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Extracts from the Internet | ||
Spiral spin-liquid1 December 2016S. Gao (Paul Scherrer Institute and University of Geneva, Switzerland) with colleagues used the methods of diffuse neutron scattering and neutron diffraction to unambiguously show for the first time the presence in a MnSc2S4 crystal of a spiral-spin liquid, i.e. a vortex-like distribution of neighboring atomic spins. The existence of such structures was predicted in 2007 in the theoretical paper by L. Balents (University of California, Santa Barbara, USA) et al. The neutron scattering by MnSc2S4 crystals revealed the correlations, typical of the spiral-spin liquid, in spin distribution in the form of a spiral surface. These experimental data are well reproduced in the model with J1–J2-Hamiltonian. Some evidence of the existence of spiral-spin liquids had been obtained before but with a lower confidence then in the new work of S. Gao et al. Source: Nature Physics, online-publication of 24.10.2016 Photoionization on attosecond scale1 December 2016M. Ossiander (Max Planck Institute of Quantum Optics, Germany) and his colleagues were the first to observe helium atom photoionization with time resolution of less than an attosecond (1 as=10-18 s) exposed to laser pulses of extreme UV range. After leaving the atoms the electrons were scattered by subsequent pulses and were registered by an electron spectrometer. The electron ionization dynamics was determined by their spectrum with resolution up to 0.85 as. Either a single electron participates in the ionization process, or two electrons at the intermediate stage pass over to the excited state. Important at the latter case are many-electron interaction effects, i.e., electron correlations. The propagation of the wave packet escaping from the electron to the field of atomic nucleus was reconstructed by the experimental data. The measured ionization characteristics are in good agreement with the results of the solution of Schrödinger equation for the helium atom. The electron was found to escape 4 to 6 as before passage of the laser pulse maximum. Thus, the exact time reading of the ionization process is established. Source: Nature Physics, online publication of 07.11.2016 Ultrafast Fano resonance1 December 2016The Fano resonance (the interference of two wave processes) was predicted theoretically to exist in the helium atom ionization owing to the presence of two ways of ionization. Either one electron immediately leaves the helium atom, or two electrons first pass over to 2s2p a twice excited state, and then one of the electrons transfers the energy to the second electron (owing to the Coulomb interaction) and leaves the atom (the process is called autoionization). These two cases are indistinguishable in their final result, and therefore they undergo quantum interference (the Fano resonance). A. Kaldun (Max Planck Institute for Nuclear Physics, Germany) with colleagues observed this resonance by way of absorption spectroscopy — by absorption of laser radiation by helium atoms. At first the helium atoms were exposed to laser pulses of extreme UV range and were excited and then after a short time lag the atoms were irradiated by a strong (≈ 1013 W cm-2) IR pulse. When the time lag was long, the Fano resonance was clearly pronounced because of interference with the direct (one-electron) ionization channel, which showed up in the characteristic asymmetric shape of the UV radiation absorption line. On the contrary, if the time lag was short, the second pulse immediately caused atom ionization, which resulted in Fano resonance breakage. The dependence of the line shape on the time lag characterized the development of the Fano resonance in time. Thus, it was the first successful attempt to control the Fano resonance on the time scale of the order of femtoseconds. Source: Science 354 738 (2016) Tracks of superheavy nuclei in meteorites1 December 2016The aim of OLIMPIYA, the experiment initiated by V.L. Ginzburg and started by V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry in collaboration with P.N. Lebedev Physical Institute (FIAN), is the search for and identification of the tracks of heavy and superheavy atomic nuclei in olivine crystals from stone-iron meteorites-pallasites employed as natural track detectors. Heavy nuclei could have been synthesized in supernova outbursts, and if long-lived (belong to the so-called “stability island” in the case of superheavy nuclei) they could have flown to the solar system in the composition of galactic cosmic rays to leave the tracks in meteorites. The nucleus transfers energy to the atoms causing defects in the meteorite crystal lattice in the shape of an extended track. After chemical etching these tracks become visible on an optical microscope. The time of exposition for meteorites — millions of years – is incomparably longer than for ordinary detectors on satellites or aerostats. It was G.N. Flyorov who proposed to use meteorites as natural cosmic ray detectors, and the first works of the kind were fulfilled in JINR. As distinct from the previous studies, in JINR experiments it is not only the track length L that is measured but also the etching velocity Vetch along the tracks without a preliminary thermal annealing that typically results in unpredictable L variations. With the help of the series of exposures to fast ions on the Darmstadt accelerator of heavy ions and IMP accelerator (China) the dependence of the nuclear charge Z on L and Vetch was calibrated. Marjalahti and Eagle Station meteorites were studied. In layer-by-layer scanning the tracks of nuclei were found with the help of the automated measuring complex PAVICOM designed in FIAN under the guidance of N.G. Polukhina. As a result, the charge spectrum of nuclei with Z>40 in the cosmic ray composition was measured on the basis of 11647 processed tracks. 384 nuclei with charges Z>75, including ten nuclei-actinoids with 90<Z<103 were identified. Three nuclei with charges Z=119+10-6 were also identified. These nuclei might belong to the “stability island”. Such superheavy nuclei cannot now be technologically obtained on ground-based accelerators. Researchers from MISiS, MEPhI, Kurchatov Institute, JINR and their colleagues from China and Germany took part in the OLIMPIA experiment. (See also Phys. Usp. 53 805 (2010).) Source: Astrophys. J. 829 120 (2016) |
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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