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| Issue 4, 2024 |
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Extracts from the Internet | ||
B meson decay1 May 2003Decays of B mesons into two baryons have been observed for the first time at the KEK laboratory in Japan. Previously, only three- and four-particle decays of B mesons, with mesons present among the decay products, have been observed. The B meson consists of a heavy anti-b-quark plus another, lighter quark. The new experiment has recorded B meson decays into an antiproton and a Lambdac+ particle consisting of u, d, and c quarks. The measured probability of such a decay is 2.19x10-5, an order of magnitude less than for three-particle decays. Among several proposed theories of B meson decay that which involves Regge poles describes experimental data most accurately. Source: Phys. Rev. Lett. 90 121802 (2003) Properties of deformed nanotubes1 May 2003It has been predicted by a number of studies that carbon nanotubes, when subjected to a mechanical force, should change their conductivity due to a change in the energy gap in the energy spectrum of the electrons. Depending on the orientation of the hexagonal grid of graphite relative to the nanotube axis, either a decrease or an increase in conductivity should occur. T W Tombler and his colleagues indeed observed a more than an order-of-magnitude reduction in conductivity in their experiments in 2000, but it was unclear whether this was related to the gap width or was due to other reasons. Now E.D. Minot and his colleagues at Cornell University have carried out a new experiment in which nanotubes, with electrodes attached to their ends, were deformed (i. e. stretched or bent) by the tip of an atomic force microscope. In some cases, not only a reduction, but an increase in conductivity was observed. The tip of the microscope also served as a gate for controlling the potential across the central part of the nanotube and the current through the tube. From the way the tube resistance depended on the voltage, it is the change in the energy gap width which dominates changes in nanotube conductivity. Source: Phys. Rev. Lett. 90 156401 (2003) Tunable Photonic Crystals1 May 2003Certain types of photonic crystals (structures with periodically varying index of refraction, liquid suspensions being an example) can change their properties under the influence of external forces, but this process takes a long time to complete. J Qi and his colleagues at Brown University have created photonic crystals which take only milliseconds to be rearranged structurally. The crystals were fabricated from the so-called holographic-polymer dispersed liquid crystals (H-PDLCs). The samples were illuminated by four coherent laser beams. At the peaks of the interference pattern obtained, liquid crystal droplet formed, to constitute a superlattice. The drops could change their index of refraction under the influence of an electric field, this changing the spectrum of radiation transmitted by the crystal. The new photonic crystals may find a number of practical applications according to the authors. Source: Physics News Update, Number 633 Magnetic levitation in cold oxygen1 May 2003In magnetic levitation experiments a non-uniform magnetic field creates a force which balances the weight of a diamagnetic body used. The levitation force increases significantly if the body is immersed in a paramagnetic medium. In previous experiments, gaseous oxygen at high pressures was used as a medium. Now L Eaves and his colleagues at Nottingham University in Great Britain have developed a method for achieving levitation in an open vessel at atmospheric pressure. In this way heavy diamagnetic (lead and gold) samples and even light paramagnetic materials could be suspended in a relatively weak magnetic field. In the same experiment, a regular pattern of peaks was found to form on the surface of liquid oxygen at fields in excess of 17 T. The surface of this shape minimizes the sum of the magnetic and surface energies. Previously such patterns were observed only on the surfaces of synthetic ferromagnetic liquids. Source: Nature 422 579 (2003) A gamma-ray burst from a supernova1 May 2003There has been evidence in recent years that al least some of gamma-ray bursts are related to supernova explosions. N Butler and his colleagues have obtained new important data of this kind using the orbital Chandra X-ray Observatory. The X-ray afterglow of the gamma-ray burst GRB 020813 was observed for 21 hours by the HETE spectrometer. The team observed an increased content of chemical elements characteristic of supernova explosions: in particular, the spectral lines of silicon and sulphur ions were seen. Compared to observations by other telescopes, the lines were detected with high confidence, the probability of statistical fluctuation being less than 0.01%. From the Doppler shift of the lines it is found that the ions move at about 1/10 the speed of light, probably through the expanding supernova shell. The narrow width of the lines suggests that the ions formed in a small volume of space. Most probably the shell was illuminated by a narrow beam of fast particles emitted by the accretion disk around a black hole that formed from a supernova. Calculations show that the supernova exploded about two months before the gamma-ray burst occurred. The observations support the so-called supra-nova model proposed by M Vietri and Stella in 1999. The basic features of the model are the explosion of a very massive star and the collimation of a beam of particles near a black hole. Source: http://arxiv.org/abs/astro-ph/0303539 |
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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