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| Issue 11, 2024 |
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Extracts from the Internet | ||
Test of the equivalence principle1 January 2008The experiment of S. Schlamminger and coworkers at the Center for Experimental Nuclear Physics and Astrophysics of the University of Washington have carried out a new test of the equivalence principle with the highest accuracy achieved so far. They used a rotating torsion balance (pendulum) with four test masses of titanium and beryllium fixed on it in a horizontal-dipole configuration. The pendulum was carefully insulated from external perturbations, its quality factor was Q = 5000 ± 200, and local gradients of the field of gravitation were compensated out. The team measured differences in the accelerations of the test masses in the direction of gravitating masses (the hill and other features of landscape around the laboratory) at a distance of one meter and longer as the orientation of the pendulum was varied in the horizontal plane. If accelerations were different, the pendulum would feel an additional force that would produce a shift in vibration frequency. No such force, and therefore, no violation of the equivalence principle was detected at the accuracy level of this experiment. The limit on the acceleration difference obtained here was Δa = (0.6 ± 3.1) × 10-15 ì ñ-2, which improves the previous result by roughly an order of magnitude. This value implies a new limit on the Etvesh parameter that characterizes a possible violation of the equivalence principle, η = (0.3 ± 1.8) × 10-13. If we take into account only the effect in the direction of the center of the Galaxy, the new restriction becomes η = ( - 4 ± 7) × 10-5; it is important to extract this contribution to test modifications of modified theories of gravitation. Source: arXiv:0712.0607v1 Energy gap in graphene1 January 2008Two groups of researchers succeeded in creating independently specimens of graphene with a gap in the energy spectrum of electrons (an interval between the valence and conduction bands). This result opens a prospect of practical application of graphene-based semiconductors in microelectronics. Graphene is a one-atom thick two-dimensional sheet of carbon atoms. A single sheet of graphene has no gap in its electron spectrum; however, it was predicted theoretically that more complex graphene systems composed of several sheets or graphene layers interacting with the surrounding atoms may have a gap. Researchers at the Berkeley National Laboratory and the University of California used the method of x-ray angle resolved photoemission spectroscopy and discovered an energy gap of 0.26 eV in a graphene film on silicon carbide substrate. Researchers are of the opinion that the energy gap is created as a result of deviation from symmetry in the arrangement of atoms in graphene's crystal lattice when it interacts with the substrate. As the number of graphene layers on the substrate increases, the gap gets narrower, and it disappears if the number of sheets is greater than four. A. Castro Neto (Boston University) and his colleagues measured the energy gap in a two-sheet graphene placed between two electrodes. The gap width can be controlled by varying the potential difference. The mechanism of gap creation in a graphene bilayer lies in the rise of electric field of excess electrons in one sheet and excess holes in the other, and they form electron-hole pairs with nonzero effective mass. When an external magnetic field is imposed, cyclotron resonances are formed whose frequency is a function of quasiparticle mass. The researchers measured these masses and established that as the potential difference increased from zero to 100 V, the energy gap grew from zero to 0.15 eV. This possibility of controlling energy gap width can be used for creating novel electronic devices, such as variable-frequency tunable semiconductor lasers. Sources: Phys. Rev. Lett. 99 216802 (2007), Berkeley Lab News Release Powerful photoelectric effect1 January 2008A.A. Sorokin, S.V. Bobashev (Ioffe Physico-Technical Institute) and their colleagues in Germany set up the FLASH experiment (Hamburg) to study the photoelectric effect with extreme-ultraviolet photons (UV wavelength — 13.3 nm) at ultra-high radiation intensity. A beam from a free-electron laser was focused by spherical multilayer mirrors to a spot 3×350µm inside a vessel with gaseous xenon so that record intensity was achieved in this UV range — 1016 W cm-2. Ionization was studied as a function of intensity. The experiment revealed an unexpectedly high degree of ionization - up to 21 electrons were knocked out of xenon atoms by UV light. The existing theoretical models do not give an adequate description of the experimental results obtained at the indicated-above wavelength and radiation intensity. Sources: Phys. Rev. Lett. 99 213002 (2007) Jets from cores of galaxies1 January 2008Jets of particles in galactic cores are generated along the axes of accretion discs in the space around black holes. A group of researchers of Pune University in India used radio telescopes to observe an elliptic galaxy CGCG 049-033 and discovered in it a jet longer than any other known jet: 1.5 million light years. Jets typically form symmetric pairs but the visible length of the second jet of the CGCG 049-033 is much shorter than the first one. This may be a consequence of the second jet pointing away from our Galaxy. An interesting feature of the first jet is the strong polarization of the radio emission generated in it. This may be a result of a strong magnetic field wrapped around the jet. It is possible that this strong magnetic field efficiently confines particles in the jet and explains its record length. Another interesting jet-related phenomenon was found in a system of two galaxies ÇÑ321 at a distance of 20,000 light years from each other. In this case the jet streaming from one galaxy happened accidentally to be aimed at a neighbor galaxy and to affect its interstellar environment. The energy of the jet dissipates on collision with the matter of the galaxy, and the jet itself gets bent and dispersed. The system ÇÑ321 was observed using several optical, x-ray and radio telescopes. The characteristics of jet dispersion made it possible to conclude that the jet reached the other galaxy relatively recently, colliding with it only a million years before astronomers started to observe it. Sources: arXiv:0712.0543v1, physorg.com |
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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