The electronic structure of the Ni_{2}MnGa Heusler alloy has been investigated for martensitic transformation β_{1} → β'_{1} → β"_{1} → β'''_{1} by the self-consistent TB-LMTO method. The distortion influences the shape of the densities of states. The β_{1} is the most stable phase. We present the values of total and local magnetic moments for all phases of Ni_{2}MnGa alloy.
The influence of the electron-positron interactions on the momentum density of annihilation quanta in real metals is discussed. The role of momentum dependence of two-particle electron-positron correlations is set forth. The effective densities describing electron-positron correlations are presented for valence electrons in simple metals.
Co-Ag systems have recently attracted considerable attention in application oriented research of ferromagnetic thin films and multilayers. Due to low miscibility of Ag in Co it is easy to fabricate thin Co-Ag films of granular structure, promising as a high magnetoresistance material. It was also shown that using modern technology it is possible to deposit thin films of Co-Ag forming metastable homogeneous alloys for compositions far exceeding the solubility limit. In the present paper results of electronic structure calculations by the tight-binding linear muffin-tin orbital method are reported. Computations were done for hypothetical uniform ordered Co_{100-x}Ag_{x} alloys assuming proper extended unit cells to simulate concentrations of x=25, 12.5, 6.25, 3.125 at.%. The results were used to calculate the photoemission spectra. The calculated photoemission spectra are in fair agreement with experimental data supporting the conclusion of existence of a homogeneous though metastable alloy in the dilute limit. On the contrary, superposition of pure Co and Ag spectra implied for precipitate of Ag in Co cannot explain the experimental data.
La_{1-x} Ca_{x}MnO_{3} perovskites show semiconducting properties in the paramagnetic range. The gap is reduced near x=0.33, where ferromagnetic ordering is observed. The ferromagnetic ordering then induces a semiconductor-metal transition, and gives rise to a giant magnetoresistance effect. The ground state electronic structure calculations were done with KKR-CPA method for hypothetical cubic and ferromagnetic LaMnO_{3} and CaMnO_{3} compounds, as well as for disordered La_{1-x}Ca_{x}MnO_{3} (x=0.33, 0.4, 0.5) alloys with real crystal data. As a result, we get a magnetic moment per formula 4.00μ_{B} and 3.00μ_{B} and half-metallic behaviour for end-compounds, respectively. In the ferromagnetic region a linear decrease in the magnetic moment of La_{1-x}Ca_{x}MnO_{3} is observed, together with the decrease in the gap width for spin-down carriers, if doping Ca in La_{1-x}Ca_{x}MnO_{3}. A simple model is developed, which describes magnetic and transport properties as resulting from an exchange-induced band-crossing semiconductor-metal transition, as for instance in EuO.
Methods of extracting the full shape of the electron-positron momentum density from experimental angular correlation of positron annihilation radiation curves are discussed. The proposed analysis of experimental data allows more reliable verification of the form of electron-positron enhancement factors near the Fermi surface in simple metals.
The UGe system crystallizes in the orthorhombic ThIn-type structure. The uranium atoms occupy three crystallographic sites with interatomic U-U distances like below and above the Hill limit. The band structure has been calculated by using the density functional theory and generalized gradient approximation. Ab initio calculations were performed based on the full-potential local-orbital minimum-basis code. Calculations showed that all three types of uranium atoms are magnetically ordered with antiparallel alignment of the magnetic moments. For uranium atoms with an interatomic distance below the Hill limit magnetic moments are significantly reduced due to hybridization effects.
Resonant photoemission spectra of Sn_{1-x}Gd_{x}Te (x=0.02 and 0.08) measured for the photon energy range 142 to 151 eV show the valence band density of states distribution and the Gd 4f derived maximum. The energy position of the J=0 component of the Gd 4f maximum was determined and used as a measure of the Gd 4f shell binding energy. The electrostatic model of core level shifts was used to interpret the difference in the Gd 4f binding energies observed for x=0.02 and x=0.08.
The intermetallic compounds Yb_{x}Gd_{1 - x}Ni_5 crystallize in the hexagonal CaCu_5-type structure. Based on wide ranging SQUID-type magnetometer, it was shown that the saturation magnetization decreases with growing concentration of ytterbium. The opposite tendency was observed for the Sommerfeld coefficient obtained in the heat capacity measurements. These results are confirmed using ab initio band structure calculations.
The metallic behavior of the band gap of intermetallic compounds has large applications in superconductivity, nickel-metal hydrides batteries, semiconductors, and heating materials. The presence of transition elements makes them more attractive for magnetic applications. In this work we studied the structural, electronic, chemical bonding, and magnetic properties of binary intermetallic compounds XY_3 (X = Al, Ga and Y = V, Nb, Cr, Mo). These compounds were investigated by using full potential linearized augmented plane wave plus local orbitals method. The exchange correlation potential of generalized gradient is used. Our calculated lattice constants are in good agreement with experimental values. The band structures of these compounds are purely overlapping across the Fermi level. The bonding is mainly covalent in these compounds. The density of states of the compounds shows that the major contribution arises from d-states of anions. The investigation carried out shows that the most of these compounds have ferromagnetic nature, while few are diamagnetic. On the basis of this study it is expected that these compounds can be used as a best moulds for future study on similar compounds.
Interdependences between plane projections of densities ρ(p) for various crystallographical structures are derived from the conditions of both the consistency and symmetry of projections. Some additional relations are obtained by treating plane projections as line projections of ρ̃_{L}(p) (ρ̃_{L} is a line projection of ρ(p)) and using the consistency and symmetry conditions for the line projections. The relations found can be utilized for both an improvement of experimental spectra and a verification of various techniques used for e.g. correcting Compton profiles.
We present some relations between experimental spectra representing line projections of electronic densities in the momentum space. All spectra which are the projections of the same density, must be interdependent. It can be derived from the consistency condition and symmetry of the line projections. The knowledge of these dependences, found in the paper, can be utilised for an improvement of experimental data as e.g. two-dimensional angular correlation of positron annihilation spectra.
The advent of synchrotron sources has led to an increasing availability of high resolution Compton profiles J(p_{z}) and a consequent renewed interest in the reconstruction of the corresponding full momentum densities ρ(p). We present results of applying a new method in which the radial parts of ρ(p) and the measured profiles are expressed in terms of the Jacobi polynomials. The technique is demonstrated using model projections that correspond to Mg and Gd spectra. Reconstructed densities, being in very good agreement with model ones, are a very good performance of our new reconstruction algorithm.
We study the electronic structure of the hexagonal DyCo_5 and DyCo_3B_2 compounds. The magnetic moments and the band structures were calculated by ab initio self-consistent tight binding linear muffin-tin orbital method within the atomic sphere approximation. These compounds crystallize in a hexagonal structure having the P6/mmm space group. The substitution of cobalt by boron atoms changes the local environment of remaining Co atoms and leads to the decrease in the local magnetic moments as well as in the Curie temperature.
Electron correlation is responsible for finite lifetimes of excited electrons in crystals. Lifetime energy dependence can be obtained for infinite jellium model and only very recently the first results for an infinite crystal have been evaluated (GW approximation). Here, a phenomenological approach, based on Green functions, is presented. Broadening of local densities of electron states as well as that of angular-resolved photoemission (ARUPS) peaks and very-low-energy-electron diffraction (VLEED) profiles, due to the imaginary component of the optical potential is reviewed and interpreted. Anisotropy of electron damping on crystal surfaces has been found in VLEED as a result of electron channeling along the densely packed (111) surface atomic planes in fcc crystals. Interpretation of peak widths in VLEED-and ARUPS-profiles provides a mean to learn about damping of electrons, excited on crystal surface.
UCo_{4}B is a member of uranium intermetallic borides family which crystallizes in the hexagonal CeCo_{4}B structure. The experimental data suggest the occurrence of the spin-fluctuation behaviour of UCo_{4}B. In this paper we present the results of band structure calculations using the ab-initio tight binding linear muffin-tin orbital method. We found good agreement between the experimental X-ray photoemission spectroscopy valence band spectrum and the ab-initio results.
Ellipsometric studies of NdMnO_3 single crystals of orthorhombic symmetry were carried out in the spectral range 0.5-5.0 eV. Experimental data, which were obtained on the (001)_{pc}-type planes of pseudo-cubic system, were analyzed in the model of biaxial crystal. For the first time, three componentsε_x,ε_y,ε_z of the effective dielectric function for manganites of orthorhombic symmetry were determined. From ellipsometric data, the spectra of optical conductivity and loss function were also calculated and considered. The fine structure of the spectra and optical anisotropy was the basis for discussion of the microscopic origin of the optical transitions responsible for the optical features. The electronic excitations due to dipole-forbidden spin-allowed transitions of the d-d-type in Mn-ions, f-f-type in Nd-ions and charge-transfer 2p(O)-3d(Mn) transitions were taken into account. The data for NdMnO_3 were compared with those obtained for other related undoped and doped single crystals of perovskite-type structure, LaMnO_3, (LaBa)(MnCo)O_3 and (LaCa)CoO_3.
ThCo_4B compound crystallizes in the hexagonal CeCo_4B type structure. The electronic structure is calculated based on full-potential local-orbital full-relativistic method. The ab initio calculations showed that small magnetic moments (≈0.1 μ_B/atom) are formed on Th and B atoms, antiparallel to the moments on Co atoms equal to 1.55 and 0.43 μ_B/atom for Co(2c) and Co(6i), respectively. The densities of states at the Fermi level are equal to 1.3 and 5.4 states/(eV spin f.u.) for spin up and down, respectively. These values are predominated by Co(3d) electrons.
LaNi_{5-x}Mₓ (M = Al, Co) alloy thin films were prepared onto oxidised Si(100) substrates in the temperature range of 285-700 K using UHV magnetron co-sputtering. The surface chemical composition and valence bands of all the alloy thin films were measured in situ, immediately after deposition, transferring the samples to an UHV analysis chamber equipped with X-ray photoelectron spectroscopy. Results showed that the shape of the valence bands measured for the polycrystalline samples is practically the same compared to those obtained theoretically from ab initio band structure calculations. On the other hand, the X-ray photoelectron spectroscopy valence bands of the nanocrystalline thin films (especially LaNi₄Co) are considerably broader compared to those measured for the polycrystalline samples. This is probably due to a strong deformation of the nanocrystals. Therefore, the different microstructure observed in polycrystalline and nanocrystalline alloy thin films leads to significant modifications of their electronic structure.
The electronic structures of the half-Heusler isostructural compounds TiPtSn, ZrPtSn and HfPtSn were calculated and measured applying the X-ray photoemission spectroscopy. The (Ti, Zr, Hf)PtSn compounds have gaps between the occupied valence band and the empty conduction band, calculated as about 0.75, 1.12, and 1.09 eV, respectively. The calculations were done by the full-potential local orbitals method in the framework of the local spin-density approximation and partly also by the full-potential linear muffin-tin orbitals method by the LmtART code. Experimental X-ray photoemission spectra were measured using photons of energy of 1486.6 eV. The experimental and calculated spectra match quite well except a small shift in the energy scale.
A metal-insulator transition, Mott transition, in layered materials 1T-TaS_{x}Se_{2-x} was investigated by cryogenic scanning tunneling microscopy/ spectroscopy. At 77 K, tunneling spectra in the insulating phase showed a conduction band with almost half filling, which becomes narrower as x decreases. Around the transition point x≈1.4 at 77 K, we observed a sign of gap opening without an overshooting peak at zero bias, supporting the Mott localization picture in which a carrier number vanishes at the transition point. From the site-specified scanning tunneling spectroscopy measurements, furthermore, electrons were found to localize at the charge density wave crest positions. In 1T-TaS_{2}, we have also found that both metallic and insulating phases coexist in a nanometer scale just above the transition temperature, 180 K. >From the minimum size of the insulating region, the coherence length of Mott insulating state was evaluated to be≈5 nm.
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