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| Issue 12, 2024 |
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Extracts from the Internet | ||
Test of electric charge conservation1 January 2016The law of electric charge conservation is one of the fundamental physical principles and was checked in many physical experiments. Nevertheless, it cannot be excluded that in some very rare processes the electric charge is not conserved. In Borexino experiment in Gran Sasso National Laboratory (Italy) whose main goal is solar neutrino registration, a search was implemented for a possible charge nonconservation due to the decay of electrons into neutrinos and photons e- → γ+νe in atoms of a liquid organic scintillator. The 256-keV gamma rays emitted under the decays might interact with the electrons in the scintillator substance and cause light flashes registered by photomultipliers. According to the data accumulated during 408 days no e- decays were registered within the accessible accuracy, but the absence of observed events yielded a new bound >6.6×1028 years on the e- lifetime in the e- → γ+νeprocess. This limit is by two ordered of magnitude better than that obtained in the 1998 experiment which preceded Borexino. For the check of the electric charge conservation see also the paper by L.B. Okun’ Sov. Phys. Usp. 32 543 (1989). Source: Phys. Rev. Lett. 115 231802 (2015) Gamma-ray spectroscopy of 4ΛHe nuclei1 January 2016The charge symmetry stating that strong interaction in atomic nuclei does not depend on the nucleon charge is of paramount importance for the properties of many nuclei, but is an approximate symmetry. The experimentally measured difference of the binding energy of 3H and 3He nuclei caused by the charge symmetry breaking agrees well with the results of computations. However, in the case of supernuclei 4ΛHe and 4ΛH with Λ-hyperons in their composition the experiment gives thrice as large a value as that predicted theoretically. To find out statistical significance of this difference, new experiments were needed. J-PARC E13-1st collaboration used the gamma-ray spectroscopic method to examine 1+ → 0+ transitions between the levels of 4ΛHe nuclei born in 4He(K-,π-)4ΛHe reactions. The effect of charge symmetry breaking turned out to depend substantially on spins and to be rather large for the state 0+ and small for 1+. Comparison of the energy difference 1406 ± 2(stat) ± 2(syst) keV of levels 1+ and 0+ in the 4ΛHe nucleus obtained in these experiments with analogous measurements carried out earlier for 4ΛH confirmed the difference between the experimental data and the available theoretical calculations. Source: Phys. Rev. Lett. 115 222501 (2015) Low-voltage tunnel field-effect transistor1 January 2016The limit has now been reached when a further miniaturization of ordinary field-effect transistors is obstructed because of the worsening of their characteristics. Moreover, the operating voltage of the transistor cannot be lowered by diminishing its size. The elaboration of transistors operating on new principles, for example, using quantum tunneling effect seems promising. D. Sarkar (the University of California in Santa Barbara, USA) with colleagues fabricated a tunnel field-effect transistor with operating voltage of 0.1 V and the mean value of the inverse logarithmic steepness of 31.1 mV per decade during the channel current variation by four orders of magnitude at room temperature, the minimum value being only 3.9 mV per decade. This result was attained by creating a channel of MoS2 in the form of a bilayer 1.3 nm thick on the doped germanium layer serving the source. The heterostructures obtained has the thinnest tunnel gap and a large tunneling-contact area. The transistor operates through quantum tunneling of electrons from the valence band of germanium to the MoS2 conduction band. The control voltage applied to the gate causes a relative shift of these bands and, accordingly, the tunnel current changes. The transistor was fabricated using the methods of electron-beam lithography and gas-phase chemical deposition. Graphene-based tunnel field-effect transistors were designed earlier, but they did not have the above-mentioned band-to-band tunneling for which reason the inverse logarithmic steepness below 60 mV per decade could not be obtained. The energy dissipation in the new transistor is by 90% lower than that of conventional transistors, which makes the new technology fairly economic and promising for application in microelectronics. Source: Nature 526 91 (2015) Vortices in granular medium1 January 2016G. Combe (3SR Laboratory, Grenoble, France) with colleagues analyzed the statistical properties of granular-medium particles to establish that in some cases the velocity fluctuations of particles in the medium are vortex-like and resemble turbulent vortices in fluids. A quasi-two-dimensional array of wooden discs from 3 to 30 mm in diameter placed between two transparent plates was chosen as an investigated medium. A spatial redistribution of discs under the effect of rising pressure upon external compression of the medium was examined by photographic survey. Along with observation of vortices, confirmed for the first time was the dependence α=2/(3-q) between the value of the exponent α in the law of displacement of particles of the medium, ⟨x2⟩ ∝ tα, and the value of q characterizing the statistical particle distribution, which was predicted by C. Tsallis and D.J. Bukman. A transition from an anomalous diffusion regime to the Brownian behavior was also observed. Source: Phys. Rev. Lett. 115 238301 (2015) Magnetic structures in the vicinity of a supermassive black hole in the center of the Galaxy1 January 2016M.D. Johnson (Harvard-Smithsonian Center for Astrophysics, USA) with colleagues used the prototype of the “Event Horizon Telescope” to reveal the ordered magnetic structures near the event horizon in a Galactic Center supermassive black hole. The Event Horizon Telescope is a system of radio telescopes situated on different continents of the Earth and operating as a single radio interferometer with a superlong base. The described observations were conducted at 1.3-millimeter wavelength by four telescopes of the system, and by 2020 their number is planned to reach 13. Spatially resolved were the regions of polarized emission on scales of about 40 ang. μs, which corresponds to about six black-hole gravitational radii. The presence of such regions is indicative of the existence on this scale of a spatially ordered magnetic field in which the observed radio emission is generated by electrons by the synchrotron mechanism, the emission characteristics corresponding to the magnitude of the magnetic field several tens of G. Variability of the radio signal on time intervals less than an hour was also recorded. An ordered magnetic field in the interior of the accretion disc might be due to its extension to a toroidal configuration or, in another model, due to magnetic flux accumulation near the event horizon. Source: Science 350 1242 (2015) |
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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