The atomic and electron structure of ZrO2

The atomic structure of amorphous and crystalline zirconium dioxide (ZrO₂) films is studied using X-ray diffraction and extended X-ray absorption fine structure techniques. The electron structure of ZrO₂ is experimentally determined using X-ray and UV photoelectron spectroscopy, and the electron energy band structure is theoretically calculated using electron density functional method. According to these data, the valence band of ZrO₂ consists of three subbands separated by an ionic gap. The upper subband is formed by the O2p states and Zr4d states; the medium subband is formed by the O2s states; and the narrow lower subband is formed predominantly by the Zr4p states. The bandgap width in amorphous ZrO₂, as determined using the electron energy loss spectroscopy data, amounts to 4.7 eV. The electron band structure calculations performed for a cubic ZrO₂ phase point to the existence of both light (0.3m ₀) and heavy (3.5m ₀) holes, where m ₀ is the free electron mass. The effective masses of band electrons in ZrO₂ fall within (0.6–2.0)m ₀.     

The Bahcall corrections to electron capture and the Hartree-Fock rearrangement

The Bahcall correction factorsX ^L/K andX ^M/L forL/K andM/L electron capture ratio respectively are calculated including the Hartree-Fock (HF) rearrangement. The rearrangement takes into account that the atom after the electron capture is not in the HF ground state. The selfconsistent single electron orbits for the excited states with a hole in theK,L orM shell are calculated using the Herman-Skillman and the Froese HF codes. The rearrangement changes the correction factor into the right direction, but the reduction is too small. Only for very light atoms the alteration is larger than 1% and therefore it is in general smaller than the present experimental errors.     

The electron capture decay of 106mAg

The electron capture decay of 8.4d ^106mAg has been investigated by means of a 4π internal source scintillation spectrometer and a Ge(Li) detector. The L/K capture ratios to the 2952 and 2757 keV excited states of ¹⁰⁶Pd have been measured: (L/K)₂₉₅₂ = 0.203 ± 0.003 and (L/K)₂₇₅₇ = 0.1457 ± 0.0010. The value of Q = 3053 ± 3 keV has been deduced from these L/K ratios.     

Direct measurement of electron spin-lattice relaxation time of paramagnetic centers and non-linear modulated responses in HTSC

The report consists of two parts. 1. The method is described to measure directly the electron spin-lattice relaxation timeT ₁ in high-T _c superconductors (HTSC). The technique is based on registering the oscillating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. TheT ₁ values of the Cu²⁺ centers in YBa₂Cu₃O_6+x (x=0.24?0.9) and Y₂BaCuO₅ materials are measured and found to be about 1.4·10^?9 s independently ofx and temperature in the range 77–300 K. Besides much longer relaxation is displayed in degradated materials. The data obtained can be considered as an argument in favor of the “green-phase” origin of the “high-temperature” EPR spectra in the YBaCuO HTSCs. 2. Non-linear responses of the microwave (10¹⁰ Hz) and rf (10⁵–10⁷ Hz) absorption in HTSC materials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at microwaves the effect is caused by non-linear interaction with shielding supercurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the Josephson weak links and the thermo-assisted flux creep.     

Photoinduced electron transfer in meso-nitrophenyl-substituted porphyrins and their chemical dimers

It is shown that steric interactions of volume substituents in the β-positions of pyrrole rings and the nitro group in mono-and di-meso-phenyl-substituted of octaethylporphyrins and their chemical dimers containing the electron-acceptor NO₂ group in the ortho-position of the phenyl ring at 295 K favor the direct overlap of molecular orbitals of the interacting subunits, resulting in the efficient quenching of fluorescence due to the direct electron transfer from the S₁ level to the lower-lying state via the “through-space” mechanism. The electron transfer in these compounds in nonpolar media (the rate constant k _et ^S =(3.2–9.5)×10⁹ s^?1 is nonadiabatic, whereas in strongly polar solvents (k _et ^S =2×10¹¹ s^?1) the adiabatic effects can be manifested. In compounds containing the NO₂ group in meta-or para-positions of the phenyl ring, the nonadiabatic electron transfer from the S ₁ level occurs less efficiently both in polar [k _et ^S =(0.2–5)×10¹⁰ s^?1] and nonpolar media [k _et ^S =(0.1–1.0)×10⁷ s^?1]. In this case, the electron transfer involves molecular orbitals of phenyl (the “through-bond” mechanism), and its efficiency depends on the orbital electron density in the meta-and para-positions of the phenyl ring. Based on the experimental data obtained and analysis of the electron transfer within the framework of the Marcus theory, the energy scheme of relaxation processes of the electronic energy in the compounds under study involving charge transfer states is suggested.     

New correlated electron physics from new materials

Many important advances in the physics of strongly correlated electron systems have been driven by the development of new materials: for instance the filled skutterudites MT₄X₁₂ (M=alkali metal, alkaline earth, lanthanide, or actinide; T=Fe, Ru, or Os; X=P, As, or Sb), certain lanthanide and actinide intermetallic compounds such as URu_2-xRe_xSi₂ and CeTIn₅ (T=Co, Rh, or Ir), and layered oxypnictides and related materials. These types of complex multinary d- and f-electron compounds have proven to be a vast reservoir of novel strongly correlated electron ground states and phenomena. In these materials, the occurrence of such a wide range of ground states and phenomena arises from a delicate interplay between competing interactions that can be tuned by partial or complete substitution of one element for another, as well as the application of pressure, and magnetic fields, resulting in rich and complex electronic phase diagrams in the hyperspace of temperature, chemical composition, pressure and magnetic field. It seems clear that this type of “materials driven physics” will continue to play a central role in the development of the field of strongly correlated electron systems in the future, through the discovery of new materials that exhibit unexpected phenomena and experiments on known materials in an effort to optimize their physical properties and test relevant theories.     

Spontaneous symmetry breaking approach to La2CuO4 properties: Hints for matching the Mott and Slater pictures

Special solutions of the Hartree-Fock (HF) problem for Coulomb interacting electrons described by a simple model of the Cu-O planes in La₂CuO₄ are presented. One of the mean field states obtained, is able to predict some of the most interesting properties of this material, such as its insulator character and the antiferromagnetic order. The natural appearance of pseudogaps in some states of this material is also indicated by another of the HF states obtained. These surprising results follow after eliminating spin and crystal symmetry restrictions usually imposed on the single particle HF orbitals, by employing the rotational invariant formulation of the HF scheme originally introduced by Dirac. Therefore, it is exemplified here, how up to know considered strong correlation effects, can be described by improving the HF solution of the considered system. In other words, it has been argued, that defining correlation effects as the ones shown by the system and not predicted by the HF best (lowest energy) solution, allows to explain important, up to know considered as strong correlation properties, as simple mean field ones. The discussion also helps to clarify the role of the antiferromagnetism and pseudogaps in the physical properties of the HTSC materials and indicates a promising way to start conciliating the Mott and Slater pictures in the physics of the transition metal oxides and other strongly correlated electron systems.     

Band engineering of ZnS by codoping for visible-light photocatalysis

Codoping is demonstrated as an efficient approach to narrow the band gap of ZnS and enhance its photocatalytic activity. Herein, we perform the density-function theory calculations of ZnS by codoping of X (N, F) with transition metals (TM = V, Cu). The band gap is reduced in four different types of codoped ZnS. In particular, Cu_ZnF_S codoping, a charge-compensated donor–acceptor pair, leads to an about 32 % reduction of the energy gap, thus extending the absorption edge to visible-light region. The band gap reduction is due to the upshift of the top valence band comprised with the delocalized hybridizing levels of Cu 3d and S 3p states, and the downshift of the bottom conduction band consisting of F 2s states. Moreover, the larger value of m _e*/m _h* in Cu_ZnF_S–ZnS would result in a lower recombination rate of the electron–hole pairs. Both band gap reduction and low recombination rate are critical elements for efficient light-to-current conversion in codoped ZnS. These findings raise the prospect of using codoped ZnS with specifically engineered electronic properties in a variety of photocatalytic applications.     

Excitation of optically forbidden transitions in argon and neon by electron impact

A radial electron monochromator and analyser system employing several new features has been used to study low energy differential inelastic electron scattering by ground state argon and neon atoms. Angular distributions for a number of optically inaccessible states are reported at energies between 30 and 100 eV and over a range of scattering angles from 0–90°. In particular, results are shown for the metastable states (³P₂, ³P₀) of argon as well as for various J levels in the parity forbidden bands (ns²np⁶ → ns²np⁵(n + 1)p) for argon and neon.     

Nuclear charge radii of Al and Si from elastic electron scattering between 25 and 60 MeV

Elastic electron scattering cross sections of²⁷Al and Si (natural isotopic mixture) have been measured relative to carbon. The rms charge radiiR _m , deduced with partial wave calculations, are (3.01±0.05) fm for²⁷Al and (3.06±0.05) fm for Si, in good agreement with results from muonic X-ray energies. The values given are those for a Fermi charge distribution with skin thickness 2.5 fm; harmonic oscillator shell model distributions yield radii smaller by 0.03 fm. The ratioR _m (²⁷Al)/R_m(Si) is 0.984±0.016.     

Phase change of CoTi and Co3Ti induced by MeV-scale electron irradiation

This study investigates equilibrium-to-nonequilibrium solid phase transitions induced by MeV-scale electron irradiation in B2-CoTi and L1₂-Co₃Ti intermetallic compounds by means of high-voltage electron microscopy. Under MeV-scale electron irradiation, B2-CoTi transforms into a body-centered cubic solid solution through chemical disordering and eventually transforms into an amorphous phase. The critical temperature for amorphisation is found to be 110 K. L1₂-Co₃Ti also exhibits chemical disordering at temperatures below 700 K. However, its amorphisation does not occur even at a low temperature of 20 K. The dominant factor in these solid phase transitions is discussed in terms of the Gibbs free energy.     

Threshold electron impact excitation of Cl2

A high resolution electron impact threshold spectroscopy technique was used to examine the excitation of Cl₂ in the 2–14 eV region. This study complements previous photon absorption and emission measurements, because it is capable of detecting transitions which are optically forbidden. In the region up to 7.5 eV, broad dissociative structures are correlated with optically active valence states, although relative intensities in the threshold spectrum differ considerably and indicate a substantial contribution from the optically forbidden transitions. At 7.46 eV a series of 5 equidistant sharp peaks is detected and interpreted as arising from the² π _g Feshbach resonance, which differs from the ground state positive ion Cl ₂ ⁺ by a pair of Rydberg electrons: (4sσ)². The decay channels responsible for the appearance of the resonance in a threshold spectrum are discussed and it is suggested that they include several valence states of the (2431) and (2341) configurations, whose potential energy curves cross the Cl ₂ ^? ,²π_g curve in the region of energy at which the resonance state is formed. At higher incident electron energies and up to ionisation, Rydberg states predominate, starting with (2430) 4s^3,1 π _g states detected for the first time. The absence of broad peaks above 8 eV and the irregular appearance of Rydberg bands is consistent with the strong Rydberg-valence configuration mixing proposed by Peyerimhoff and Buenker. Where our resolution permits comparison, good general agreement is found with recent synchrotron radiation absorption measurements of optically allowed transitions.     

Recent results on multiple ionization and fragmentation of negatively charged fullerene ions by electron impact

Employing the dynamic crossed-beams technique, the absolute cross sections of the electron-impact multiple ionization and fragmentation of mass-selected negatively charged fullerene ions C _m ^? → C _m?n ²⁺ (m=60, 70, 84; q=1, 2, 3; n=0, 2, 4) were measured. The electron energy varied from the respective threshold up to 1 keV. A scaling law was observed for the cross-section magnitude as a function of the fullerene size m and the charge state q of product ions. The data indicate that different mechanisms account for the detachment of an extra electron from the negatively charged fullerene and the formation of a positively charged ion, respectively. Moreover, the multiple ionization of a fullerene anion is found to be a sequential process. A novel ionization mechanism is proposed which might be expected to be valid for all negatively charged molecular or cluster ions able to shield the attached electron from the incident electron.     

Radiative recombination of ions and nuclei in electron cooling systems

Experimental data on rates for the radiative recombination of nuclei (from helium to uranium) and various ions in interaction with an electron beam in electron cooling systems are reviewed. An analysis of the experimental data has yielded the dependence of the radiative recombination rate on the relative electron energy appreciably differently than the theoretical models obtained earlier by H. Kramers and R. Schuch. In addition, it is shown that the radiative recombination rate of nuclei in the experiment depends on the transverse electron energy as T _?? ^?0.82 ,which is also different from the results of the calculations by the theoretical model proposed by M. Bell and J. Bell. Experimental data on the cooling of ions in intermediate charge states are analyzed and the dependence of the radiative recombination rate on the charge state of the ion (electron-shell configuration) is shown. For some ion charge states, the rate of the process is of a resonance character. Loss to radiative recombination in the electron cooling system of the NICA Booster is evaluated for the Au³²⁺, Au³³⁺, Au⁵⁰⁺, and Au⁵¹⁺ ion beams. Limitations imposed on the Au79+ beam lifetime by radiative recombination in the electron cooling system of the NICA Collider are analyzed. Possible ways to decrease the radiative recombination rate of nuclei by selecting the parameters of the electron cooling system for the NICA Collider are proposed.     

Effective electron mass in high-mobility SiGe/Si/SiGe quantum wells

The effective mass m* of the electrons confined in high-mobility SiGe/Si/SiGe quantum wells has been measured by the analysis of the temperature dependence of the Shubnikov-de Haas oscillations. In the accessible range of electron densities, n _s , the effective mass has been found to grow with decreasing n _s , obeying the relation m*/m _b = n _s /(n _s ? n _c ), where m _b is the electron band mass and n _c ≈ 0.54 × 10¹¹ cm^?2. In samples with maximum mobilities ranging between 90 and 220 m²/(V s), the dependence of the effective mass on the electron density has been found to be identical suggesting that the effective mass is disorder-independent, at least in the most perfect samples.     

Detection limits in Auger electron spectroscopy

The sources of background noise in Auger electron microanalysis are analyzed in order to evaluate the minimum detectable concentration x_m and the minimum number of detectable atoms y_m that can be reached. The best choices of operating conditions (the energy E₀, intensity I₀, and spot size d₀ of the incident electron beam, and the duration of the experiment t_e) are deduced for bulk and thin film analysis. The main results are: (i) The choice of E₀ is not very stringent, at least when E₀ ? 5E_i(A), where E_i(A) is the ionization energy, (ii) For a given electron dose received by the sample, x_m is improved by the use of the largest incident spot size while y_m is improved by the use of the finest spot size. The results also hold for other microanalytical techniques such as electron energy loss spectroscopy or electron probe microanalysis. (iii) Chemical identification of a single atom will be possible on samples able to tolerate very large electron doses by using incident electron beams 10 nm or less in diameter. The expected performance of a coincidence technique first suggested by Wittry is also discussed.     

Systematics of vector meson-proton scattering

The systematics of Vp scattering (V = ?, ω, φ, ψ) is studied using unitarity to relate the elastic and inelastic amplitudes. Assuming that φ and ψ are pure strange and charmed quark states, respectively, φp and ψp elastic amplitudes are shown, in the first approximation, to be generated by diffractive inelastic states. This leads to the relations B_ψ ≈ B_φ ≈ B_ω/2 ≈ B_?/2 for the elastic differential cross section slopes. The approximate m_V² is related to a similar suppression in inelastic Vp cross sections and, more speculatively, to the m_A² suppression of the hadronic production cross sections σ(pp → A).     

纳米金属Tm的电子浓度研究

电子浓度是与金属的宏观特性相关的重要参数.反射光谱和霍尔效应分别是得出电子浓度和载流子浓度的基本实验.两个纳米稀土金属铥Tm样品(样品1,平均粒径100nm,样品2,平均粒径10nm)的红外---紫外反射光谱实验表明,金属铥Tm表面的反射光学性质具有金属的特征,6s能带具有与碱金属相近的电子浓度n_p,数值分别为2.434×10²⁸/m³和1.701×10²⁸/m³.而样品的霍尔效应实验测得金属Tm的载流子是电子-空穴型的,载流子浓度n_H仅分别为8.032×10²⁴/m³和7.679×10²⁴/m³,仅仅是费米面附近的电子-空穴状态.另外,铥Tm的电导率比半导体的大3个量级.晶粒纳米化使电子浓度n_p减小,电导率σ减小,载流子浓度减小,而霍尔系数R_H增大.     

Identification of Cu(I) and Cu(II) oxides by electron spectroscopic methods: AES,ELS and UPS investigations

The externally prepared black-coloured copper oxide (T? 700 K, P_O2 ? 100 torr) on a Cu(100) surface is identified by electron spectroscopy as CuO. Compared to the red-coloured Cu(I) oxide (in situ oxidation at T ? 400 K, P_O2 ? 0.5 torr, ～ 10⁹ L), the He(I)- excited photoemisson from CuO reveals characteristic shake-up satellites 10–12 eV below E_F and a broadened emission from overlapping oxygen-induced 2p and Cu 3d states. From the AES and ELS results, in correlation with the data from core electron spectroscopy, chemical shifts of Cu 2p, Cu 3s and Cu 3p in CuO to higher binding energy and decreases in binding energy of the oxygen-induced states were deduced. The unoccupied electron states of Cu at 5 and 7.5 eV above E_F — postulated from the ELS results — are preserved in Cu₂O and CuO compounds. Annealing of the Cu(II) oxide at 670 K is accompanied by decomposition into Cu₂O due to the solid-state reaction following the scheme: 2CuO → 1/2 O₂ + Cu₂O.