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| Issue 12, 2024 |
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Extracts from the Internet | ||
Casimir – Lifshitz force in repulsion mode1 February 2009In 1961 E.M. Lifshitz, I.Ye. Dzyaloshinskii and L.P. Pitaevskii, Adv. Phys. 10 165 (1961) (see also Uspekhi. fiz. nauk 73 381 (1961) (in russian) ) formulated the conditions at which the Casimir – Lifshitz force between two plates becomes repulsive: it is necessary that the dielectric permittivity of the intermediate dielectric layer be lower than that of one of the plates but higher than that of the other. The Casimir – Lifshitz force in repulsion mode was measured in a number of experiments but only at distances not larger than several nm where the Casimir effect works in the Van der Waals mode and the contribution of intermolecular forces is large. F. Capasso, J. Munday and A. Parsegian were the first to conduct detailed measurements of the Casimir – Lifshitz force at distances from 20 to 300 nm in which the effect of repulsion stemming from the distortion of the spectrum of zero quantum oscillations is directly measurable. They observed the interaction between a sphere 40 µm in diameter coated with a thing gold layer and a quatz plate in liquid bromobenzene. Using a small sphere instead of the second plate proved to be very convenient as this removed the need in the complicated procedure of setting the plates parallel to one another at a small separating distance. Moreover, this method allows determining the force from the velocity of the sphere, and determining this velocity by measuring the displacement of the beam of light reflected from the sphere. Preliminary calibration was carried out far from the quartz plate (where the Casimir effect is very low): various velocities of motion through the liquid were put in correspondence with hydrodynamic forces. The measured force of repulsion of the sphere from the plate was in good agreement with calculations in terms of the Lifshitz – Dzyaloshinskii – Pitaevskii theory. The quartz plate was replaced in the control experiment with a gold plate; as was expected, in this case the sphere was attracted to the plate. Source: Nature 457 170 (2009) Symmetry of energy gap in Ba0.6K0.4Fe2As21 February 2009Experiments show that the energy gap in high-temperature cuprate superconductors (the bonding energy of a Cooper pair) has different signs on different regions of the Fermi surface. Angle-resolved photoelectron spectroscopy has not confirmed the existence of a similar asymmetry of the energy gap in iron-based superconductors such as Ba0.6K0.4Fe2As2. In fact, however, these superconductors cannot be described in terms of the Bardeen – Cooper – Schrieffer theory in which the gap is assumed to be symmetrical. A hypothesis was advanced that the result is negative because the spectroscopic techniques used to study these materials are not sensitive to the phase of the wave function of electrons that carries the symmetry information. A.D. Christianson and his colleagues conducted new studies of the energy gap in Ba0.6K0.4Fe2As2 by using inelastic neutron scattering which makes it possible to measure the phase. The characteristic effect on the magnetic moments of neutrons established that the symmetry of the gap in Ba0.6K0.4Fe2As2 differs from the d-symmetry in cuprate superconductors and is most likely of the type s± in which case electrons split into groups with the opposite phase of the wave function. In this case the pairing of electrons is created through antiferromagnetic fluctuations. Source: Nature 456 930 (2008) Stability of coherence of laser light1 February 2009Ì. Bellini (University of Florence, Italy) and his coworkers confirmed a theoretical prediction of R. Glauber (1963) stating that removal of individual photons from a laser beam leaves the beam in coherent quantum state. A laser beam was passed through two optical splitters. The first splitter split the beam in two and their interference gave the measure of the degree of coherence. One of these beams was sent through the second splitter with very little splitting efficiency which allowed the authors to single out individual photons from this beam. These photons were recorded by a detector that could be triggered by single photons. In accord with R. Glauber's theoretical prediction it was found that extraction of individual photons did not destroy the coherence of the laser beam. Furthermore, Ì. Bellini and his coworkers developed a technique for adding single photons to a beam; in this manner they confirmed that the operations of extracting and injecting of photons are noncommutive. Source: http://physicsworld.com/cws/article/news/37106 Quantum cascade laser1 February 2009K.J. Franz (Princeton University) and his colleagues were the first to discover in 2007 that the quantum cascade laser they built generated, in addition to the "normal" beam, a laser beam at the second frequency but of lower power (see Appl. Phys. Lett. 90, 091104 (2007)). The lasing area of the laser in question consists of tens of layers of various semiconductors, each only a few atomic layers thick. Later the same team of researchers obtained further interesting results. It was found that the radiations at the two frequencies are in anticorrelation which is caused by a "competition" for charge carriers that can take part in the radiation at both the principal and the second frequency: as temperature increases, the output power of the second beam grows while that of the first one decreases. A possible explanation of the discovered effect was also suggested. It is assumed that the second beam is generated by electrons with momenta k = p / $\hbar$ ≈ 3.6 × 108 m-1 in nonequilibrium states while quasi-nonequilibrium electrons with zero momenta are responsible for the first beam. Lasers emitting via the new mechanism may find useful practical applications. The nearest task facing the researchers is to suppress the emission of the laser beam at the principal frequency so as to achieve emission of the second type only. Sources: Nature Photonics 3 930 (2008); http://engineering.princeton.edu/news/laser_08 Fast-moving stars1 February 2009A group of astronomers led by R. Sahai using NASA's Hubble Space Telescope found 14 young stars moving at huge velocities through the interstellar gas and creating a bow shock ≈1011-1012 km in size, leaving behind a trace of glowing gas. The shock wave is produced by the collision of the powerful stellar wind with the surrounding gas. The stars were found to be fairly young — not older than a million years or so — and have masses not more than eight time the solar mass. Hubble's observations yielded the shape and structure of shock waves. The stars move at about 180000 km h-1, which is roughly five times higher than the characteristic velocities of ordinary young stars. It is assumed that these stars get such high velocities from slingshot ejection out of a stellar cluster in a close flyby, or from gravitational interaction between two binary stars or a binary and a single star, or when the second component of a pair exploded as a supernova. Such fast-moving stars were first detected by the IRAS telescope at the end of the 1980s; however, the stars observed then by the IRAS were considerably more massive. Source: http://hubblesite.org/newscenter/archive/releases/2009/03/full/ |
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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