Studies of the microstructure of a superconducting Nb<Subscript>3</Subscript>Sn-Band obtained from (Cu/Nb)/Cu12Sn nanocomposite

A multilayer layer from the superconducting compound Nb₃Sn was obtained by the thermal treatment of composite (Cu/Nb)/Cu12Sn conductor consisting of Cu-12% Sn and Cu/Nb interlayers consisting in turn of nanosized copper and niobium layers. The conductor was coated with copper from the outside, which served as a stabilizer. The microstructure of the cross section of the composite band in dependence on the volume ratio of Cu/Nb and bronze interlayers and the microstructure of the Cu/Nb interlayers themselves in dependence on the annealing temperature were investigated. The optimum ratio of composite components was as follows: t _Nb N _Nb ≈ 0.288t _CuSn N _CuSn, where t and N are the thickness and number of the niobium and bronze layers, respectively. Trying to maintain the optimum design of the conductor forced us to increase the bronze volume content relative to the content of Cu/Nb interlayers, which had a negative impact on the composite microstructure. This resulted in disruptions of Cu/Nb interlayers. A technique has been developed for producing the (Cu/Nb)/Cu12Sn composite as a precursor to the band from the Nb₃Sn compound with allowance for the first experiment’s shortcomings.     

Coexistence of superconductivity and ferromagnetism in ferromagnet/superconductor proximity structures

The Nambu spinor Green's function approach is applied to calculating the density of states (DOS) and superconducting order parameter in normal-metal/insulator/ferromagnet/superconductor (NM/I/FM/SC) junctions. It is found that the s-wave superconductivity and ferromagnetism can coexist near the FM/SC interface, which is induced by proximity effect. On the SC side, the spin-dependent DOS appears both within and without the energy gap. On the FM side, the superconducting order parameter displays a damped oscillation and the DOS exhibits some superconducting behavior. The calculated result for the DOS in FM for “0 state” and “π state” can reproduce recent tunneling spectra in Al/Al₂O₃/PdNi/Nb tunnel junctions. Received 1st July 2002 Published online 19 November 2002     

Laser Ionization and Penning Trap Mass Spectrometry – A Fruitful Combination for Isomer Separation and High-precision Mass Measurements

We have demonstrated for the first time that element-selective laser ionization in combination with ultra-high resolution mass spectrometry can be used to prepare isomerically pure ion ensembles. Together with β–γ coincidence studies this method allowed a determination of the low-energy structure and the unambiguous identification of triple β-decaying isomerism in ⁷⁰Cu. By selective resonant ionization and measurement of the masses of these three states using ISOLTRAP at ISOLDE/CERN with a relative uncertainty of δm/m ≈ 5 ⋅ 10⁻⁸ a clear state-to-mass assignment was possible which resolved the assignment puzzle in ⁷⁰Cu.     

Dislocation structure of α<Subscript>2</Subscript>/β two-phase Ti-21.4Al-5.6Nb alloy

An electron microscopic analysis of the dislocation structure of two-phase α₂/β Ti-21.4 at % Al-5.6 at % Nb alloy deformed at ambient temperature was performed. It was established that after deformation, the microstructure of the α₂ phase consists of mobile a-superdislocations in the basal and prism planes and of 2c + a-superdislocations in the pyramidal planes. Some dislocations with the Burgers vector [0001] in the prism planes were also observed. Deformation transfer from the α₂ to the β phase occurs only due to a-super-dislocations with a Burgers vector corresponding to the Burgers orientation relations between these phases. The reasons for the increase in plasticity in two-phase Ti₃Al with Nb alloys are discussed.     

Background structure in the Heidelberg-Moscow experiment on the search and investigation of double beta decay of 76Ge1

Independent analysis of the spectra from the Heidelberg-Moscow experiment has been carried out. A direct comparison of the peak intensities in the spectra allows for the conclusion that in the energy range up to 3200 keV the background is due to the detection of γ rays from the decay of trace impurities of anthropogenic and cosmogenic nuclides occurring in the apparatus between the HPGe detectors and the main (Pb and Cu) shielding. The ²²⁶Ra, ²¹⁴Pb, and ²¹⁴Bi γ rays observed in the spectra are shown to arise in the ²²⁶Ra rather than ²³⁸U decay chain. The estimates of the expected intensities of the weak ²¹⁴Bi γ rays in the spectrum region around Q _ββ(⁷⁶Ge) = 2039 keV do not contradict the observation of the peak at 2039 keV as claimed by Klapdor-Kleingrothaus and coauthors. The text was submitted by the authors in English.     

Low energy ion beams by laser interaction

We developed an ion accelerator with a double accelerating gap system supplied by two power generators of different polarity. The ions were generated by laser ion source technique. The laser plasma induced by an excimer KrF laser, freely expanded before the action of accelerating fields. After the first gap action, the ions were again accelerated by a second gap. The total acceleration can imprint a maximum ion energy up to 160 keV per charge state. We analysed the extracted charge from a Cu target as a function of the accelerating voltage at laser energy of 9, 11 and 17 mJ deposited on a spot of 0.005 cm². The peak of current density was 3.9 and 5.3 mA for the lower and medium laser energy at 60 kV. At the highest laser energy, the maximum output current was 11.7 mA with an accelerating voltage of 50 kV. The maximum ion dose was estimated to be 10¹² ions/cm². Under the condition of 60 kV accelerating voltage and 5.3 mA output current the normalized emittance of the beam measured by pepper pot method was 0.22 π mm mrad.     

<Emphasis Type="Italic">J</Emphasis>/<Emphasis Type="Italic">ψ</Emphasis> suppression and <Emphasis Type="Italic">p</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript> spectra in RHIC and LHC energy collisions

In a hydrodynamic model, we have studied J/ψ production in Au+Au/Cu+Cu collisions at RHIC energy, GeV. At the initial time, J/ψ’s are randomly distributed in the fluid. As the fluid evolves in time, the free streaming J/ψ’s are dissolved if the local fluid temperature exceeds a threshold temperature T _J/ψ . Sequential melting of charmonium states (χ _c , ψ ^′ and J/ψ), with melting temperatures , T _J/ψ ≈2T _c and feed-down fraction F≈0.3, explains the PHENIX data on the centrality dependence of J/ψ suppression in Au+Au collisions. J/ψ p _T spectra and the nuclear modification factor in Au+Au collisions are also well explained in the model. The model however overpredicts the centrality dependence of J/ψ suppression in Cu+Cu collisions by 20–30%. The J/ψ p _T spectra are underpredicted by 20–30%. The model predicts that in central Pb+Pb collisions at LHC energy,  GeV, J/ψ’s are suppressed by a factor of ∼10. The model predicted a J/ψ p _T distribution in Pb+Pb collisions at LHC is similar to that in Au+Au collisions at RHIC.     

Properties of superconducting Nb/Al/Nb/Al?AlOx?Al?AlOx?Al/Nb/Al/Nb tunnel junctions

Modified geometry (MG) devices, Nb/Al/Nb/Al−AlO_x−Al−AlO_x−Al/Nb/Al/Nb, have been fabricated and investigated in comparison with the basic geometry (BG) double-barrier Nb/Al−AlO_x−Al−AlO_x−Al/Nb devices. The enhancement of the critical temperature in the Al film is found to be weaker for the MG devices as compared with the BG devices at temperatures nearT=4.2 K but stronger at lowT. Indication of an enhancement of dc Josephson critical current density,j _c , at bias voltageV≠0 as compared withj _c (V=0) has been observed in the MG devices for the first time.     

Transport properties of Nb/Cu multilayers in 2D and 3D state

Transport properties of Nb/Cu multilayers were measured along and across layers. 3D-2D crossover occurs both when the temperature is belowT _2D and when the magnetic field parallel to layers exceeds the ‘breaking field’,H _br.     

High refractive index and transparent heavy metal oxide glassy thin films

Heavy metal oxide thin films of the ternary system Nb₂O₅–GeO₂–PbO have been prepared by pulsed laser deposition in an O₂ environment from either glassy or crystalline bulk samples. The range of ([Pb]+[Nb]) content in which the films are optically homogeneous and transparent is much broader (0.5–1.0) than that of the bulk samples considered in the present work (0.55–0.62). The imaginary part of the refractive index is very low in all cases (k<10^-3), whereas the real part increases linearly with the ([Pb]+[Nb]) content up to values as high as 2.35. The optical energy gap has been found to be strongly dependent on [Pb], whereas it is almost independent of [Nb]. This dependence is discussed in terms of the role of Pb and Nb as network modifiers or formers. Received: 5 August 2002 / Accepted: 8 August 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +34-91/564-5557, E-mail: j.gonzalo@io.cfmac.csic.es     

Reaction of β-carotene with nitrite anion in a homogeneous acid system. An electron paramagnetic resonance and ultraviolet-visible study

Ultraviolet and visible spectroscopy was applied to characterize and to measure the concentration of β-carotene dissolved in a dioxane and water mixture. The reaction of β-carotene in the presence of nitrite anion and acid medium was studied at different temperatures. The reaction systems were homogeneous and were kept anaerobic. Pseudo-first-order rate constants in respect of β-carotene were measured in the range from 293 to 313 K and pH 5.8 ± 0.2. The energy of activation was calculated to be E _a = 67.2 ± 3.4 kJ/mol. We interpolate a value that may have biological interest, k _β-carotene(310 K) = (9.70±0.78) · 10⁻³ s⁻¹, in the presence of 9.3 · 10⁻³ M nitrite anion. Electron paramagnetic resonance spectroscopy was applied to characterize and quantify a persistent intermediate radical generated in the reaction system described. The recorded spectra showed triplet-type signals with a peak-to-peak value of 12.7 G. Nearly the same triplet radical-type intermediates were detected when studying the following reaction systems in pure dioxane: nitrogen dioxide (NO₂)/β-carotene, nitric oxide (NO)/β-carotene and NO/NO₂/β-carotene. Therefore, we proposed that the nitrogen oxides have also been intermediates in the reaction system of β-carotene, nitrite anion and acid medium, in the dioxane and water mixture. A mechanism was proposed and checked by employing the chemical kinetics simulation. The explanations developed would lead to a better understanding of the behavior of carotenoids in the presence of nitrite anion and nitrogen oxides.     

Simulation of interaction of radiation with nanoparticles

Enhancement of microwave radiation at wavelength λ ∼ 10 cm in a cavity is simulated using the system of constituent equations derived earlier. It is shown that a radiation energy density of W ∼ 1000 J/m³ can be attained. Pumping of the medium containing conducting nanoparticles is carried out with a stationary electric field. The required mass concentration of nanoparticles and pumping field are estimated. A method of obtaining active medium using a statitionary electric field for enhancement of microwave radiation in a wavelength range of λ ∼ 10 cm is proposed. In this method, extended conducting nanoparticles should be sputtered.     

X-ray attenuation around <Emphasis Type="Italic">K</Emphasis>-edge of Zr,Nb, Mo and Pd: A comparative study using proton-induced X-ray emission and <Superscript>241</Superscript>Am gamma rays

Mass attenuation coefficients (μ/ρ) for Zr, Nb, Mo and Pd elements around their K-edges are measured at 14 energies in the range 15.744–28.564 keV using secondary excitation from thin Zr, Nb, Mo, Rh, Pd, Cd and Sn foils. The measurements were carried out at the K _α and K _β energy values of the target elements by two techniques: (1) Proton-induced X-ray emission (PIXE) and (2) ²⁴¹Am (300 mCi) source. In PIXE, 2 MeV proton-excited X-rays were detected by a Si(Li) detector. In the second case, X-rays excited by 59.54 keV photons from the targets were counted by an HPGe detector under a narrow beam good geometry set-up with sufficient shielding. The results are consistent with theoretical values derived from the XCOM package and indicate that the PIXE data have better statistical accuracy.     

Beta decay and other processes in strong electromagnetic fields

We consider effects of the fields of strong electromagnetic waves on various characteristics of quantum processes. After a qualitative discussion of the effects of external fields on the energy spectra and angular distributions of the final-state particles as well as on the total probabilities of the processes (such as decay rates and total cross sections), we present a simple method of calculating the total probabilities of processes with production of nonrelativistic charged particles. Using nuclear β decay as an example, we study the weak- and strong-field limits, as well as the field-induced β decay of nuclei stable in the absence of the external fields, both in the tunneling and multiphoton regimes. We also consider the possibility of accelerating forbidden nuclear β decays by lifting the forbiddeness due to the interaction of the parent or daughter nuclei with the field of a strong electromagnetic wave. It is shown that for currently attainable electromagnetic fields all effects on total β-decay rates are unobservably small.     

Exponential Potentials and Attractor Solution of Dilatonic Cosmology

We present the scalar-tensor gravitational theory with an exponential potential in which Pauli metric is regarded as the physical space-time metric. We show that it is essentially equivalent to coupled quintessence (CQ) model. However for baryotropic fluid being radiation there are in fact no coupling between dilatonic scalar field and radiation. We present the critical points for baryotropic fluid and investigate the properties of critical points when the baryotropic matter is specified to ordinary matter. It is possible for all the critical points to be attractors as long as the parameters λ and β satisfy certain conditions. To demonstrate the attractor behaviors of these critical points, We numerically plot the phase plane for each critical point. Finally with the bound on β from the observation and the fact that our universe is undergoing an accelerating expansion, we conclude that present accelerating expansion is not the eventual stage of universe. Moreover, we numerically describe the evolution of the density parameters Ω and the decelerating factor q, and computer the present values of some cosmological parameters, which are consistent with current observational data.     

String cosmology in LRS Bianchi type-II dusty Universe with time-decaying vacuum energy density Λ

A model of a cloud formed by massive strings is used as a source of LRS Bianchi type-II with time-decaying vacuum energy density Λ. To construct string cosmological models, we have used the energy–momentum tensor for such strings as formulated by Letelier (1983). The high nonlinear field equations have been solved for two types of strings: (i) massive string and (ii) Nambu string. The expansion θ in the model is assumed to be proportional to the shear σ. This condition leads to A = βB ^m , where A and B are the metric coefficients, m is a constant and β is an integrating constant. Our models are in accelerating phase which is consistent with the recent observations of supernovae type-Ia. The physical and geometrical behaviour of these models are also discussed.     

Defects in nanocrystalline Nb films: Effect of sputtering temperature

Thin niobium (Nb) films (thickness 350-400 nm) were prepared on (1 0 0)Si substrate in a UHV chamber using the cathode beam sputtering. The sputtering temperature T_s was varied from 40 up to 500 °C and the influence of the sputtering temperature on the microstructure of thin Nb films was investigated. Defect studies of the thin Nb films sputtered at various temperatures were performed by slow positron implantation spectroscopy (SPIS) with measurement of the Doppler broadening of the annihilation line. SPIS was combined with transmission electron microscopy (TEM) and X-ray diffraction (XRD). We have found that the films sputtered at T_s = 40 °C exhibit elongated, column-like nanocrystalline grains. No significant increase of grain size with T_s (up to 500 °C) was observed by TEM. The thin Nb films sputtered at T_s = 40 °C contain a high density of defects. It is demonstrated by shortened positron diffusion length and a high value of the S parameter for Nb layer compared to the well-annealed (defect-free) bulk Nb reference sample. A drastic decrease of defect density was found in the films sputtered at T_s ≥ 300 °C. It is reflected by a significant increase of the positron diffusion length and a decrease of the S parameter for the Nb layer. The defect density in the Nb layer is, however, still substantially higher than in the well-annealed reference bulk Nb sample. Moreover, there is a layer at the interface between the Nb film and the substrate with very high density of defects comparable to that in the films sputtered at T_s < 300 °C. All the Nb films studied exhibit a strong (1 1 0) texture. The films sputtered at T_s < 300 °C are characterized by a compressive macroscopic in-plane stress due to lattice mismatch between the film and the substrate. Relaxation of the in-plane stress was observed in the films sputtered at T_s ≥ 300 °C. The width of the XRD profiles of the films sputtered at T_s ≥ 300 °C is significantly smaller compared to the films sputtered at lower temperatures. This is most probably due to a lower defect density which results in reduced microstrains in the films sputtered at higher temperatures.     

Balmer Hβ vs. Dβ line intensity in the collisions of H+, H2 +, H3 +, D+, D2 +, and D3 + ions with “realistic” metal surface at energies below 1 keV

The intensities of hydrogen H_β and deuterium D_β spectral lines of the Balmer series were measured as a function of collision energy when H⁺, H₂ ⁺, H₃ ⁺, D⁺, D₂ ⁺, and D₃ ⁺ ions impinged on Al, Ti, Cu, Mo, W, and Pb targets. The collision energies were kept in the 100–1000 eV range. The target surface was contaminated with hydrocarbons from the vacuum pumping system and possibly also by oxygen molecules due to limited vacuum conditions. At projectile velocities above 200 km/s the luminescence of backscattered deuterium atoms is about 30–50% weaker than that of hydrogen atoms.     

Emission of quasi-thermal ions from a solid bombarded by cluster ions

Comparative studies of the emission of quasi-thermal atomic and cluster ions from V, Nb, Ta, Au, and In targets bombarded by cluster ions Au _m ⁻(m = 1–9), as well as from Si and Bi targets bombarded by cluster ions Au _m ⁻(m = 1–9) and Bi _m ⁻(m = 1–5), with energy E ₀ ranging from 6 to 21 keV are carried out. In the case of bombardment by heavy cluster ions, the fraction of the quasi-thermal component in the energy spectra of sputtered atomic ions reaches 50 (for V, In, and Au), 70 (Nb), or more than 90% (Ta). In addition, quasi-thermal ions play a considerable part in the emission of small cluster ions Au₂⁺, In₂⁺, In₃⁺, and Bi _n ⁺(n = 2–7). The results of the generalizing investigation favor the presence of thermal spike conditions at cluster bombardment and their appreciable contribution to the emission of atomic and small cluster ions.