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| Issue 12, 2024 |
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Extracts from the Internet | ||
Measurement of masses of superheavy nuclei1 October 2012To measure the masses of heavy atomic nuclei, an indirect method of studying fragments of their α-decays is typically used; however, this method leads to large errors in the case of superheavy nuclei. Masses of the nuclei 252,253,254No and 255,256Lr were measured with high accuracy at the Helmholtz Center for the Study of Heavy Ions in Darmstadt (Germany) by measuring the cyclotron frequencies of their motion in a magnetic field. The nuclei produced at the GSI accelerator were captured into a Penning trap and rotated in the magnetic field. Cyclotron frequencies measured by the resonance technique were compared in the same trap with the cyclotron frequency of ions 133Cs+ whose mass is known very accurately. Several dozens of nuclei were studied in this manner and their masses were measured to within 15 keV. The knowledge of the masses of nuclei makes it possible to draw model-independent conclusions on their binding energy which results in their mass defect. The binding energy is largely determined by the effects of filling of nucleonic shells which also affect the stability of nuclei; consequently, the results obtained may help to clarify the issue of the “island of stability” — the area on the “charge vs. number of neutrons” diagram in which nuclei are conjectured to possess sufficiently long lifetimes. Source: Science 337 1207 (2012) The uncertainty principle for measurements1 October 2012The Heisenberg uncertainty relation is written in the case of the measurement process in the form ε(A)η(B)≥h/(4π) where ε(A) and η(B) are measurement errors of variables A and B in such a way that ε(A) and η(B) are typically associated with the quantum uncertainties σ(X)=(⟨ψ|X2|ψ⟩-⟨ψ|X|ψ⟩2)1/2 in the quantum state of the system ψ, although this identification has never been rigorously proved. Another formulation of the uncertainty principle in Robertson's general form is σ(A)σ(B)≥ ⟨ψ|[A,B]|ψ⟩/2 where [A,B], the commutator of operators, is an exact consequence of quantum mechanics. However, in 2003 Ì. Ozawa proved in a theoretical paper that this last expression cannot be applied in a straightforward manner to the process of measurement in the general case and that a more correct uncertainty relation for measurements takes the form ε(A)η(B)+ε(A)σ(B)+σ(A)η(B)≥ ⟨ψ|[A,B]|ψ⟩/2. This expression does not forbid, in principle, for two conjugated variables to be measured with the product of uncertainties less than h/(4π). In most cases, this clarification is not essential but the result may change if the measurement technique is in some way correlated with the state of the measured object. This conclusion is confirmed by L.A. Rozema (University of Toronto, Canada) and his colleagues in the experiment on quantum teleportation of qubit states built of the polarization states of photons. The states of a correlated pair of photons were determined prior to their strong interaction with the measuring device using the so-called weak measurements. As a result, the accuracy of measurements better than allowed by the uncertainty principle in Heisenberg's formulation has indeed been achieved. The conclusion is that even though the above results leave the fundamental principles of quantum mechanics unperturbed, they represent an important clarification of the meaning of the uncertainty principle for the process of measurement. These results may prove important for quantum cryptography, as well as for gravitational wave interferometers operating at the very limit of measurement accuracy. Source: arXiv:1208.0034 [quant-ph] The study of electronic bonding in single molecules1 October 2012L. Gross (IBM laboratory in Zurich) et al were able for the first time to experimentally distinguish between individual electron bonds in a single organic molecule. Single molecules were already studied using atomic force microscopes before but it was only possible to characterize their chemical properties for the molecule as a whole.The new experiment used an atomic force microscope with a CO molecule at the very apex of the tip. Theoretical calculations by the density functional method allowed the researchers to identify effects capable of distinguishing between bonds of different order, strength and length. It turned out that increased density of the electron cloud or higher bond order lead to stronger repulsion owing to Pauli's exclusion principle, which results in higher image contrast in the microscope (greater frequency shift in tip vibrations). Furthermore, higher-order bonds look shorter on the images due to the changed tilting of the CO molecule at the tip apex. Thanks to these attributes, it was possible to distinguish between electron bonds differing in order by 0.2 and in length by only 0.03 A. This method led to successful characterization of electron bonding in molecules of polycyclic aromatic hydrocarbons as well as in fullerene molecules C60. Source: Science 337 1326 (2012) Plasmon needle beams1 October 2012J. Lin and his colleagues in the U.S.A., Singapore and France were able to generate surface plasmons (quasiparticles corresponding to collective oscillations of the electron gas) in the form of a thin (needle) beam propagating along the metal surface to distances up to 80µm without noticeable diffraction. The experiment confirmed the validity of new plasmonic solutions of Maxwell's equations found by members of the same research team. Plasmons were generated by a grid consisting of two sets of parallel grooves on the surface of a gold film, intersecting at an angle of 10° between normals to the grooves. Two plane plasmon waves were excited by laser pulses and sent in the directions of the normals. The interference of these waves led to the formation of a needle beam with a profile calculated as a product of a cosine and a Gaussian, hence the name cosine-Gauss plasmon beams. Plasmons propagated along a straight line along the gold surface and were observed with an optical near-field microscope. Generation of needle plasmons may find applications in plasmon nanoelectronics as a way to reduce losses in signal transmission. Source: Phys. Rev. Lett. 109 093904 (2012) Gas cloud near the black hole1 October 2012A gas cloud has recently been discovered near the center of our Galaxy; it gradually approaches the central black hole Sgr A*. The closest approach to the minimum distance of 10-3 pc could be observed by mid-2013, at the same time as the anticipated growth of the x-ray activity due to the accretion of the gas. It is already obvious that the cloud is being deformed by the gravitational tidal forces of the black hole. The nature of the cloud and the cause of its movement in the direction of the black hole are not yet known. A collision of two stars was discussed as a possible scenario. R.A. Murray-Clay and A. Loeb of Harvard-Smithsonian Center for Astrophysics suggested a new hypothesis. In their model, the cloud is a protoplanetary accretion disk of gas and dust mixture around a weak (and hence invisible) star. The star and its disk were initially in a ring-shaped cluster of young stars whose inner edge lied at a distance 0.04 ps from Sgr A*. After a gravitational scattering on another star, the disk and the star changed the orbit and began to approach the black hole. Tidal forces and the background UV radiation cause the expansion of the disk which leaks gas and thus forms the visible cloud. Given that hundreds of exoplanets around other stars were found in recent years, the new hypothesis appears quite plausible. The R.A. Murray-Clay - A. Loeb model predicts a certain form of growth in the luminosity of the cloud as it approaches the black hole. This prediction could be tested fairly soon in the new year. Source: Nature Communications 3 1049 (2012) |
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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