Deuterium-deuterium nuclear cross-sections in insulator and metallic environments

The three-dimensional Thomas-Fermi (TF) model is used to simulate the variation of the d + d → t + p cross-section at low impact energies, when the target deuterium nucleus is embedded in metallic or insulator environments. The comparison of the computational results to recent experiments demonstrates that even though the TF model can explain some increase in the low-energy cross-section for metallic host, a full explanation of the experimental results is still lacking. Possible reasons for the disagreement are discussed.     

Domain wall motion in ferromagnets modelled by a quintic complex Ginzburg-Landau equation

A quintic complex Ginzburg-Landau equation is derived from a Landau-Lifshitz-Gilbert equation and is used to describe the magnetization dynamics in a one-dimensional uni-axial ferromagnet. Trough the use of suitable approximations, we derive the magnetic solitary wave excitations solutions which have pulse-like shapes. Subsequent numerical simulations reveal domain wall propagation.     

Strong electron-phonon coupling in Be<Subscript>1−x</Subscript>B<Subscript>2</Subscript>C<Subscript>2</Subscript>: ab initio studies

Several structures for off-stoichiometric beryllium diboride dicarbide Be_1−xB₂C₂ have been designed, and their properties studied from first-principles density functional methods. Among the most stable phases examined, the layered hexagonal structures are shown to exhibit various features in the electronic properties and in the lattice dynamics reminiscent of the superconducting magnesium diboride and alkaline earth-intercalated graphites. For substoichiometric composition x ∼ 1/3, the system is found metallic with a moderately strong electron-phonon coupling through a predominant contribution arising from high frequency streching modes modulating the σ-bonding of the B-C network, and a weaker contribution at medium frequency range of the phonon spectra, arising from the intercalent motion coupled to the π-bonding states. Further, anharmonicities emerging from the proximity of the Fermi level to the σ-band edge, contributes to reduce the phonon softening hence stabilizing the structure. All these effects appear to combine favourably to produce a high temperature phonon-superconductivity.     

Nucleon charge exchange on the deuteron: A critical review

The existing experimental data on the d(n, p)nn and d(p, n)pp cross-sections in the forward direction are reviewed in terms of the Dean sum rule. It is shown that the measurement of the ratio of the charge exchange on the deuteron to that on the proton might, if taken together with other experimental data, allow a direct construction of the np → np scattering amplitude in the backward direction with few ambiguities. Communicated by T. Hennino     

Indirect magnetic interaction in the “net fractal” systems

A localized spin system of fractal symmetry with indirect exchange between them is considered. We define a specific class of fractals as the “net fractals” which display multidimensional logarithmic periodicity. Basing on this property we model the effective indirect exchange interaction by the conventional RKKY exchange with the logarithmic coordinates playing role of the real space ones. Finally, we discuss the case of non-ideal “net fractals” in which fractional dynamics of the electrons is expected. In this case we show that RKKY exchange integrals are given by the formulas derived under assumption that a system has a fractional spectral dimension.     

Modelling of Lévy walk kinetics of charged particles in edge electrostatic turbulence in tokamaks

We model and discuss the possible types of motion that charged particles may undergo in a stationary and spatially periodic electrostatic potential and a homogeneous magnetic field. The model is considered to be the simplest approximation of more complex phenomena of plasma edge turbulence in tokamaks. Therein, low frequency turbulence appears in the plasma edge, resulting in a fluctuation of the electron density, and also in the generation of a turbulent electrostatic field. Typical parameters of this turbulent electrostatic field are an electrical potential amplitude of 10–100 V and wave numbers k≈10³ m^-1. In our model, we consider these regimes, together with a homogeneous magnetic field with a magnitude of 1 T. We investigate the dynamics of singly-ionized carbon ions – a typical plasma impurity – with kinetic energies on the order of 10 eV. Besides the obvious Larmor and drift motions, a motion of random-walk and of Lévy walk character appear therein. All of these types of motion can play an important role in the modelling of the anomalous diffusion of particles from the plasma edge turbulence region. The dynamics mentioned will cause an inevitable escape of energetic particles and thus of power loss from the thermonuclear reactor. Moreover, Lévy walk kinetics represents a very interesting kind of kinetics, currently of great interest, which was previously not so often discussed.     

Neutron- and muon-induced background in underground physics experiments

Background induced by neutrons in deep underground laboratories is a critical issue for all experiments looking for rare events, such as dark matter interactions or neutrinoless ββ decay. Neutrons can be produced either by natural radioactivity, via spontaneous fission or (α, n) reactions, or by interactions initiated by high-energy cosmic rays. In all underground experiments, Monte Carlo simulations of neutron background play a crucial role for the evaluation of the total background rate and for the optimization of rejection strategies. The Monte Carlo methods that are commonly employed to evaluate neutron-induced background and to optimize the experimental setup, are reviewed and discussed. Focus is given to the issue of reliability of Monte Carlo background estimates. We dedicate this work to the memory of our friend and colleague Nicola Ferrari, who prematurely passed away in July 2006.     

Noise spectra of microcavity-emitting field in the linear regime

Our work presents a theoretical study on the behavior of the fluctuation statistics of the field emitted from a semiconductor microcavity containing a quantum well. We derive an analytical expression of the noise spectra in non-resonant pumping. Dynamical behaviors of the noise spectra depending on the thermal bath and the detuning between exciton and cavity frequencies are discussed.     

Adhesion of binary giant vesicles containing negative spontaneous curvature lipids induced by phase separation

We report the adhesion of binary giant vesicles composed of two types of phospholipids, one has negative spontaneous curvature which tends to bend toward the head group and the other has zero spontaneous curvature. In a homogeneous one-phase region, the giant vesicles do not adhere to each other, whereas in a coexisting two-phase region, the giant vesicles show adhesion. A fluorescence microscope observation reveals that the adhesion takes place through the domains rich in phospholipids having negative spontaneous curvature. We propose a phase separation induced hemifusion model where two apposed monolayers of adjacent vesicles are hemifused in order to reduce the bending energy of monolayers with negative spontaneous curvature and the boundary energy between the domains and matrix. We provide a strong evidence for the hemifusion model by lipid transfer experiments.     

A mechanistic study of oxygen atom transfer from N-sulfonyloxaziridine to enolates

Enolate additions to chiral N-sulfonyloxaziridines providing enantiomerically enriched α-hydroxy carbonyl compounds is a reaction of importance, yet a clear understanding of the factors governing stereoinduction in these transformations remains ambiguous. This is despite, previous computational studies, one by Bach et al. employing truncated model systems exploring oxygen atom transfer to an unsubstituted lithium enolate and another by our own group. In clarifying this reactivity we report here a computational study examining oxygen atom transfer from 1-S-(+)-(10-camphorsulfonyl)oxaziridine, viz., archetypal Davis chiral oxaziridine to substituted Li, Na, K enolates offering improved mechanistic understanding. From this investigation, a revised model is offered revealing the metal cation, chelation effects and sterics as decisive stereocontrolling factors in enolate additions to chiral N-sulfonyloxaziridines affording enantiomerically enriched α-hydroxy carbonyl compounds.