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| Issue 12, 2024 |
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Extracts from the Internet | ||
Superfluididty of a Fermi gas1 September 2004It has already been reported in (Phys. Usp. 174 531 (2004)) on the possible observation of a superfluid fermion gas in an experiment by J Thomas and colleagues on the vibrations of a magneto-optically trapped gas cloud. An independent, methodologically different experiment by R.Grimm and his colleagues of the University of Innsbruck, Austria, also discovered a superfluid fermion gas. The gas under study was that of 6Li atoms, cooled to a temperature of 500 nK in a magnetic field. The atoms first formed Cooper pairs and made a transition to the BEC state. Varying the magnetic field strength caused the condensate to transform into a strongly interacting Fermi gas - until the Cooper pairs ultimately broke up under the action of a radio frequency radiation. Measuring the frequency of the radio waves absorbed by the gas provided the magnitude of the energy gap (i.e., the pair binding energy) and made it possible to trace its growth with temperature. The presence of a gap is important evidence for the superfluidity of the fermion gas. The results of the experiments are in good agreement with the theoretical calculations. Source: http://physicsweb.org Glowing nanotubes1 September 2004In 2003, P.Avouris and his colleagues at the IBM Research Center in New York saw carbon nanotubes glowing when an electric current was passed through them. The reason was that the counterpropagating flows of electrons and holes (electron vacancies) recombined in the narrow transverse layer. Now, in their new experiment, the same team used a microscope and an infrared camera to monitor the position of the bright spot, and a control electrode, to move the spot along the tube. The current through the tube (about 50 mkm in length) was passed by means of two electrodes attached to its ends. The third, control (or gate) electrode was separated from the nanotube by a thin insulating layer, making the entire device similar to a field transistor. To the gate electrode, a voltage of 0 to 40V was applied. Because electrons and holes propagate diffusively from nanotube ends, the gate voltage turned out to affect exactly where the charges meet to recombine and thus to produce glow. Varying the potential allowed the bright spot to be moved along the entire length of the nanotube connecting the electrodes. With this technique, T.Hertel suggests, the surface defects of nanotubes can be explored by observing the way the bright spot passes through the defects. Source: Phys. Rev. Lett. 93 076803 (2004) The origin of peaks in laser spectra1 September 2004Dye lasers or random lasers are terms referring to devices that use ground liquids or suspensions instead of a single crystal with reflecting faces. Such lasers emit light in all directions and over a wide range of frequencies. A moot question, until recently, was why do high intensity peaks of unpredicted frequencies appear in the absorption spectra of such lasers. D.Wiersma of the European Laboratory for Nonlinear Spectroscopy in Florence and his colleagues were able to find an answer after a number of experiments and computer simulations. Their laser had as its active medium an alcohol solution of dye to which the light-scattering powder of zinc oxide was added. To excite the laser, short pulses from the pumping flashlamp were used. Of those photons forming the radiation continuum, most underwent only a few scattering in the medium. As it turned out, a small part of the photons underwent 100 to 1000 scattering before escaping from the medium. It is precisely those photons which stimulated powerful peaks at certain frequencies. Source: Phys. Rev. Lett. 93 052903 (2004) Controlling the state of individual atoms1 September 2004Researchers from Switzerland and Sweden have developed a technique with which single electrons can be added to or removed from individual gold atoms. The manipulations were performed with a tip of a scanning tunneling microscope at temperatures of 5 to 60 K. Gold atoms were absorbed on the surface of insulator sodium chloride deposited only three atomic layers thick on a copper crystal. When the STM tip potential of +0.6V gave rise to a tunnelling current, a gold atom gained an additional electron and thus became an ion - as the STM image clearly showed. Under a potential of -1V, an atom lost an electron and returned to its original state. Source: http://physicsweb.org Determining the mass of a single star1 September 2004Ground-based and Hubble space telescope observations have provided the first high-precision measurement of the mass of a single star (other than the Sun). The year 1993 witnessed a microlensing event in which a star in our Galaxy gravitationally focused light from a star in the Large Magellanic Cloud. At that moment, however, the source and the lens were both along the line of sight and impossible to resolve spatially. After several years of its travel through the Galaxy, the lens star moved enough angular distance from the source star for both objects to be seen as separate stars by a telescope. Besides, it proved possible to measure the parallax of the lens star - and so to place it at about 1,800 light years from Earth. Also, from the way the event took place, it was established that the lens is a single star, rather than a member of a multiple star system. By combining the microlensing and parallax data, the lens star was calculated to have a mass of 0.097+-0.016 times the mass of the Sun, making it a class M red dwarf star. Source: http://hubblesite.org, astro-ph/0405124 |
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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