Study of the Nonlinear Response of Superconductors in the Microwave Band Using a Local Technique

Using a local technique, we study the microwave radiation power P₃(T,P,H_dc) at the triple frequency of the main signal as a function of the temperature T, the input power P, and the external magnetic field H_dc perpendicular to the superconductor surface for YBa₂Cu₃O₇ (YBCO) films, monocrystals, and polycrystals and for Nb films. The most distinctive feature of the temperature dependence P₃(T) of nonlinear response of superconductors is a maximum of the nonlinearity near the critical temperature T_c. Spatial distributions of the third-harmonic power are obtained for YBCO films at various temperatures. These distributions are indicative of a nonuniform distribution of T_c over the superconductor surface. Additional nonlinearity maxima are discovered in the dependence P₃(T) for YBCO films and monocrystals at temperatures about 2T_c/3. These maxima are probably related to the existence of several superconducting phases with different critical temperatures. For Nb films, the second nonlinearity maximum in the dependence P₃(T) appears only in the presence of an external magnetic field. The experimental data are interpreted within the framework of a two-fluid model of a superconductor, which takes into account the phenomenological nonlinear relationship between the vector potential A and the supercurrent j_s(A). The origin of nonlinearity in the studied superconductors is discussed.     

Search for direct CP violation in nonleptonic decays of charged Xi and lambda hyperons

A search for direct CP violation in the nonleptonic decays of hyperons has been performed. In comparing the product of the decay parameters, alpha(Xi)alpha(Lambda), in terms of an asymmetry parameter, A(XiLambda), between hyperons and antihyperons in the charged Xi-->Lambdapi and Lambda-->ppi decay sequence, we found no evidence of direct CP violation. The parameter A(XiLambda) was measured to be 0.012+/-0.014.     

Nature of the short wavelength excitations in vitreous silica: An X-Ray brillouin scattering study

Spontaneous emission in photonic crystals with anisotropic three-dimensional dispersion relation is studied. If the upper level is below a characteristic frequency omega(1), or above omega(2), or between omega(1) and omega(2), the radiation is a localized field with a frequency in the band gap, or a propagating field with a frquency in the band, or a diffusion field, respectively. An analytical expression for the Lamb shift is obtained. The Lamb shift for the current case is small compared to that in an ordinary vacuum or in one- or two-dimensional photonic crystals due to lower density of states.     

Nickel nanostructured materials from liquid phase photodeposition

Liquid Phase Photo-Deposition (LPPD) technique has been used to obtain both colloidal particles and thin films of metallic and chloride nickel from solutions of only precursor Ni(acac)₂ (acac=2,4-pentandionato). Metallic nickel was obtained from ethanol solutions by direct nickel(II) photoreduction at 254 nm and by acetone sensitised reaction at 300 nm. In this latter process the rate was higher than in the first one. NiCl₂ was formed from CCl₄ solution by a solvent-initiated reaction. TEM analysis, performed on colloidal particles of nickel, showed that their dimensions are in the range 2–4 nm. The films did not present carbon contamination and were characterized by AFM, XPS and GIXRD. Metallic films consisted of particles of 20–40 nm that are the result of the aggregation of smaller crystallites (4–5 nm). Larger agglomerations (around 200 nm) have been observed for NiCl₂ films.     

New results on the semidiscrete Boltzmann equation for a binary gas mixture

Summary A plane semidiscrete model of the Boltzmann equation for a binary gas mixture with molecular collisions ruled by the hard-spheres interaction potential is described. After establishing a model, a theorem demostrating the global existence of mild solutions of the initial-value problem is given and the propagation of unidimensional shock waves examined.

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Sommario Si propone un modello semidiscreto piano dell'equazione di Boltzmann per una miscela binaria con collisioni molecolari soggette al potenziale di interazione delle sfere rigide. Costruito il modello, si dà un teorema di esistenza globale di soluzioni generalizzate per il problema di Cauchy, e si analizza la propagazione di onde d'urto unidimensionali.

     

Free-carrier generation in aggregates of single-wall carbon nanotubes by photoexcitation in the ultraviolet regime

We present evidence for the generation of free carriers in aggregated single-wall carbon nanotubes by photoexcitation in the energetic range of the π→π(*) transition associated with the M saddle point of the graphene lattice. The underlying broad absorption culminating at 4.3 eV can be fit well with a Fano line shape that describes strong coupling of a saddle-point exciton to an underlying free electron-hole pair continuum. Moreover, it is demonstrated that transitions in this energetic region autoionize into the continuum by detecting features unique to the presence of free charges in the transient transmission spectra of the continuum-embedded second sub-band exciton, S(2).     

Modeling triplet flavin-indole electron transfer and interradical dipolar interaction: a perturbative approach

A benchmark biochemical reaction is here theoretically investigated by means of a perturbative approach in order to model each reaction step. The reaction is the flavin-indole electron transfer, involving also a spin-state relaxation of the ionic complex. The whole reaction path is modeled and the kinetics of the process is studied. The dipolar interaction between the two radicals is explicitly considered during the dynamic evolution of the system in order to investigate the proper conditions for the triplet-to-singlet transition to occur.     

A novel derivation of collision theory rate constants for a bimolecular reaction

A novel exact derivation for the kinetic constant of a bimolecular reaction according to three well-known models of collision theory is reported. The use of probability density functions, with the introduction of a novel geometry to study the collision process, allows simplifying the derivation to such an extent, that, for a proper statistical background, it is is possible to recover the line-of-centers and the angular dependent line-of-centers models in no more than a couple of lectures. Although the derivation is introduced to recover the temperature-dependent kinetic constant, it is shown that the energy-dependent reactive cross section can be recovered as well. A possible confusion in the interpretation of the pre-exponential factor is commented.     

Blocking sets and semifields

We find a relationship between semifield spreads of PG(3,q), small Rédei minimal blocking sets of PG(2,q²), disjoint from a Baer subline of a Rédei line, and translation ovoids of the hermitian surface H(3,q²).