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| Issue 11, 2024 |
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Extracts from the Internet | ||
Parity violation in ytterbium atoms1 September 2009Parity violation (the invariance of the properties of systems under mirror reflections) was first detected in 1957 in the asymmetry of the ejection of electrons from decaying cobalt-60 atoms in magnetic field. The record amplitude of this effect was observed in cesium atoms. D. Budker and coworkers at the Berkeley National Laboratory (USA) measured the degree of parity violation in atoms of 174Yb, which proved to be approximately 100 times stronger than in cesium atoms. Transitions were observed between ytterbium atomic levels 6s2 1S0 → 5d6s 3D1 that corresponded to the wavelength of the absorbed photons of 408 nm. These transitions are forbidden by selection rules; in other words, they occur with very low probability. Transitions to upper levels in a beam of ytterbium atoms were excited by laser pulses in crossed electric and magnetic fields. The time rate of these excitation transitions of atoms was measured by observing the fluorescent emission from spontaneous transitions of atoms to lower levels. The time rate of atomic transitions in the left-hand-oriented configuration of fields was higher than in the case of right-handed orientation. Parity violation arises as a result of mixing of levels of different parity due to the weak interaction in the external electric field. So far the accuracy achieved in the experiment is only 14% but it proved sufficient for detecting the record-high parity in 174Yb. Future experimental measurement of parity violation in various isotopes of nuclei and in transitions between levels of hyperfine splitting could provide new data on the distribution of neutrons in nuclei and perhaps could find effects go ing beyond the Standard Model of elementary particles; they will also lead to measurements of the anapole moment of nuclei (see Phys. Usp. 40 1161 (1997)). Source: Phys. Rev. Lett. 103 071601 (2009) Mechanism of growth of carbon nanotubes1 September 2009Researchers at University of Lyon (France) and the Rice University (USA) established that as new atoms join a carbon nanotube in the course of its growth, the nanotube revolves around is axis. This mechanism of nanotube growth was predicted in a theoretical paper by B. Yakobson. Field emission microscope was used to observe nanotubes in the process of catalyzed growth, with the FEM tip moving along the growth zone of the nanotube. The image of the nanotube was projected onto a phosphorus-covered screen and the processes taking place were video-recorded. In one of the experiments a nanotube made 180 rotations around its axis during 11 min of observation. Another interesting result was the observation that the nanotube revolved stepwise, not smoothly, doing one complete turn in approximately 24 steps. Atoms join the rotating nanotube in pairs and form a helical structure. Unique mechanical and electronic properties of carbon nanotubes hold great promise for the future in designing new superstrong materials and elements of microelectronics. Clarifying the mechanism of nanotube growth may help control this process in industrial-scale production. Source: Nano Lett. 9 2961 (2009) Transparent aluminum1 September 2009J. Wark (Oxford University, Great Britain) and coworkers observed a transition in aluminum to a phase transparent to UV radiation in response to irradiation by laser pulses. Aluminum foil was exposed to pulses of the most powerful in the world x-ray laser FLASH operating in Hamburg. The radiation flux density for 92 eV photons over a small area of the foil reached 1016 W cm-2. The light of the laser produced single ionization of practically all aluminum atoms by knocking out electrons off the L shells, without destroying the crystal structure; this kept the specimen transparent to UV photons for 40 fs. Source: Nature Physics 5 693 (2009) Nanolaser went over the diffraction limit1 September 2009Ì.Ò. Hill (Technical University of Eindhoven, The Netherlands) and his colleagues in The Netherlands and the USA designed a microscopic laser whose transverse dimensions are below the diffraction limit of the radiation it emits. The device consists of alternating semiconducting InP/InGaAs/InP structures of square cross section, 90 to 350 nm thick, bounded on both sides by dielectric layers of SiN 20 nm in thickness; the entire structure was coated with an outer layer of silver. Epitaxy, electron-beam lithography and some other methods were used to create the laser. Electric current was passed through special contacts connected to the semiconductor, and electrons and hole were injected into the structure. The layered structure forms a waveguide through which gap plasmon modes can propagate, undergoing reflections from silver layers at waveguide boundaries — like light does in the Fabry – Perot interferometer. The device can generate laser radiation with wavelength of about 1500 nm while being less that a quarter of wavelength thick. The diffraction limit is overcome both because the wavelength in a dielectric is shorter than that in vacuum and also because photons are transformed into surface plasmons in metal layers. The generation of laser light was recorded even at room temperature despite the fact that the device is more efficient if it is cooled considerably. Nanolasers may find applications in, for example, computers to transfer signals between components of microelectronic circuits and thus greatly speed up their work. Source: Optics Express 17 11107 (2009) Massive compact galaxy in early Universe1 September 2009P. van Dokkum (Yale University, USA) and his colleagues measured on the 8-meter Gemini telescope the dispersion of stellar velocities in the 1255-0 galaxy. The galaxy is observed at redshift z=2,186, i.e. in the epoch when the age of the Universe was a mere 3 billion years. However, with the mass of the galaxy ≈ 1011 mass of the Sun, stars in it move with velocity dispersion of 510+165-95 km s-1, which is approximately 2.5 times greater than the velocity dispersion in typical present-day galaxies. The size of the galaxy 1255-0 is however, six times smaller than that of present-day elliptical galaxies of the same mass. It is not clear so far what mechanism formed such dense galaxies and what was their evolution path. It is conceivable that in the time since such galaxies were formed, they merged with other surrounding galaxies and acted as seeds of very dense central regions of today's giant galaxies. It is also possible that central supermassive black holes formed very early in such galaxies. Source: Nature 460 717 (2009) |
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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