Characteristic functions in the statisticalR-matrix theory

The characteristic functions of the statistical Wigner'sR-matrix for the case of one-channel scattering in competition with multichannel radiative capture have been drown. Different approximations and their relevance in neutron physics applications are discussed.Communicated by: X. Campi     

Considerations about Z-scan sensitivity improvement: theory versus experiments

This paper reports an experimental demonstration of the improvement of the Z-scan technique’s sensitivity. It is shown that this sensitivity can be multiplied by a factor equal to almost 400 with the help of simple binary diffractive elements. Such a possibility was actually predicted theoretically in one of our previous papers. In this study, the interpretation is investigated in a wider context taking into account the definition of the signal normalisation as introduced by Z-scan and the well-known eclipsing Z-scan (EZ-scan) experiments. In particular, advantages and drawbacks are compared, by looking at the normalised or the unnormalised aperture transmission.     

Electron diffraction study of various lattice defects in a Bi<Subscript>4</Subscript>Ge<Subscript>3</Subscript>O<Subscript>12</Subscript> crystal

Lattice defects in a scintillation detector made of Bi₄Ge₃O₁₂ (BGO) could severely impact detector efficiency via non-radiative transfer of electron excitation, thus making thorough investigations of these defects highly important. Here we present a combined experimental and theoretical study of two- and three-dimensional defects in a Czochralski-grown BGO crystal. Upon examination by transmission electron microscopy the selected-area electron diffraction (SAED) patterns in two neighboring parts of the specimen reveal different kinds of two- and three-dimensional defects. Three sub-grains misoriented at 2.47° with reference to each other and probable presence of stacking faults lying in {011} planes were observed in the first examined local area. The SAED image taken from an area in the close neighborhood is much more complicated and is explained in terms of the superposition of reflections from: (i) a partially textured GeO₂ second-phase inclusion; (ii) the basic lattice of BGO and (iii) a superlattice-like structure based on the BGO lattice. The atomic structure of such a superlattice-like structure was theoretically modeled and the corresponding simulated SAED patterns were found to be in good agreement with the experimentally observed one.     

Submicrometer-MOS capacitor with ultra high capacitance biased by Au nanoelectrodes

The ultimate limits of size of the current metal-oxide-semiconductor capacitors can be overcome by preparation of three-dimensional devices that can vertically be biased using one-dimensional metal nanostructures. Here, we present a general and efficient approach to the assembly and integration of Au nanocrystals into functional nanoelectrodes of three-dimensional submicrometer-MOS (0.35 μm²) capacitors, presenting an ultra high capacitance (24±1 pF). The Au nanocrystals were directly produced into a nanoporous template of anodized aluminum oxide that was evaluated, and the electrical characterization of this device corroborates the formation of the MOS capacitor. Flat band voltage is independent of sweep voltage range, and negligible hysteresis of capacitance-voltage curves is observed when sweep voltage ranges from positive to negative and turned again to positive bias. In addition, experimental results match theoretical analysis and indicate the presence of free surface charges stored in the Au nanostructures. The demonstrated ability to control the assembling of the nanocrystals and the results of electrical characterization indicate that the embedded Au nanoelectrodes have a high potential for memory applications based on three-dimensional devices.     

Adiabatic theory of particle trapping due to Coulomb crossing of one-dimensional betatron resonance

In the presence of space charge forces, synchrotron oscillations result in periodic modulation of the space charge tune shift, periodic crossing of betatron resonances, and particle trapping in resonance islands. The trapping effect for one-dimensional resonance is considered using classical perturbation theory and the “frozen core” approach to calculation of space charge forces. The beam losses and emittance growth are analyzed for arbitrary order resonance; the numerical results are given for the third-order resonance. The text was submitted by the authors in English.     

Purity of rubrene single crystals

Liquid chromatography connected with mass spectroscopy reveals that the oxidized form of rubrene is the major impurity in commercial powder of rubrene as well as in rubrene single crystals. One form of rubrene impurity can be transformed into the other. In solution, rubrene undergoes photo-oxidation completely until the red color of the rubrene solution disappears. Single crystals, due to compact packing of molecules and the required molecular shape change during oxidation, oxidize only on the surface.     

Electrical performance of Ti/Pt thin films on glass-ceramic substrates after high temperature loading

In this study, the influence of post-deposition annealings (PDA) up to temperatures of T _PDA=700°C on the room-temperature resistivity of e-beam evaporated titanium/platinum (Ti/Pt) bi-layers on low temperature co-fired (LTCC) substrates covered with a glass encapsulate is investigated. The thickness of the platinum top layer is varied between 24 and 95 nm (titanium film thickness: 5 nm) and between 23 and 90 nm (titanium film thickness: 15 nm), respectively. In the “as-deposited” state and up to post-deposition annealing temperatures of T _PDA=450°C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. When applying, however, solely the Fuchs-Sondheimer model for evaluation, the effective mean free path for electrons is substantially above the value reported for crystalline platinum at room temperature. Compared to similar investigations on smooth Si/SiO₂ substrates yielding interpretable results within this theoretical approach, this is due to the increase of the thickness-dependent fraction in film resistivity which is strongly affected by the enhanced LTCC/glass surface roughness. At T _PDA>600°C, diffusion of titanium into the platinum top layer and the roughening of the LTCC/glass substrate dominate the electrical behavior, both causing an increase in film resistivity above average. In contrast to Si/SiO₂ substrates, thermal induced grooving effects in the Pt top layer play a minor role as the temperature coefficients of expansion of metallization and glass-ceramic substrate match better and the effective temperature difference for stress generation is lower due a glass softening temperature of about 450°C.     

Cooperative downconversion and near-infrared luminescence of Tb<Superscript>3+</Superscript>–Yb<Superscript>3+</Superscript> codoped lanthanum borogermanate glasses

In the present paper, we investigate the near-infrared (NIR) luminescence of Tb³⁺–Yb³⁺ codoped lanthanum borogermanate (LBG) glasses under visible and ultraviolet light excitation. The results indicate that NIR quantum cutting occurs through cooperative energy transfer from Tb³⁺ to Yb³⁺ ions when only 4f ⁸ levels of Tb³⁺ ions are excited in the wavelength region of 300–490 nm. The highest quantum efficiency under the excitation ⁵ D ₄ level of Tb³⁺ at 484 nm is 146%. Ultraviolet excitation that populates the charge transfer band (CTB) of Yb³⁺ near 270 nm does not result in quantum cutting as the fast nonradiative decay from CTB to ² F _5/2 level dominates. These materials are expected to be used as a converting layer for silicon solar cells to enhance their efficiency by splitting each high-energy photon into two NIR photons.     

Microstructural characterization of ultrafine-grain interstitial-free steel by X-ray diffraction line profile analysis

This paper highlights the microstructural features of commercially available interstitial free (IF) steel specimens deformed by equal channel angular pressing (ECAP) up to four passes following the route A. The microstructure of the samples was studied by different techniques of X-ray diffraction peak profile analysis as a function of strain (ε). It was found that the crystallite size is reduced substantially already at ε=2.3 and it does not change significantly during further deformation. At the same time, the dislocation density increases gradually up to ε=4.6. The dislocation densities estimated from X-ray diffraction study are found to correlate very well with the experimentally obtained yield strength of the samples.     

Potential scattering in Dirac field theory

We develop the potential scattering of a spinor within the context of perturbation field theory. As an application, we reproduce, up to second order in the potential, the diffusion results for a potential barrier of quantum mechanics. An immediate consequence is a simple generalization to arbitrary potential forms, a feature not possible in quantum mechanics.     

Rapid-thermal-anneal-based internal gettering for?germanium-doped Czochralski silicon

The internal gettering (IG) effects involved with a rapid thermal anneal (RTA) in germanium-doped Czochralski silicon (GCz-Si) wafer have been investigated. It was found that germanium doping could enhance the oxygen precipitation in bulk while shrinking the denuded zone width near the surface through pre-RTA at high temperature plus low–high temperature conventional furnace anneals. Rapid cooling rate after RTA was clarified to be beneficial for oxygen precipitation for GCz-Si wafer. It was suggested that the germanium doping could increase the vacancy concentration in Cz-Si during RTA by forming the germanium–vacancy complexes. In contrast to that in Cz-Si wafer, the smaller-sized higher-density oxygen precipitates were presented in the nucleation anneals, then followed RTA pretreatment while more oxygen precipitates survived during ramping processes after nucleation anneals in the GCz-Si wafer. Enhanced heterogeneous nucleation and reduced critical radius of precipitates associated with the germanium–vacancy complexes have been proposed for the oxygen precipitation enhancement.     

Projected Tamm Dancoff theory for diabolical pair transfer in rotating nuclei

Angular momentum projected Tamm Dancoff theory in a realistic configuration space is used to investigate the occurrence of diabolical points and the connected Berry phase in the rotational spectra of well deformed Yb- and Hf-nuclei. Specific nuclei are predicted, where we expect diabolic pair transfer.Dedicated to Prof. Dr. H.J. Mang on the occasion of his 60th birthday     

Improving solar grade silicon by controlling extended defect generation and foreign atom defect interactions

Multicrystalline silicon has a high commercial potential for solar cell applications. Extended lattice defects, such as dislocations and grain boundaries, are important as recombination and storage centers for metallic impurities. Their control is essential to obtain high efficiencies of the solar cell. Important parameters for the assessment of the final efficiency of the solar cells are the distribution and structure of the defects and their impact on the lifetime of minority carriers. The current understanding of the nucleation mechanisms of the most important defects during crystal growth will be described. Interaction processes between mobile impurities and extended defects are important for the recombination activity. Measurements of the recombination behavior and the contamination level will be presented. Finally, the ramifications on the solar cell efficiency shall be discussed.     

Modified effective-range theory for low energy ${\sf e}$-N2 scattering

We analyze the low-energy e-N₂ collisions within the framework of the modified-effective range theory (MERT) for the long-range potentials, developed by O’Malley et al. [J. Math. Phys. 2, 491 (1961)]. In comparison to the traditional MERT we do not expand the total cross-section in the series of the incident momentum ?k, but instead we apply the exact analytical solutions of the Schrödinger equation for the long-range polarization potential, as proposed in the original formulation of O’Malley et al. This extends the applicability of MERT up to few eV regime, as we confirm using some simplified model potential of the electron-molecule interaction. The parameters of the effective-range expansion (i.e. the scattering length and the effective range) are determined from experimental, integral elastic cross-sections in the 0.1–1.0 eV energy range by fitting procedure. Surprisingly, our treatment predicts a shape resonance that appears slightly higher than experimentally well known resonance in the total cross-section. Agreement with the experimentally observed shape-resonance can be improved by assuming the position of the resonance in a given partial wave. Influence of the quadrupole potential on resonances is also discussed: we show that it can be disregarded for N₂. In conclusion, the modified-effective range formalism treating the long-range part of the potential in an exact way, reproduces well both the very low-energy behavior of the integral cross-section as well as the presence of resonances in the few eV range.     

Independent electron theory of multiple ionization of xenon by intense laser pulses

The intensity dependence of the multiphoton ionization spectra of Xe atoms has been investigated with an improved accuracy and well-controlled laser parameters. In particular, we have examined the ionization rates for X³⁺, X²⁻, X⁺ as functions of the laser intensity and the pressure in the target chamber. The apparatus used for these measurements is characterized by a high-energy resolution (better than 200 meV) and a completely digital acquisition system. The time-of-flight spectra clearly show the contributions of the different isotopes present in Xe gas. The laser pulses have been characterized with great accuracy by monitoring the energy, pulse width and divergence shot by shot. The ionization rates of the different ions have been used for testing the basic assumption of the Geltman theory of multiple ionization based on the single electron ionization model. We have found that for the small intensity range investigated the quantity (dXe ⁺/dI)·(dXe ³⁺/dI)/(dXe ²⁺/dI)² appears to be quite close to the value 0.5 predicted by this model.     

Isotopic dependence of electron screening in fusion reactions

The fusion reactions⁶Li(p, )³He,⁶Li(d, )⁴He, and⁷Li(p, )⁴He have been studied over the c.m. energy rangeE=10 to 1450 keV. Each reaction involved the use of hydrogen projectiles and LiF solid targets as well as Li projectiles and hydrogen molecular gas targets. In all cases the effects of electron screening on the low-energy fusion cross sections (exponential enhancement) have been observed; the effects are somewhat stronger in the case of atomicp ord projectiles compared to the case of molecularH ₂ orD ₂ gas targets. If isotopic effects on electron screening are negligible, all three reactions should exhibit the same enhancements for each set of experimental techniques. The measurements confirmed this expectation to a large extent.Supported in part by the Landesamt Nordrhein-Westfalen (IVA5-10600387), the Deutsche Forschungsgemeinschaft (Ro429/ 18-2 and Ro429/21-1), and the Comision Interministerial de Ciencia y Tecnologia (AEN90-0932)     

A field-theoretic model for Hodge theory

We demonstrate that the four-dimensional (4D) ((3+1)-dimensional) free Abelian 2-form gauge theory presents a tractable field-theoretical model for the Hodge theory where the well-defined symmetry transformations correspond to the de Rham cohomological operators of differential geometry. The conserved charges, corresponding to the above continuous symmetry transformations, obey an algebra that is reminiscent of the algebra obeyed by the cohomological operators. The discrete symmetry transformation of the theory represents the realization of the Hodge duality operation that exists in the relationship between the exterior and co-exterior derivatives of differential geometry. Thus, we provide the realizations of all the mathematical quantities, associated with the de Rham cohomological operators, in the language of the symmetries of the present 4D free Abelian 2-form gauge theory.     

Phenomenological analysis of intermediate mass fragments emission in alpha induced reactions

Intermediate mass fragment emission in induced reactions on Al and Ti at 60 and 104 MeV, respectively has been analysed using phenomenological moving source model. Fragment emission at backward angles is found to be well explained by a single fusion-like source, whereas for forward angles an additional intermediate velocity source is required to explain the data. The relative contribution of the intermediate velocity source decreases sharply with angle for lower energies and it extends significantly to larger angles in the higher energy reaction. The fractional momentum transfer for the fusion-like source is in agreement with the corresponding Viola systematics value in the case of +Al reaction, whereas for +Ti reaction, it is larger than that obtained from the Viola systematics. The degree of incompleteness (in momentum transfer) is found to be fragment dependent; incompleteness being more for lighter fragments and vice-versa.