X-ray natural circular dichroism in copper metaborate

The local electronic structure of copper ions in a copper metaborate CuB₂O₄ crystal is studied on the ESRF synchrotron using X-ray absorption polarization-dependent spectroscopy. The X-ray natural circular dichroism near the K absorption edge of copper is measured in the direction that is perpendicular to crystal axis c. The data obtained indicate the presence of hybridized p–d electronic states of copper. Theoretical calculations are used to separate the contributions of the two crystallographically nonequivalent positions of copper atoms in the unit cell of CuB₂O₄ to the absorption and X-ray circular dichroism spectra of the crystal.     

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.     

Crystal structure and electronic states of Co and Gd ions in a Gd<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>CoO<Subscript>2.85</Subscript> single crystal

X-ray diffraction and X-ray absorption near edge structure (XANES) spectra have been measured at the Со K-edge and Gd L ₃-edge in GdCoO₃ and Gd_0.4Sr_0.6CoO_2.85 cobaltites. The effect of Sr substitution on the crystal structure and electronic and magnetic states of Co³⁺ ions in a Gd_0.4Sr_0.6CoO_2.85 single crystal has been analyzed. The XANES measurements at the Co K-edge have not showed a noticeable shift of the absorption edge with an increase in the concentration of Sr. This indicates that the effective valence of cobalt does not change. An increase in the intensity of absorption at the Gd L ₃-edge is due to an increase in the degree of hybridization of the Gd(5d) and O(2p) states. The effect of hole doping on the magnetic properties results in the appearance of the ferromagnetic component and in a significant increase in the magnetic moment.     

Dependence of the X-Ray Dosimetric Parameters of AgGaS<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2–2<Emphasis Type="Italic">x</Emphasis></Subscript> Single Crystals on Their Composition

It is demonstrated that AgGaS_2xSe_2–2x single crystals grown by the chemical transport method are characterized by high X-ray conductivity and X-ray sensitivity coefficients at room temperature. The X-ray dosimetric parameters of crystals are compared. For example, the X-ray conductivity coefficient of AgGaS₂ samples varies within the range of 0.22–3.20 min/R at effective radiation hardness V _a = 25–50 keV and dose rate E = 0.75–78.05 R/min. Single-crystalline AgGaSe₂ samples have higher X-ray dosimetric coefficients than AgGaS₂-based samples. The X-ray conductivity coefficient of AgGaSe₂ varies within the range of 1.2–8.5 min/R at effective radiation hardness V _a = 25–50 keV and dose rate E = 0.75–31.3 R/min. The dose dependences of steady-state X-ray current in AgGaS_2xSe_2–2x are determined.     

Low-Temperature Schottky Anomalies and the Magnetic State of the <Emphasis Type="Italic">p</Emphasis> Electrons of Oxygen in Substituted Gd<Subscript>0.4</Subscript>Sr<Subscript>0.6</Subscript>CoO<Subscript>3 – δ</Subscript> Cobaltites

The XANES spectra (X-ray absorption near-edge spectra) at the K edge of Co and the L₃ edge of Gd in polycrystalline Gd_0.4Sr_0.6CoO_{3 – δ} rare-earth oxides with an ordered and disordered distribution of Gd³⁺ and Sr²⁺ cations over the A sites in the crystal lattice are measured. The results of XANES measurements do not reveal a noticeable shift in the absorption edge with increasing Sr concentration as compared to the GdCoO₃ parent composition. The measured temperature dependences of the heat capacity of polycrystalline ordered and disordered samples and a single-crystal ordered Gd_0.4Sr_0.6CoO_2.85 sample exhibit two Schottky anomalies. These anomalies are thought to be related to the high-spin state of the Co³⁺ ions in the pyramidal environment caused by oxygen deficiency and to the magnetic state of oxygen p electrons induced by the doping-assisted generation of a hole in the 2p state. The absence of a noticeable shift in the absorption edge and the presence of two Schottky anomalies support the fact that the charge state of cobalt remains unchanged in the compounds under study.     

Electronic and Magnetic States of Pr and Mn in the Pr<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> Films Studied by XANES and XMCD Spectroscopy

The spectral dependences of X-ray absorption near-edge spectroscopy (XANES) and X-ray magnetic circular dichroism (XMCD) and the field dependences of XMCD near the K edge of Mn and the L_2,3 edges of Pr in the Pr_0.8Sr_0.2MnO₃ and Pr_0.6Sr_0.4MnO₃ films at T = 90 K are studied. The spectral dependences point to a mixed valence state of Mn and Pr in the films. It is found that, as compared to XANES, XMCD is more sensitive to the valence state of Pr⁴⁺. The field dependences of XMCD point to ferromagnetic behavior of Mn ions and the Van Vleck paramagnetism of Pr ions, which makes a significant contribution to the total magnetization of the films. It is shown that as the Sr concentration increases, the XMCD intensity at the K edge of Mn increases, which indicates a growth of the total magnetic moment of the film due to an increase in the 4p–3d hybridization.     

Optical properties of mechanically compressed K<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of a uniaxial mechanical compression (σ_m ≤ 100 bar) on the spectral dependences (300–800 nm) of the birefringence Δn _i and refractive indices n _i of K₂SO₄ crystals is studied. The electronic polarizabilities, refractions, and parameters (λ_0i , B _1i ) of ultraviolet oscillators of mechanically compressed crystals are calculated. It is shown that the dispersions of Δn _i(λ) and n _i(λ) are normal and sharply increase near the absorption edge. It is found that the uniaxial compression changes the value of the dispersions dΔn _i/dλ and dn _i/dλ rather than their character. It is ascertained that the simultaneous action of the compressions σ_x and σ_z, as well as of σ_y and σ_z, leads to the appearance of new isotropic states in the K₂SO₄ crystal, which manifests itself in the equality of corresponding birefringences. It is shown that the baric dependences n _i(σ) are determined by the change in the density of oscillators (～30%), by the shift of the absorption edge and effective band maximum and by the change in the oscillator strength (～70%).     

Numerical simulation of the spectrum of X-ray natural circular dichroism in the CsCuCl3 crystal

The possibility of observing x-ray natural circular dichroism (XNCD) in enantiomorphous CsCuCl₃ crystals has been considered. The spectra of XNCD in CsCuCl₃ have been numerically simulated for the K, L1, L2, and L3 x-ray absorption edges of copper, cesium, and chlorine using FDMNES and LMTO programs. It has been shown that the largest value of dichroism signal can be expected near the copper and chlorine L1 edges.     

Exciton luminescence of YAlO<Subscript>3</Subscript> single crystals and single-crystal films

The paper reports on a study of exciton luminescence in single crystals (SCs) and single-crystal films (SCFs) of YAlO₃, which have substantially different concentrations of vacancy-type and substitutional defects, under excitation by synchrotron radiation near the fundamental absorption edge. The radiative annihilation of excitons in SCFs was shown to occur primarily at regular perovskite lattice sites and to be accompanied by luminescence in a band peaking at λ_max = 295 nm with τ = 5.2 ns. In contrast to SCFs, the radiative exciton decay in YAlO₃ SCs takes place predominantly near vacancy-type defects (F⁺ and F centers) and is accompanied by luminescence in the bands at λ_max = 350 nm (τ = 2.5 ns) and 440 nm (τ₁ = 1.9 ns, τ₂ = 30 ms). Photoexcitation in the 175-nm band of YAlO₃ SCs revealed photoconversion of the centers F → F⁺.     

Population and current noise in semiconductor laser junctions

The population noise in a semiconductor laser is calculated by means of the quantum mechanical Langevin method. The resulting population noise is given by 〈δ N _c ² 〉=(T _c/2) (rate in+rate out)+K(¯n), whereN _c is the total number of electrons in the conduction band in the active region,T _c is a relaxation time. The first expression is the usual shot noise term. The transition rates are the sum of the rates due to the light field, the pumping and the spontaneous emission. The last termK(¯ n) is caused by the light field fluctuations;¯n is the mean number of photons in the laser mode.K(¯ n) consists of two parts: a) The main part is proportional to the intensity noise of the light field, which increases below but near threshold and gets constant above threshold. b) There is a second term due to the fact that parts of the fluctuations of the population and of the light field are correlated. — The noise spectrumS _I(ω) of the junction currentI is calculated for low frequencies. Beyond the usual shot noise termS _I(0)=2eI, additional noise is found in and above the threshold region, a) mainly because of the fluctuations of the light field in the laser mode and b) to a small amount, because the absorption processes due to the laser photons weaken the forward current, which is carried by emission processes, while the absorption noise adds to the emission noise.     

Preparation and characterization of cerium doped LaCoO<Subscript>3</Subscript> perovskite

Cerium doped lanthanum cobaltite perovskites La_1–x Ce _x CoO₃ with x = 0, 0.2, 0.4 were prepared by the sol-gel method (calcined for 5 h at 750°C) and characterized by X-ray diffraction, X-ray absorption, energy dispersive X-ray spectroscopy and Brunauer–Emmett–Teller surface area analysis. The results showed that the cerium doping promoted the structural transformation of LaCoO₃ from rhombohedral into cubic structure. High specific surface area and small crystallite size were achieved at x = 0.2. The X-ray absorption results confirmed the formation of compound La_1–x Ce _x CoO₃.     

Resonant Raman scattering in superconducting Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript>x</Subscript>BiO<Subscript>3</Subscript>

The effect of the photon energy of the exciting laser radiation on the Raman spectra of Ba_1?xK_xBiO₃ with x=0.25, 0.40, and 0.50 is studied. An increase in the laser wavelength from 488 to 750 nm scarcely affects the amplitudes and frequencies of the spectral lines in the Raman spectra of the nonsuperconducting compound with x=0.25. For the optimally doped (x=0.40) and overdoped (x=0.50) superconducting compounds, a substantial increase in the line intensity and a considerable shift of the characteristic frequencies are observed. This result suggests that, in the whole range of superconducting compositions 0.37≤x≤0.50, the local symmetry of the Ba_1?xK_xBiO₃ crystal lattice differs from the perfect cubic symmetry, which should take place according to the literature data. The fact that resonance phenomena are observed when the laser photon energy is shifted toward the optical gap testifies to the presence of local electron pairs in the whole range of superconducting compositions 0.37≤x≤0.50 and is evidence in favor of the superconductivity mechanism proposed for Ba_1?xK_xBiO₃ on the basis of the X-ray absorption studies in our previous paper.     

Effect of Nitrogenation and Hydrogenation on the Magnetic Properties and Structure of the Sm<Subscript>2</Subscript>Fe<Subscript>17</Subscript> Alloy: Analysis of XMCD Data

Changes in the local magnetic and structural properties of Sm₂Fe₁₇ alloys at nitrogenation and hydrogenation of samples have been studied by the X-ray magnetic circular dichroism (XMCD) technique at the Fe K absorption edge and Sm L₃ absorption edge using synchrotron radiation. The results have been discussed in comparison with X-ray diffraction data and macroscopic vibration magnetometry measurements. The observed changes in XMCD spectra indicate a noticeable effect of nitrogenation on the local magnetic properties of sublattices of both iron and samarium, whereas hydrogenation of samples makes a small effect. The mentioned effects have been analyzed and discussed in terms of the effect of nitrogen (N) and hydrogen (H) interstitial atoms on Sm 5d and Fe 4p electronic states. The effect of nitrogenation is larger than the effect of hydrogenation because the volume expansion of the crystal lattice of initial Sm₂Fe₁₇ in the case of nitrogenation is larger than that in the case of hydrogenation. The studied local magnetization curves for samarium and iron sublattices in magnetic fields up to 17 T also indicate a strong increase in the magnetocrystalline anisotropy at nitrogenation.     

On correlators for high multiplicity events

We present the results of the study of the energy correlators K₂(n) and K₃(n) and their ratio R₃(n) as a function of the hadron multiplicity at the LHC. The PYTHIA generator has been used. PYTHIA predicts that R₃(n) is not dependent on multiplicity. K₂(n), K₃(n), and the R₃(n) ratio can be studied at ATLAS.     

Optical and electrical properties of β-Tl<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript>x</Subscript>InS<Subscript>2</Subscript> (0 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 0.015) single crystals

The peculiarities of fundamental optical absorption, thermally stimulated conductivity, and depolarization currents in β-Tl_{1 ? x} Cu_xInS₂ (0 ≤ x ≤ 0.015) single crystals have been investigated in the temperature range 4.2–300 K. It is found that the temperature coefficient of the band gap E _g changes near the temperature of the structural phase transition.     

Die absolute Intensität der durch Elektronenstoß in einer dicken Cu-Antikathode erzeugtenKα-Strahlung

The numberN _Kα ^dir (produced) ofKα-photons produced by electron-bombardment in a thick target of copper per incident electron has been measured absolutely with the Ross-filter method and relatively with the crystal-spectrometer method in the energy-region up to the four times theK-ionization energyE _K . The result can be presented in the following empirical form:N _Kα ^dir (produced) = 4π·

     

Electronic energy structure and x-ray spectra of wide-gap AlN and BN crystals and B<Subscript>x</Subscript>Al<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N solid solutions

The electronic energy structure of 2H and 3C AlN and BN crystals and B_xAl_1?xN solid solutions is calculated on the basis of the local coherent potential method using the cluster version of the MT approximation and the theory of multiple scattering. The features of the electronic structure of 2H-AlN crystals are compared with x-ray K and L absorption and emission spectra of aluminum and nitrogen. An interpretation of these features is given. The concentration dependences of the width of the upper subband of the valence band and the band gap in B_xAl_1?xN solid solutions (x = 0.25, 0.5, 0.75) are investigated. Charge transfer from aluminum to nitrogen atoms is shown to occur and increase with boron doping in both crystallographic modifications.     

Optical effects caused by coincidence between the energies of the plasma oscillations and the band-to-band transition in bismuth crystals doped with an acceptor impurity

The energy of plasma oscillations of free charge carriers in bismuth crystals ?ω_p can be qual to the band gap at the L point of the Brillouin zone E _gL as a result of doping with an acceptor impurity. Variation in the edge shape and splitting of the minimum in the plasma reflection are observed in experimental studies of reflection under normal incidence of radiation on the crystal. An analysis of the totality of available experimental data shows that the above special features are caused by interaction of elementary excitations (such as the plasma oscillations) with band-to-band transitions. It became possible for the first time to ascertain the composition of the bismuth crystals for which the condition ?ω_p=E _gL is satisfied and observe the variation in the characteristics of the plasma oscillations of free charge carriers, which occurs as a result of electron-plasmon interaction.     

Effect of electron irradiation on the galvanomagnetic properties of In<Subscript>x</Subscript>Bi<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>3</Subscript> semiconductor single crystals

The effect of 5-MeV electron irradiation of p-In_xBi_2?xTe₃ single crystals (x=0, 0.04, 0.07), performed at 250 K, on the galvanomagnetic properties of the crystals was studied. The irradiation was shown to change the conduction from the p to the n type. Annealing at temperatures of 310–390 K restores the conduction to the p type. The reversal of the conduction type and variation of the carrier concentration can be accounted for by an increase in the concentration of charged point radiation defects produced in In_xBi_2?xTe₃ by irradiation. Electron irradiation of p-type Te single crystals reduces the electrical resistivity without reversing the conduction type. Annealing restores the original properties almost completely.     

Two-Photon Absorption of Nonchain HF Laser Radiation in Germanium Single Crystals

Some results of experimental and numerical studies on the transmission of a nonchain high-frequency (HF) laser beam through germanium (Ge) single crystals of differing thickness and specific resistance are presented. Based on the experimental data for the HF(DF) lasing spectrum, the coefficient of two-photon absorption in germanium has been estimated as K₂ = 55 ± 10 cm/GW at λ = 2.8 μm. The results are in good agreement with theory. The developed experimental data-processing software has enabled the numerical study of the transmission of a high-power beam with λ = 2.6–3 μm through Ge at any laser-pulse-impact moment. It has been shown that thin germanium coatings can efficiently smoothen the distribution of energy over the beam aperture for high-power laser radiation with λ = 2.6–4 μm.