Deuteron Momentum Distribution in KD2HPO4

The momentum distribution in KD2PO4(DKDP) has been measured using neutron Compton scattering above and below the weakly first order paraelectric-ferroelectric phase transition(T=229K). There is very litte difference between the two distributions, and no sign of the coherence over two locations for the proton observed in the paraelectric phase, as in KH2PO4(KDP). We conclude that the tunnel splitting must be much less than 20mev. The width of the distribution indicates that the effective potential for DKDP is significantly softer than that for KDP. As electronic structure calculations indicate that the stiffness of the potential increases with the size of the coherent region locally undergoing soft mode fluctuations, we conclude that there is a mass dependent quantum coherence length in both systems.Comment: 6 pages 5 figure

Angular distribution studies on the two-photon ionization of hydrogen-like ions: Relativistic description

The angular distribution of the emitted electrons, following the two-photon ionization of the hydrogen-like ions, is studied within the framework of second order perturbation theory and the Dirac equation. Using a density matrix approach, we have investigated the effects which arise from the polarization of the incoming light as well as from the higher multipoles in the expansion of the electron--photon interaction. For medium- and high-Z ions, in particular, the non-dipole contributions give rise to a significant change in the angular distribution of the emitted electrons, if compared with the electric-dipole approximation. This includes a strong forward emission while, in dipole approxmation, the electron emission always occurs symmetric with respect to the plane which is perpendicular to the photon beam. Detailed computations for the dependence of the photoelectron angular distributions on the polarization of the incident light are carried out for the ionization of H, Xe$^{53+}$, and U$^{91+}$ (hydrogen-like) ions.Comment: 16 pages, 4 figures, published in J Phys

Electron source with a multi-apertured plasma emitter

In the present study, we investigated the energy efficiency of an electron source with a multi-aperture plasma emitter where the generated beam is extracted into the atmosphere through a thin metal foil. The boundary of the plasma produced in this type of emitter is stabilized with a fine metal grid. To prevent the loss of electrons at the circle-holed support grid of the extraction foil window, a metal mask with holes of smaller diameter arranged coaxially to the support grid holes is put on the emission grid. Thus, the electron beam is a superposition of beamlets formed by individual electron emitting units with the plasma boundary stabilized by the fine metal grid. The efficiency of current extraction from the acceleration gap into the atmosphere reached 75% with respect to the gap current, making possible to increase the average power of the extracted electron beam. With a 200-kV accelerating voltage, a 16-A current in the acceleration gap, and 40 µs FWHM pulse duration, 4 kW of the average beam power was extracted into the atmosphere from the acceleration gap. With the geometric transparency of the support grid of the extraction foil window equal to 56%, this made 65% of the beam power in the gap. Further increasing the beam power was limited by the power of the high-voltage power supply

Superconducting joining of melt-textured Y-Ba-Cu-O bulk material

The Tm-Ba-Cu-O solder can be successfully used to produce a superconductive joint between MT-YBCO parts. The peculiarities of solidification, phase formation, structure transformations and electromagnetic properties of MT-YBCO soldered with TmBa2Cu3O7-d are discussed.Comment: PS of 6 pages text and 5 figures, presented at ICMC'2000, Brasi

Investigation of the stability of the electron source with a multi-aperture plasma emitter generating a large cross-section electron beam

An experiment was performed to investigate the electric strength of the high-voltage acceleration gap of an electron source with a multi-aperture plasma emitter generating a beam of large cross section (750•150 mm{2}) extracted into the atmosphere through a thin metal foil. It has been shown that the use of a mask in the plasma emitter which partitions the overall emission region to produce a plurality of small-cross-section beamlets, so that the extracted beam is a superposition of beamlets formed by individual emission units whose plasma boundary is stabilized by a fine metal grid, increases the electric strength of the high-voltage acceleration gap. This is of critical importance in cases where the electron source is operated in a repetitive pulse mode at high average power of the beam. In addition, an increase in the electric strength of the acceleration gap is promoted by that the modernized cathode assemblies of the plasma emitter are arranged normal to the axis along which electrons are extracted into the acceleration gap

Lithium-induced phase transitions in lead-free Bi0. 5Na0. 5TiO3 based ceramics

Lithium-substituted 0.95[0.94(Bi0.5Na(0.5–x)Lix)TiO3–0.06BaTiO3]–0.05CaTiO3 materials include the polar rhombohedral R3c and the weakly polar tetragonal P4bm phases. On increasing lithium content, the (R3c/P4bm) phase ratio decreased, while the rhombohedral and tetragonal lattice distortions remained the same. The temperature corresponding to the shoulder in the dielectric permittivity shows no clear shift with respect to lithium substitution because of the rhombohedral distortion remaining constant. Electrical poling produced an increase of the rhombohedral phase fraction together with a rise of the rhombohedral and tetragonal distortion. This confirmed the occurrence of a phase transition from the weakly polar to the polar phase during electrical poling. Four peaks found in the current–electric field (I–E) loops are related to reversible electric field induced transitions. By studying the temperature dependence of the current peaks in the I–E loops, it was found that the minimum temperature where these electric field induced transitions take place decreases with increasing lithium substitution

High capacity in G protein-coupled receptor signaling.

G protein-coupled receptors (GPCRs) constitute a large family of receptors that activate intracellular signaling pathways upon detecting specific extracellular ligands. While many aspects of GPCR signaling have been uncovered through decades of studies, some fundamental properties, like its channel capacity-a measure of how much information a given transmission system can reliably transduce-are still debated. Previous studies concluded that GPCRs in individual cells could transmit around one bit of information about the concentration of the ligands, allowing only for a reliable on or off response. Using muscarinic receptor-induced calcium response measured in individual cells upon repeated stimulation, we show that GPCR signaling systems possess a significantly higher capacity. We estimate the channel capacity of this system to be above two, implying that at least four concentration levels of the agonist can be distinguished reliably. These findings shed light on the basic principles of GPCR signaling