Yu. I. Kuz’min

Sm2Fe17 and Tm2Fe17: electronic structure, magnetic and optical properties

For two intermetallics, the results of experimental measurements of optical and magnetic properties together with calculations in the frame of the LSDA+U method are reported. The calculations of the electronic structure allow one to interpret the curves of optical conductivity extracted from direct experimental ellipsometry measurements of optical constants. A detailed analysis shows that, whereas experimental curves of the optical conductivity for both compounds are similar to each other, the contributions of various interband transitions are slightly different. Also band structure calculations give magnetic structure and values of magnetic moments for each ion in Sm2Fe17 and Tm2Fe17 as well as the total one in good agreement with experimental values if the orbital moment is taken into account. Obtained optical theoretical and experimental results demonstrate good agreement with each other for both intermetallics.

Specific features of the behavior of the optical properties of TbNi5 − xCux intermetallic compounds

The optical properties of the TbNi5 − xCux intermetallic compounds have been investigated in the spectral range 0.08–5.64 eV by the ellipsometric method. It is shown that substitution of nickel for copper atoms leads to a significant change in the frequency dependence of the optical conductivity; this change is related to modification of the electronic spectrum. The formation of a new interband absorption band has been revealed, whose intensity increases with an increase in the copper content. The concentration dependences of the plasma and relaxation frequencies of conduction electrons in the compounds under study are determined. Self-consistent calculation of the electronic structure of the TbNi5 binary compound has been performed in the approximation of local electron spin density. The electron density of states for two spin projections and the optical conductivity of this compound have been calculated.

Optical conductivity and magnetic parameters of the intermetallic compounds R2Fe17−xMx (R=Y, Ce, Lu; M=Al, Si)

Abstract Optical properties of the intermetallic compounds R 2 Fe 17− x M x (R=Y, Ce, Lu; M=Al, Si; x =0, 1.7) were investigated in the energy range 0.083–5.64 eV. It was shown that the Curie temperature enhancement for substitutional alloys is accompanied by a decrease in the plasma frequency of the conduction electrons, which indicates a decrease in the density of states at the Fermi level N ( E F ). This correlation was explained using the spin-fluctuation model of Mohn and Wohlfarth. From the analysis of recent results on the electronic, structural and magnetic properties of the R 2 Fe 17− x M x compounds, a supposition is made about the connection of N ( E F ) with the distance between Fe ions in the ‘dumbbell’ crystal site.

Optical absorption and structure of energy bands of GdNi5 − xCux intermetallic compounds

Optical properties of intermetallic compounds GdNi5 − xCux (x = 0, 0.5, 1, 1.5, and 2) have been studied in a spectral range from 0.22 to 15 μm using the ellipsometry method. The substitution of copper for nickel has been found to lead to local changes in the interband optical conductivity spectra. A new quantum-absorption band, whose intensity increases substantially with increasing copper content, has been found at 3–4 eV. The relaxation and plasma frequencies of conduction electrons, which were calculated using data on the optical parameters, also depend on the concentration. Self-consistent calculations of the electronic structure of the intermetallic binary GdNi5 compound have been performed using the LSDA + U method. The density of electronic states for two spin projections and the frequency dependence of the interband optical conductivity of the compound have been calculated.

Analysis of dispersion of long-wavelength optical phonons in zinc with the help of light scattering

The temperature dependences (5–300 K) of the Raman spectra of E2g phonons and optical constants in zinc single crystals are measured in the excitation energy range 1.4–2.54 eV. It is found that phonon damping decreases upon an increase in the wavelength of exciting radiation. The obtained results are compared with the dependence of the phonon width on the excitation energy (the probed wave vector of the excitations under investigation), which are presented for the first time for the transition metal osmium, as well as with the calculated electron-phonon renormalization of damping, taking into account the actual distribution of wave vectors.

The statistical distribution of magnetic critical currents determined by HTSC film morphology

An analysis is made of the geometric and morphological properties of YBCO superconducting films and of their effect on the magnetic and transport phenomena. A study is presented of the statistical characteristics of critical currents derived from the variation of trapped magnetic flux induced by passing a pulsed transport current. It is shown that the critical currents in the materials under study have a specific statistical distribution, whose main properties are due to the morphology of their structure and can be determined by geometric-probability analysis. The superconducting film is considered as a percolation system. An empirical normal-phase cluster-area distribution function was used to derive the distribution function of magnetic critical currents, which describes adequately the experimental data on how transport current affects trapped magnetic flux. The critical current for transition of a film to the resistive state has been calculated.

Influence of aluminum impurity on the electronic structure and optical properties of the TbNi5 intermetallic compound

The electronic structure of the TbNi5 − xAlx intermetallic compounds (x = 0, 1, 2) is calculated in the local electron density approximation with the correction to strong electron correlations in 4f shell of terbium ions. Spectral properties of these compounds are measured by ellipsometry in a wavelength range of 0.22–16 μm. Frequency dependences of optical conductivity in the region of interband optical absorption are interpreted based on the results of calculations of electron densities of states. The relaxation and plasma frequencies of conduction electrons are determined.

Electronic structure of the intermetallic compounds Ce2Fe17 and Ce2Fe15.3 M 1.7 (M = Al, Si): Experiment and theory

The electronic structure of the intermetallic compounds Ce2Fe17 and Ce2Fe15.3M1.7 (M = Al, Si) is theoretically calculated in the local-spin-density approximation. It is shown that a considerable increase in the Curie temperature upon introduction of Al and Si impurities is accompanied by a proportional increase in the exchange interaction parameters Jij within the Heisenberg model for iron ions in the 6c crystallographic positions. The experimental optical conductivities of the compounds under investigation are interpreted in terms of the calculated electronic structure. The experimental valences obtained for the cerium ions from an analysis of the Ce L3 x-ray absorption spectra indicate that cerium in these intermetallic compounds is in intermediate valence states.

Evolution of the electronic structure and optical spectra of intermetallides DyNi5 − xCux under changes of concentration

Spin-polarization calculations of the electronic structure of intermetallic compounds DyNi5 − xCux (x = 0, 1, 2) are performed in the local spin density approximation with strong electron correlations in the 4 f shell of a rare-earth ion taken into account (the LSDA + U). Spectral properties of these materials are studied by the optical ellipsometry method in the wavelength range of 0.22–16 μm. It is established that the optical absorption spectra are significantly modified upon partial substitution of nickel by copper atoms. The experimental dispersion dependences of optical conductivity in the region of interband light absorption are interpreted on the basis of calculations of electronic state densities.

Role of Fe and Co in optical conductivity and electronic structure of TbNi4Fe and TbNi4Co

Optical properties of two intermetallic compounds TbNi4Fe and TbNi4Co have been studied employing ellipsometry in a spectral range 0.22–15 μm to reveal their characteristic features in comparison with the parent compound TbNi5. The electronic structure of TbNi4Fe and TbNi4Co was calculated within the LSDA + U method (local spin density approximation with Hubbard U-correction). Based on the calculated electronic structure results, the theoretical optical conductivity was calculated and used to interpret experimental conductivity in the range of interband optical absorption.