Coherent neutrino-nucleus scattering
1 October 2017
The international collaboration COHERENT including Russian researchers from ITEP, MEPhI, and MIPT performed an experiment at the Oak Ridge National Laboratory (USA) in which the
coherent neutrino-nucleus scattering was registered for the first time. Under the coherent scattering predicted in the theoretical paper by D.Z. Freedman in 1974, low-energy neutrinos interact simultaneously with all nucleons of the nucleus because the interaction-carrying Z boson has the de Broglie wavelength exceeding the nucleus size. Although the coherent scattering cross section proportional to the squared number of neutrons in the nucleus is large compared to the scattering cross section by individual nucleons, the coherent scattering could not be registered earlier because of the low-energy recoil nuclei. The detector used in the COHERENT experiment only contained 14.6 kg of the low-background scintillator CsI[Na] (sodium-doped cesium iodide). The detector was placed in a deep cellar with a low background from neutrons and cosmic rays and was exposed to neutrinos produced in the interaction between proton pulses from the accelerator and a mercury target. Coherent scattering was registered at a confidence level of 6.7 σ. Owing to the detector compactness the method used to register coherent neutrino-nucleus scattering can find practical application in nuclear reactor monitoring.
Source: Science 357 1123 (2017)
Mechanism of structureless phase transition in (TMTTF)2PF6
1 October 2017
S. Kitou (Nagoya University, Japan) et al. used the method of synchrotron X-ray diffraction to clarify for the first time the mechanism of phase transition in an organic molecular conductor (TMTTF)2PF6 near the temperature of 67 K when the crystal gets over from the state of Mott isolator to the charge-ordered state and then to the Peierls spin phase. Since no changes in the crystal structure were earlier observed in this case, this transition was called structureless. In spite of 40 years of research, the mechanism of this transition remained unknown. In the new experiment, a synchrotron X-ray source was used for the structural analysis. The intrinsic electron coupling of atoms in molecules and the coupling with surrounding molecules were characterized by the inverse Fourier analysis. The study showed that the phase transition is due to the formation of a two-dimensional Wigner crystal, to a change in the bond length and to a 0.20e charge transfer between two neighboring TMTTF molecules in a dimer.
Source: Phys. Rev. Lett. 119 065701 (2017)
Magnesium rechargeable batteries with a nanostructured cathode
1 October 2017
The first prototype of magnesium-based rechargeable batteries was demonstrated by D Aurbach et al. in 2000. These batteries are safer than and not so expensive as lithium ones, but yield to them in capacity. H.D. Yoo (Houston University, USA) et al. modified the magnesium rechargeable battery by using another electrolyte and an improved nanostructured cathode. This time, instead of the former Mg2+ the charge carriers were MgCl- ions. For the battery to function no Mg-Cl bond scission is needed that was earlier necessary for Mg2+ release, which resulted in the energy barrier lowering in chemical reactions from 3 eV to 0.8 eV. The new cathode was coated with titanium disulfide TiS2, and the distance between TiS2 layers was increased from 5.69 Å to 10.86 Å through implantation (intercalation) of organic molecules. This made the penetration of MgCl ions in the cathode much easier and speeded up their diffusion. As a result, the capacity of the magnesium battery was heightened four times up to 400 mA h g-1. The capacity increase results in a good productivity upon recharging.
Source: Nature Commun. 8 339 (2017)
Interatomic Coulomb decay
1 October 2017
In some high-energy processes, so-called “hole atoms” appear in which electrons are located on external orbitals, the internal levels being unoccupied. It is known that in the interactions with matter “hole atoms” are able to transfer to the ground state within several fs, but the mechanism of fast energy effluence by the electrons remained unclear. R.A. Wilhelm (the Institute of Applied Physics, Austria and the Institute of Ion Beams Physics and Materials Research, Germany) with colleagues showed that this mechanism is the “interatomic Coulomb decay” during which the electrons of a hole atom interact simultaneously with several neighboring atoms. The experiment was performed with multiply charged Ar16+ and Xe30+ ions flying though a graphene layer. At a distance of several Å from the graphene surface the ions entrap electrons from it and are partially neutralized. The entrapped electrons have high energies, and therefore they find themselves predominantly on the external levels. The flight of the “hole atoms” thus produced through a graphene layer takes ≈ 1 fs, but within this time the electrons pass over to the internal orbitals. The outlet electrons were registered by an electrostatic analyzer. Comparison of the results of the present study with the ab initio calculations showed that responsible for the energy effluence effect in this case is the interatomic Coulomb decay, whereas the other mechanisms make a negligible contribution. The interatomic Coulomb decay can also take place in biological tissues upon atom deexcitation induced by ionizing radiations.
Source: Phys. Rev. Lett. 119 103401 (2017)
Magnetic field in a distant galaxy
1 October 2017
S.A. Mao (Max Planck Institute for Radio Astronomy) et al. measured the magnetic field in the galaxy as distant as 4.6 billion light years. The galaxy was observed in the frequency band of 1-8 GHz by VLA radio telescopes in New Mexico. It is a strong gravitational lens producing two images of a distant quasar. A model of electron density distribution in the galaxy was developed and the difference of Faraday rotation of polarization planes in the two images was measured. The measurement of the difference helped exclude the influence of background fields on the line of sight. Using these data the magnitude of the large-scale magnetic field in the galaxy was found to be of the order of several µG. In its magnitude and configuration the field in the distant galaxy is similar to the field in our Galaxy and in other nearby galaxies. This is indicative of the fact that strong magnetic fields appeared in galaxies earlier than it was regarded. The mechanism of magnetic field origination in the early Universe has not yet been definitely clarified, but a universal mechanism of their strengthening to high values is a magnetic dynamo in moving plasma. The observed magnetic field characteristics in the investigated galaxy are well consistent with the dynamo theory.
Source: Nature Astronomy 1 621 (2017)
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.