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| Issue 12, 2024 |
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Extracts from the Internet | ||
Superconductivity in multiwalled nanotubes1 March 2006I Takesue and his colleagues from Japan studied the superconducting properties of multi-walled carbon nanotubes, i. e., of ones composed of concentric layers of carbon atoms. The superconducting transition temperature of these tubes, Tc, was measured to be about 12 K, 30 times lower than for their single-walled counterparts. These latter have their superconductivity greatly suppressed due to the formation of a Tomonaga-Luttinger liquid phase, in which an additional repulsion between electrons breaks up the Cooper pairs. The Japanese experiment proceeded by synthesizing multi-walled nanotubes on a substrate of porous aluminum oxide, cutting all of them off at the same level using ultrasound and etching techniques, and then evaporating silver electrodes on the resulting cuts. This ensured that the electrodes be in electrical contact with all the inner layers of carbon atoms - unlike previous experiments, where the electrodes made contact only with outer layers and the nanotubes did not superconduct. In the new experiment, with all of the carbon layers connected by electrodes and electrically active, superconductivity turns out to overwhelm the Tomonaga-Luttinger effect. The magnitude of the superconducting gap is exactly as predicted by the Bardeen-Cooper-Schrieffer theory, and the critical current - the current above which the material's superconductivity breaks down - has a temperature dependence described by the Ginzburg-Landau formula. Source: Phys. Rev. Lett. 96 057001 (2006) Nuclear molecules1 March 2006M Freer of the University of Birmingham in the UK and his colleagues have established that the nucleus of 10Be consists of two alpha-particles and two neutrons in the space between them - a diatomic molecular structure in a sense, with the neutrons creating attractive forces between the alpha-particles. The cluster structure is found in many nuclei, but that in 10Be is the most manifest one. Such `nuclear molecules' measure a few fermis across (1fermi=10-15m), and have a lifetime of only 10-21s. The 10Be nuclei were produced by bombarding a gaseous 4He target with a beam of 6He nuclei, preliminarily produced in proton-lithium collisions. The nuclear molecules were found to decay into the same original 4He and 6He. Measuring the kinematics of these decay products provided the rotational characteristics of the nuclear molecules, from which their structure was inferred. Source: Physics News Update, Number 762 Electron velocity in a conductor1 March 2006M. Drescher, N. Kaplan, and E. Dormann have performed an experiment at Jerusalem University to measure the average velocity of electrons in a conductor. Because electrons undergo scattering, their average velocity (i. e., the current velocity) and instantaneous velocity are not the same thing. The team used a technique known as magnetic resonance imaging to study the electron spin motion in a magnetic field that varies along the conductor. The researchers started with a zero-electric-current calibration procedure, by measuring the radio echo from the spins they had made precess in phase using radio pulses, and then repeated their measurements with a current flowing through the conductor. From the observed shape of the echo the researchers determined the average electron velocity as a function of current and found it to be in good agreement with the predictions of classical theory. The major challenge in these measurements lies in the random nature of spin precession. For the specific conductors - salt crystals - used in the Jerusalem experiment, the randomization time was of order of a few microseconds. It is not yet clear whether metals, with their nanosecond randomization times, can be used in such experiments. Source: Phys. Rev. Lett. 96 037601 (2006) Ball lightning in the lab?1 March 2006The formation in the laboratory of spherically shaped, glowing, lightning-ball-like gaseous structures has been investigated by E Jerby and V Dikhtyar of Tel Aviv University. Unlike gas discharge experiments, the Tel Aviv researchers were concerned with the formation of hot plasma balls near a solid - specifically silicate - surface, a small portion of which was heated and evaporated by a radio-frequency pulse. The experiment involved directing a 600-watt magnetron pulse into a rectangular waveguide, inside which the silicate sample and a metal needle pointing to it were placed, an arrangement that produced high-intensity radiation between the needle's tip and the sample's surface. To see what is going on, special holes had been made in the waveguide's walls. What the researchers saw was that a ball of partially ionized gas formed under the tip in the region where fast melting and evaporation processes took place, which lifted itself from the surface and then either was hanging above it for some time or moved along the waveguide over a distance of 0.5 m or so at 0.3ms-1. The balls absorbed radiation very efficiently from the waveguide. The concentration of electromagnetic radiation in a ball is believed to be due to the plasmon resonance phenomenon - as in the previously proposed models of the natural fireball. There are, however, substantial, mostly scale related, differences between the laboratory balls and their natural counterparts, which move much faster, are larger in size, and live longer. Source: Phys. Rev. Lett. 96 045002 (2006) A new class of neutron stars1 March 2006Astronomers using Australia's Parkes radio telescope have detected 11 radio transient sources of a new type in the Galactic plane, which go invisible for a long period of time after emitting for 2-30 ms. With the `dark spell' lasting 2 to 30 ms depending on the source, the observation of these sources is limited to less than 1 s per day. Ten of these eleven sources were found to emit in a periodic pattern - with a period of 0.4-7.0 s - strongly suggesting that the new sources are spinning neutron stars. On the other hand, because no signs of orbital motion are seen in the signals from them, these neutron stars should be lone rather than members of binary star systems. Given that radio transients are very difficult to detect, the Galaxy may actually contain several times as many such neutron stars as conventional, continuously emitting pulsars. The Parkes findings may change the existing supernova rate estimates and call for developing models of how neutron star radiate. Source: Nature 439 817 (2006) |
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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