Augmented Spherical Wave Research Articles

Ferromagnetism and temperature-dependent electronic structure of HCP gadolinium

We use a combination of a many-body model analysis with an ab initio band structure calculation to derive the temperature dependent electronic quasiparticle structure of the rare-earth metal Gadolinium. As a local-moment system Gd is properly represented by the ferromagnetic (multiband) Kondo-lattice model (s-f (d-f) model). The single-particle part of the model-Hamiltonian is taken from an augmented spherical wave (ASW) band calculation. The proposed method avoids the double counting of relevant interactions by exploiting an exact limiting case of the model and takes into account the correct symmetry of atomic orbitals. The a priori only weakly correlated 5d conduction bands get via interband exchange coupling to the localized 4f levels a distinct temperature dependence which explains by a Rudermann-Kittel-Kasuya-Yosida (RKKY) -type mechanism the ferromagnetism of Gd. We get a self-consistently derived Curie temperature of 294.1 K and a T=0-moment of 7.71 $\mu_{\rm B}$, surprisingly close to the experimental values. The striking induced temperature-dependence of the 5d conduction bands explains respective photoemission data. The only parameter of the theory (interband exchange coupling J) is uniquely fixed by the band calculation.

Chemical Pressure and Hydrogen Insertion Effects in CeNiIn.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Extended moment formation and magnetic ordering in the trigonal chain compound Ca 3Co 2O 6

The results of electronic structure calculations for the one-dimensional magnetic chain compound Ca 3Co 2O 6 are presented. The calculations are based on density functional theory and the local density approximation and used the augmented spherical wave (ASW) method. Our results allow for deeper understanding of recent experimental findings. In particular, alternation of Co 3d low- and high-spin states along the characteristic chains is related to differences in the oxygen coordination at the inequivalent cobalt sites. Strong hybridization of the d states with the O 2p states lays ground for polarization of the latter and the formation of extended localized magnetic moments centered at the high-spin sites. In contrast, strong metal–metal overlap along the chains gives rise to intrachain ferromagnetic exchange coupling of the extended moments via the d 3z 2–r 2 orbitals of the low-spin cobalt atoms.

Chemical pressure and hydrogen insertion effects in CeNiIn

Electronic and magnetic properties of the intermediate valence indide CeNiIn and of its ferromagnetic hydride are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method treating cerium 4f band as valence states. The influence of the insertion of hydrogen is addressed from volume expansion whereby the major effects on the changes of the chemical bonding and on the onset of magnetization on cerium are inferred. Hydrogen insertion is illustrated by the plot of the electron localisation function ELF. Magnetic calculations for the expanded and hydrogen inserted systems point to a finite local moment carried by 4f (Ce) states.

Calculated electronic properties of the mixed perovskite oxides: CaCu 3T 4O 12 (T=Ti, Cr, Mn, Ru) within the DFT

Electronic and magnetic properties of the mixed perovskites CaCu 3T 4O 12 (T=Ti, Cr, Mn, Ru) are calculated ab initio starting from the experimental crystal data. Using the self-consistent band-structure augmented spherical wave (ASW) method built within the local spin-density approximation (LSDA) to the density functional theory (DFT), calculations of the density-of-states (DOS) and of the covalent-energy ( E cov translating the chemical bonding) are performed. From the analyses of the results of the chemical bonding and the DOS at the top of the valence band (VB), an interpretation of the experimental physical properties evidenced experimentally particularly the large dielectric constant for the Ti member and valence degeneracy between Cu and Ru is proposed.

Structure and electronic properties of new model dinitride systems: a density-functional study of CN 2, SiN 2, and GeN 2

The dinitrides CN 2, SiN 2, and GeN 2 in assumed pyrite-type structures are studied by means of density functional theory using both ultrasoft pseudopotentials and the augmented spherical wave (ASW) method. The former two materials constitute the large- x limit of the broader class of CN x and SiN x compounds, which are well known for their interesting mechanical and electronic properties. For CN 2 a large bulk modulus B 0 of 405 GPa was determined. While SiN 2 is found to be a wide band gap compound, the calculated gaps of CN 2 and GeN 2 are considerably smaller. The trends in structural and electronic properties, as e.g., bond lengths, band gaps and covalency are well understood in terms of the interplay of different types of bonding.

Octahedral tilting in ACu 3Ru 4O 12 (A=Na, Ca, Sr, La, Nd)

The perovskite-like compounds ACu 3Ru 4O 12 (A=Na, Ca, Sr, La, Nd) are studied by means of density functional theory based electronic structure calculations using the augmented spherical wave (ASW) method. The electronic properties are strongly influenced by covalent-type bonding between transition metal d and oxygen p states. The characteristic tilting of the RuO 6 octahedra arises mainly from the Cu–O bonding, allowing for optimal bond lengths between these two atoms. Our results provide a deeper understanding of octahedral tilting as a universal mechanism, applicable to a large variety of multinary compounds.

Electronic band structure calculations on the antiferromagnetic ternary compounds Ce3Ni2X7 (X = Ge and Sn) and CeNiGe3

The electronic structure of the cerium intermetallic systems Ce3Ni2X7 (X = Ge and Sn) and CeNiGe3, which are structurally related and antiferromagnetically ordered, are examined both experimentally using X-ray absorption spectroscopy at the Ce LIII-edge as well as theoretically by local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation of the 4f(Ce) states on the chemical bonding and magnetic behaviour of these ternary compounds is discussed with reference to the densities of states (DOS) projected on to the atomic sites. From this we propose a mechanism to explain the respective behaviours of the cerium atoms at the different crystallographic sites. The magnetic properties are discussed from a spin-only viewpoint; spin–orbit coupling effects are inferred from previous calculations.

Ab initio study of the Fe intra- and inter-layer magnetic order in Fe/Ir(001) superlattices

An ab initio study of the magnetic behaviour of strained Fe/Ir(001) superlattices is presented using the first principle Augmented Spherical Wave method. First, the intralayer magnetic order of the Fe layers is examined as a function of their tetragonal distortion induced by the superlattice structure. It is shown that (i) an antiferromagnetic order can not occur, (ii) a ferrimagnetic phase, characterized by interfacial Fe magnetic moments opposite to the others, is allowed in a large domain of distortions, (iii) first order transitions toward the ferromagnetic (non magnetic) phase occur when the tetragonal distortion is (respectively) reduced at constant atomic volume (in plane lattice parameter) and (iv) when the Fe thickness increases, the ferromagnetic/ferrimagnetic transition is shifted to higher tetragonal distortions. Second, the interlayer magnetic couplings are determined in Fe5Irn(001) superlattices with the Fe layer in the relaxed body centered cubic structure with n = 1 to 11 monolayers. It is shown that the coupling energy decreases surprisingly slowly (like 1/n) with increasing n with a period of approximately 3.2 monolayers, which corresponds nicely to the period of the Ir induced polarisation. Finally, these results are discussed in relation with experimental studies.

Effets chimiques et magnétovolumiques compétitifs dans les nitrures d’insertion

An analysis of the chemical and magnetovolume effects due to nitrogen insertion is carried out in order to establish trends for the Fe–N system. Computations are based on the density functional theory and use is made of the augmented spherical wave method. Taking the Fe 4N nitride as a representative member, the chemical bonding study shows that nitrogen selectively contracts covalent bond with one of the two iron sublattices, with the consequence of lowering the magnetic moment. The magnetovolume effects are examined for a comparative study of the volume-dependent magnetic moments within Fe 4N as with respect to γ-Fe, in its two magnetic states ‘high-moment–large volume’ and ‘low moment–low volume’. The insertion of nitrogen ‘prepares’ the volume in such a manner in the nitride that the two states are simultaneously present at the two Fe crystal sites. The trends for an increase of the Fe atomic volume within the series Fe 8N, Fe 4N, Fe 3N, Fe 2N should favour the enhancement of the magnetisation; this is, however, hindered by the Fe–N chemical bond, involving an increasing number of Fe sites leading to a lowering of the moments.

Ab Initio Calculations of Magneto-optical Effects

The full-potential linearized augmented-planewaves (FLAPW) method is often regarded as a benchmark-setting approach among the electronic structure methods. Here we present ab initio calculations of polar magneto-optical Kerr effect spectra using the FLAPW method. A comparison to a different electronic structure method, the augmented spherical waves (ASW) scheme, demonstrates the independence of the results on a particular numerical procedure. The materials studied here include transition metals and their alloys, viz. Fe, Ni, Co, CoPt, and FeAu, the uranium compounds US, URhAl, and UCu2P2, and the transition-metal oxide CrO2.

Electronic structure ofCeNi2Ge2investigated by angle-resolved photoemission and density-functional calculations

We present a detailed investigation of the electronic band structure of the heavy Fermion compound ${\mathrm{CeNi}}_{2}{\mathrm{Ge}}_{2}$ by means of angle resolved photoemission spectroscopy (ARPES). Measured with high angular and energy resolution, the experimental data from the (001) surface display several narrow bands close to the Fermi energy. The nature of these bands was identified by a comparison with augmented spherical wave (ASW) band structure calculations based on density functional theory (DFT) in the local density approximation (LDA). In contrast to the results of previous photoemission measurements on in situ prepared films of ${\mathrm{CeNi}}_{2}{\mathrm{Ge}}_{2}$ we demonstrate that the investigated bands show strong dispersion, in agreement with the theoretical results. In addition we show that the orbital character of the states close to the Fermi level is mainly Ni $3d$ and that an ``unusual giant Kondo resonance'' does not exist in the photoemission spectra.

Electronic band structure of single-crystal and single-layerWS2:Influence of interlayer van der Waals interactions

The valence band structure of the layered transition metal dichalcogenide ${\mathrm{WS}}_{2}$ has been determined experimentally by angle resolved photoelectron spectroscopy and theoretically by augmented spherical wave band structure calculations as based on density functional theory. Good agreement between experimental and calculated band structure is observed for single crystal ${\mathrm{WS}}_{2}.$ An experimental band structure of a single layer was determined from an electronically decoupled film prepared on a single crystalline graphite substrate by metal-organic van der Waals epitaxy. The polarization dependent photoemission selection rules of the single layer film are appropriate for a free standing film. The experimental single layer band structure shows some differences compared to band structure calculations using bulk atomic positions within the layer. We conclude that relaxation of the single layer occurs as a consequence of the missing interlayer interactions leading to close agreement between electronic structure of the single layer and single crystal. As a consequence of the missing interlayer interactions the valence band maximum for the single layer is located at the K point of the Brillouin zone.

Pressure Induced Phase Transition in TiB

In situ high pressure x-ray diffraction and electrical resistanceexperiments on TiB have been carried out by using a diamond anvil cell device.The results revealed that the sample undergoes a first-order phasetransition at pressures of 3.5-5.0 GPa and 4.0-5.5 GPa for the x-raydiffraction and electrical resistance experiments, respectively. Theparameters of the state equation are calculated before and after thephase transition and compared with the values calculated by Mohn et al. [J.Phys. C: Solid State Phys. 21 (1988) 2829] using the augmented spherical wave method.

Investigation of the bonding and magnetic properties of U3Cu4Ge4 and structurally related systems

Electronic and magnetic properties of the ferromagnets UCu2Ge2 (122), UGe2 (122), and of the newly found U3Cu4Ge4 (344) intermetallic system are self-consistently calculated within the local spin density functional theory using the augmented spherical wave (ASW) method. The influence of hybridisation on the chemical bonding and magnetic behaviour is discussed from the densities of states (DOS) as well as from the crystal orbital overlap population (COOP) leading to suggest comparative influences between electronic effects of the hybridisation as with respect to the structural changes when the constituents of (122) and (12) are built within U3Cu4Ge4. From the spin-only magnetic results the important contribution of the orbital effects is analysed.

Electronic band structure of the layered compoundTd−WTe2

We have studied the electronic structure of the layered compound $\mathrm{Td}\ensuremath{-}{\mathrm{WTe}}_{2}$ experimentally using high-resolution angle-resolved photoelectron spectroscopy, and theoretically using density-functional based augmented spherical wave calculations. Comparison of the measured and calculated data shows in general good agreement. The theoretical results reveal the semimetallic as well as metallic character of $\mathrm{Td}\ensuremath{-}{\mathrm{WTe}}_{2};$ the semimetallic character is due to a 0.5 eV overlap of Te $5p$- and W 5d-like bands along $\ensuremath{\Gamma}\ensuremath{-}Y,$ while the metallic character is due to two classical metallic bands. The rather low conductivity of $\mathrm{Td}\ensuremath{-}{\mathrm{WTe}}_{2}$ is interpreted as resulting from a low density of states at the Fermi level.

Resonant inelastic scattering at intermediate X-ray energies

Abstract We describe resonant inelastic X-ray scattering (RIXS) experiments and magnetic circular dichroism (MCD) in X-ray fluorescence performed in the 3–5 keV range. The examples chosen are X-ray fluorescence MCD of FeRh and RIXS experiments performed at the L 3 edge of Ce. FeRh is antiferromagnetic at room temperature but has a transition to the ferromagnetic state above 400 K. The Rh MCD signal is confronted to an augmented spherical wave calculation. The experiment confirms the predicted spin polarization of the Rh 4 d valence states. The RIXS measurements on Ce compounds and intermetallics address the problem of mixed valency especially in systems where degeneracy with the Fermi level remains small. Examples are taken from the 2 p →(4 f 5 d ) +1 followed by 3 d →2 p RIXS for a highly ionic compound CeF 3 and for almost γ-like CeCuSi.

Noncollinear magnetic order in U 3Bi 4

Results are presented of calculations of the magnetic properties of U 3Bi 4 in the framework of the local density functional approximation (LDA). The calculations are carried out with a modified version of the augmented spherical wave (ASW) method that takes into account self-consistently the noncollinearity of the magnetization inside the atomic spheres and uses the full shape, instead of the spherically symmetric, intra-atomic potential. The new technique is discussed. The calculations for U 3Bi 4 are put into the larger context of the series of compounds U 3X 4 (X=P, As, Sb, Bi) which allows us to exhibit important trends in their magnetic properties. Thus we discuss in particular: the variation of the magnitude of the magnetic moment, the change of the direction of the magnetization axis and the regularities of the magnetocrystalline anisotropy in the series. We argue that U 3Bi 4, just as the other compounds in the series, belongs to the class of systems for which the noncollinearity of the magnetic structure is a necessary consequence of the symmetry of the system. The latter is analyzed and related to the results of the first-principles calculations of the complex magnetic structure in U 3Bi 4. We compare our results with other calculations and with available experimental data.

Spin spiral ground state ofγ-iron

Using density-functional theory we calculate the magnetic ground-state properties of $\ensuremath{\gamma}\ensuremath{-}\mathrm{Fe}$ for a large set of spiral vectors and lattice constants. The effective single-particle equations are solved by means of an advanced version of the augmented spherical waves method which takes into account the full-shape potential and the intra-atomic noncollinearity of the magnetization. Together with the generalized gradient approximation the experimentally determined spiral magnetic ground state is reproduced successfully. Symmetry properties of the intra-atomic noncollinearity of the magnetization are analyzed and illustrated for spiral magnetic structures. We conclude that $\ensuremath{\gamma}\ensuremath{-}\mathrm{Fe}$ is an itinerant electron system possessing well-defined atomic moments.

Embedded Peierls instabilityand the electronic structure of MoO2

Molybdenum dioxide crystallizes in a monoclinic structure whichdeviates only slightly from the rutile structure and ischaracteristic of several early transition metal dioxides. Wepresent results of all-electron electronic structure calculations based on density functional theory within the local densityapproximation and using the augmented spherical wave method.The electronic properties of MoO2 are dominated by stronghybridization of O 2p and crystal-field-split Mo 4d stateswith bands near the Fermi energy originating almost exclusively fromMo 4d t2g orbitals. In additional calculations for ahypothetical high-symmetry rutile structure these bands separateinto quasi-one-dimensional d∥ states pointing along the rutile c-axis and the rather isotropically dispersing π* bands.On going to the monoclinic structure, the characteristic metal-metaldimerization causes strong splitting of the d∥ bandsinto bonding and antibonding branches which embrace the nearlyinert π* bands at EF. As a consequence,large portions of the Fermi surface are removed. According to ourcalculations the monoclinic structure of MoO2 thusresults from a Peierls-type instability of the d∥bands in the presence of, but still rather unaffected by, anembedding background of π* states. Our work has strongimplications for the current understanding of VO2 and thestriking metal-insulator/structural transition displayed by thismaterial.