X-ray spectra and electronic band structure of nitrogen in Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solid solutions

The electronic energy structure of the valence band and the x-ray absorption near edge structure (XANES) region of nitrogen in Al _x Ga_1?x N solid solutions and binary crystals of gallium nitride GaN and aluminum nitride AlN are calculated using the local coherent potential method and the cluster version of the muffin-tin approximation within the framework of the multiple scattering theory. It is demonstrated that the calculated electron densities of states correlate with the nitrogen K x-ray emission and nitrogen K x-ray absorption spectra. The electronic energy structure of the top of the valence band and the XANES region in Al _x Ga_1?x N solid solutions are compared with those in the binary crystals of the GaN and AlN nitrides, and an interpretation of their specific features is proposed. An analogy is drawn between the evolution of the electronic energy structure of the valence band and the XANES region in the alloys under investigation and the evolution of the electronic band structure in the Al _x B_1?x N and B _x Ga_1?x N alloys. General trends in the transformation of the structure and variations in properties of these alloys are discussed.     

Terahertz radiation by spontaneous polarization fields in InN

Terahertz (THz) radiation is measured from InN excited by femtosecond optical pulses at 790 nm. The InN sample is grown by metalorganic chemical vapor deposition. The InN surface shows typical grain like morphology often observed in N-polar InN. The polarity of the THz radiation field from InN is opposite to that from p-InAs, whose radiation mechanism is dominated by the photo-Dember effect, and is same as that from p-GaAs indicating that the dominant radiation mechanism in InN is the drift current induced by the internal electric field at low-density excitation. In addition, since no azimuthal angle dependence of the THz radiation is observed, the optical rectification effect is ruled out. The lattice constant c=5.7091 Å for the InN epilayer is determined by θ–2θ scan of X-ray diffraction. This implies that the InN epilayer is fully relaxed. Therefore, there is no piezoelectric polarization, and the internal electric field in the InN epilayer results from spontaneous polarization. According to the direction of the spontaneous polarization, the N polarity of the InN sample is confirmed.     

Study of local atomic and electronic structure of titanium-containing forsterite based on analysis of X-ray absorption spectra

The local atomic structure of titanium-containing forsterite is investigated based on analysis of X-ray absorption near-edge structure (XANES) spectra. The Ti K-XANES spectra of the mineral under study are calculated for several possible structural models. Theoretical analysis of the X-ray absorption spectra is performed based on the full-potential finite-difference method. The local geometry of the titanium environment is determined by comparing the experimental Ti K-XANES spectrum with the theoretical spectra calculated for different structural models. The structural model where titanium atoms substitute for silicon ones is found to be most likely for titanium-containing forsterite. The calculated partial densities of electron states of titanium near the bottom of the conduction band of titanium-containing forsterite are analyzed.     

The nature of a selective peak in the BK-absorption edge spectrum and the electronic energy band structure of the crystal 3C BN0.99

A new interpretation of the nature of the resonance in the quantum-yield K spectra of boron in the crystal 3C BN is proposed. This interpretation is based on calculation of the electronic energy band structure of the nonstoichiometric boron nitride 3C BN_0.99, which is carried out by the local coherent potential method in the multiple-scattering approximation. The tops of the valence band and of the XANES range of nonstoichiometric and perfect crystals of boron nitride are compared with the x-ray photoelectron spectrum of 3C BN and the BK-absorption edge spectrum. The electronic states near the BK-absorption edge are modeled and discussed for the relaxed and metastable states caused by the formation of vacancies in the nitrogen sublattice.     

Local electronic structures at selected sites of intermetallic perovskites Mn3MeX (Me=divalent metal,X=N,C)

Summary We have measured the XANES (X-ray absorption near edge structure) spectra on the manganese and metal MeK-edges of the intermetallic perowskites Mn₃MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) to investigate the distribution on the partial density of the empty states. The information on the local electronic structure at the two differents sites, obtained from the spectra in the first 10 eV above the onset, is discussed in terms of one-electron band model. The relative shape changes in the spectra between the different compounds are explained by the different magnitude of hybridization between the electronic states of different atoms and/or the filling of the empty band by the electrons supplied by the substituted metal. Moreover, information about the local geometrical structure has been obtained from the (10÷50) eV range of the spectra.

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Riassunto La distribuzione della densità parziale degli stati vuoti nelle perovskiti intermetalliche Mn₃MeX (X=N, Me=Mn, Cu, Zn, Ga, Sn; X=C, Me=Ga, Sn) è stata studiata mediante spettroscopia XANES (X-ray absorption near edge structure) alle soglieK del Mn e del metallo Me. Le informazioni sulla struttura elettronica locale di entrambi i siti, ottenute dall’analisi dei primi 10 eV degli spettri XANES, sono discussed in termini di un modello proposto. Il cambiamento relativo della forma negli spetttri dei diversi composti è posto in relazione alla differente ibridizzazione tra stati di atomi diversi e/o dal riempimento degli stati della banda vuota con elettroni forniti dal metallo sostituente. Sono state anche ottenute informazioni sulla struttura geometrica locale analizzando gli spettri nell’intervallo da 10 a 50 eV.

     

Multiple scattering approach to Co K-edge XMCD and XANES spectra in Gd–Co alloys

We have measured and analyzed Co K-edge X-ray absorption near edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) near edge spectra in crystalline and amorphous GdCo₂ alloys. We have used a semi-relativistic full multiple scattering approach to the analyses of the XMCD spectra. A general formula is obtained which is applicable to randomly oriented systems in space. Useful information is obtained on both the electronic and geometric structure around a Co atom. The difference in XANES and XMCD spectra between crystalline and amorphous GdCo₂ is well explained by models referring to the anomalous X-ray scattering result, where three Gd atoms in the second shell are removed away. The calculated XANES are not so sensitive to the electronic structure, whereas the calculated XMCD spectra are rather sensitive to the spin polarization on Co atoms. The result shows that the spin polarization on Co atoms in GdCo₂ alloys is smaller than that in Co metals.     

Numerical study of the extinction matrix for a plate crystal

Energy and polarization characteristics of optical radiation passed through a semitransparent plate crystal with a preset orientation in space are studied numerically within the framework of the method of physical optics. Results of calculations of the extinction matrix elements versus particle size, refractive index, crystal orientation, and incident radiation wavelength (from 0.5 to 15 μm) are presented. It is demonstrated that K₁₁, K₁₂, and K₃₄ are most informative among the elements of the extinction matrix. It is established that the first of them is most sensitive to changes in the microphysical and orientational parameters of the particle when the angle of radiation incidence on the plate changes from 0 to 20°, and the nondiagonal elements are most sensitive when the angle of radiation incidence is greater than 40°. The characteristics of the total field scattered near the forward direction are determined. It is established that their dependence on the physical parameters of the crystal is most strong at scattering angles smaller than or equal to 4° and wavelengths from the IR range (in particular, from the atmospheric transparency window).     

高压下闪锌矿InN光电性质的密度泛函研究

 采用密度泛函理论,计算了闪锌矿型InN在压力下的结构、力学性质和光学性质,结果显示,随着压强的增大晶格常数减小。给出了零压下C₁₁、C₁₂、B、C_s、C₄₄的值及至70 GPa压力下弹性常数随压强的变化关系。结果表明,C₁₁、C₁₂、B随压强增大而增大,C_s、C₄₄随压强增大而减小,计算结果与现有实验和理论结果符合较好。在价带区,InN的分态密度（PDOS）有两个带,且在费米面附近密度很小,显示其倾向于形成稳定结构并且导电性较差。对闪锌矿型InN在高压下的光学性质研究发现,导带电子向高能方向偏移,而价带电子向低能方向偏移,结果导致能带间隙增大,光吸收谱在压力的作用发生了“蓝移”。研究结果对认识高压下闪锌矿型InN的结构、电学及光学性质具有重要意义。     

Local electronic structure around Mn and Ga in Mn-doped GaN films showing room temperature ferromagnetism

Local electronic structures around Ga and Mn in Mn-doped GaN film with T_c of 940 K are investigated by K X-ray absorption near edge structure (XANES) analysis. It was found that the shape of the Ga XANES spectrum is remarkably similar to that of the un-doped GaN film indicating that the local electronic structure around Ga is not disturbed with Mn doping. As for the Mn XANES spectra, obvious pre-edge peaks were observed: the fine structures in the pre-edges correspond with calculated Mn 3d partial density of states which predict impurity band formation with the Fermi energy stays in the spin-up band. These findings imply that Mn 3d levels stay within the gap with the Fermi energy stays in the spin-up band.     

Effect of local atomic phase separation in the x-ray absorption near edge structure spectroscopy of FeSexTe1−x

Ab initio X-ray absorption near edge structure (XANES) calculations for FeSe_xTe_1−x, using a structural model that combines FeSe and FeTe phases at the nanoscale, are compared with Fe K-edge XANES measurements in the “pre-edge” region. The important aspects of this model are (i) magnetic order in the FeTe phase; (ii) Se and Te atoms placed randomly in both FeSe and FeTe crystallographic positions and; (iii) the two distinct distances for Fe–Se and Fe–Te of the bulk phases. The calculated spectra reproduce the observed increase of spectral weight in the experiments on FeSe_xTe_1−x with Se concentration. This is consistent with an inhomogeneous local electronic structure of FeSe_xTe_1−x. Additionally, we have calculated projected electronic density of d-states for the Fe atom, revealing increased spectral weight in the “pre-edge” region of the XANES spectra, which correlates with the increase in Se concentration. The decrease of calculated Fe d-density of states for the Fermi level, N(ε_F), for high Te content is consistent with the suppression of superconductivity in the title system.     

Investigation of the temperature dependence for the spectra of supercooled water in the middle infrared

The temperature dependence of the bending ν₂, combination ν₂ + ν _L , and stretching (ν₁, ν₃, 2ν₂) absorption bands in the infrared spectra of supercooled water with a temperature-change step Δt from 2 to 2.5°C was studied using an advanced infrared Fourier spectrometer. It was found that the frequency of the maximum of the stretching absorption band (2700–3700 cm^?1) decreases with the reduction of the water temperature from ?0.5 to ?5.0°C. The frequency of the maximum of the combination absorption band (2130 cm^?1) increases with the reduction of the water temperature in a range from ?3.0 to ?5.0°C. The frequency of the maximum of the absorption band of bending oscillation (1640 cm^?1) is invariable with a reduction of the water temperature from ?0.5 to ?5.0°C.     

X-ray spectroscopic identification of garnet from the placer deposits of the Taman peninsula

Garnet from recent placer deposits of the Taman peninsula has been investigated by energy-dispersive X-ray fluorescence microanalysis and X-ray absorption near-edge structure (XANES) spectroscopy. Energy-dispersive X-ray fluorescence microanalysis showed that the chemical composition of the garnet under study corresponds to pyrope-almandine-spessartine series. The Fe K-edge X-ray absorption spectra of garnet have been recorded using a Rigaku R-XAS laboratory spectrometer. Iron K-edge XANES spectra for two iron-containing garnet minals (components), almandine and andradite, have been calculated using the full multiple-scattering and finite-difference methods. Based on a comparison of the experimental and theoretical Fe K-XANES spectra, it is concluded that recent magnetite-garnet placer deposits of the Taman peninsula contain garnet in the form of almandine.     

Electroreflectance of strontium titanate

The room temperature electroreflectance (ER) measurements for cubic SiTiO₃ are reported with particular emphasis on experimental conditions important for consistent reproducible lineshapes. This study includes for the first time a determination of the position of zero band perturbation (flatband) by making use of the even field nature of the ER lineshapes. The ER spectra are dependent upon dc bias, indicating the importance of a knowledge of the flatband position. A Kramers-Kronig analysis is performed and the lineshapes are reported in terms of ΔR/R, Δε₁ and Δε₂. The results in terms of Δε₂ show that the strongest direct transition occurs near 3.8 eV. The weak structure in the 3.4 eV region is due to either indirect transitions or weak direct transitions. Our unpolarized flatband ER spectra give rise to structures which for (110) samples are resolved into contributions from [001] and [12&#x0304;10] polarized spectra. In addition to the large oscillations observed above the band gap, weaker oscillatory structure is observed in the range 1.5 to 2.8 eV in the polarized spectra of (110) faced samples. The oscillations for [11&#x0304;0] polarization are 180° out of phase with those for [001] polarization.     

Polarization parameters of elastic pp scattering in the angular range 22.5°–42.5° at an energy of 1 GeV and partial-wave analysis of data

The polarization properties of elastic scattering are studied experimentally for pp interaction at 1 GeV. The respective experimental results are presented for the spin-transfer parameter K _n00n and the polarization P _n000 at c.m. angles of 22.5°, 27.5°, 32.5°, and 42.5° and for the analyzing power A _000n at c.m. angles of 22.5° and 27.5°. The experiment employed an unpolarized proton beam from the accelerator of the Petersburg Nuclear Physics Institute, a polarized proton target, and a high-resolution magnetic spectrometer. The data from this experiment are analyzed and compared with the predictions of partial-wave analyses.     

X-ray absorption near edge structure in solid Kr and KrF2

X-ray absorption near edge structure (XANES) spectra of gaseous Kr, solid Kr and solid KrF₂ have been obtained and compared to differentiate between continuum resonances near the edge and bound state transitions. By comparing experimental and ab initio (MSW-Xα) calculated spectra the influence of environment as well as molecular bonding on XANES is demonstrated.     

Calculation of the X-Ray emission K and L 2,3 bands of metallic magnesium and aluminum with allowance for multielectron effects

A procedure is proposed to calculate the shape of the characteristic X-ray emission bands of metals with allowance for multielectron effects. The effects of the dynamic screening of a core vacancy by conduction electrons and the Auger effect in the valence band are taken into account. The dynamic screening of a core vacancy, which is known to be called the MND (Mahan-Nozeieres-De Dominics) effect, is taken into account by an ab initio band calculation of crystals using the PAW (projected augmented waves) method. The Auger effect is taken into account by a semiempirical method using the approximation of a quadratic dependence of the level width in the valence band on the difference between the level energy and the Fermi energy. The proposed calculation procedure is used to describe the X-ray emission K and L _2,3 bands of metallic magnesium and aluminum crystals. The calculated spectra agree well with the experimental bands both near the Fermi level and in the low-energy part of the spectra in all cases.     

Photoemission investigation of hexagonal BN: Band structure and atomic effects

Photoemission valence band spectra and the polarization dependent photoyield at the B K-edge are compared to calculated densities of states. Excitonic effects at the edge are discussed. For the decay of exciton and ionized core the influence of atomic effects is shown.     

Subband transitions in AlxGa1−xN /GaN/ AlxGa1−xN and AlxGa1−xN /InN/ AlxGa1−xN single quantum wells

We have calculated the spectral regime of subband transitions in Al_xGa_1−xN/GaN and Al_xGa_1−xN/InN single quantum wells. We used a simplified model to account for the internal electric fields, which modify the shape of the quantum well. Some of the parameters for these materials have not yet been firmly established. Therefore, we carried out the analysis for the extremes of the reported values of conduction band discontinuities and band gaps (in the case of InN). This analysis shows that the spectral regime of interband transitions for 1–4 nm thick wells has wavelengths above 0.5 μm for AlGaN/InN and above 0.8 μm for AlGaN/GaN and both heterostructures cover several μm wavelengths. The spectral variation with alloy composition is less pronounced in the Al_xGa_1−xN/InN single quantum wells due to the higher electric field present across the InN quantum well as compared to GaN. The results of these calculations are in good agreement with more rigorous theoretical approaches and available experimental values for Al_xGa_1−xN/GaN.     

Einfluß des elektrischen Feldes auf die fundamentale Absorptionskante von Bariumtitanat

The shift of the absorption edge of BaTiO₃ caused by an electric field has been measured in function of temperature and field strength. It shows a sharp maximum at the Curie pointT _C near 120°C, a linear field dependence belowT _C, and a quadratic field dependence aboveT _C. Furthermore this shift has the opposite sign to the Franz-Keldysh effect and is one or two orders of magnitude larger. This new effect is correlated with the electric polarization and is not induced by electromechanical lattice deformations. It is shown in a simple band model, that the relative displacement between the titanium and the oxygen sublattices induces an increase of the band gap, which agrees qualitatively with the experimental results.     

The growth temperatures dependence of optical and electrical properties of InN films

InN films grown on sapphire at different substrate temperatures from 550°C to 700°C by metalorganic chemical vapor deposition were investigated. The low-temperature GaN nucleation layer with high-temperature annealing (1100°C) was used as a buffer for main InN layer growth. X-ray diffraction and Raman scattering measurements reveal that the quality of InN films can be improved by increasing the growth temperature to 600°C. Further high substrate temperatures may promote the thermal decomposition of InN films and result in poor crystallinity and surface morphology. The photoluminescence and Hall measurements were employed to characterize the optical and electrical properties of InN films, which also indicates strong growth temperature dependence. The InN films grown at temperature of 600°C show not only a high mobility with low carrier concentration, but also a strong infrared emission band located around 0.7 eV. For a 600 nm thick InN film grown at 600°C, the Hall mobility achieves up to 938 cm²/Vs with electron concentration of 3.9 × 10¹⁸ cm⁻³. Supported by the National Basic Research Program of China (Grant No. 2006CB6049), the National Natural Science Foundation of China (Grant Nos. 6039072, 60476030 and 60421003), the Great Fund of the Ministry of Education of China (Grant No. 10416), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20050284004), and the Natural Science Foundation of Jiangsu Province of China (Grant Nos. BK2005210 and BK2006126)