Extracts from the Internet


A search for a neutrinoless double-beta decay

Some atomic nuclei undergo a double-beta decay when two neutrons in a nucleus simultaneously decay into two protons, two electrons and two antineutrinos. However, also discussed is the hypothetic possibility of a double-beta decay without emission of neutrinos. The presence of such a decay would mean nonconservation of the lepton number, which is forbidden in the Standard Model, but is predicted in its extensions. The neutrino must then be a Majorana fermion, i.e. its own antiparticle. The neutrinoless double-beta decay is being sought, in particular, in the CUORE experiment in the National Gran Sasso Laboratory (Italy), where an array of low-temperature TeO2 crystals is used. Thermal bursts due to 130Te → 130Xe decays are being sought. The neutrinoless double-beta decay has not been revealed at the current level of precision, and the limit is obtained on the decay half-life T1/2 > 1.5×1025 years. This limit is comparable with the limit T1/2 > 5.3×1025 years obtained earlier for 76Ge nuclei in the low-background experiment GERDA performed in Gran Sasso with the participation of Russian scientists. Source: arXiv:1710.07988 [nucl-ex]

Electronic topological Lifshitz transition in YbAl3

Using angle-resolved photoemission spectroscopy, S. Chatterjee (Cornell University, USA) with colleagues discovered that valence-band structure fluctuations (transitions between two valence configurations Yb2+(4f14) and Yb3+(4f13)) due to the interaction between localized and delocalized electrons appearin a thin YbAl3 film upon temperature or pressure variation. The results of measurements were compared with calculations by the density functional method with allowance for relativistic effects and spin-orbital interaction. The fluctuations were found to lead to variation of the Fermi surface topology, which corresponds to the phase transition of order 2½ (“Lifshitz transition”) which was considered theoretically By I.M. Lifshitz in 1960. For the Fermi surface geometry see the paper by M.I. Kaganov and I.M. Lifshitz in Physics-Uspekhi 22 904(1979). Source: Nature Communications 8 852 (2017)

Fraction of atoms in Bose – Einstein condensate

R. Lopes (the University of Cambridge, Great Britain) with colleagues were the first to confirm experimentally what was theoretically predicted by N.N. Bogolyubov in 1947 (see UFN 93 564 (1967)) about the fraction of interacting Bose-atoms that pass over to the composition of Bose – Einstein condensate. The momentum distribution of 39K atoms and their fraction in the condensate was measured via two-photon Bragg scattering after the atomic trap potential was switched off. The measurements were performed for different values of the pair interaction of atoms regulated using Feshbach resonance. The results were well consistent with the Bogolyubov theory. Source: Phys. Rev. Lett. 119 190404 (2017)

Dzyaloshinskii – Moriya interaction in dielectrics

Dzyaloshinskii – Moriya interaction in the form of a vector product of atomic spins occurs when symmetry under reflection in a crystal is locally violated. The possibility to control this interaction is of importance for future applications in spintronics. Such kind of control as applied to metal alloyshas already been demonstrated. G. Beutier (Grenoble Alpes University, France) with colleagues observed the Dzyaloshinskii – Moriya interaction in MnCO3, FeBO3, CoCO3, and NiCO3 ferromagnets which are dielectrics. The Dzyaloshinskii – Moriya interaction occurs in them due to the oxygen layerstructure twisting. Phase-sensitive X-ray magnetic diffraction was employed in the study that gave good quantitative correlation with “first principles” calculations reproducing both the sign and value of the Dzyaloshinskii – Moriya interaction. Russian scientists from MSU, UrFU (Ekaterinburg) and A.V. Shubnikov Institute of Crystallography, FSRC “Crystallography and Photonics” RAS (Moscow) took part in the research. Source: Phys. Rev. Lett. 119 167201 (2017)

Muon Radiography

K. Morishima (Nagoya University, Japan) et al. used the muon radiography method to discover a30-m-long void in the Khufu's Pyramid. The muons produced in the atmosphere by cosmic rays are able to pass through a thick layer of matter, and hence a registration of muons makes it possible to reveal inner inhomogeneities (like radiogram), which is important for remote investigation of mines, volcanoes and other objects. The pyramid was studied using nuclear photoemulsion films exposed in voids inside the pyramid during several months. The density distribution in the pyramid was determined and a new void found from the muon track positions in the films. Then these results were confirmed by scintillation and gas detectors. The idea of employing cosmic rays in geological survey was first put forward by P.P. Lazarev (the founder and the first editor-in-chief of our journal) in 1926 and was widely developed in practice in the USSR and abroad. At the present time this method has been revived using nuclear photoemulsionsin Russia by researchers from FIAN and SINP MSU (see the article in the present issue of Physics-Uspekhi). Source: Nature, online publication of November 2, 2017

Gamma-ray halo around pulsars

The cosmic detector PAMELA has recently discovered an excess of positrons e+ with energies > 10 GeV. As a possible explanation, along with dark matter particle annihilation the emission of e+ by nearby pulsars was considered. A.U. Abeysekara (the University of Utah, USA) with colleagues investigated the extended halos of gamma-ray emission with energy of 8-40 TeV around pulsars Geminga and PSR B0656+14 using the Cherenkov telescope HAWC and considered the hypothesis that these halos are produced by the same e- and e+ flows as those that give the e+ excess observed on the Earth. The gamma-ray emission might have been generated by the emitted e- and e+ during their inverse Compton scattering by background photons. However, the observed gamma-ray emission spectrum turned out to be incompatible with this model for much more e+ are registered than might reach the Earth and the shape of the calculated energy spectrum of e+ (a peak) differs from the observed power-law spectrum. Thus, the excess positrons cannot have been emitted by the indicated pulsars and must be of another origin. Source: Science 358 911 (2017)

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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.

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