Beta-decay of 71Co and 73Co

A decay spectroscopy study of the neutron-rich cobalt isotopes has been performed using fragmentation of a ⁸⁶Kr³⁶⁺ beam and the new LISE2000 spectrometer at GANIL. For ⁷¹Co and ⁷³Co, the -delayed radiation has been observed for the first time, and the half-lives were found to be 79(5) ms and 41(4) ms, respectively. Features of the decay are discussed qualitatively in terms of nuclear models.     

Influence of the oxidation temperature on the fabrication process of silicon dioxide aperture tips

The thermal oxidation of structured silicon surfaces was successfully used to reproducibly define apertures of approximately 100 nm in silicon dioxide tips at reduced oxidation temperatures. In this paper we theoretically investigate the oxidation process in more detail, describing the rheological behavior of silicon dioxide as a Maxwell fluid with non-linear viscosity. For this purpose numerical calculations of the oxidation process of trench-like silicon structures were performed. Contrary to former assumptions, our theoretical results indicate that oxide-growth retardation is more effective at raised oxidation temperatures. This is experimentally confirmed in the case of trench structures. The more pronounced oxide retardation at elevated temperatures is exploited to obtain apertures in silicon dioxide tips of 60 nm for oxidation temperatures of 1100 °C. Received: 8 April 2002 / Accepted: 11 April 2002 / Published online: 4 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-561/804-4136, E-mail: oester@physik.uni-kassel.de     

Phase Transitions and Spatio-Temporal Fluctuations in Stochastic Lattice Lotka–Volterra Models

We study the general properties of stochastic two-species models for predator-prey competition and coexistence with Lotka–Volterra type interactions defined on a d-dimensional lattice. Introducing spatial degrees of freedom and allowing for stochastic fluctuations generically invalidates the classical, deterministic mean-field picture. Already within mean-field theory, however, spatial constraints, modeling locally limited resources, lead to the emergence of a continuous active-to-absorbing state phase transition. Field-theoretic arguments, supported by Monte Carlo simulation results, indicate that this transition, which represents an extinction threshold for the predator population, is governed by the directed percolation universality class. In the active state, where predators and prey coexist, the classical center singularities with associated population cycles are replaced by either nodes or foci. In the vicinity of the stable nodes, the system is characterized by essentially stationary localized clusters of predators in a sea of prey. Near the stable foci, however, the stochastic lattice Lotka–Volterra system displays complex, correlated spatio-temporal patterns of competing activity fronts. Correspondingly, the population densities in our numerical simulations turn out to oscillate irregularly in time, with amplitudes that tend to zero in the thermodynamic limit. Yet in finite systems these oscillatory fluctuations are quite persistent, and their features are determined by the intrinsic interaction rates rather than the initial conditions. We emphasize the robustness of this scenario with respect to various model perturbations.     

On adaptive time stepping for large-scale parabolic problems: Computer simulation of heat and mass transfer in vacuum freeze-drying

The work is motivated by the problem of freeze-drying, which is a process of dehydrating frozen materials by sublimation under high vacuum. In particular, it concerns the mathematical modelling and computer simulation of the heat and mass transfer with the core in solving the time-dependent nonlinear partial differential equation of parabolic type.     

Magnetic moments of hindered rotators by off‐center impurity ions

Off‐center impurity ions in solids often perform rotations around their regular lattice sites. Unlike quasifree rotors with rotational line spectra subject to textbook attention in quantum mechanics, the off‐center species are hindered rotors with spectra quantized in rotational bands. These bands occur because of tunneling through barriers arising along the orbital path. For an off‐center ion rotating along a planar orbit, such as the Li⁺ impurity nearest‐neighboring an F center in alkali halide, the hindered rotation will give rise to specific magnetic moments that couple to and quantize external magnetic fields normal to the orbital plane. We present a simple theory and estimates of Li⁺ magnetic dipoles and rotational bands to find conditions for an experimental verification. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Preparation,crystal structure and infrared spectroscopy of the new compound Rb4Be(SeO4)2(HSeO4)2·4H2O

A new compound, Rb₄Be(SeO₄)₂(HSeO₄)₂·4H₂O, crystallizes in a comparatively wide concentration range from mixed beryllium rubidium selenate solutions (from solutions containing 29.06 mass% beryllium selenate and 25.75 mass% rubidium selenate up to solutions containing 12.53 mass% beryllium selenate and 55.32 mass% rubidium selenate).Rb₄Be(SeO₄)₂(HSeO₄)₂·4H₂O crystallizes in the acentric orthorhombic space group Pmn2₁ (a = 32.607(4), b = 10.676(2), c = 6.069(1) Å, V = 2112.8 ų, Z = 4, R1 = 0.047 for 4059 F_o > 4σ(F_o) and 311 variables). The crystal structure is composed of Be(H₂O)₄ tetrahedra arranged in layers at x = 0 and x = ½, alternating with broad layers built up from SeO₄ and HSeO₄ selenate tetrahedra and Rb cations. The beryllium–water layers are linked to the rest of the structure via hydrogen bonds only. The H₂O molecules as well as the OH molecules of the acid HSeO₄ groups form strong to very strong hydrogen bonds with donor–acceptor distances between 2.58 and 2.74 Å.Vibrational spectra (infrared and Raman) of Rb₄Be(SeO₄)₂(HSeO₄)₂·4H₂O are presented and discussed in the region of the fundamentals of both the selenate and the beryllium tetrahedra (skeleton motions) as well as in the region of the OH vibrations at ambient and liquid nitrogen temperature (LNT). The appearance of four Raman bands corresponding to ν₁ of the selenate ions reflects the existence of four crystallographically different selenate tetrahedra in the structure. The spectroscopic experiments reveal that the ν₁ modes of the selenate ions appear at higher frequencies than some components of ν₃. Bands of an AB doublet structure (2950, 2390 cm^?1) arising from the OH stretching modes of the HSeO₄^- ions are recognized in the infrared spectra. The appearance of two infrared bands (1308, 1250 cm^?1) corresponding to δ(OH) (in-plane bending modes of the OH groups) confirms the structural data regarding the existence of two crystallographically different OH groups. The water librations are also briefly commented. The appearance of a band at a comparatively large wavenumber (1013 cm^?1) corresponding to rocking librations of the water molecules indicate that strong hydrogen bonds are formed in the title compound.     

Continued fractions of primitive recursive real numbers

A theorem, proven in the present author's Master's thesis 2 states that a real number is ‐computable, whenever its continued fraction is in (the third Grzegorczyk class). The aim of this paper is to settle the matter with the converse of this theorem. It turns out that there exists a real number, which is ‐computable, but its continued fraction is not primitive recursive, let alone in . A question arises, whether some other natural condition on the real number can be combined with ‐computability, so that its continued fraction has low complexity. We give two such conditions. The first is ‐irrationality, based on a notion of Péter, and the second is polynomial growth of the terms of the continued fraction. Any of these two conditions, combined with ‐computability gives an (elementary) continued fraction. We conclude that all irrational algebraic real numbers and the number π have continued fractions in . All these results are generalized to higher levels of Grzegorczyk's hierarchy as well.     

Ice-mediated coating of macroporous cryogels by carbon nanotubes: a concept towards electrically conducting nanocomposites

Electrically conducting super-macroporous polymer/carbon nanotubes (CNTs) composites have been fabricated with the aid of cryogenic treatment. CNTs are selectively deposited onto the inner surface of interconnected polymer walls surrounding large air-filled pores. The present concept provides highly conducting nanocomposites at a very low nanotubes content.     

Contactless Spectroscopy of Deep Levels in Semiconducting Materials: Gaas

Abstract

The surface photo voltage (SPV) and photocurrent (PC) transients as a result of the excitation by the short high-intensity light pulses from semiconductor's intrinsic absorption spectral region are investigated in semi-insulating GaAs. It is shown that the mathematical convolution of SPV transients and arbitrary form double-pulse integrator (lock-in, double-boxcar) in a wide temperature range allows to receive the deep-level (DL) spectrum without the need to form electrical contacts to the crystal investigated. The use of such a procedure while scanning the crystal surface with a light spot at a temperature, corresponding to some DL maximum in the spectrum, makes possible the con tactless determination of this DL density distribution profile along the scanning direction.