5 eV. The symmetric characteristics of conduction bands are of t2g in principle. The energy gap between conduction bands and fully occupied oxygen 2p bands is 1.9 eV. The band dispersions near Fermi energy display complicated structures. Furthermore, the N( EF) and ycal are 11.1 (states/eV/f.u.) and 26.7 mJ/mol.K2 determined numerically by LDA calculation, respectively. It is insufficient to clarify the origin of local moment in LiV2O4 from plain LDA calculations. In addition to the above LDA calculation, we also found a LSDA solution of LiV2O4 that is lower in total energy than that of LDA calculation. Similarly, LSDA + GGA calculation yields almost the identical result as that in LSDA. We conclude that the mechanism responsible for heavy fermion properties in LiV2O4 might be somewhat different from the plain Kondo mechanism in conventional 4f and 5f heavy fermion compounds and perhaps the quantum transition might play an adequate role in heavy-ferrnion behaviors in LiV2O4.

Effect of pressure on the magnetic susceptibility of CeCo 2

The magnetic susceptibility, χ, of CeCo 2 compound has been measured under pressure up to 2 kbar at T=78 and 300 K , and a pronounced pressure effect, d ln χ/ dP≃−5 Mbar −1 , is revealed. In order to analyze the experimental magnetovolume effect, d ln χ/ d ln V , the volume-dependent electronic structure of CeCo 2 has been calculated ab initio by employing a relativistic full-potential LMTO method. The calculations have brought out strong hybridization of the itinerant 4f (Ce) and 3d (Co) states, which leads to a filling of the peculiar bonding band. It is demonstrated that the magnetic properties of CeCo 2 are consistent with itinerant behavior of the 4f electrons of cerium. The large volume derivative of the effective electron–electron interaction parameter, d ln I/ d ln V≃−1.7 , implies a predominant role of the short-range correlations, in accordance with the Hubbard model.

All-electron GW approximation with the mixed basis expansion based on the full-potential LMTO method

We present a new all-electron, augmented-wave implementation of the GW approximation using eigenfunctions generated by a recent variant of the full-potential LMTO method. The dynamically screened Coulomb interaction W is expanded in a mixed basis set which consists of two contributions, local atom-centered functions confined to muffin-tin spheres, and plane waves with the overlap to the local functions projected out. The former can include any of the core states; thus the core and valence states can be treated on an equal footing. Systematic studies of semiconductors and insulators show that the GW fundamental bandgaps consistently fall low in comparison to experiment. Also the quasi-particle (QP) levels differ significantly from other, approximate methods, in particular those that approximate the core with a pseudopotential, or those that include valence states only.

Calculation of thermal expansion of iron-aluminides with transition metal additives

ABSTRACTThe addition of transition metal elements can significantly modify physical properties of intermetalic compounds. We studied the influence of Molybdenum and Vanadium additives on thermal expansion coefficient (CTE) of Fe3Al and FeAl over the wide range of temperatures. The site preference of both transition metals was determined by full-potential LMTO method within the grandcanonical formalism. At low temperatures CTEs were found directly from the FP-LMTO calculations by incorporating them into the Debye model of a solid. The obtained thermal expansion for pure Fe3Al and FeAl is within 10% of its experimentally measured values. At high temperatures we performed Molecular Dynamics (MD) simulations based on our many-body atomistic potentials. The parameters were fitted to reproduce the total energy of a crystal under various types of deformations obtained by FP-LMTO method and were tested with respect to different structures and vacancy formation energies. Our calculations show that addition of V decreases the CTEs of both iron-aluminides, while the addition of Mo makes Fe3Al DO3 structure unstable.

First-Principles Calculation of Lattice Dynamics of bcc and hcp Iron Under High Pressures

Lattice dynamics and electron-phonon coupling are calculated for non-magnetic hcp iron at 100 GPa by a first-principles linear response full-potential LMTO method. Superconducting transition temperature is estimated to be less than 0.5 K. For ferromagnetic bcc iron lattice dynamical calculations are performed for 9.8 GPa as well as for ambient pressure. The results of calculation reproduce well the pressure dependence of phonon dispersion curves observed by neutron scattering measurements.