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| Issue 12, 2024 |
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Extracts from the Internet | ||
Antiprotonic helium1 January 2003The term antiprotonic helium refers to a helium atom in which one of the electrons is replaced by an antiproton. This is achieved in an experiment by passing an accelerator-produced beam of antiprotons through a helium medium. The antiprotonic helium was first discovered in 1991, although its existence had been predicted theoretically as early as 1964 by Kondo. This exotic atom is interesting in that it has an unusual energy level structure. The antiprotonic helium exhibits fine and hyperfine level splittings, due to the interaction of the magnetic moments of the antiproton and the electron. In 1998 D Bakalov (Bulgaria) and V I Korobov (JINR, Dubna) showed theoretically that each of the two energy levels of the hyperfine doublet must in turn split into two close sublevels. This results from the interaction of the spin magnetic moments of the antiproton and the electron with the antiproton's orbital magnetic moment. This splitting has now been discovered for the first time by a group of European and Japanese researchers headed by E Widmann. To obtain atoms of antiproton helium, a CERN accelerator was used. The presence of the two sublevels was registered by quantum transitions between them. Using a laser, the overpopulation of the upper level was achieved, and then quantum transitions to the lower level occurred under the action of microwave radiation. The frequency of the microwave radiation was chosen such as to cause allowed transitions between the sublevels. The resulting population of the levels was measured by a second laser, which caused transitions from the metastable levels to short-lived ones, thus leading to a rapid annihilation of the antiproton and producing a burst of radiation. By varying the frequency of the laser pulses it was possible to determine the shape of the transition lines and to measure the energies of the states involved. The experiment has confirmed, to a high degree of precision, the predictions made by D Bakalov and V I Korobov. Source: Phys.Rev.Lett. 89 243402 (2002) A new superconductor1 January 2003Researchers from the Los Alamos National Laboratory (US), the University of Florida (US) and the Institute for Transuranium Elements in Karlsruhe, Germany, have discovered that the alloy of plutonium, cobalt, and gallium PuCoGa5 becomes a superconductor when cooled to a temperature below Tc=18.5K. This temperature is about an order of magnitude higher than the superconducting transition temperature in the so-called heavy-fermion systems, compounds based on uranium and cerium. The plutonium-based alloy may therefore belong to a new class of superconducting materials. It was also found that the critical current and critical magnetic field that destroy superconductivity have relatively large values. The superconductivity of the PuCoGa5 alloy is due to the complex electron structure of the plutonium atom according to the authors of the experiment. Source: Nature 420 297 (2002) The Beliaev effect1 January 2003N Katz and colleagues in Israel have examined collisions between elementary excitations (quasiparticles) and atoms in a Bose- Einstein condensate in the previously unexplored regime of the continuous energy spectrum of the quasiparticles. It was observed that the collision rate decreased with decreasing energy. This effect was theoretically predicted by S T Beliaev in 1958. The decrease in the collision rate is explained by a quasiparticle decaying into two or more lower-energy quasiparticles and may occur at almost zero temperature. At higher temperatures, Landau damping usually predominates. The experimental setup used was a quadrupole-type magnetic trap in which 105 atoms of 87Rb were confined in the state of a Bose-Einstein condensate. Quasiparticles were generated by laser radiation modulated at sound frequency. The scattering of the quasiparticles was registered by atoms that flew out of the condensate after undergoing a recoil in a scattering event. The study of the Beliaev effect is important for understanding quantum correlations between quasiparticles and for creating `atom lasers.' Source: Phys.Rev.Lett. 89 220401 (2002) Magnetic cooling1 January 2003The underlying mechanism of the well-known magnetic cooling technique (adiabatic demagnetization) is one is which the internal energy of the paramagnetic material goes to misaligning the magnetic moments of the particles as the magnetic field is decreased. Now O Waldmann and his colleagues at Erlangen-Nurnberg University in Germany have for the first time encountered the opposite situation, in which the material is cooled when the magnetic field is increased. The team studied crystals whose molecule, NaFe6 , consists of a ring of six iron atoms and a sodium atom at the centre. Such ring structures attract the attention of researchers because they open the possibility of coherent quantum tunneling and because they hold promise of quantum computer applications. As a result of the interaction of the spin magnetic moments of the iron atoms with the magnetic field, the NaFe6 molecules have two energy levels which cross when the external magnetic field is Bc=12T. As the magnetic field is varied around Bc, the crystal exhibits hysteresis. If the rise of the magnetic field starts from below Bc, the crystal cools. Source: Phys.Rev.Lett. 89 246401 (2002) A galaxy with two active nuclei1 January 2003Astronomers using the ACIS spectrometer onboard the Chandra X-ray Observatory have made detailed observations of the nuclei of NGC 6240 galaxy, which has great brightness in the infrared and X-ray portions of the spectrum. The presence of two nuclei about 3000 light years apart was established earlier using optical telescopes. It was also known that at least one of the nuclei is active. New Chandra observations have now shown that indeed both nuclei are active. The nuclei have the X-ray spectrum which is characteristic of single active galactic nuclei and which may result from the accretion of matter onto supermassive black holes. Given its irregular shape, the galaxy NGC 6240 is the result of the merger of two lower-mass galaxies. Additional evidence for this is the high rate of star formation, which process could have been initiated by tidal forces. The scientists estimate that in a few hundred million years two supermassive black holes in the galaxy NGC 6240 will merge into one black hole. The observation of the galaxy NGC 6240 is important for building models for the formation of galaxies and black holes. It is not yet clear which process dominates the growth of the black holes, their merger following the merger of galaxies or the mass increase due to the accretion of matter. If mergers of black holes are frequent enough, the gravitational waves they produce can be registered by gravitational wave detectors within the next few years (see Physics-Uspekhi 43 691 (2000)). Source: http://arxiv.org/abs/astro-ph/0212099 |
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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