The band structure and electronic properties of sulphur nitride polymer

A semiempirical calculation of the energy band structure of (SN)_x has been made on a tight-binding model with three p orbitals per atom. An important feature is that the Fermi level crosses two overlapping conduction bands. Measurements are reported of the optical transmission spectrum between 0.2 and 4.0 eV in thin films, the free carrier reflectivity in thick films, and the hydrostatic pressure dependence of the conductivity to 15 kbar. The calculated band structure accounts for experimental results connected with interband transitions (optical absorption) and intraband effects (metallic conductivity, reflectivity, specific heat).     

Reflectivity of Tm monochalcogenides: No evidence of valence fluctuations

The reflectivity of metallic TmS, TmSe and semiconducting TmTe has been measured between 0.03 eV and 12 eV and has been interpreted as 4fⁿ-4f^n?15d and p-d interband transitions and coupled plasmon modes.     

Optical anisotropy of LiNbO3 and KNbO3 in the interband transition region

The reflectivity of single domain crystals of LiNbO₃ and KNbO₃ has been measured using linearly polarized light up to 10 eV. The Kramers-Kronig analysis reveals a large anisotropy of the optical constants in the interband transition region. It is shown that the large electro-optic effects observed in these compounds are due to transitions in a relatively narrow spectral region above the band edge only.     

Low temperature photoconductivity of ZnIn2Se4 and CdIn2Se4

The photoconductivity of ZnIn₂Se₄ and CdIn₂Se₄ single crystals has been studied at 4.2 K. The spectra are compared to reflectivity spectra and relevant structures are interpreted in terms of interband transitions. The absence of excitonic transitions is discussed in connection with the crystalline perfection.     

Electronic structure of uranium monochalcogenides and uranium monopnictides

Single crystals of the uranium chalcogenides US, USe, UTe and the pnictides UAs and USb have been cleaved under high and ultrahigh vacuum conditions and the near normal incidence reflectivity has been measured in situ from 0.03 to 12 eV. In the case of US the spectral range of the measurement was further extended down to 0.0017 eV. The various optical functions (?₁, ?₂, σ, E_loss, n_eff) have been deduced by a Kramers-Kronig analysis. Typical features of all spectra are weak intraband contributions and the extension of strong interband transitions to very low photon energies. Highly damped plasmons are found at 4.25 eV for US, 3.5 eV for USe nad 2.2 eV for UTe. For the pnictides UAs and USb the corresponding values are 2 eV and 1.6 eV, respectively.The apparent contradiction between the presence of a large number of conduction electrons and only very weak intraband contributions is resolved within the spectral exclusion model of Keller. In this self-consistent cellular multiple scattering calculation the coupling of the narrow 5f states and the much wider 6d states produces a dip in the density of d states near E_F where the f electrons form a resonance state. Quantitative agreement is obtained between theory and experiment for US. For all investigated compounds empirical energy level schemes are derived.The reflectivity measurements down to 1.7 meV for US reveal the existence of a further excitation at 39 meV which is assigned to a transverse optical phonon mode. This unique observation of an optical phonon in a direct reflectivity measurement of a metal supports the picture of a small density of conduction electrons at E_F. Further arguments in favor of the spectral exclusion model are presented and the question of the localization of thr 5f electrons in various uranium compounds is discussed.     

Electronic structure and optical properties of LaNiO3: First-principles calculations

Using a first-principles method, we investigate the electronic structure and optical properties of rhombohedral LaNiO₃. The total density of states shows that there is no band gap and bulk LaNiO₃ is metallic. There is a strong hybridization between Ni and O orbits near the Fermi level, suggesting that the metallic nature of LaNiO₃ mainly originates from Ni 3d states and La atoms have no noticeable contribution to this. The absorption coefficient of LaNiO₃ is one order of magnitude less than that of nickel in the lower energy region (0–5 eV), and the interband optical transitions are mainly derived from O 2p and Ni 3d states. In reflectivity spectrum of LaNiO₃, there are three main reflectance peaks located at 0 eV, 15.6 eV and 22.9 eV, respectively. In the visible–ultraviolet energy range, the reflectivity of LaNiO₃ remarkably decreases with the increasing photon energy and the value is always smaller than that of nickel in the region.     

Non-stoichiometric metals with strongly varying electron concentration: Gd-monochalcogenides and LaS

The reflectivity of single crystals of Gd monochalcogenides and of LaS has been measured at 300 K in the spectral region between 0.03 and 12eV. Special attention has been given to the chemical analysis of the materials and the determination of the deviation from stoichiometry. The optical constants have been determined by means of a Kramers-Kronig relation. A coupled mode of plasmons with interband transitions has been observed. The separation into interband transitions and free electron behaviour permitted the determination of the number of free carriers and their effective mass. It can be shown that the carrier concentration changes much more drastically than the stoichiometry. By comparison with LaS the position of the 4f⁷ levels of GdS was found to be 9 eV below the Fermi level.     

Optical properties of metallic glasses

Optical reflection spectra of metallic glasses (Pd₈₁Si₁₉ and Pd₈₄Si₁₆) were measured in the spectral range 0.03 to 12 eV and the optical constants were determined by Kramers-Kronig analysis. The experimental reflectivity spectra of glassy Pd₁₈Si₁₉ are similar to the one obtained for Au₈₁Si₁₉ prepared by getter sputtering in argon. However, the energy ranges where intra- and interband optical transitions occur are quite different for the two alloys.     

Intraband and interband optical transitions in Zn3AS2

Absorption measurements were made on single crystals and thin films of Zn₃As₂; within the photon energy range of 0.12–1.16 eV at temperatures of 300, 80 and 5 K and reflectivity was measured in the range of 1.0–5.5 eV at 300 K. Absorption below the fundamental edge has been interpreted as a process involving three mechanisms: (i) free-carrier absorption, (ii) intraband transitions between levels in the valence band, and (iii) direct transitions from valence levels to the acceptor level/band. The fundamental absorption edge has been ascribed to direct interband transitions from three valence levels to one conduction level. An isotropic three-level Kane band model has been used to interpret the experimental data, modified by introducing the light-hole level split from the heavy-hole level due to the tetragonal crystal field. A reasonable fit of the model to the experimental results has been obtained in the region of both intraband and interband absorption for the following set of parameters: E_g = 0.985 eV, Δ_SO = 0.30 eV, Δ_CF = 0.05 eV, m^*_hh = 0.36 m₀ and P = 4 × 10^?10eVm (at 300 K). A proposed Zn₃As₂ energy-band model near the Γ point is described to interpret the observed absorption.     

Electronic excitations and luminescence of SrMgF4 single crystals

The electronic and crystal structures of SrMgF₄ single crystals grown by the Bridgman method have been investigated. The undoped SrMgF₄ single crystals have been studied using low-temperature (T = 10 K) time-resolved fluorescence optical and vacuum ultraviolet spectroscopy under selective excitation by synchrotron radiation (3.7–36.0 eV). Based on the measured reflectivity spectra and calculated spectra of the optical constants, the following parameters of the electronic structure have been determined for the first time: the minimum energy of interband transitions E _g = 12.55 eV, the position of the first exciton peak E _n = 1 = 11.37 eV, the position of the maximum of the “exciton” luminescence excitation band at 10.7 eV, and the position of the fundamental absorption edge at 10.3 eV. It has been found that photoluminescence excitation occurs predominantly in the region of the low-energy fundamental absorption edge of the crystal and that, at energies above E _g , the energy transfer from the matrix to luminescence centers is inefficient. The exciton migration is the main excitation channel of photoluminescence bands at 2.6–3.3 and 3.3–4.2 eV. The direct photoexcitation is characteristic of photoluminescence from defects at 1.8–2.6 and 4.2–5.5 eV.     

Analysis of reflectivity spectrum and band structure of Bi12GeO20

The fundamental reflectivity spectrum in the energy range 1.5–8.5 eV is reported for Bi₁₂GeO₂₀ (BGO) single crystals. From a Kramers-Kronig analysis the optical constants of BGO are computed. An energy level scheme for interband optical transitions is proposed. A value of 0.7 eV for the spin-orbit splitting of the valence band is suggested. Some features of the band structure are discussed, and the correlations between the band structures of BGO and A₂^VB₃^VI-compounds are indicated. The experimental premises for possible simplifications of a theoretical calculation of the band structure are given.     

Electroreflectance and reflectance study of 2H-MoSe2

We report the first electrolyte electroreflectance (EER) study of 2HMoSe₂ in the energy range 0.7 to 6 eV. The reflectivity (R) was also measured in the range of 0.7 to 9.5 eV and the real and imaginary parts of the dielectric constant were determined using the Kramers-Kronig relations. All the measurements were done at room temperature. Single crystals of 2H-MoSe₂ were grown using the direct combination of the elements in a sealed silica tube combined with chemical vapor transport using Br as the transport agent. The EER spectrum exhibits sharp structure in the vicinity of the excitonic transitions A, A′, B and B′ as well as higher lying interband transitions; the transition energies are determined with better accuracy than has been possible to date. A comparison of the R and EER spectra helped identify the various features in the EER spectrum.     

Fundamental optical properties of α-RuCl3

The fundamental optical properties in the paramagnetic phase of α-RuCl₃ are studied at different temperatures in the photon energy interval 0.03 to 10 eV. Infrared reflectivity spectra show a transverse optical frequency at 0.038 eV (32 μm) for an E_u mode (E⊥c, in plane atomic displacements). The absorption spectra in the energy range 0.2 to 1 eV reveal three bands (0.29, 0.51, 0.71 eV) attributed to d-d electronic transitions. Reflectance and thermo-reflectance measurements indicate the onset of the charge-transfer transitions at 1.1 eV and show structure at 1.85, 2.55, 3.05, 4.5 eV. The marked reflectivity peak at 5.2 eV is probably related to p(Cl) → s(Ru) band-to-band transitions.     

Optical properties of the ternary compound Tl Sb S2

We present an investigation of the near band-gap optical properties of TlSbS₂ between 2 and 300 K. We use both transmission and reflectivity measurements. The resolution of the first exciton line permits to obtain an accurate determination of the temperature coefficients of both the direct band-gap E₀ and the second threshold E₁. The absorption curves have been fitted according to the Toyozawa's model. We find a strong interaction with a phonon mode of energy 22 meV for both the E₀ and E₁ thresholds. The low temperature reflectivity spectra reveal clearly several direct transitions in the range 1.5–5.5 eV. All these structures have been identified as transitions between the highest valence band and the lowest conduction band.     

Optical constants of Cu(In0.7Ga0.3)5Se8 and Cu(In0.4Ga0.6)5Se8 crystals

Variable angle spectroscopic ellipsometry has been applied to characterize the optical constants of bulk Cu(In_0.7Ga_0.3)₅Se₈ and Cu(In_0.4Ga_0.6)₅Se₈ crystals grown by the Bridgman method. The spectra were measured at room temperature over the energy range 0.8-4.4 eV. Adachi’s model was used to calculate the dielectric functions as well as the spectral dependence of complex refractive index, absorption coefficient, and normal-incidence reflectivity. The calculated data are in good agreement with the experimental ones over the entire range of photon energies. The parameters such as strength, threshold energy, and broadening, corresponding to the E₀, E_1A, and E_1B interband transitions, have been determined using the simulated annealing algorithm.     

Dielectric function of YBa2Cu3O7-δ between 50 meV and 50 eV

Electronic excitations with polarization parallel to the CuO₂-planes in single-crystalline YBa₂Cu₃O₇ and YBa₂Cu₃O₆ have been investigated by optical reflectance and by highenergy electron energy-loss spectroscopy in transmission in the energy ranges from 50 meV to 6 eV and 0.2 eV to 150 eV, respectively. From a combination of these experimental data the dielectric function, the reflectivity and the optical conductivity have been obtained in a wide energy range. The observed spectra are interpreted in terms of optical phonons, free-carrier absorption, transitions across the charge-transfer gap, further interband transitions, low-lying core-level excitations, excitonic transitions and valence conservingd-d transitions within the Cu 3d shell combined with O 2p intraband transitions. The charge carrier plasmon and the 4 eV-excitation in YBa₂Cu₃O₆ show quadratic dispersions in momentum transfer. From the dispersion constant of the plasmon a mean value of the Fermi velocity of the charge carriers parallel to the CuO₂ planes has been derived.     

Superconductivity of the graphite intercalation compounds KHgC8 and RbHgC8

Superconductivity was observed in the graphite intercalation compounds, KHgC₈ and RbHgC₈, using an AC induction technique. The transition temperatures were 1.90K and 1.44K for KHgC₈ and RbHgC₈ respectively. A full Meissner effect was observed for KHgC₈ with a temperature dependant anisotropy in critical field with a value of 25 ± 5 at T_c.     

Fundamental optical properties of Cd1-xMnxSe single crystals

The absorption near the fundamental edge of Cd_1-xMn_xSe was measured in the composition range 0<x<0.3 at room and liquid nitrogen temperature with the electric field of the radiation parallel and perpendicular to the hexagonal axis. An exponential dependence of the absorption coefficient versus photon energy was found, and a linear dependence of energy gap E₀ on composition was obtained. The room temperature reflectivity measurements in the energy range 2.5–5.2eV, for two polarization of light were performed, and a linear dependence of the interband transitions energies vs. alloy composition was found.     

On the 3d electron contribution to the electronic structure of tetrahedral I–III–VI2 compounds

The reflectivity of single crystals of (CuInSe₂)_1?x(2ZnSe)_x which have the chalcopyrite structure for x ? 0.43 and the sphalerite structure for x ? 0.48 was measured. A band near 3 eV was observed for all compositions in the chalcopyrite structure but was not seen in the sphalerite phase. Its abrupt disappearance from x = 0.43 to x = 0.48 indicates that it may not be due to transitions from the copper d-levels to the lowest conduction band but may be associated with interband pseudo-direct transitions which become allowed by zone folding.     

Influence of impurity hydrogen on the structure and properties of bulk Li and pressure effects

The structure and properties of a 16-atom body-centered cubic lithium cell with an interstitial hydrogen atom are studied using a pseudopotential-plane-wave method within the density functional theory at 0 K and high pressures. The host lattice is dramatically distorted by the introduction of H. Although the hydrogen atom is stable at the tetragonal site in perfect bcc host lattice, it favors the octahedral site formed by six non-equivalent Li atoms after full relaxation of the cell, showing P4/mmm symmetry within the pressures ranging from 0 to 6 GPa. The lattice ratio (a/c) changes irregularly with external pressure at about 3 GPa. The hydrogen band lies in the bottom of the valence band, separated by a gap from the metallic bands, illustrating the electronegativity of hydrogen. High reflectivity in the low frequency area induced by the impurity hydrogen is observed when only interband transitions are taken account of. A dip in reflectivity due to parallel band transitions is observed at ∼0.4 eV. Another dip at ∼4.3 eV appears when external pressure increases over 4 GPa.