Optical properties and electronic structure of europium chalcogenides

The normal incidence reflectivity of europium chalcogenide single crystals and of Gd doped EuO has been measured at room temperature in the spectral region from 250 μm to 12 eV and has been analyzed in terms of the optical constants. In addition, in a reduced spectral region from 0.5 to 6 eV, the optical constants have been evaluated by means of a polarimetric method, as well above as below the magnetic ordering temperature. To enhance the resolution of the magneto-optical transitions, a modulation technique has been applied with a magnetic field as modulating parameter. The Kramers-Kronig relation has been used to analyze the normal incidence reflectivity and the magnetoreflectance spectra in terms of, respectively, the optical constants and the changes in the real and imaginary part of the dielectric response function. For Gd doped EuO the Kramers-Kronig analysis has revealed plasmon and coupled plasmon-phonon modes. The interband transitions of the europium chalcogenides are discussed within the framework of recent APW and OPW energy band calculations. On the other hand we have derived an energy level scheme of the europium chalcogenides from our optical data.     

Optical properties and band structure of CdSb

The optical constants of CdSb were determined for photon energies up to 2 eV from reflectivity measurements and by using the Kramers-Kronig dispersion relation. The experimental set-up for the reflectivity measurements is described. Possible locations of the observed direct transitions are found by the application of the selection rules. It is shown how these results, together with some results regarding the transport properties of CdSb, can be used to obtain information about the structure of the valence and conduction bands.     

Vacuum ultraviolet reflectivity and band structure of SrTiO3 and BaTiO3

The room temperature near-normal incidence reflectance spectra for SrTiO₃ and BaTiO₃ have been measured in the energy region 10 to 32 eV. The optical constants for the region 0 to 32eV have been derived from a Kramers-Kronig analysis by including earlier measurements for the low energy region. For SrTiO₃ five peaks in the Ti3d derived conduction band density of states have been found on the basis of experimental data only, neglecting excitonic effects. The energies of these peaks are in excellent agreement with peaks in the density of states calculated by Mattheiss. The results for BaTiO₃ could not be interpreted as unambiguously in such a scheme. Alternatively, some of the transitions could be interpreted as being due to core level excitons.     

Optical properties of zinc telluride in vacuum ultraviolet region

Reflectance spectra in the photon energy range between 15 and 150eV were measured on zinc telluride single crystals by using synchrotron radiation. A peak associated with transitions from the 3d core level of Zn to the conduction band was observed. A sharp doublet related to the 4d of Te was also observed. The optical constants including the complex index of refraction and the complex dielectric constant were determined on the basis of the Kramers-Kronig analysis.     

Optical constants of gold and silver clusters in the spectral range between 1.5 eV and 4.5 eV

Optical constants of gold and silver clusters of various sizes are determined from measurements of their optical extinction in the range from 1.5 eV to 4.5 eV photon energy. For this purpose, it is shown that the optical extinction by spherical clusters satisfies a Kramers-Kronig relation, yielding the second quantity needed for the determination of the complex dielectric constant ε of the cluster material. The dielectric constant is then obtained applying a generalized Newton-Raphson iteration method on the measured extinction data and the Kramers-Kronig data. The results show a clear dependence on cluster size, and also deviations from bulk dielectric constants in the range of the interband transitions. From the various ε-data, a “bulk” dielectric constant of gold and silver is obtained, which fits the positions of the cluster plasmons more precisely than bulk dielectric constants from the literature.     

Magnetooptik und elektronische Struktur der magnetisch ordnenden Europiumchalkogenide

The absorption coefficient and the interband Faraday rotation of EuS, EuSe and EuTe thin films have been measured as function of the photon energy (1–6 eV), the temperature (2.7–300 K) and the applied magnetic field (0–11.5 kOe). In addition a magnetic field modulation technique has been developed, with a resolution of 2 ? 10^?4 deg. This allows the measurement of the Faraday rotation in fields of only 100 Oe, which is important for metamagnetic samples with low critical fields. A Kramers-Kronig transformation of the Faraday rotation leads to the circular dichroism and from these two quantities and the optical constants the off-diagonal elements of the conductivity tensor have been computed. From a comparison of this experimental result with values obtained from a modified atomic model, we deduce the character of the involved transitions and the spin polarization of the occupied ground states (4f ⁷,p(anion)). In addition the ratio of exchange splitting to band width of the empty 5d final state can be evaluated. The fine structure of the first main peak is discussed in terms of Kasuya's coupling scheme between the 4f ⁶ multiplett and the excited 5d electron. In the antiferromagnetic EuTe the temperature dependence of the Faraday rotation does not follow the net magnetization of the sample for all photon energies, but some transitions show a “ferromagnetic” behavior. This is interpreted in Slater's model of the magnetic Brillouin zone.     

Spectra of optical functions and transitions in diamond

Spectra of a complete set of optical functions are obtained for three different diamond samples in the region from 0 to 32 eV. The calculations were performed by the Kramers-Kronig method using experimental reflection spectra. Special features and differences of the spectra of optical functions of the three samples are analyzed. Based on the method of joined Argand diagrams, the spectra of the dielectric constant are decomposed into elementary components for the first time, and their energies and oscillator strengths are determined. The component structure is in good agreement with the theoretical spectrum of the dielectric constant and the expected spectrum of the interband transitions.     

Structure and bonding of hydrocarbon plasma generated carbon films: An electron energy loss study

Electron energy loss spectroscopy has been used to study the structure and the bonding in hard amorphous carbon films (aC:H) prepared by plasma decomposition of benzene. EXAFS spectra indicate an amorphous and not a microcrystalline structure for aC:H films. The near-edge structure of the carbon K-amsorption edge reveals a considerable amount of π bonding in the aC:H films. The optical constants were derived by a Kramers-Kronig analysis of the loss functions for energies less than 40 eV. From the sum rule for ε₂, we derive that $\text{1}\text{3}$ of the carbon atoms are trigonally bonded and 273 of the carbon atoms are tetrahedrally bonded.     

Optical properties of the red bronze K0.33MoO3

The optical reflectivity of the red bronze K_0.33MoO₃ has been measured on single crystals in the spectral energy range between 0.03 and 12 eV at temperatures from 4 K to 300 K using polarized light. The optical constants have been determined by means of a Kramers-Kronig analysis; the data are interpreted that this compound is a 0.5 eV energy gap semiconductor with very strong anisotropy in the infrared and visible energy range.     

Synchrotron reflectivity spectra and the optical properties of CuGaS2 crystal

For a CuGaS₂ crystal, the polarized spectra of complete sets of optical functions in the energy range of 0 to 24 eV are obtained for the first time. The calculations are performed using synchrotron reflectivity spectra for E ∥ c and E ⊥ c polarizations in the energy ranges of 0 to 24 eV and of 1 to 5 eV by means of computer programs using the Kramers-Kronig relations. The main features of the spectra of permittivity, characteristic electron loss, and other optical functions are determined. The band maximum energies for the volume and surface plasmons are obtained, and their independence from the type of polarization is found.     

Spectra of the complete set of CdBr2 optical functions in a broad energy range of 3 to 30 eV

The spectra of the complete set of CdBr₂ optical functions at 35 K in the energy range 3–30 eV are obtained using experimental reflectivity spectra and the Kramers-Kronig relations. The main features of the structures of the optical spectra, including the energies of volume and surface losses, are determined. The results of an analysis of the effective number of electrons participating in transitions are presented.     

Electron energy loss studies in solids; The transition metal dichalcogenides

The theory of characteristic electron energy losses is discussed in terms of the electronic band structure of a solid. The relationship between the observed plasmon energies, the average interband energy gap and the background dielectric constant of the solid is developed.

The transmission energy loss spectra of a number of the layer-type transition metal dichalcogenides, MX₂, where M=Zr, Hf, Nb, Ta, Mo and W and X=S and Se, have been measured in the range of 0–50 eV. In the experiments, a beam of 50 keV electrons is incident along the c-axis of the crystals and electrons inelastically scattered through an angle of 1 m radian are selected for energy analysis. This ensures that the momentum transfer and hence the electric vector for the excitations lies in the basal plane of the crystal (E⊥c).

Kramers-Kronig analysis has been applied to the energy loss data to deduce the complex dielectric function of each material. From this function, all other ‘optical’ constants, such as the reflectivity, and the oscillator integral function and joint density of states function have been calculated.

The results give substantial support to the existing band model for the family of materials and, in addition, provide the basis for a quantitative understanding of the band structure of individual compounds.     

Electronic structure of GdP as determined from optical data

The growth of large single crystals of GdP of various stoichiometry has permitted for the first time accurate measurements of the reflectivity for photon energies from 0.03 to 12 eV. By means of a Kramers-Kronig analysis it was possible to separate intra- and interband transitions. The latter ones have provided valuable information about the structure of the 5d conduction band, its crystal field splitting and the position in energy of the 4f levels of Gd³⁺. The variation of the plasma resonance with deviations from stoichio-iometry does not exclude a semimetallic behavior of GdP.     

Optical properties of CdF<Subscript>2</Subscript> in a wide energy range

For either of the two reflection spectra of cadmium difluoride that are known from experiments, a complete set of the fundamental optical functions is calculated in the energy range 4–45 eV with the Kramers-Kronig relationships. The basic features of the optical spectra are established, and a hypothesis for their origin is suggested based on the known theoretical results for the band structure.     

Optical excitation of trapped charges in quartz, potassium feldspars and mixed silicates: The dependence on photon energy

Optical excitation of trapped charges has been studied by stimulating them with photons of energies from 1.8 to 2.7 eV and measuring the resulting luminescence at higher photon energies. The samples studied were quartz, potassium feldspar and fine-grained silicate extracts from natural sediments. The initial luminescence intensity and its decay as the traps are emptied have been used to gain information about the traps contributing to the emission. For the quartz studied the data are well explained by excitation from a single trap. The excitation spectra for potassium feldspars and mixed silicates show evidence of two different traps, the deepest of which is seen to give rise to the most luminescence. Its optical trap-depth may be more than 2.4 eV while the depth of the shallow trap appears to be about 2.1 eV. In nature, the deeper trap will probably not be well bleached during deposition in all aqueous environments, and for optical dating of water-laid sediments selective sampling of the shallow trap photons with energies less than 2.3 eV for stimulation may prove beneficial.     

纳米纤锌矿氮化硼的同步辐射真空紫外反射光谱与光学性质

对冲击波法合成的纳米纤锌矿氮化硼（ｗＢＮ）进行了同步辐射真空紫外区的反射光谱测量，得到波长范围１００一２４０ｎｍ区间的反射光谱．利用Ｋｒａｍｅｒｓ－Ｋｒｏｎｉｇ关系计算获得了它的光学常数和介电函数谱，并导出了光电导谱．获得了纳米ｗＢＮ的光学禁带宽度实验值，为８．７±０．５ｅＶ．将以上结果和最新的理论计算进行了比较．观察到了经不同退火温度处理的纳米ｗＢＮ样品结构及尺寸变化对反射光谱和光学性质的影响． 关键词：     

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 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.     

LaFeAsO单晶各向异性光学性质的第一性原理研究

使用密度泛函第一性原理研究了高温超导体LaFeAsO各向异性的光学性质。在描述光学性质的计算原理和计算方法的基础上, 计算了LaFeAsO的态密度、光电导谱、反射谱以及电子能量损失谱。光电导谱中, x方向与z方向有着很大差别, 在沿x方向的第一个带间吸收峰出现在1.3 eV处, 沿z方向出现在1.5 eV处; 在反射谱与电子能量损失谱中, x方向与z方向的特征峰位置在能量较高处都是相互吻合的。分析认为, 主要是电子在Fe原子之间的各个态间的跃迁所引起。考虑到温度效应对其光学性质的影响, 在计算光学矩阵元时, 加入Lorentz展宽δ=0.10 eV。本文的研究结果, 可为实验制备以及材料性质的研究提供有价值的参考。     

Absolute efficiency and dispersion of Raman scattering by phonons in silicon

We have measured the absolute Raman efficiency for first order scattering of the optical zone center phonon with two different incident photon energies (1.16 and 0.94 eV) by comparison with the Raman scattering from diamond. A value significantly lower than the experimental data published previously in this region has been found. For the first order scattering no dispersion of the scattering efficiency has been observed at the indirect gap, in contrast to the second order scattering which shows a pronounced resonance behaviour in this energy range.