Search Results (66 total)

Photoelectron spectra from (100) and (111) surfaces of aluminum in the photon energy range 44–100 eV are measured and calculated within an ab initio one-step theory. Dispersion of spectral structures is interpreted in terms of unoccupied electronic structure of a semi-infinite crystal. The energy dependence of complex self-energy is derived from the experiment. The lifetimes of the (100) and (111) surface states and the photon energy dependence of the intensity of photoemission from the surface states are determined. A broad spectral structure is experimentally observed at the (111) surface, which disappears at the room temperature. It is tentatively interpreted as a surface resonance.

The electronic structure, optical spectra, and x-ray magnetic circular dichroism (XMCD) of CoS₂ were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. Densities of valence states, and orbital and spin magnetic moments are analyzed and discussed. The origin of the optical and XMCD spectra in the CoS₂ compound is examined. The band-by-band decomposition of the optical conductivity spectrum is presented and the interband transitions responsible for the prominent structures in the spectra are identified. The calculated results are compared with available experimental data.

The x-ray magnetic circular dichroism (XMCD) spectra of CeFe₂ at the Ce L_2,3, M_4,5, Fe K, and L_2,3 edges are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation. The origin of the XMCD spectra in the compound is examined. The core hole effect in the final states has been investigated using a supercell approximation. It improves the agreement between the theory and the experiment at the Ce M₅ edge. However, it has a minor influence on the shape of the Ce L_2,3 XMCD spectra.

The outstanding discrepancy between the measured and calculated (local-density approximation) Fermi surfaces in the well-characterized, paramagnetic Fermi liquid Sr₂RhO₄ is resolved by including the spin-orbit coupling and Coulomb repulsion. This results in an effective spin-orbit coupling constant enhanced 2.15 times over the bare value. A simple formalism allows discussion of other systems. For Sr₂RhO₄, the experimental specific-heat and mass enhancements are found to be 2.2.

Soft x-ray absorption spectra and their x-ray magnetic circular dichroism (XMCD) have been theoretically studied at the transition-metal L_2,3 thresholds of the Heusler-type Fe_2−xV_1+xAl. The electronic structure of Fe_2−xV_1+xAl upon changing the content x was theoretically investigated from first principles by using the fully relativistic Dirac linear-muffin-tin-orbital band structure method and the scalar relativistic Korringa–Kohn–Rostoker method within the coherent potential approximation generalized to treat disorder in multicomponent complex alloys. Densities of valence states and spin and orbital magnetic moments are analyzed and discussed. The origin of the XMCD spectra in the Fe_2−xV_1+xAl compound is examined. The calculated results are compared to the experimental data.

GdN is a system with a strongly correlated electronic structure and a low concentration of free charge carriers. The x-ray magnetic circular dichroism (XMCD) spectra of GdN at the Gd L_2,3, M_4,5 and N K edges are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the LSDA+U method. The origin of the XMCD spectra in the compound is examined. The core-hole effect in the final states has been investigated using a supercell approximation. The final-state interaction improves the agreement between the theory and the experiment at the Gd M_4,5 and N K edges, however, it has a minor influence on the shape of the Gd L_2,3 XMCD spectra. We found also a strong influence of the surface on the x-ray absorption spectrum at the N K edge.

Mn₃ZnC possesses a magnetic phase transition at T_t=233 K from a ferromagnetic phase to a ferrimagnetic one with a noncollinear magnetic structure. The transition is accompanied by a structural change from cubic to tetragonal. The experimentally measured x-ray magnetic circular dichroism (XMCD) at the Mn K edge shows a drastic change at the magnetic phase transition, which is associated with the appearance of the noncollinear magnetic structure. The electronic structure and XMCD spectra of the Mn₃ZnC were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method for both the high-temperature cubic and low-temperature tetragonal noncollinear phases. Densities of valence states, spin, and orbital magnetic moments are analyzed and discussed. The origin of the XMCD spectra in the Mn₃ZnC compound is examined. The calculated results are compared with the experimental data.

The experimental difficulties of observing Fe²⁺∕Fe³⁺ charge order in Fe₂OBO₃ and Fe₃O₄ are different and are considered separately. Charge order in Fe₃O₄ has a complex arrangement but is long-range coherent, as evidenced by a recent structure determination and two resonant x-ray diffraction studies. Long-range charge order has not been observed crystallographically in Fe₂OBO₃, although there is substantial indirect evidence for Fe²⁺∕Fe³⁺ ordering over shorter distances. Further support is provided by LSDA+U calculations on a doubled unit cell of Fe₂OBO₃, which shows a charge separation similar to that in Fe₃O₄, with a large t_2g subshell occupancy difference that is partially screened by Fe-O covalency.

The electronic structure of the low-temperature (LT) monoclinic magnetite Fe₃O₄ is investigated using the local spin density approximation (LSDA) and the LSDA+U method. The self-consistent charge-ordered LSDA+U solution has a pronounced [001] charge density wave character. In addition, a minor [001/2] modulation in the phase of the charge order (CO) also occurs. While the existence of CO is evidenced by the large difference between the occupancies of the minority spin t_2g states of “2+” and “3+” Fe_B cations, the total 3d charge disproportion is small, in accord with the valence-bond-sum analysis of structural data. Weak Fe orbital moments of ∼0.07μ_B are obtained from relativistic calculations for the CO phase which is in good agreement with recent x-ray magnetic circular dichroism measurements. Optical, magneto-optical Kerr effect, and O K-edge x-ray absorption spectra calculated for the charge-ordered LSDA+U solution are compared to corresponding LSDA spectra and to available experimental data. The reasonably good agreement between the theoretical and experimental spectra supports the relevance of the CO solution obtained for the monoclinic LT phase. The results of calculations of effective exchange coupling constants between Fe spin magnetic moments are also presented.

The electronic structure and x-ray magnetic circular dichroism linear muffin-tin orbital spectra of IrMnAl were investigated theoretically from first principles using the fully relativistic Dirac LMTO band structure method. The electronic structure is obtained with the local spin-density approximation. We found that the induced magnetic moment at the Ir site depends strongly on the local environment. An important feature of IrMnAl is a very small magnetic moment on the Mn site of about 0.4μ_B which is much smaller than the moment of (2–5)μ_B found in ferromagnetic Mn compounds with either L2₁ or C1_b crystal structure. The small value of the net magnetic moment in IrMnAl is associated with the noncollinear magnetic structure and possible Mn intersite disorder. The total energies for different spin spirals are calculated, and the ground-state magnetic structure is identified. In addition, the variation of the magnetic moment as a function of the spiral structure is studied.

II-VI semiconductor nanostructures exhibit interesting optical properties due to quantum confinement of their charge carriers. Here, we discuss the assembly of nanostructures of CdS, CdSe, and CdTe using buffer-layer-assisted growth with Xe buffers. Both compact clusters and ramified wires can be synthesized by varying the Xe buffer layer thickness. Analysis of the nanostructure size distributions and densities makes it possible to calculate their diffusion parameters on the desorbing Xe. Clear differences in the effective activation energies for diffusion for CdS, CdSe, and CdTe can be attributed to differences in London dispersion interactions. Photoluminescence measurements indicate changes from 3D to 2D confinement as compact particles are replaced by ramified wires. Laser power dependent measurements yield the low temperature exciton lifetime, and temperature dependent measurements indicate that optical phonons play a dominant role in the decay of the signal above 50 K and defect states play a dominant role below 50 K.

Spontaneous desorption of Cl, Br, and I from n- and p-type Si(100)-(2×1) was studied with scanning tunneling microscopy at temperatures of 620–800 K where conventional thermal bond breaking should be negligible. The activation energies and prefactors determined from Arrhenius plots indicate a novel reaction pathway that is initiated by the capture of electrons which have been excited by phonon processes into Si-halogen antibonding states. This configuration is on a repulsive potential energy surface, and it is sufficiently long lived that desorption can occur, constituting phonon-activated electron-stimulated desorption. Surprisingly, the Arrhenius plots for differently doped samples crossed and, above a critical temperature, the reaction with the largest activation energy had the highest rate. This is explained by large entropy changes associated with the multiphonon nature of the electronic excitation. For Cl desorption from p-type Si, these entropy changes amounted to 34k_B. They were 19k_B, 13k_B, and 8k_B for Br desorption from p-type, lightly doped n-type, and heavily doped n-type Si, respectively. The desorption rates for I were nearly three orders of magnitude larger than the rates observed for Cl and Br. Here, the Si-I antibonding states overlap the conduction-band minimum, so that conduction-band electrons with this energy can be captured by the Si-I antibonding states. Together, these results reveal that a complex relationship exists between phonons and electronic excitations during chemical reactions at surfaces.

The electronic structure, x-ray absorption (XAS), x-ray magnetic circular dichroism (XMCD), and magneto-optical spectra of the doped Heusler alloys Co₂Cr_1−xFe_xAl (x=0, 0.125, 0.25, 0.375, 0.5, 0.625, 0.75, and 1) were investigated theoretically from first principles, using the fully relativistic Dirac linear-muffin-tin-orbital band structure method. It was shown that the concentration and arrangement of Fe atoms in the lattice play the leading role in the formation of the magnetic properties of the compounds, determining to a considerable extent the magnetic states of Co and Cr atoms. The spin polarization of electron states at the Fermi energy is quite high (about 95%) for the compositions between x=0 and x=0.4, owing to the presence of high peaks of majority-spin Cr and Co d density of states at the Fermi energy. Further increasing of Fe content leads to the abrupt decreasing of the spin polarization of carriers. The band structure calculations in the LSDA approximation reproduce quite well the shape of the XAS and XMCD spectra at the Co, Fe, and Cr L_2,3 edges. The energy dependence of the matrix elements affects strongly the values of both the spin and the orbital magnetic moments derived from the calculated XAS and XMCD spectra using the sum rules. Our calculations show that the magneto-optical measurements are sensitive to changes in the structure of compounds (doping, disorder, etc.) accompanied by changes of the magnetic states of atoms.

The electronic structure of the mixed-valence Sm and Eu pnictides R₄X₃ (R=Sm, Eu; X=As, Sb, Bi) and samarium and europium chalcogenides Sm₃X₄ (X=S, Se, Te), Eu₃S₄ were investigated theoretically from first principles, using the fully relativistic Dirac LMTO band structure method. The electronic structure is obtained with the rotationally invariant LSDA+U method. We find in Sm₄Bi₃ as a generic feature a very rigid pinning of the energy of Sm 4f hole states close to the top of the pnictide p band and the Fermi level pinned to those hole states. Eu₄Bi₃ was found to be a semimetal with low density of states at the Fermi level. The main trends in the electronic structure of the sequence of Sm₄X₃ and Eu₄X₃ compounds (X=As, Sb, or Bi) are discussed. A detailed comparison of the electronic and magnetic structures of Sm₃S₄ and Eu₃S₄ compounds is also presented.

Charge ordering in the low-temperature monoclinic structure of iron oxoborate (Fe₂OBO₃) is investigated using the local spin density approximation (LSDA)+U method. While the difference between t_2g minority occupancies of Fe²⁺ and Fe³⁺ cations is large and gives direct evidence for charge ordering, the static “screening” is so effective that the total 3d charge separation is rather small. The occupied Fe²⁺ and Fe³⁺ cations are ordered alternately within the chain which is infinite along the a direction. The charge order obtained by LSDA+U is consistent with observed enlargement of the β angle. An analysis of the exchange interaction parameters demonstrates the predominance of the interribbon exchange interactions which determine the whole L-type ferrimagnetic spin structure.

Co₂NbSn undergoes a structural transition at T_s=233 K from the cubic Heusler Fm3m high-temperature phase into an orthorhombic low-temperature lattice of Pmma symmetry. Furthermore, the system exhibits a magnetic transition at T_c=116 K from para to ferromagnetism. The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of the Heusler alloy Co₂NbSn were investigated theoretically from first principles, using the fully relativistic Dirac LMTO band structure method for the crystal structures corresponding to both the high temperature and low temperature phases. The origin of the XMCD spectra in the compounds is examined. The effect of Ni substitution in the series (Co_1−xNi_x)₂NbSn on the electronic structure and magnetic ordering is also investigated.

Charge and orbital ordering in the low-temperature monoclinic structure of magnetite (Fe₃O₄) is investigated using the local spin density approximation with Coulomb interaction correction method. While the difference between t_2g minority occupancies of Fe_B²⁺ and Fe_B³⁺ cations is large and gives direct evidence for charge ordering, the screening is so effective that the total 3d charge disproportion is rather small. The charge order has a pronounced [001] modulation, which is incompatible with the Anderson criterion. The orbital order agrees with the Kugel-Khomskii theory.

Physical vapor deposition onto rare gas buffer layers leads to the spontaneous formation of clusters. During the thermal desorption of the buffer, these clusters diffuse and aggregate into larger structures, a process known as buffer-layer-assisted growth and desorption assisted coalescence. We studied the effect of buffer thickness and the rate of buffer desorption on the extent of this aggregation for Ag, Au, Cu, Pd, Co, and Ni particles on a solid Xe surface. On the basis of these experiments, results from Monte Carlo simulations and the existing theoretical models for cluster–cluster aggregation, we report for the first time the Arrhenius parameters for nanoparticle slip-diffusion. The effective activation energies range from 0.12 for small Ag clusters (few hundred atoms) to 0.60 eV for ramified Ni islands (millions of atoms), and the giant pre-exponential factors were found to differ by many orders of magnitude. Significantly, the pre-exponential factors follow a Meyer–Neldel-type dependence on the corresponding effective activation energy, with a characteristic Meyer–Neldel energy of 6.9 meV. This energy is associated with the phononic excitations in solid Xe that are responsible for nanostructure mobility. This dependence should be a characteristic feature of nanoparticle diffusion.

The optical and magneto-optical (MO) spectra of the lanthanum monochalcogenides are investigated theoretically using the fully relativistic Dirac LMTO band-structure method. The LSDA calculations with shifted La 4f empty states describe quite well the shape and magnitude of the LaS MO spectra in an external magnetic field and to a lesser extent the MO spectra of LaSe. On the other hand, theory fails to describe the broader spectral structures in LaTe between 3 and 5 eV. The origin of the Kerr rotation in these compounds is examined.

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of heavy-fermion compounds UPt₃, URu₂Si₂, UPd₂Al₃, UNi₂Al₃, and UBe₁₃ are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with a generalization of the LSDA+U method. The origin of the XMCD spectra in the compound is examined.

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of UCoAl, URhAl, and UPtAl were investigated theoretically from first principles, using the fully relativistic Dirac LMTO band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA) as well as with a generalization of the LSDA+U method which takes into account the nondiagonal (in spin indexes) occupation matrix of localized electrons. The origin of the XMCD spectra in the compounds is examined.

The electronic structure, magneto-optical and x-ray magnetic circular dichroism (XMCD) spectra of UFe₂ were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with a generalization of the LSDA+U method which takes into account the nondiagonal occupation matrix (in spin indexes) of localized electrons. The origin of the Kerr rotation and XMCD spectra in the compound is examined.

Physical vapor deposition of a wide range of materials on rare-gas solids leads to spontaneous cluster formation. Desorption of the rare-gas buffer causes the clusters to aggregate, a process known as buffer-layer-assisted growth. We have studied the extent of aggregation and the size distribution of Au nanostructures as a function of the buffer composition (Xe, Kr, and Ar) and thickness, using transmission electron microscopy to image them after buffer desorption and delivery to amorphous carbon substrates. For small compact Au nanostructures (less than ∼5 nm mean radius, <~3×10⁴ atoms), the diffusivity varies strongly with size and even increases with average size in a limited range. This enhanced diffusion phenomenon is attributed to self-heating during coalescence. It is most important for small particles and is more evident on Kr than on Xe because of weaker interface coupling. In the limit of large ramified Au nanostructures (exceeding ∼10 nm mean radius, >~2×10⁵ atoms), the diffusivity scales as the inverse of the contact area, in agreement with molecular dynamics simulations of fast slip diffusion of nanocrystals on incommensurate surfaces. Motion is driven by phonons of the cluster and substrate, and is controlled by friction between a cluster facet and the buffer surface. A simple model is proposed that explains the observed exponential dependence of cluster size on buffer thickness. In this model, the growth kinetics are controlled by competition between the rate of cluster diffusion and the rate of buffer depletion.

A generalization of the local-density approximation + U (LDA+U) method which takes into account that in the presence of spin-orbit coupling the occupation matrix of localized electrons becomes nondiagonal in spin indices is used to study the electronic structure of UPd₃ and UPd_1.5Pt_1.5. For both compounds LDA+U calculations give a solution with two localized U 5f electrons. Their energy position agrees well with the binding energy of a U 5f peak observed in photoemission experiments. The calculations also reproduce the shift of the peak position toward the Fermi level upon Pt substitution. The LDA+U results are compared to the results of LDA calculations for ThPd₃ and ThPd_1.5Pt_1.5 and their dependence on the crystal structure is analyzed.

The electronic structure of charge-ordered magnetite (Fe₃O₄) below the Verwey transition and Mn-, Co-, or Ni-substituted Fe₃O₄ are investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with the so-called LSDA+U approach, for which the charge ordering is found to be a stable solution in contrast to a metallic state given by the LSDA. The x-ray absorption spectra as well as the x-ray magnetic circular dichroism spectra at the K, L_2,3, and M_2,3 edges for transition metal sites are calculated. A good agreement between theory and experiment is obtained.