1 January 2020
In 2016, A.J. Krasznahorkay (the Institute of Nuclear Research, Debrecen, Hungary) and co-authors reported in their experiment an anomaly in angular correlations of electrons and positrons born in intranuclear transitions in 8Be nuclei (see Phys. Rev. Lett. 116 042501 (2016)). This anomaly was interpreted as the birth and decay of a new particle called X17 particle of mass ≈ 17 MeV. This result attracted much attention, as X17 might explain, e.g., dark matter or divergences in the measurements of the anomalous magnetic moment of a muon, or else, it might be a boson – carrier of a new force. In the updated experiment with new detectors, A.J. Krasznahorkay and co-authors verified and specified the former results with 8Be and performed new measurements with 4Íå nuclei. They examined an electromagnetically forbidden transition 0-→0+ in the nuclei of 4Íå produced in bombardment of a hydrogen target by a proton beam. The background mostly due to cosmic rays was measured a fortnight before and a fortnight after the experiment and was subtracted from the results of measurements. The additional peak corresponding to the production of the new particle of the same mass ≈ 17 MeV as that of 8Be nuclei was observed both in the energy spectrum with 7.1 σ statistical significance and in e+e- angular correlations with 7.2 σ significance. Particles with presupposed properties of X17 have recently been sought in the CERN experiment NA64. No new particles were registered, but the range of their interaction with electrons was substantially limited. Nevertheless, a wide allowed region still remains. The hypothesis concerning the birth of X17 particle can be verified in several independent experiments to be performed in the near future.
Source: arXiv:1910.10459 [nucl-ex]
Spin heat engine
1 January 2020
The investigation of microscopic and quantum heat engines is of great interest for nanotechnologies. A thermodynamic cycle has recently been realized with a unit ion in a trap. However, the heat energy exceeded greatly the distances between quantum energy levels, which made this heat engine very largely classical. In the new experiment, J.P.S. Peterson (University of Waterloo, Canada) with colleagues managed to attain a regime in which the heat engine requires quantum description. The NMR spectroscopy method was used to investigate the quantum version of Otto cycle in CHCl3 solution in acetone. Spins ½ of 13C nuclei served as a working substance of heat engines and the nuclear spins of 1H were used as heat carriers. The hot and cold reservoirs were represented by respectively high- and low-frequency radio waves. The Otto cycle stages were also initiated by radio pulses. Although the working substance of heat engines consisted of 1017 molecules, they behaved independently because of their weak interaction, and therefore only averaged characteristics were studied in the experiment. The measurements showed that the heat engine does operate at a quantum level with the determining role of quantum fluctuations. Its efficiency equals 42 %, which is close to the upper limit of 44 % for the Otto cycle, and the work done in one cycle at a unit spin amounted to several peV.
Source: Phys. Rev. Lett. 123 240601 (2019)
Quantum analog of Penrose inequality
1 January 2020
In the gravitation theory, several important theorems are known for whose validity a light-like (isotropic) null energy condition (NEC) (see UFN 184 137 (2014)
[Phys. Usp. 57 128 (2014)]) is needed violated by some types of the so-called exotic matter. One of these theorems is the Penrose inequality relating the minimum body mass to the area covering its trapped surface. Researchers from USA and Spain (R. Bousso, A. Shahbazi-Moghaddam, and M. Tomasevic) clearly showed that the Penrose inequality is also violated in the presence of exotic quantum matter and extended it to the quantum case. The main idea of their approach lies in the substitution of the generalized entropy on the light sheet for the trapped surface area. Since the generalized entropy can be expressed in terms of quantum degrees of freedom, this method may appear to be useful in formulation of the theory of quantum gravity. In the new work, the authors also introduced a quantum analog of expansion of a surface and gave a new definition of the trapped surface. These definitions were used to formulate the quantum analog of the Penrose inequality and to demonstrate its application on simple examples.
Source: Phys. Rev. Lett. 123 241301 (2019)
An electric waveguide in graphene
1 January 2020
The feasibility for the information contained in quantum states of electrons to be transmitted through nanowires is limited to a small electron free path length in nanowires. In graphene (a carbon layer one atom thick), charges behave as Dirac massless quasi-particles and can be carried over large distances, but the creation of a strictly directed beam of these particles remained problematic. A. Cheng (Harvard University, USA) with co-authors used a carbon nanotube as a guiding device. Graphene was placed between the insulator layers with a nanotube above one of the layers. Between the nanotube and graphene, the potential difference was maintained that created a potential well along the nanotube. Dirac fermions could move in graphene along this tube in the waveguide single-mode regime. This configuration is conceptually similar to optical fiber through which photons are transported. The experiment demonstrated fermion transport at a distance of 500 nm bounded by the device size. The nanotube simultaneously served as a charge sensor used for measurements that confirmed the device operation as a waveguide. For graphene electronics see the review in
UFN 188 1249 (2018) [Phys. Usp. 61 1139 (2018)].
Source: Phys. Rev. Lett. 123 216804 (2019)
Torsional Alfven waves
1 January 2020
Researchers from Norway, Great Britain and France were the first to directly observe torsional Alfven waves upon a solar flare. Torsional Alfven waves were theoretically predicted to participate in energy transfer between different layers of solar atmosphere, however the earlier observations provided only indirect evidence of their existence. These waves are difficult to observe because the plasma density and, accordingly, its luminosity in torsional oscillations are homogeneous. P. Kohutova, E. Verwichte and C. Froment investigated the flare of December 9, 2015 with plasma stream ejection in the solar corona (for coronal processes see UFN 178 1165 (2008)
[Phys. Usp. 51 1123 (2008)]
and UFN 189 905 (2019)
[Phys. Usp. 62 847 (2019)]). Energy release in a magnetic tube in the course of magnetic field line reconnection generated torsional motion in the plasma. The IRIS data with Doppler shift measurements in silicon and magnesium lines were used, which made it possible to observe both the time evolution of plasma velocity and the velocity distribution across the stream. The data obtained corresponded to the torsional Alfven waves propagating at a velocity of 140 km s−1.
Source: arXiv:1912.03954 [astro-ph.SR]
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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