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| Issue 10, 2024 |
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Extracts from the Internet | ||
Antihypertriton1 April 2010Antihypertritons — hypernuclei composed of antiprotons, antineutrons and anti-Λ-hyperons — have for the first time been obtained in the Relativistic Heavy Ion Collider in the Brookhaven National Laboratory. Hypernuclei are nuclei that contain a hyperon (a baryon containing an s-quark). Hypernuclei were for the first time obserbed in 1952 in cosmic rays. Two gold ion beams with center-of-mass energy of 200 GeV per one nucleon collision were collided in this RHIC experiment; this achieved temperature and density reached in the Universe in the first microseconds of its life. As the quark--gluon plasma cooled, quarks merged into hadrons and then into various nuclei that were identified from the products of their decays. So far 70 ± 17 antihypertritons as well as 157 ± 30 hypertritons were recorded at the confidence level 4.1 σ. The antihypertriton mass is about 3 GeV; its lifetime measured in the RHIC, 182 ( + 89 - 45 ) ps, is close to the lifetime of the free Λ-hyperon. This registration of antihypertritons expands the (N,Z,S) table of nuclei observed so far to an octet with the number of neutrons N < 0 (antineutrons), nucleus charge Z < 0 and strangeness S < 0. Russian scientists from the Russian Federation State scientific center ITEP, JINR, National Research Nuclear University MEPhI and State scientific center IHEP participated in the international collaboration STAR which conducted this experiment. Source: Science 328 58 (2010), arXiv:1003.2030v1 [nucl-ex] Superconductivity of picene1 April 2010A team of researchers in Japan led by Y. Kubozono (Okayama University) reported the discovery of superconductivity in cyclic organic compound C22H14 (known as picene) doped with atoms of alkali metals. Superconducting transition was identified by registering an abrupt jump in magnetic susceptibility of the specimen as its temperature was lowered. Specimens with dopant concentration from x = 2.6 to x = 3.3 potassium atoms per one molecule of C22H14 became second-kind superconductors and their critical temperature Tc correspondingly grew from ≈ 6.5 K to 18 K. Superconductivity of picene with Tc ≈ 6.9 K was also observed after doping with rubidium at x = 3.1. Among organic superconductors known at the moment, the highest critical temperature Tc = 38 K was found in fullerene C60 doped with cesium atoms. Source: Nature 464 76 (2010) Cryogenic electron emission1 April 2010Í.Î. Meyer (Indiana University, USA) carried out a new study of the effect of emission of individual electrons in darkness from the surface of photomutiplier cathodes. At high temperatures, ordinary thermoelectronic emission occurs and obeys the Richardson law; as temperature decreases towards 220 K, the rate of ejection of electrons decreases. However, it was noticed about 50 years ago that on further cooling the rate of emission again starts to climb. This effect still lacks theoretical explanation. In Í.Î. Meyer's experiment, different models of photomultipliers with bi-alkaline cathodes were placed in a container which was cooled from the outside with liquid nitrogen and helium from room temperature down to 4 K. In contrast to thermoelectronic emission, the rate of cryogenic emission per unit area of cathode was independent of the photomultiplier model and did not correlate with electric field intensity at the cathode surface. It has been established that even though electrons are emitted one by one, successive events of emission often formed correlated groups, called bursts; burst lengths showed power-law distribution and the mean rate of burst emergence at 81 K was 4.2 Hz. Further cooling increased emission rate owing both to increased frequency of bursts and to greater number of electrons in individual bursts. Í.Î. Meyer hypothesizes that there exists some mechanism of capture and recombination of electrons in the cathode material and suggests an empirical model which is in good agreement with the obtained data. Various hypotheses concerning the mechanism of cryogenic emission were proposed in the past, taking into account thermionic emission, electric fields, radioactivity and cosmic rays but so far none were able to provide a satisfactory explanation. Source: Europhys. Lett. 89 58001 (2010) Delocalization of electrons in metallic glasses under high pressure1 April 2010Metallic glasses are metallic alloys not having crystal structure. Their unique mechanical and electromagnetic properties may find numerous useful technical applications. In 2007 it has been discovered that the volume of metallic glasses Ce55 Al45 and La32Ce32Al16Ni5Cu15 decreases under high pressure. This has been a surprise since it was believed that the compressibility of metallic glasses is very low in view of their maximum density packing of atoms. The decrease in the volume was explained theoretically in terms of changes in the structure of electron shells of cerium atoms: by delocalization of 4f-electrons. A team of researchers in China and US led by Qiao-shi Zeng conducted a new experiment which provided confirmation of this theoretical model. Electronic properties of the metallic glass Ce75Al25 at pressures from 1.5 to 5 GPa produced on a diamond anvil were investigated by absorption x-ray spectroscopy techniques. The synchrotron x-ray source APS of the Argonne National Laboratory was used. As pressure rose from 1.5 to 5 GPa the specimen volume decreased gradually by 8.6%, while the specific feature of the spectrum characteristic of 4f-electrons grew in strength. Source: Phys. Rev. Lett. 104 105702 (2010) Gravitational lens and cosmological parameters1 April 2010S.H. Suyu and his colleagues in Germany, US and the Netherlands obtained from their observtion of gravitational lensing of galaxies the values of cosmological parameters whose accuracy approaches that of the best alternative methods. The lens Â1608+656 (a pair of interacting galaxies) creates four images of a more remote radiogalaxy lying on the line of sight. Relative delays of radio signal arriving from the source galaxy along four paths have been mesured earlier with radio telescopes. Knowing the distribution of mass in the lens, it is possible to determine from this data the geometry and dynamics of expansion of the Universe. The Hubble Space Telescope observations and a careful analysis of the structure of the lens Â1608 + 656 and its surroundings made it possible to improve the accuracy of this method of calculation of cosmological parameters by a factor of nearly two. Taken together with the data of the WMAP satellite collected over five years, these results allowed the authors to confine the curvature parameter of the Universe (its deviation from the flat model) to the interval -0.031 < Ωk < 0.009, which is close to the accuracy of measurements in supernovas of type Ia. Assuming flat geometry, the result for the Hubble constant is H0 = 69.7 ( + 4.9 - 5.0 ) km s-1 Mpc-1, and the parameter for the equation of state of dark energy is w = p/ρ = - 0.94 ( + 0.17 - 0.19 ). Source: Astrophysical Journal 711 201 (2010), arXiv:0910.2773v2 [astro-ph.CO] |
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics. Permanent editor is Yu.N. Eroshenko. It is compiled from a multitude of Internet sources. | |
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