Extracts from the Internet

Chemical element 118

Nuclei of the still-to-be-named chemical element with atomic number Z = 118 and mass number A = 294 has been discovered by a Russian team at the Joint Institute of Nuclear Research in Dubna in collaboration with colleagues from the Lawrence Livermore National Laboratory in the US. The nuclei of element 118 were produced in collisions of a beam of ⁴⁸Ca ions with a target of ²⁴⁹Cf. In the course of the experiment, three such nuclei were unambiguously identified by their characteristic decay chain into lower mass daughter nuclei. As a preliminary work, a series of experiments have been performed in the last few years looking at the synthesis and the decay properties of elements 112, 114, and 116. In particular, collisions ²⁴⁵Cm + ⁴⁸Ca producing element 118 nuclei were studied in detail, providing an improved decay schemes for superheavy nuclei and yielding the optimized technique for producing element 118. In the Dubna experiment, which used about 4×10¹⁹ ions ⁴⁸Ca cyclotron-accelerated to 215 MeV, the nuclei of element 118 were produced in the reaction ²⁴⁹Cf + ⁴⁸Ca with a cross section of 0.5×10^-36sm² and had a half-life of 0.89 ms. Creating superheavy elements is in particular of interest for testing the existence of the so-called “stability island”, a theoretically predicted group of nuclei with nearly filled nucleon shells. Source: Phys. Rev. С 74 044602 (2006)

Bose-Einstein condensation of magnons and polaritons

A team of researchers from Germany, Ukraine, and the US has for the first time observed the Bose-Einstein condensation of magnons (spin wave quasiparticles or quanta) in an yttrium-iron compound at room temperature. The magnons were excited by short microwave pulses in a thin film placed in a magnetic field. The magnon concentration achieved in the experiment was high enough for the magnons even to Bose condense at room temperature. The spectrum of the magnons was measured from the way they scattered laser light photons. Soon after the pulse was over, all the magnons disappeared - except for one narrow spectral peak which corresponded to the Bose-Einstein condensate and remained stable for quite a long time. Source: Nature 443 430 (2006)

The term polariton refers to the combination of an electron-hole pair (exciton) with a photon it binds. Although polaritons may have been Bose condensed in semiconductors according to reports, the previous experiments were not definitive as they did not measure polarization and spatial coherence. Now a new experiment by J.Kasprzak and his colleagues from France and the UK have remedied this omission. The researchers excited polaritons in a microcavity in a CdTe crystal using laser light and used an interferometer to study the emission spectrum of the polaritons. The result was a polarization and coherence picture characteristic of a Bose- Einstein condensate. The findings of the experiment can be used to develop a `polariton laser', according to the team. Source: Nature 443 409 (2006)

Melting hysteresis of nanocrystals

Unlike macroscopic samples, the melting mechanism of nanosized crystals is to a large extent determined by surface effects. In particular, implanting nanocrystals in the bulk of a different crystalline material increases their melting point because the crystal lattice surrounding them acts to suppress atomic vibrations near the nanocrystal's surface. The previous belief was that this effect does not occur if the surrounding medium is amorphous (for example, glass) rather than crystalline. Now Berkeley National laboratory researchers have studied the way in which Ge nanocrystals implanted in the bulk of silicate glass SiO₂ melt and solidify. The phase state and melting temperature of the nanocrystals were determined from whether the electron diffraction technique does or does not produce a diffraction pattern (so implying a crystal and a melted particle, respectively). It was found that the melting process is strongly hysteretic. While melting occurs at a temperature 199K higher than for the free macroscopic samples of Ge, the subsequent cooling of the melted nanocrystals required cooling to 256K below the free Ge melting point. This result, although surprising at first, was explained by a melting model which accurately includes all the material parameters as well as those describing melting process dynamics and the geometries of the nanocrystal and glass surfaces that interact with each other. Source: Phys. Rev. Lett. 97 155701 (2006)

Formation of massive stars

In the current view, stars start forming as a result of high- density gas clouds being compressed by gravity, and once the central core of a star has formed, the star's mass continues to grow due to the surrounding matter accreting onto it. However, the detailed nature of this process still remains in many respects obscure - especially for stars much more massive than the Sun. The radiation emitted by a young star exerts an outward pressure which hampers the matter accretion/mass growth process for stars about eight times the mass of the Sun. As a way out, a number of models were suggested, including a merger of several stars and non- spherical accretion. Now observations made with the VLA radio telescope of G24.78+0.008, a star about 20 times as massive as the Sun, has added weight to the latter model. Specifically, VLA astronomers measured the Doppler shifts of spectral lines from ammonia molecules to determine the speed of gas flows near the star. For the first time, three types of motion were found to simultaneously exist: a toroidal gas cloud orbiting the star, the inward (starward) motion of matter, and gas outflow from the star along the axis of rotation - all this indicating that the disk accretion is at work here, a scenario which does not seriously prevent gas from falling onto the star and increasing its mass. Source: Nature 443 427 (2006)