Size effect in nanocrystalline manganites La<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript><Emphasis Type="Italic">A</Emphasis><Subscript>x</Subscript>MnO<Subscript>3</Subscript> (<Emphasis Type="Italic">A</Emphasis>=Ag,Sr)

Nanocrystalline samples of the manganites La_0.9Ag_0.1MnO₃, La_0.7Ag_0.3MnO₃, and La_0.7Sr_0.3MnO₃ were synthesized through pyrolysis and isothermally annealed. The atomic, subatomic, and magnetic structures of these manganites were studied using magnetic, x-ray, and neutron diffraction measurements. Increasing the annealing temperature from 600 to 1200°C coarsens the grains from 30–40 to 600–700 nm in size. All the samples studied have rhombohedral structure and are ferromagnets. The Curie temperature decreases for the samples doped by silver and increases for the samples doped by strontium as the anneal temperature is increased. The magnetization of the Mn ions increases with nanoparticle size in all the three systems, which indicates the presence of a size effect.     

Sign reversals of the carriers and the volume change in Al-Cu-Fe quasicrystalline alloys

New results of dilatometric experiments with rapidly quenched Al-Cu-Fe quasicrystalline alloys in the course of their thermal annealing are presented. It is established that the icosahedral (I) phases with different types of carriers exhibit different signs of volume changes with ordering. The observed effect is a direct experimental proof of the fact that structural defects in icosahedral quasicrystals are electrically active centers.     

On the existence of a cycle in an asymmetric model of a molecular repressilator

In this paper, a nonlinear six-dimensional dynamic system, which is a model of functioning of a simple molecular repressilator, is considered. Sufficient conditions for the existence of a cycle C in the phase portrait of this system are found. An invariant neighborhood of C, which retracts to C, is constructed.     

A model of a neuron with oscillatory activity below the excitation threshold

We propose a dynamic model of a neuron with spontaneous periodic oscillations below the excitation threshold. Such neurons, in particular, play an important role in the problem of coordination of motions by the brain specifying the universal rhythm of muscular contractions. The model is constructed on the basis of the known model dynamic systems and is described by a system of fourth-order differential equations. A good qualitative agreement between the model dynamics and experimental data for the actual neurons is obtained. This work was presented at the Summer Workshop “Dynamic Days” (Nizhny Novgorod, June 30–July 2, 1998). Lobachevsky State University of Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 12, pp. 1623–1635, December, 1998.     

Accurate lifetime measurements for the noble gases by the electron beam alignment technique

Rotational rainbow scattering has been measured in the systems K- and Rb-CO₂ at collision energies 0.17≦E≦0.56 eV and CMS angles 110°≦?≦170°. The ellipsoidal hard shell anisotropy of the normal (repulsive) potentials is evaluated to be (c?a)=0.67Å in K?CO₂, (c?a)=0.63Å in Rb?CO₂, averaged over a slight energy dependence. Below an apparent activation thresholdE _a?0.23 eV, nearly equal in both systems, the scattering largely conforms to that of diatomic molecules, indicating linear, rigid rotor type behaviour as the dominant collisional response of triatomic CO₂. AboveE _a characteristic, eventually drastic distortions set in. They are probably caused by intermediary charge transfer which activates a complex, coupled vibrational, rotational motion inM ⁺-CO ₂ ^? , altering the outcome of the collisions altogether. The simple model of a partially absorptive, hard shell is used to find that some gross features of the distortions are consistent with distinct properties of the acceptingelectronic orbital 6a ₁ in CO ₂ ^? .