Calculation of absorption coefficient for the ν2 and ν3 bands of sulfur dioxide

Absorption coefficients S/d of sulfur dioxide were calculated in the ν₂ and ν₃ bands for a temperature range from 300 to 1200°K. Calculations were carried out for such vibrational transitions that their integrated absorption coefficients α at T°K are greater than 0.1% of the total integrated absorption coefficients α^obs observed at 300°K. Rotational lines with intensities greater than 10^-6 cm^-2 atm^-1 STP were included in the calculation. The line intensities were averaged over small wavenumber intervals of Δω = 10 cm^-1 throughout the bands to obtain the absorption coefficients S/d. They were compared with the experimental values of other investigators and good agreement was obtained.     

X-ray and UV photoelectron spectra of the oxides NbO2, MoO2 and RuO2

HeI and Mg Kα_1,2 valence band photoelectron spectra of polycrystalline samples of NbO₂, MoO₂ and RuO₂ are reported. A marked increase is observed in the intensity of the metal 4d structure, relative to that due to oxygen 2p electrons, on changing from X-ray to UV excitation. The superior resolution of the 4d signals in the HeI spectra reveals the presence of the Fermi edge in the metallic oxides MoO₂ and RuO₂. In addition, the HeI spectrum of MoO₂ shows a splitting of the metal 4d signal, confirming established ideas concerning the electronic structure of such materials.     

Tuning the dispersion of 4f bands in the heavy-fermion material YbRh2Si2

Localized Yb 4f and itinerant Rh 4d states are subject to substantial hybridization effects in the heavy-fermion material YbRh₂Si₂. The proximity to the Fermi level and the high anisotropy in k space naturally raise questions regarding the role of these hybridization effects for the observed, unusual physical properties. Using angle-resolved photoemission spectroscopy (ARPES) we found that the non-dispersive behavior of the localized Yb f states is broken around the Γ point due to interaction with approaching Rh 4d bands. The intriguing point here is that the hybridization strength turns out to be systematically tunable by electron doping of the material. Gradual deposition of silver atoms onto the atomically clean, silicon terminated surface of YbRh₂Si₂ leads to transfer of Ag 5s charge into the Rh 4d bands. This substantially changes the energy overlap, and thus the hybridization strength, between the interacting Yb 4f and Rh 4d bands in the surface and subsurface region. The shown possibility to control the variation of the f-d hybridization at the surface of heavy-fermion materials may also be helpful for other ARPES studies on the diverse phenomena in electron-correlated materials.     

Pressure dependence of elastic constants of orthorhombic Li2Ge7O15 above and below the ferroelectric transition

The pressure derivatives of the elastic constants c_ij of orthorhombic Li₂Ge₇O₁₅ have been determined at 293 K by the method of pressure-induced shifts of resonance frequencies of thick plates at ca. 15 MHz in the range between 0 and 1500 bar. Approaching the transition at ca. 630 bar, all P_ij = dc_ij/dp (i, j = 1, 2, 3; p pressure) develop strongly negative values. At higher pressures a similar behaviour with reversed sign is observed. The pressure derivatives of the pure “shear resistances” c₄₄, c₅₅, and c₆₆ depend only slightly on pressure even in the vicinity of the transition. The main interactions driving the transition are of the totally symmetric type. The values dK^?1/dp (K volume compressibility) deviate strongly from the quasi-invariant value of ca. 5 observed in almost all stable crystals (dK^?1/dp = ? 1750 at 620 bar and 1380 at 700 bar). The anomalous piezoelastic behaviour reflects the anomalous thermoelastic behaviour: negative P_ij in the low pressure (high temperature) phase correspond to positive T_ij = d log c_ij/dT (T temperature) and vice versa.     

Quadratic electrostrictive effects in NaCl and KA1(SO4)2·12H2O derived from stress dependence of dielectric constants

The equivalence of quadratic electrostrictive coefficients and certain stress derivatives of dielectric constants provides a chance for an independent determination of the tensor of quadratic electrostriction. The experimental values obtained by the aid of stress-induced shifts of the dielectric constants allow a clarification of the contradictory results of different authors on NaCl and KA1(SO₄)₂·12H₂O. The new values for 293 K are: NaCl: d₁₁₁₁ = 0.68, d₁₁₂₂ =-0.215, d₁₂₁₂ = 0.12; KA1(SO₄)₂·12H₂O: d₁₁₁₁ = 0.63, (d₁₁₂₂ + d₁₂₁₁)/2 = -0.15, d₁₂₁₂ = 0.46 (Unit: 10^-20 m² V^-2). These crystals exhibit a mechanical dilatation parallel to an electric field and a contraction in perpendicular directions. This is accompanied by by a small increase in volume. Further, it is shown that certain relations between different electrostrictive effects of second order given in literature are erroneous in respect to sign..     

Measured and predicted band model parameters of BF3(v3) at high temperatures

Two experimental techniques were used to determine the high temperature infrared absorption coefficients of the v₃ band of BF₃. A high temperature flow tube fitted with a multipass infrared absorption cell was used to determine the BF₃ absorption coefficients at 295, 585, 870, 1150, and 1440K. BF₃ was also used in a flame emission-absorption apparatus to determine the BF₃ absorption coefficients at 2400 K.A generalized formulation for predicting the S/d and 1/d band model parameters of symmetric top perpendicular bands of MX₃ molecules is presented. The effect of including nuclear spin statistics in computing the S/d and 1/d band model parameters is considered. It is shown that nuclear spin statistic?s effect only the 1/d parameter and are significant only for the case of zero nuclear spin. A number of practical considerations, which arise in designing a computer algorithm to calculate the S/d and 1/d parameters, are discussed.Comparisons of the theoretical and experimental BF₃ absorption coefficients are made. Good agreement between theory and experiment at all temperatures was obtained by adjusting only two vibrational energy coupling constants, _χ23 and _χ34. Predicted S/d and 1/d band model parameters at 300, 600, 900, 1200, 1500, 2000, 2500, and 3000 K are presented.     

Esca studies of Ga,As, GaAs,Ga2O3, As2O3 and As2O5

The binding energies of Ga 3d, As 3d, Ga L₃M₄,₅M₄,₅ and O 1s in Ga, As, GaAs, Ga₂O₃, As₂O₃ and As₂O₅ are reevaluated by means of ESCA. The calibration lines of the C 1s and the Au 4f_{$\text{7}\text{2}$} gave different binding energies for the compound materials. In order to determine the absolute binding energies, the chemical shifts in Auger and photoelectron lines from a layered structure composed of thin layer oxide and substrate of a defined material were used. An energy calibration curve, E(Ga 3d) vs. ΔE(GA LMM - Ga 3d), was found to be useful for determination of binding energies in the material which contains gallium. In the case of the GaAs sample, both the chemical etching and the ion bombardment effects on the chemical structure of the GaAs surface are also discussed.     

On the properties of the 1s-forms of TaSe2 and TaS2

The 1s-forms of TaSe₂ and TaS₂ with octahedral coordination of the metal are diamagnetic; 1s-TaS₂ is a semiconductor at low temperature. The diamagnetism is explained by taking account of spin-orbit coupling which leads to a ground state with zero magnetic moment (g = 0). This spin-orbit coupling stabilizes the d¹ configuration of the metal with respect to d² + d⁰. Thus, it can be understood that 1s−TaS₂ is semiconducting, while isostructural VSe₂ is metallic. Similarly, BaTaS₂ and BaTaSe₃ are semiconductors, but BaVS₃ is metallic.     

Mechanism of electron beam poled SHG in 0.95GeS2·0.05In2S3 chalcogenide glasses

Utilizing the Maker fringe method, SHG was observed in the 0.95GeS₂·0.05In₂S₃ chalcogenide glass irradiated by the electron beam and the intensity of SH increases with the enhancement of beam current from 15 to 25 nA. According to Raman spectra of the as-prepared and the irradiated one, no distinct micro-structural transformation was found. In this work, the built-in charge model was founded to interpret the poling mechanism of electron beam irradiation, the emission of the secondary electrons and Auger electrons results in the formation of positive region and the absorbed electrons form negative region. The positive region was situated near the poling surface, and the negative region was much deeper than the positive region. Between the two opposite charged regions, a strong space-charge electrostatic field, E_dc, is created, which leads to the nonzero χ⁽²⁾ in the 0.95GeS₂·0.05In₂S₃ glass. The emission of backscattered electrons does no contribution to the formation of E_dc.     

Pressure dependence of phase transitions in K2SnCl6 from NQR frequency measurements

The influence of hydrostatic pressure 0 ? p ? 4 kbar on the ³⁵Cl NQR in K₂SnCl₆ was studied in the temperature range 238 K ? T ? 300 K. The phase transition temperatures T_C1 and T_C2 were determined from changes in the NQR line pattern.The phase boundaries in the p-T diagram are straight lines in the region studied. The pressure coefficients are given by dT_C1/dP = 1.35 (10) K kbar^?1 and dT_C2/dP=?1.25 (20) K kbar^?1.     

Covalent distribution of spin in V2O3:O17 nuclear resonance

O¹⁷ nuclear magnetic resonance has been observed in metallic V₂O₃ with frequency shifts from (?0.10 ± 0.02)-(?0.05 ± 0.02) per cent between 170 and 460°K respectively, a linewidth of 37 ± 5 oe and spin-lattice relaxation rate 1/T₁ ≈ 60 sec^?1 at 296°K. From these quantities, covalency parameters f_s/2S = ? 0.35 × 10^?3 and ?_π/2S ≈ ? 0.07 are calculated. One of the two vanadium 3d electrons in the antiferromagnetic state below the 170°K metal-insulator transition is inferred to lie in a non-magnetic state, while covalent charge transfer augments the spin moment of the other 3d electron to the observed 1.2 μ_B.     

Self-broadening, self-shift and self-mixing in the ν2, 2ν2 and ν4 bands of NH3

Using Fourier-transform spectra (Bruker IFS 120 HR, resolution ≈0.004 cm⁻¹) of NH₃ in nine branches of the ν₂, 2ν₂ and ν₄ bands, self-broadening and self-shift as well as self-mixing coefficients have been determined at room temperature (T=295 K) for more than 350 rovibrational lines located in the spectral range 1000–1800 cm⁻¹. A non-linear least-squares multispectrum fitting procedure, including line mixing effects, has been used to retrieve successively the line parameters from 11 experimental spectra recorded at different pressures of pure NH₃. The accuracies of self-broadening coefficients are estimated to be better than 2% for most lines. The mean accuracies of line-mixing and line-shift data are estimated to be about 15% and 25%, respectively. The results are compared with previous measurements and with values calculated using a semiclassical model based upon the Robert–Bonamy formalism that reproduces rather well the systematic experimental J and K quantum number dependencies of the self-broadening coefficients.The results concerning line mixing demonstrate a large amount of coupling between the symmetric and asymmetric components of inversion doublets mainly in the ν₄ band. The line mixing parameters are both positive and negative. More than two thirds of the lines studied here have a positive shift coefficient. However, for most of them the shift coefficients are negative in the 2ν₂ band. They are positive for the R branch of the ν₂ band and for the ^PR and ^RP branches of the ν₄ band. For the other branches they are both positive and negative. Some components of inversion doublets illustrate a correlation between line mixing and shift phenomena demonstrated by a quadratic pressure dependence of line position.     

Electronic structures of MRhO2, MRh2O4, RhMO4 and Rh2MO6 on the basis of X-ray spectroscopy and ESCA data

X-ray O Kα, Rh Mγ and a series of M Lα emission spectra, ESCA spectra of the valence and inner levels, and O K and Rh M_III quantum-yield spectra for X-ray photoemission of the rhodium double oxides MRhO₂ (M = Li, Na, K), MRh₂ O₄ (M = Be, Mg, Ca, Sr, Ba, Co, Ni, Cu, Zn, Cd, Pb), RhMO₄ (M = V, Nb, Ta) and Rh₂MO₆ (M = Mo, W) have been measured and the dependence of electronic structure on the metal M analysed. For all compounds the inner part of the valence band corresponds to O 2p_σ + O 2p_π + Rh 4d states, while the outer part corresponds to Rh 4d. The valence band is separated from the conduction band by a narrow gap of width less than 1 eV. The first empty band, near the bottom of the conduction band, is formed by Rh 4d states, followed by a band due to vacant O 2p states.     

Theoretical study of trigonal Fe center in SrCl2:Fesystem

The presence of Fe³⁺ centers with trigonal symmetry in chlorinated SrCl₂ crystal is an interesting phenomenon. By diagonalizing the complete energy matrices for a d⁵ configuration ion in a trigonal ligand-field and simulating the EPR low-symmetry parameters D and (a−F) simultaneously, the local lattice structure around trigonal Fe³⁺ center in SrCl₂:Fe³⁺ system has been studied. It is shown that Nistor et al.'s viewpoint about replacement is right, but the Cl⁻ ion along 〈111〉 axis around the Fe³⁺ center is replaced not by an O²⁻ ion but by some negative ion with effective charge to be less than that of Cl⁻ ion. Our results indicate that when the ratio of the effective charge of the negative ion to that of Cl⁻ ion is 0.8 as well as the distortion angle of the upper triangle is Δθ=−4.682°, the EPR parameters D and (a−F) can be explained satisfactorily.     

Superconductivity in porous MgB2

Porous magnesium diboride samples have been prepared by the heat treatment of a pressed mixture of Mg and MgB₂ powders. It was found that linked superconducting structure is formed down to the minimum normalized density γ_c=d/d₀≅0.16 (percolation threshold), where d is the density of MgB₂ averaged over the sample, d₀=2.62 g/cm³ is the X-ray density. Lattice parameters and critical temperature of the porous sample decrease with increasing porosity (decreasing γ) and T_c2≅32 K is minimal at γ_c. The grain boundaries in the porous samples are transparent for the current and J_c∼3×10⁵ A/cm² in self field at T=20 K in the samples with γ∼0.24.     

Pressure dependent optical absorption edge shift and compressibility of CdCr2Se4 single crystals

The pressure shift of the optical absorption edge (dE_g/dp = (1.1 ± 0.1) × 10^?6 eV bar^?1) and the compressibility (κ = (1.3 ± 0.) × 10^?6 bar^?1) of single crystalline CdCr₂Se₄ have been measured at ambient temperature. These data suggest an interpretation of the fundamental absorption in terms of either p → p interband or p → localized d charge transfer transitions, but exclude excitations involving s-band states.     

Photoelectrical memory effect in ZnIn2Se4

The compound ZnIn₂Se₄ with n-type conductivity is shown to exhibit electro-optical memory effect and negative resistance effect similar to those previously reported for ZnIn₂S₄. At low temperature ZnIn₂Se₄ material will present a high conductivity during and after illumination by light of energy greater than the band gap. This high conductivity state can be quenched in three different ways (i) by heating the sample until it reaches room temperature, (ii) by illumination with monochromatic light of appropriate energy, (iii) by an electric field. In the latter case the material exhibits a negative resistance effect in the transition between the low and high conductivity conditions. These charge storage phenomena have been explained by assuming the presence of a level, localized in the forbidden gap, which is twofold negative charged and presents a repulsive barrier to the recombination of electrons.     

Dark-current relaxation in EuGa2S4 single crystals

The results of the investigation of dark-current relaxation in EuGa₂S₄ single crystals are reported. The depth and concentration of the traps are found to be E_t = 0.79 eV and N_t = 1.64 × 10¹⁴ cm^?3, respectively.The charge accumulation region (d_c = 3.3 × 10^?5 cm) and contact capacitance (C_c = 1.23 × 10^?10 F) are also estimated.     

Sign of the electric field gradient at 57Fe nuclei in Eu3Sc2Fe3O12 garnet

The ⁵⁷Fe Mössbauer effect in conjunction with an external magnetic field H_ext has been used to measure the signs of quadrupole coupling constants Δ at the octahedral (a) and tetrahedral (d) iron sites in the Eu₃Sc₂Fe₃O₁₂ garnet. It is shown that application of H_ext to the sample at temperatures T above the Néel temperature T_N slows down the relaxation effects, whose influence on the spectra depends upon the temperature difference T-T_N. By comparing a spectrum measured at 203.0 K and in H_ext=62.4 kOe with theoretical spectra computed for different combinations of Δ(a), Δ(d) signs, it is concluded that Δ(d) is negative. The sign of Δ(a) could not be determined.     

Luminescence study of the 4f5d configuration of Nd in LiYF4, LiLuF4 and BaY2F8 crystals

Ultraviolet fluorescence of Nd³⁺ ions induced by triphotonic excitation process was studied in Nd-doped LiYF₄, LiLuF₄ and BaY₂F₈ crystals using a technique of time-resolved spectroscopy. The observed ultraviolet luminescence was due to transitions between the bottom of 4f²5d configuration and 4f³ states of Nd³⁺ ions. Narrow emission lines superposed to the broadband emissions were observed. A detailed analysis of luminescence spectrum revealed that the narrow emissions are due to parity and spin allowed radiative transitions from the Stark levels of ⁴K_11/2(5d) state created by the electrostatic interaction between the 5d electron and the two electrons of the 4f² configuration. The narrow emissions are related to the high spin state (S=3/2) which gives f-f characteristics to the f-d broadband emissions. The narrow emissions superposed to the wide emission correspond to 18%, 34% and 43% of the integrated broadband emission at 262 nm observed in LiYF₄, LiLuF₄ and BaY₂F₈ crystals, respectively. Although the 5d-4f² interaction is observed to be weaker than 5d-crystal field interaction, it is stronger enough to select only the radiative transitions from 4f²5d configuration to 4f³ states that preserves the total spin S=3/2.