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| Issue 12, 2024 |
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Extracts from the Internet | ||
Two-loop QED corrections1 January 2006The first measurement of two-loop corrections to the Lamb shift has been made at the Lawrence Livermore National Laboratory. The Lamb shift, i. e., the shift of electronic energy levels in an atom, is due to the interaction of electrons with virtual electron-positron pairs and photons that appear from the vacuum. To accurately calculate this shift requires that interactions between various types of virtual particles be taken into account which, in terms of the Feynman diagrammatic technique, means adding loop contributions to the probability of the process. In the hydrogen atom, the relative two-loop correction to the Lamb shift is as small as 10-6, which is beyond current measurement abilities. In the highly ionized atoms of heavy elements, however, the strong electric field of the nucleus increases these corrections considerably. The Livermore team studied the Lamb shift in uranium ions that had only three electrons left. In these ions the corresponding corrections are already as large as about one-third percent of the total effect. The measurements were made by accumulating ions in a trap and detecting the photons emitted in transitions. The two-loop corrections were measured to an accuracy of about 10% according to the team. Source: Physics News Update, Number 756 Josephson junction capacitance1 January 2006Two independent teams of researchers have measured the electrical conductance of a Josephson junction for the first time. A Josephson junction is two layers of superconductor separated by a thin insulating film through which Cooper pairs can tunnel without losing their quantum properties. It has been theoretically predicted that when in electrical circuits, Josephson junctions have quantum capacitance in addition to the ordinary geometrical capacitance. In both experiments the quantum capacitance was measured in much the same way, by detecting the phase shift of a radio frequency signal in a resonant circuit containing Josephson junction(s) cooled to mK temperatures. The single-junction experiment by P.Hakonen and colleagues at Helsinki University and the Landau Institute of Theoretical Physics (Yu.G.Makhlin); and the double-junction experiment by P Delsing and colleagues at Chalmers University of Technology in Sweden produced results that are in good agreement with the theoretical calculations. The Josephson junction is a promising device for storing single units of information (known as qubits). The fact that quantum capacitance was measured without destroying the quantum state of the junction makes the technique potentially useful for future quantum computer technologies. Source: Phys. Rev. Lett. 95 206806 (2005); Phys. Rev. Lett. 95 206807 (2005) Pseudogap in a ferromagnet1 January 2006According to the Bardeen-Cooper-Schreiffer theory of superconductivity, the energy gap in a superconducting material is associated with the binding energy of the electron Cooper pairs. However, a similar spectral feature, known as a pseudogap, is also observed in high-temperature superconductors above the superconducting transition temperature. N.Mannella and colleagues first found the pseudogap in the energy spectrum of the La1.2Sr1.8Mn2O7 manganite, a material which is not a high-temperature superconductor. The interesting point about La1.2Sr1.8Mn2O7 is that close to the Curie temperature it makes an abrupt transition from the ferromagnetic metallic state to the paramagnetic non-conducting state. The spectrum of La1.2Sr1.8Mn2O7 in its ferromagnetic phase was measured at Berkeley National Lab using the technique known as angle-resolved photoemission spectroscopy (ARPES). Along with the pseudogap, team also observed an anisotropy in the material's spectrum in momentum space, which is another characteristic feature of a high-temperature superconductor. The findings imply that the pseudogap is not specific to high-temperature superconductors but is in fact a more general phenomenon yet to be explained theoretically. Source: Nature 438 474 (2005) Positronium molecules1 January 2006Positronium is a hydrogen-like system consisting of an electron and a positron. A.Mills and colleagues from the University of California may have discovered bound molecular states of positronium atoms. In their experiment, positrons traveling in a beam through porous silicon dioxide caught silicon atom' electrons to form positronium atoms which remained trapped within the pores for some time before positron-electron annihilation took place. The photons emitted in the annihilation process were registered by a detector. Under the conditions of high-dense positronium gas as obtained in the experiment, frequent positronium-positronium collisions led to frequent transitions from the angular-momentum-one state to short-lived angular-momentum-zero state, thus increasing the annihilation rate. It was found that at high density of positronium in the original beam the annihilation rate was four times higher than predicted by theoretical calculations. One possible explanation for this is the presence of defects in the material. It is also hypothesized that when at high concentrations, positronium atoms start to interact with one another to form a short-lived bound state, the molecule of positronium. Because experiment cannot as yet establish for certain which theory is correct, further investigations are needed. In the future, plans are to experiment on the creation of the Bose-Einstein condensate of positronium molecules and possibly to develop the electron-positron annihilation laser. Source: Phys. Rev. Lett. 95 195006 (2005) Laser nuclear fusion1 January 2006A new test experiment for a fusion technology based on the use of a high-power laser has been performed at the Lawrence Livermore National Laboratory. In this experiment, light from 192 lasers was passed through a hole into a sphere whose inner surface was covered with gold. The resulting heating caused the gold to emit powerful x-ray radiation which was focused to the center of the sphere. So far the researchers have only been concerned with the device's X-ray production and focusing properties. Future plans are to obtain a 100-fold increase in laser power and to place a deuterium-tritium capsule at the center of the sphere. The heating and evaporation of the capsule's shell will produce a large pressure rise in its core, causing fusion reactions to occur. It is believed that the Livermore idea can potentially be the basis for future fusion electric power plants. Source: Physics News Update, Number 755 Galaxy cluster lensing1 January 2006A new Hubble Space telescope study has been made of the galaxy cluster MS 1054-0321 observed at a redshift z=0.83 (corresponding to the time when the Universe was about half its current age). Using data on the weak gravitational lensing of the line of sight galaxies provided unprecedented accuracy in determining the distribution of mass in the cluster. In particular, in addition to the three main clumps of dark matter (or hidden mass), a few smaller clumps were seen as satellites around the cluster. Comparison with Chandra's X-ray images of the same cluster showed that only two of the three larger clumps are luminous in X-rays and that the X-ray maxima are off the centers of the dark matter clumps, presumably due to the gas pressure driven motions of the hot matter. The new finding are important for understanding the structure and formation of galaxy clusters. Source: astro-ph/0508044 |
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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