Effective charge in the II–VI and III–V compounds with zincblende or wurtzite type structure

The effective charge in the II–VI and III–V compounds was analyzed by using a linear chain model. On the assumption that the ionic lattice is immersed in a cloud of valence electrons with the dielectric constant of ?(∞) = n²?₀ (n: refractive index, ?₀ = 8·85 × 10^?12f/m), the effective charge on an ion is equal to 2e in the II–VI compounds and to e in the III–V compounds (e: electronic charge), respectively. These values of the effective charge are just n times the Szigeti charge.Although direct connection between neighboring atoms is the main part of the binding force, the fact can not be neglected that the second nearest neighbor atoms are connected by sharing some of valence electrons between them. The electrons in valence bonds contribute to the refractive index and are estimated to be e and 2e per atom in the II–VI and III–V compounds, respectively.     

Penning ionization electron spectroscopy of CO,HCl, HBr and of triatomic molecules N2O,NO2, CO2, COS and CS2

The energy of electrons released in ionization of di- and triatomic molecules by He (2¹S, 2³S), Ne and Ar (³P_{0, 2}) metastable atoms was measured. Attention was concentrated on the determination of peak shifts and peak form in order to elucidate new features of the mechanism of ionization with respect to ionization of atoms. In ionization by He (2¹S) atoms, asymmetric peak forms and greater shifts towards lower electron energy were found than in ionization by He (2³S) atoms. A tentative explanation for peak shifts in terms of orientation of molecules during the collision and of molecular orbitals involved in ionization was proposed. Effects due to J values of metastable atoms and molecular ions were described. The ionization of electronegative molecules (NO₂) probably takes place already at large particle separation because of mixing with the X⁺ABC^? ionic state which, on recombination, ionizes into X + ABC⁺ + e.     

Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect

Double perovskite, (Sr_1−xNd_x)₂FeMoO₆, was doped with electrons through partial substitution of divalent Sr by trivalent Nd (0≤x≤0.2). The Fe valence and the degree of B-site order were probed by ⁵⁷Fe Mössbauer spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms occupying wrong sites, i.e. antisite disorder. It had very little effect on the Fe valence: a small but visible increase in the isomer shift was seen for the mixed-valent Fe^II/III atoms occupying the right site indicating a slight movement towards divalency of these atoms, which was more than counterbalanced by the increase in the fraction of antisite Fe atoms with III valence state. It is therefore argued that the bulk of the electron doping is received by antisite Mo atoms, which - being surrounded by six Mo^V/VI atoms-prefer the lower IV/V valence state. Thus under Nd substitution, the charge-neutrality requirement inflicts a lattice disorder such that low - valent Mo^IV/V can exist.     

FLAPW-GGA calculations of the influence of Mn, Fe, and Co impurities on the electronic and magnetic properties of layered oxychalcogenides LaCuSO and LaCuSeO

The electronic and magnetic properties of oxychalcogenides LaCuSO and LaCuSeO with a layered ZrCuSiAs-type structure doped with impurity atoms M = Mn, Fe, and Co have been predicted using the first-principles FLAPW-GGA method. It has been shown that a partial substitution of 3d ^{n < 9} metal atoms for copper atoms in the structure of the initial matrix leads to the transition of the oxychalcogenides (nonmagnetic semiconductors) to the state of a magnetic half-metal with 100% spin polarization of near-Fermi electrons. In this case, the magnetic and conducting properties of the LaCu_{1 ? x} M _x S(Se)O systems are determined by the states of the [Cu₂(S,Se)₂] blocks with magnetic impurities separated by nonmagnetic semiconducting [La₂O₂] blocks.     

Interpretation of auger energy shifts of silicon in solid silicon compounds

Chemical shifts of Auger transitions and photoelectron binding energies of silicon have been measured and interpreted using the quasi-atomic approach. The Si KL_2,3L_2,3 and L_2,3V₁V₁ Auger transitions and the binding energies of Si 2p and of the valence electrons at the maximum of the density of states V₁ have been investigated in solid silicon and in the compounds SiC, Si₃N₄, SiO₂, Na₂SiF₆ and T₃Si (T = V, Cr, Mn, Fe, Co, Ni). The relaxation-energy shift ΔR^ea_S(2p, 2p) describing the polarization effect (final-state effect) has been evaluated by AES and XPS measurements. Furthermore, the extra-atomic relaxation energy R^ea_D(2p) of the 2p electrons has been determined experimentally for silicon atoms in differing environments. This allows estimation of the potential parameter V(2p) describing the potential effect (initial-state effect). In general R^ea_D(2p) was found to be more sensitive to changes in chemical bonding than V2p). The behaviour of the quasi-atomic Si V₁ electrons seems to be the converse.     

First-principles study on the structural and electronic properties of LiB and its hydrides (Li2BnHn, n=5, 8, 12, LiBH4)

Structural, electronic and vibrating properties of LiB and its hydrides (Li₂B_nH_n, n=5, 8, 12, LiBH₄) were calculated by the first-principles using density functional theory in its generalized gradient approximation. The calculated results are in good agreement with experimental studies. The deviation between theory and experimental results are also discussed. With the increasing of H atoms in range of 5-12, the band gap energy increases and the width of the conduction band decreases. Comparing with LiB, the band gap of LiBH₄ is broadened, which indicates the enhancement of Li-B and Li-H bond strength. Valence electrons mainly transfer from Li atoms to B and H atoms. As a result, Li atoms are thought to be partially ionized as Li⁺ cations. There is little contribution of Li orbital to the occupied states, resulting in Li-H and Li-B bond exhibiting an ionic nature, and B-H bond showing a covalent nature.     

Spectroscopic study of the formation of excited strontium ions upon interaction of electrons with metastable atoms

The interaction of electrons with metastable Sr atoms, as a result of which the latter are simultaneously ionized and excited, is investigated experimentally. The magnitudes of the interaction cross sections (reaching ≈5 × 10^?20 m² for the 5² S _1/2-3² P _3/2 transition in Sr II) and their energy dependences are determined. Possible mechanisms of this interaction are discussed.     

Spinpolarisation und Winkelverteilung der Elektronen nach Anregung verschiedener Zustände des Hg-Atoms

At energies above ～ 50 eV, electrons scattered inelastically from mercury atoms show a similarity to elastically scattered electrons in both spin polarization and angular distribution. This similarity vanishes at primary energies below ～ 30 eV for the 6p′³ P ₁-channel (11.0 eV energy loss) and below～ 25 eV for the 6¹ P ₁-channel (6.7 eV energy loss).     

Superconductivity recurrence and charge carrier compensation effect in La2−xSrxCu0.94M0.06O4 (M = Fe,Co and Ni) system

In this paper, the substitution effect of Sr for La in three non-superconducting system La_2−xSr_xCu_0.94M_0.06O₄ (M = Fe, Co and Ni) have been systematically studied. As the concentration of Sr increases to around 0.20, the superconductivity is recovered. The peak at 695 cm⁻¹ in the infrared spectra which is assigned to the stretching mode of oxygen atoms in plane induced by M³⁺ doping is suppressed by Sr increasing, demonstrating that the holes introduced by the Sr substitution have been injected into the CuO₂ plane and compensate the localized electrons. These results suggest that the superconductivity recurrence is caused by the carrier compensation. The main origin of superconductivity suppression by the magnetic ions Fe/Co/Ni is the carrier localization effect.     

Magnetotransport in modulation-doped In0.53Ga0.47As/In0.52Al0.48As heterojunctions: in-plane effective mass,quantum and transport mobilities of 2D electrons

Shubnikov–de Haas (SdH) and Hall effect measurements, performed in the temperature range between 3.3 and 20 K and at magnetic fields up to 2.3 T, have been used to investigate the electronic transport properties of lattice-matched In_0.53Ga_0.47As/In_0.52Al_0.48As heterojunctions. The spacer layer thickness (t_S) in modulation-doped samples was in the range between 0 and 400 Å. SdH oscillations indicate that two subbands are already occupied for all samples except for that witht_S =  400 Å. The carrier density in each subband, Fermi energy and subband separation have been determined from the periods of the SdH oscillations. The in-plane effective mass (m ^* ) and the quantum lifetime (τ_q) of 2D electrons in each subband have been obtained from the temperature and magnetic field dependences of the amplitude of SdH oscillations, respectively. The 2D carrier density (N₁) in the first subband decreases rapidly with increasing spacer thickness, while that (N₂) in the second subband, which is much smaller thanN₁ , decreases slightly with increasing spacer thickness from 0 to 200 Å. The in-plane effective mass of 2D electrons is similar to that of electrons in bulk In_0.53Ga_0.47As and show no dependence on spacer thickness. The quantum mobility of 2D electrons is essentially independent of the thickness of the spacer layer in the range between 0 and 200 Å. It is, however, markedly higher for the samples with a 400 Å thick spacer layer. The quantum mobility of 2D electrons is substantially smaller than the transport mobility which is obtained from the Hall effect measurements at low magnetic fields. The transport mobility of 2D electrons in the first subband is substantially higher than that of electrons in the second subband for all samples with double subband occupancy. The results obtained for transport-to-quantum lifetime ratios suggest that the scattering of electrons in the first subband is, on average, forward displaced in momentum space, while the electrons in the second subband undergo mainly large-angle scattering.     

Internal conversion of valence shell electrons: Measurement and analysis for the 10.84 keV transition in206Bi

Conversion electrons of the 10.84 keV transition in²⁰⁶Bi were measured with the iron-free magnetic spectrometer. The conversion line of the valence shell was observed to be narrower than those of the inner ones. The computer program which optimizes the line profiles given numerically is outlined and used for the spectrum analysis. The studied transition was proved to be pure M1 with the nuclear structure parameterλ=1.7±0.7. The internal conversion coefficients were calculated using the relativistic Hartree-Fock model for neutral atoms as well as various isolated ions. Altogether, 15 different valence-shell configurations were considered. The prediction for the 6s ₁ ^2/2 configuration (Bi³⁺) is in accord with the measured conversion intensities. This is consistent with probable chemical form Bi₂O₃ of bismuth atoms in the radioactive source.     

Ionization function of magnesium for electron impact

Ionization cross sections for electron impact are measured with crossed beams of Mg atoms and pulsed electrons. Total numbers of particles are determined by the light emission of excited atoms. The ionization cross section obtained for electron energies from 7 eV to 60 eV has its maximum valueq _i (max)=7.8 · 10^?16 cm² at 12 eV electron energy.     

Magnetic susceptibility and nuclear magnetic resonance of vanadium sulfides

Measurements of the magnetic susceptibility in the temperature range 4–1000°K are reported for the vanadium sulfides VS, V₃S₄-V₂S₃ and V₅S₈. At higher temperatures the susceptibility of all compounds approaches a constant, positive value, indicating a Pauli-paramagnetic contribution of itinerant electrons. At lower temperatures an increase of the susceptibility is observed. This effect is particularly pronounced in V₅S₈, and is taken as evidence for the presence of localized magnetic moments.Nuclear magnetic resonance spectra of V₅S₈ show two lines, one with a temperature-independent Knight shift, and one with a positive Knight shift which increases strongly at lower temperatures. The two lines are attributed to vanadium atoms with itinerant d electrons, and to vanadium atoms with localized d electrons, respectively. This interpretation is consistent with the magnetic susceptibility data. The difference in properties between two types of vanadium atoms in V₅S₈ can be understood by considering the crystal structure.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

Ionization by H+2. Molecular effect

The molecular effect in the ionization of inner shell electrons of aluminum atoms by energetic H⁺₂ molecules has been calculated. We distinguish between the molecular effect in Al and Al₂O₃. We conclude that in the case of Al the protons of the cluster have a definite orientation but in the case of Al₂O₃ the orientation is at random.     

Magnetic and electrical transport properties of amorphous Zr-Fe alloys

Amorphous Zr_1?xFe_x samples were prepared in the composition range 0.2 ? x ? 0.9 either by means of vapour deposition or melt spinning. The electrical resistivity was determined in the range 4.2–300 K. Negative temperature coefficients were observed in the whole concentration range. The extended Ziman theory (diffraction model) was found to be able to explain these results only if the effective valence of the Fe atoms involves not only s electrons but also d electrons. The magnetic properties and the ⁵⁷Fe Mössbauer effect of the Zr_1?xFe_x alloys were studied in the range 4.2–300 K. The Fe-rich alloys are ferromagnetic. The Fe moment vanishes in alloys of an Fe concentration lower than about 50 at%. In most alloys (x ? 0.8) the Curie temperature is below room temperature and continuously decreases with Zr concentration. By means of Mössbauer spectroscopy and magnetic measurements it is shown that compositional short-range order (CSRO) is present to a higher degree in melt-spun alloys than in vapour-deposited alloys. The effect of sign and magnitude of the heat of solution on CSRO and the magnetic properties is discussed.     

Ionization of hydrogen and deuterium atoms in thermal energy collisions with state-selected Ne(3s 3 P 2,3 P 0) metastable atoms

High resolution energy spectra of electrons and ions resulting from thermal energy collisions of hydrogen and deuterium atoms with state-selected metastable Ne(Ne(3s ³ P ₂,³ P ₀) atoms are reported. The electron spectra for Ne(³ P ₂)+H(D) are very broad: The high energy part due to formation of NeH⁺ (NeD⁺) bound states (associative ionization), amounts to about 30% of the ionizing events, whereas the dominant part of the spectrum including a prominent low-energy peak is due to Penning ionization out of a strongly-attractive entrance potential curve. Comparison of the spectra with quantum mechanical fit calculations yields fairly accurate information on this potential, in particular its well depthD _e [Ne(³ P ₂)?H,D]= 2.0(1) eV. The spectra for Ne(³ P ₀)+H, D are comparatively narrow with much lower cross sections than the one for the Ne(³ P ₂) state. The corresponding entrance channel is a weakly bound van der Waals molecule with a well depth below 0.1 eV. A perturbation calculation of the Ne(3s)+H(1s) potential energy curves at large distances explains the observed difference between the Ne(³ P ₂)+H(D) and Ne(³ P ₀)+H(D) systems. Symmetry arguments are given that the major contribution to the Ne(³ P ₂)+H(D) spectra is due to the² Σ potential.     

Electron spin resonance in the Heusler alloy YbRh2Pb

An electron spin resonance (ESR) signal was observed in a concentrated Kondo lattice, Heusler alloy YbRh₂Pb. It is attributed to the combined effect of the 4f local magnetic moments of Yb³⁺ and conduction electrons. It is shown that the significant broadening and disappearance of the ESR line at temperatures above 20 K is caused by the processes of the spin-lattice relaxation of the Yb³⁺ ions through the first excited Stark doublet with an activation energy Δ ≈ 73.5 K. A comparison of the ESR data for YbRh₂Pb and some other undoped intermetallic compounds based on ytterbium, cerium, and europium indicates that hybridized electronic states occurring as the result of hybridization between the localized 4f electrons and the collectivized conduction electrons constitute a fundamentally new source of ESR.     

Surface analysis by ion-electron spectroscopy; Silicon target

The energy spectra of secondary elections emitted from a Si(111) surface due to bombardment by 6 keV He⁺ and O⁺₂ ions have been examined. The fine structure in the spectra is explained on the basis of a novel mechanism of creation of Auger electrons at the surface. There are two stages of interaction between incoming ions and the substrate via adsorbed atoms. In the first stage, due to a level promotion mechanism, vacancies in the adsorbed atoms are created. In the second stage, Auger neutralization processes accompanied by the emission of electrons from a solid with characteristic energies take place. These electrons provide a good indication of the degree of coverage of the silicon surface with contaminant atoms. The energy losses of escaping electrons are also discussed.