Fundamental absorption edge of thin films of Bi4Si3O12

The fundamental absorption edge of thin films of Bi₄Si₃O₁₂ is investigated. On the basis of its temperature dependence, exciton-phonon interaction is investigated, which made it possible to interpret the absorption edge as the absorption of autolocalized excitons. The temperature dependence of the forbidden band width is given. I. Franko Lvov State University, 50, Dragomanov St., Lvov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 2, pp. 232–235, March–April, 1997.     

Edge absorption spectra of crystalline and glassy PbGeS<Subscript>3</Subscript>

Edge absorption spectra of crystalline and glassy lead thiogermanate (PbGeS₃) have been measured in the temperature range from 77 to 470 K. It is shown that the dependence of the absorption coefficient on the photon energy for the glassy and crystalline states in the polarization E ∥ c is described by the Urbach rule. For the crystal in the polarization E ∥ b, at T < 300 K, an almost parallel shift of the intrinsic absorption edge to lower energies occurs with an increase in temperature, whereas at T ≥ 300 K, the Urbach absorption edge is observed. The parameter σ₀, related to the electron-phonon coupling constant, and the energy ?ω_ph of the effective phonons involved in the formation of the absorption edge of crystalline PbGeS₃ are determined from the temperature dependence of the parameter of the absorption edge slope. The contributions of the dynamic and static disorders to the diffusion of the absorption edge of crystalline PbGeS₃, as well as the topological disorder of glassy PbGeS₃, have been estimated.     

Temperature‐dependent EXAFS study of the local structure and lattice dynamics in cubic Y2O3

The local structure and lattice dynamics in cubic Y₂O₃ were studied at the Y K‐edge by X‐ray absorption spectroscopy in the temperature range from 300 to 1273 K. The temperature dependence of the extended X‐ray absorption fine structure was successfully interpreted using classical molecular dynamics and a novel reverse Monte Carlo method, coupled with the evolutionary algorithm. The obtained results allowed the temperature dependence of the yttria atomic structure to be followed up to ～6 Å and to validate two force‐field models.     

The shift of the optical absorption edge in an antiferromagnetic semiconductor

Thes-f model with spinS=7/2 has been used to study the temperature dependence of the optical absorption edge of an antiferromagnetic semiconductor. Two possible antiferromagnetic structures are considered: anAB two-sublattice model and an eight-subllattice model (MnO structure). For theAB two-sublattice model the density of states has been calculated as a function of temperature. A blue shift of the absorption edge of about 0.057 eV is obtained betweenT=T _N andT=0 K for this structure (using the parameters for EuTe). The dependence of the blue shift on the bandwidth and the band filling has been studied. For small values of the band filling the blue shift is practically unchanged.In the case of eight-sublattice structure the lowest single-particle energy (the bottom of the conduction band edge) has been observed as a function of temperature. Using the parameter set suitable for EuTe, it is concluded that the eight-sublattice structure does not show any shift of the optical absorption edge when the temperature is varied.     

Optical absorption edge of As<Subscript>40–<Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript>60</Subscript> glassy alloys

The fundamental absorption edge of glassy alloys of an As–Sb–S system was studied in the temperature range 77–300 K. The spectral and thermal behavior of the fundamental absorption edge of As_40–x Sb_xS₆₀ glasses was shown to be described by an exponential function of phonon energy. The temperature and concentration behavior of the optical band gap in the studied glasses was established. The well-known Varshni relationship was used to describe the temperature dependence of the optical band gap.     

Nature of the temperature dependence of the absorption coefficient in the remote wing of the 4.3 μm CO2 band

The complex nature of the temperature dependence of the absorption coefficient beyond the edge of the CO₂ 4.3 m band — the change of the sign of the derivative of the absorption coefficient with respect to the temperature near the edge and in the remote wing of the band — is explained on the basis of the theory of spectrum line wings. The first change in the sign is due to the temperature dependence of the classical potential of intermolecular interaction; the second depends mainly on the difference in the quantum energies of molecule interaction. Therefore, a study of the dependence of the absorption coefficient on the temperature in the band wings provides information about the nature of the terms of the interacting molecules.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 42–45, December, 1987.     

Exciton-phonon interaction in Y2O3 and Sc2O3 thin films

An investigation is made of the fundamental absorption edge of Y₂O₃ thin films. Based on its temperature dependence the exciton-phonon interaction is studied, which makes it possible to interpret the absorption edge as the absorption of self-localized excitons. A number of parameters of the energy spectrum of the investigated films are determined. I. Franko State University, 50, Dragomanov St., L'vov, 290005, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 132–134, January–February, 1999.     

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.     

Thermomodulation of the aluminum L2,3 absorption edge

Thermomodulation spectra were obtained near the Al L_2,3 edge in Al films, using synchrotron radiation. These spectra, temperature derivatives of the absorption coefficient, completely resolve the two spin-orbit components. A fit to the shape of one component shows that broadening of the Fermi function is the dominant mechanism for the temperature dependence one-electron or many-body absorption process, is needed near the region of the “spike”.     

The optical absorption edge of brookite TiO2

The optical absorption edge of brookite TiO₂ was measured at room temperature, using natural crystals. The measurements extend up to 3.54 eV in photon energy and 2000 cm⁻¹ in absorption coefficient. The observed absorption edge is broad and extends throughout the visible, quite different from the steep edges of rutile and anatase. No evidence of a direct gap is seen in the range measured. The spectral dependence of the absorption strongly suggests that the brookite form of TiO₂ is an indirect-gap semiconductor with a bandgap of about 1.9 eV.     

Temperature dependence of the bandgap width in FeIn2Se4 single crystals

We have used the Bridgman method to grow single crystals of the ternary compound FeIn₂Se₄ and we have determined their composition and structure. We measured the transmission spectra in the intrinsic absorption edge region in the temperature range 20–300 K. From the transmission spectra, we determined the bandgap width and plotted its temperature dependence. We show that the E_g(T) dependence has a shape typical for semiconductor compounds.     

Temperature dependence of the Ho L2‐edge XMCD spectra of Ho6Fe23

An X‐ray magnetic circular dichroism (XMCD) study performed at the Ho L_2,3‐edges in Ho₆Fe₂₃ as a function of temperature is presented. It is demonstrated that the anomalous temperature dependence of the Ho L₂‐edge XMCD signal is due to the magnetic contribution of Fe atoms. By contrast, the Ho L₃‐edge XMCD directly reflects the temperature dependence of the Ho magnetic moment. By combining the XMCD at both Ho L₂‐ and L₃‐edges, the possibility of determining the temperature dependence of the Fe magnetic moment is demonstrated. Then, both μ_Ho(T) and μ_Fe(T) have been determined by tuning only the absorption L‐edges of Ho. This result opens new possibilities of applying XMCD at these absorption edges to obtain quantitative element‐specific magnetic information that is not directly obtained by other experimental tools.     

Optical absorption in MnIn2S4 single crystals

Optical absorption in MnIn₂S₄ single crystals has been studied. Direct and indirect optical transitions are found to occur at photon energies of 1.90?C2.16 eV in the temperature range of 80?C342 K. The temperature dependence of the band gap is determined; its temperature coefficients E _gd and E _gi are found to be ?4.84 × 10^?4 and ?6.33 × 10^?4 eV/K, respectively. The electron-phonon interaction is the main mechanism of the temperature shift of the intrinsic-absorption edge. MnIn₂S₄ single crystals exhibit anisotropy in polarized light at the absorption edge in the temperature range of 90?C190 K; the nature of this anisotropy is explained.     

Letters to the editor

Abstract

Optical absorption spectra of TeO₂ crystals, irradiated at room temperature by up to 2·10¹⁸ cm^?2 10-MeV electrons and subsequently annealed to 575K, are studied. The dependence of Urbach absorption edge parameters in TeO₂ on the electron beam fluence is discussed. The irradiation-induced near-edge broad (2.5–3.5eV) absorption band is shown to be related to oxygen vacancies, annealing at 475–525K.     

Synthesis,magneto-optics and fluorescence of the ferromagnetic semiconductor Eu3(II)SiO5

Small single crystals of the transparent ferromagnet Eu₃SiO₅ have been synthesized by high temperature chemical transport. Magnetic measurements indicate T_c = 9°K and a saturation magnetization very close to 7 μ_B/Eu ion. Crystalline samples show a very low residual optical absorption and an absorption edge near 2 eV which displays a small red shift of 20 meV on cooling below T_c. Samples, containing a small percentage of dissolved EuO clusters, show in addition an absorption band at lower energies with a temperature dependence and magnetic behavior typical for EuO. The photoluminescence of the pure compound has a single emission band near 1·9 eV with a high quantum yield. At low temperatures also the fluorescence displays a red shift similar to that of the absorption edge. The fluorescence is accompanied with photoconductivity.     

Temperature dependence of slope of exponential absorption edge and the temperature shift of absorption edge in Ge20SbxS80−x glasses

The temperature dependence of the slope exponential absorption edge of glasses Ge₂₀Sb_xS_80?x (for x = 5, 10, 20, 25) has been determined and the results described using Mahr's formula. Adopting the idea of phonons-generated microfields given by Yacobi et al., the temperature shift of exponential edge is described in the weak field approximation of Franz.     

Temperature dependence of the indirect band gap, steepness parameter and related optical constants of [Kx(NH4)1−x]2ZnCl4 mixed crystals

Optical transmittance measurements near the absorption edge of [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals, where x=0.00, 0.232, 0.522, 0.644, 0.859 and 1.00, are reported over 276–350 K range. Analysis reveals that the type of transition is the indirect allowed one. The absorption edge shifted towards lower energy with increasing temperature. It is shown that [K_x(NH₄)_1−x]₂ZnCl₄ mixed crystals with x0.644 reveal a phase transition at 319 K, this phase disappeared at high concentrations of K⁺ ions. The steepness parameter is given, its value is used to estimate the temperature dependence of the indirect energy gap. In the region of the absorption edge, the absorption coefficient obeys Urbach's rule. Urbach parameters are investigated as a function of temperature.     

Optical absorption in films of V2O5 and vanadium-phosphorus glasses

The results of an investigation of the spectral dependence of the absorption coefficient of amorphous and polycrystalline films of V₂O₅ and films of vanadium-phosphorus glasses in the 0.3–5 eV energy range are presented in this paper. It is shown that the fundamental absorption edge is determined by direct forbidden transitions; the nature of the absorption bands detected below the fundamental absorption edge is discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No.2, pp. 102–106, February, 1976.     

Nature of optical properties of GdFe3(BO3)4 and GdFe2.1Ga0.9(BO3)4 crystals and other 3d5 antiferromagnets

Influence of the partial substitution of paramagnetic Fe³⁺ ions by diamagnetic Ga³⁺ ions in the trigonal crystal GdFe₃ (BO₃)₄ on its optical and magnetic properties is studied and discussed in connection with problems common for all antiferromagnets containing 3d ⁵ ions. Polarized optical absorption spectra and linear birefringence of GdFe₃ (BO₃)₄ and GdFe_2.1Ga_0.9 (BO₃)₄ single crystals have been measured in the temperature range 85–293 K. Specific heat temperature dependence (2–300 K) and structure of GdFe_2.1Ga_0.9 (BO₃)₄ crystal have been also studied. As a result of substitution of 30% Fe to Ga the Neel temperature diminishes from 38 till 16 K, the strong absorption band edge shifts on 860 cm^-1 (0.11 eV) to higher energy and the d-d transitions intensity decreases substantially larger than the Fe concentration does. Strong absorption band edge is shown to be due to Mott-Hubbard transitions. Correlation between position of the strong absorption band edge and the Neel temperature of antiferromagnets has been revealed. Properties of the doubly forbidden d-d transitions in the studied crystals and in other antiferromagnets are explained within the framework of the model of the exchange-vibronic pair absorption, which is theoretically analyzed in detail. The model permitted us to determine the connection between parameters of d-d absorption bands (intensity, width and their temperature dependences), on the one hand, and the exchange, spin-orbit and electron-lattice interactions, on the other hand.     

Photoconductivity and the structural phase transition in SrTiO3

Detailed photoconductivity measurements have been performed in nominally pure SrTiO₃ in order to elucidate the effect of the antiferrodistorsive cubic-tetragonal phase transition. Small features in the photoconductivity’s temperature dependence in the phase transition region were found using low intensity interband UV or 514 nm light illumination. Such features are associated with a transformation of the defect system controlling the photoconductivity. At the same time, the temperature behavior of the photoconductivity spectral maximum reveals a rather unusual feature which is connected with changes in the absorption band edge structure in the phase transition region.