Thermal gradient induced sidebranching in directional solidification

The polycrystalline perovskitelike manganese oxides La_1-xA_xMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (T_c) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie temperature. Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998     

Inelastic processes in collisions of multicharged fast ions with atoms

We use the eikonal approximation to develop a general formula for the cross sections of inelastic collisions of multicharged fast ions (including relativistic ions) with atoms that is applicable within a broad range of collision energies, has the standard nonrelativistic limit, and becomes, in the ultrarelativistic case, the well-known result that follows from the exact solution of the Dirac equation. As an example we study the excitation and ionization of a hydrogenlike atom, the single and double excitation and ionization of a heliumlike atom, and multiple (up to the eighth order) ionization of the neon atom and (up to eighteenth order) ionization of the argon atom. We derive simple analytical expressions for the inelastic cross sections and establish recurrence relations linking the cross sections of ionization of different orders. Finally, we compare our results with the experimental data. Zh. éksp. Teor. Fiz. 114, 1646–1660 (November 1998)     

The orbit space of the p-subgroup complex is contractible

We show that the quotient space of the p-subgroup complex of a finite group by the action of the group is contractible. This was conjectured by Webb. Received: June 28, 1996     

Laser spectroscopy of the 1s HFS of H-like ions at a bunched beam in the storage ring ESR

The investigation of the 1s HFS provides a good possibility for testing QED effects in a combination of a strong electric and magnetic field. Here, we report about the laserspectroscopic measurements of the ground state hyperfine splitting in ²⁰⁷Pb⁸¹⁺. To handle this M1-transition in the infrared optical regime with its long lifetime, we developed a new detection technique using a bunched ion beam. For the observation of fluorescence light, a new mirror system is adapted to the emission characteristics from an ion beam at relativistic velocities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Loss and non-linearity saturation effects on the bistability of non-linear distributed feedback structures

We investigate theoretically the effects of optical absorption and saturation of the non-linear index of refraction on the bistability of non-linear distributed feedback structures (NLDFBs). By assuming that the Kerr non-linearity saturates in an exponential fashion, we obtain for the first time closed-form expressions for the so-called self-phase and cross-phase modulation terms. Our investigation shows that both absorption and, in particular, saturation significantly affect the bistable properties of this structure and in many cases eliminate this response completely. In some cases, however, saturation alters the NLDFB's transfer characteristics in a potentially useful manner. We find that weak levels of saturation may increase the contrast ratio between the intensities of the high and low bistable states. At increasing levels of saturation, where bistability is no longer observed, we find regimes where the NLDFB structure could possibly be used for optical amplification. This revised version was published online in November 2006 with corrections to the Cover Date.     

Singularities in cascade models of the Euler equation

The formation of singularities in the three-dimensional Euler equation is investigated. This is done by restricting the number of Fourier modes to a set which allows only for local interactions in wave number space. Starting from an initial large-scale energy distribution, the energy rushes towards smaller scales, forming a universal front independent of initial conditions. The front results in a singularity of the vorticity in finite time, and has scaling form as function of the time difference from the singularity. Using a simplified model, we compute the values of the exponents and the shape of the front analytically. The results are in good agreement with numerical simulations.