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| Issue 12, 2024 |
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Extracts from the Internet | ||
Low-energy solar neutrinos1 October 2007The energy spectrum of solar neutrinos created in radioactive decays of 7Be nuclei was measured for the first time in the Borexino experiment (Gran Sasso National Laboratory, Italy). 7Be nuclei are created in the 3He+4He synthesis; their decay produces emission of monoenergetic electron neutrinos νe (with energy 0.862 MeV). In the past this neutrino component was detected only integrally, by using radiochemical separation. The existing detectors that respond to the Vavilov – Cherenkov radiation due to neutrino interaction with water can only reveal neutrinos with energy above 5 MeV. The new Borexino experiment uses liquid organic scintillator. Excitation of scintillator molecules produced by elastic scattering of neutrinos on electrons results in a signal that is sufficiently strong for real time recording of low-energy neutrinos, beginning with about 200 keV. High background noise of β-decays of 14C nuclei dominates at still lower energies. The experimental setup includes structural elements and materials with very low level of radioactive contamination, fabricated using specially dedicated technologies. The scintillator was poured into a spherical nylon shell and placed inside a steel sphere surrounded with a layer of water, while the experiment as a whole was run inside a mountain tunnel. After subtraction of background events from all known processes and taking into account the effect of neutrino oscillations via the Mikheyev – Smirnov – Wolfenstein mechanism, the neutrino spectrum was found consistent, with high accuracy, with the beryllium solar neutrino in the standard model of the Sun. Source: http://arxiv.org/abs/0708.2251 Quantum Hall effect in heterostructures1 October 2007A team at the Institute of Solid State Physics (Chernogolovka, Russia) and their colleagues in Italy carried out a study of 2D electron subsystems in a heterostructure prepared as a GaAs/AlGaAs contact placed in magnetic field. This system manifests fractional Hall effect. The experiment measured the magnetic capacitance of the contact and calculated the chemical potential jump for various Landau level filling factors ν. Capacitance minima are reached at ν=1/3 and ν=2/3 while close to ν=1/2 the value assumed by the capacity is dictated by the contact geometry. Interesting specifics were observed in the behavior of chemical potential in the low-temperature limit T<1Ê when the difference between the properties of specimens with ν=1/3 and ν=2/3 was well pronounced. Thus the spin transition was observed in situations with low magnetic field in the specimen with ν=1/3. Totally spin-polarized state appears in high magnetic fields, and the chemical potential jump in the specimen with ν=1/3 grows linearly with magnetic field and tends to the value of the jump in the specimen with ν=2/3, which is caused by the electron-hole symmetry of Landau level splitting. Source: Phys. Rev. Lett. 99 086802 (2007) Molecular positronium1 October 2007For the first time in a laboratory experiment D. Cassidy, A. Mills and coworkers at the University of California obtained positronium molecules Ps2 — short-lived bound states of two Ps atoms (e--e+ pairs). Atoms of Ps were created by electron capture when a thin quartz film was bombarded by a high-intensity beam of e+. Positronium atoms Ps could survive for a short time before annihilation in quartz's microcavities. If Ps concentration was high, this time was enough for some Ps atoms to merge into Ps2 molecules which were rapidly destroyed by annihilation of the constituent particles and antiparticles. Since e- and e+ in a Ps2 molecule have identical masses, it is not possible to single out exactly two individual Ps atoms and thus is necessary to consider a bound system of four particles. In this experiment Ps2 was identified from a characteristic dependence of the intensity of annihilation emission on temperature since Ps2 molecules form with higher probability at low temperatures. Research planned for the future includes work with more powerful e+ beams to produce Bose – Einstein positronium condensate and also the application of Ps for creating coherent beams of gamma emission — gamma lasers. Source: http://www.physorg.com/news108822085.html Effect of sound on electric resistance1 October 2007It is known that switching on of magnetic field causes the electric resistance of certain manganites (oxides and hydroxides of manganese) in the vicinity of the Curie temperature to change by several orders of magnitude. It was hypothesized that this effect is caused by the nature of the interaction between electrons and phonons (quasiparticles, i.e. acoustic vibrations of crystal lattice). A. Cavalleri and his colleagues at the Oxford University carried out an experiment in which another spectacular property of manganites was revealed. A manganite specimen was subjected to a short laser pulse which excited the phonon mode of vibrations at a frequency of 17 THz. This sharply reduced the electric resistance of manganite by a factor of hundreds of thousands, with resistance returning 5 ns later to its pre-pulse value. The laser pulse duration was such (300 fs) that only one phonon mode was excited and there was no time for electrons to be heated. This allowed Cavalleri et. al to eliminate additional factors involved in heating. So far the authors do not have an exact theoretical model for the sharp drop in manganite resistance caused by acoustic vibrations. The phenomenon is observed at room temperature but it is conceivable that it has some common features with the mechanism of high-temperature superconductivity. It cannot be excluded that the effect discovered here may find practical applications in acoustic detectors for the THz range. Source: Nature 449 72 (2007) ; http://physicsworld.com/cws/article/news/31066 Supermassive black hole in NGC 1399 galaxy1 October 2007The study of the distribution and motion of stars in galactic nuclei is important for elucidating the mechanism of initial formation and growth of supermassive black holes. The structure of central stellar clusters may retain traces of mergers of black holes that followed the merger of protogalaxies. The distribution of stars also influences the rate at which they are absorbed and the rate of tidal destruction by the black hole, and also the rate of the accretion of matter onto the black hole. The Hubble space telescope was used to study the stellar kinematics close to a supermassive black hole in the giant elliptic galaxy NGC 1399 — the central galaxy of the Fornax galaxy cluster at a distance of of 21.1 Ìpc from the Sun. Strong anisotropy of velocities was observed: the tangential dispersion of stellar velocities exceeds three-fold the radial dispersion. This is a record value of tangential anisotropy in a galactic nucleus among all investigated galaxies. The best-fit dynamic model of the stellar cluster yields the black hole mass of 5.1(±0.7)×108 solar masses, which is approximately 2.5 times less than the mean value that follows from the known correlation between black hole mass and dispersion of stellar velocities at galactic centers. To interpret the results of observations, it is necessary to further develop the models of formation of supermassive black holes and galactic nuclei. One of the possible explanations of the high velocity anisotropy is that a stellar cluster is collapsing towards the center of the NGC 1399 galaxy. Source: http://arxiv.org/abs/0709.0585 |
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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