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| Issue 12, 2024 |
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Extracts from the Internet | ||
The limiting magnetic field1 July 2006In their stability analysis of the vacuum against the so-called `positronium collapse,' A.E.Shabad (P.N.Lebedev Physics Institute, RAS, Russia) and V.Usov (Weizmann Institute of Science, Israel) established that the maximum magnetic field achievable on Earth is 1042Gauss - 109 times lower than previously thought. Shabad and Usov performed standard QED calculations by applying the Bethe-Salpeter equation to a positronium - an electron-positron pair - in a magnetic field and found that as magnetic field increases, the coupling between the particles grows and ultimately causes them to fall together and annihilate - thus eliminating the energy barrier for pair production from the vacuum. Magnetic field cannot increase above 1042Gauss because when approaching this threshold it gets screened by the electron-positron pairs produced from the vacuum. This result places stringent constraints on the superconducting cosmic string and magnetic monopole theories. For example, according to existing estimates, magnetic field near a string can be as large as 1047-1048 Gauss - a value which the new theoretical finding now rules out. Source: Phys. Rev. Lett. 96 189401 (2006) Vortices found to form in a phase transition1 July 2006Tom Kibble and W.Zurek in the 1980s developed a theory that relates the cooling rate of a material to the number of topological defects that form as a phase transition occurs. The general predictions of this theory apply both to laboratory processes and the extreme conditions of mater early in the evolution of the Universe. Now experimenters at the University of Salerno, Italy, have successfully tested the Kibble-Zurek theory for the first time by studying how inhomogeneities in the quantum mechanical phase in superconducting samples shape themselves as a sample is cooled at different rates. The samples studied consisted of two niobium disks separated by a thin insulating layer and thus forming a Jefferson junction. The cooling time was varied from several milliseconds to tens of seconds and all in all, more than 100,000 cooling cycles were run. The phase distribution in the material was detected based on electric potential and current measurements. Theoretically predicted topological vortex-like defects were observed, and the probability of vortex formation was found to be proportional to the square root of the cooling rate - as the Kibble-Zurek theory predicts it should. Source: Phys. Rev. Lett. 96 205301 (2006) Bosons and fermions in a 3D trap1 July 2006Two independent research teams have developed a technique that enables ultracold degenerate gases of bosons and fermions to be accumulated simultaneously in an optical 3D trap. Experiments showed that such a mixture of gases has interesting properties - in particular, that boson atoms interact differently in the presence of fermions. The 3D trap is a `light lattice' of many interfering laser beams. S.Ospelkaus of the Institute of Laser Physics in Hamburg, Germany, and colleagues placed bosonic 87Rb and fermionic 40K atoms into this trap at a temperature of several hundreds of nanokelvin and studied the behavior of the mixture as the trap was abruptly switched off. It was found that unlike a pure gas of 87Rb atoms, in this case the presence of fermions led the 87Rb atoms to attract rather than repel one another. In a similar experiment, T.Esslinger and colleagues in Switzerland looked at the effect of 40K atoms on the superfluid properties of the Bose-Einstein condensate of 87Rb atoms and found that the presence of the fermions suppressed the condensate's phase coherence. Source: Phys. Rev. Lett. 96 180402 (2006); Phys. Rev. Lett. 96 180403 (2006) Laser goes acoustic1 July 2006The acoustic analogue of an optical laser - a device capable of amplifying and generating superhigh frequency acoustic waves - has been developed by a Britain-Ukrainian collaboration using a lattice of semiconductor layers. The lattice was designed to give rise to the stimulated emission of sound quanta (phonons) in the frequency range 0.1-1.0THz, which formed a coherent sound wave. Layer separations on the lattice faces were chosen such that the sound wave reflected from the faces similar to the way in which light reflects from mirrors in a conventional laser. As a result, a narrowly directed, superhigh frequency, high intensity sound beam was obtained. The acoustic `laser' can be used for a wide variety of scientific applications including, for example, optical modulators. Source: Physics News Update, Number 779 Properties of matter under combined high pressure and ion irradiation conditions1 July 2006A study of the combined effect of high pressure and intense ion irradiation have revealed structural changes that neither of these factors could produce alone. Researchers at the GSI laboratory in Darmstadt, Germany, placed samples of graphite or zirconium in a `diamond anvil cell' under a pressure of about 14GPa and irradiated them with a beam of uranium or gold ions at an energy of 70.0GeV for several seconds. Electron microscope observations showed that graphite transformed from a crystalline to amorphous state as a result of carbon-carbon bond transformation - in contrast to atmospheric pressure experiments, in which ions just leave tracks behind them without changing the structure of the crystal. For zirconium samples, Raman spectroscopy revealed the formation of nanoscale single crystals; and a phase transition which occurs only at a pressure of 20GPa in the absence of irradiation. The study may help to better understand the properties of some deep-earth minerals subject to the combined action of high pressure and radioactive decay products. Another potential application of this technique is to produce new metastable phases of matter. Source: Phys. Rev. Lett. 96 195701 (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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