Eine Temperaturabhängigkeit der Feldelektronenemission bei sehr geringen Stromdichten

Low current density field electron emission between 1 and 10^?6 A/cm² was measured for clean tungsten as a function of temperature between 300 and 1350° K. A strong increase in temperature dependence was found with decreasing current density. Measurements are interpreted according to calculations ofMurphy, Good andMüller. Agreement is better if the work function temperature dependence is also considered.     

Crystallization behavior of supercooled smectic cholesteryl myristate nanoparticles containing phospholipids as stabilizers

Supercooled smectic nanoparticles based on physiological cholesterol esters are under investigation as a potential novel carrier system for lipophilic drugs. The present study investigates the very complex crystallization behavior of such nanoparticles stabilized with the aid of phospholipids. Phospholipid and phospholipid/bile salt stabilized cholesteryl myristate dispersions were prepared by high-pressure melt homogenization and characterized by particle size measurements, differential scanning calorimetry, X-ray diffraction and electron microscopy. To obtain fractions with very small smectic nanoparticles, selected dispersions were ultracentrifuged. A mixture of cholesteryl myristate and the phospholipid used for the stabilization of the dispersions was also investigated by light microscopy. The nanoparticles usually display a bimodal crystallization event which depends on the thermal treatment and cannot be attributed to crystalline polymorphism. The ratio of the particle fractions crystallizing in the two successive steps strongly depends on the particle size of the dispersions. The presence of larger particles leads to an increased fraction crystallizing at higher temperature and a higher recrystallization tendency upon storage. The observed peculiarities of the crystallization behavior seem to be mainly caused by the presence of particles with different shapes (cylindrical and spherical) as observed in electron microscopy. Alterations in the composition of the nanoparticles may also play a role.     

Ultrafast Fiske effect in semiconductor superlattices

The current flowing across a semiconductor superlattice in tilted electric and magnetic fields is known to exhibit resonant enhancement, when Landau states of neighboring wells align at certain ratios of the field strengths. We show that the ultrafast version of this effect, in which coherent electron wave packets are involved, has a profound analogy to the Fiske effect in superconductor Josephson junctions and superfluid weak links, in that the coupling of the tunneling-induced charge oscillations (magneto-Bloch versus Josephson oscillations) to another oscillator (in-plane cyclotron oscillations versus external oscillator modes) opens an elastic rectifying transport channel. We explore the superlattice effect both theoretically and experimentally, and find that the transient self-induced current can be adequately modeled if the damping of both types of coupled electron oscillations is properly taken into account.     

Lanthanoid-nickel-phosphides with ThCr2Si2-type structure

The new compounds YNi₂P₂ and LnNi₂P₂ (Ln = La, Ce, Pr, Nd, Sm, Tb, Dy, Ho, Er, Tm) were prépared by sintering the elemental components in silica tubes. Well-developed crystals were obtained using tin as a flux. They crystallize with the ThCr₂Si₂ (CeGa₂Al₂)-type structure which was refined from single-crystal X-ray data for EuNi₂P₂ to a conventional R value of 0.049 for 118 unique structure factors. While the P atoms in formally isotypic EuCo₂P₂ are isolated from each other, they form pairs in EuNi₂P₂. This results in a different $\text{c}\text{a}$ ratio and an entirely different bonding situation. A comparison of cell volumes shows that Eu in EuNi₂P₂ has an intermediate valence.     

Novel sol–gel derived cellular foam: reaction of an organotrialkoxysilane with sodium hypophosphite

(3-Triethoxysilylpropyl)succinic anhydride (TESP-SA) is an organo-functional silicon compound that can be converted into a polysilsesquioxane when it is hydrolyzed and subsequently subjected to a condensation reaction at elevated temperatures (>160 °C). If this process is performed without sodium hypophosphite (SHP), a hard solid material is obtained. In contrast, the condensation reaction of TESP-SA in conjunction with SHP results in the formation of a foamed, brittle material with closed macro cells. The foam was characterized by means of various analytical methods (FT-IR, ²⁹Si MAS NMR, XRD, TG-MS, SEM).