Extracts from the Internet


Brown – Zak fermions in graphene

J. Barrier (the University of Manchester, Great Britain) and co-authors discovered a new type of quasiparticles, referred to as Brown – Zak fermions, in a graphene layer placed between two layers of insulator – hexagonal boron nitride [1]. The arising superlattice has the form of a moire pattern with a period of ≈ 14 nm. According to the calculations, for some magnetic field values the translational symmetry is recovered and the fermions must move rectilinearly (like in a zero field). The fermion electron spectrum was examined in detail by measuring the longitudinal and Hall conductivity of the sample. The spectrum shows multiple Landau levels that are fanning out. The predicted ballistic propagation of Brown-Zak fermions with mobility exceeding 106 cm2 V−1 s−1 was revealed. In a zero magnetic field, the free path of charge carriers exceeds 20 µm and is limited by the channel width. These data testify that Brown – Zak fermions are Bloch quasiparticles. However, at a low temperature some mini-fans show anomalous behavior that has not yet been explained theoretically. Brown – Zak fermions may possibly become the basis for the creation of ultrafast transistors and other electronic components. For graphene and graphene electronics see [2, 3]. [1] Barrier J et al. Nature Communications 11 5756 (2020); [2] Sheka E F, Popova N A, Popova V A Phys. Usp. 61 645 (2018); UFN 188 720 (2018) [3] Ratnikov P V, Silin A P Phys. Usp. 61 1139 (2018); UFN 188 1249 (2018)

Quantum gravity scattering amplitude

The formulation of quantum gravity theory encounters the problems of nonrenormalizability. In their theoretical paper [4], T. Draper (Radbound University Nijmegen, Netherlands) and colleagues proposed a new type of effective quantum action in which the amplitude of two different high-energy scalar particles scatter by means of the gravitational field remains finite. The action includes special type propagators, and an infinite array of interacting spin zero states and spin 2 poles occurin the amplitude. The formulated theory meets the unitarity and microcausality requirements and becomes scale-invariant with approaching the Planck energy. It does not require the introduction of nonlocalities typical of string theory. The high-energy scattering region alone has hitherto been examined, whereas the infrared divergences due to the massless character of gravitons have not been considered. The proposed new type of action may underlie the construction of the theory of various elementary processes in quantum gravity. For the history of the development of quantum gravity theory see [5] and for the unsolved problems see [6]. [4] Draper T et al. Phys. Rev. Lett. 125 181301 (2020); [5] Gorelik G E Phys. Usp. 48 1039 (2005); UFN 175 1093 (2005) [6] Kazakov D I Phys. Usp. 62 364 (2019); UFN 189 387 (2019)

Equivalence principle violation for quantum particles

The weak equivalence principle asserts that the free particle motion in a gravitational field is independent of its mass. The validity of the assertion in the quantum realm was doubted because of quantum uncertainties that exclude mentioning particle trajectories. The violation of the equivalence principle may also be due to the difference in the evolution of the wave packet of particles with different masses. J.Q. Quach (the University of Adelaide, Australia) investigated this aspect of the equivalence principle [7] using the R. Fischer’s theory in which information is expressed in an integral way in terms of the particle wave function constituting the wave packet. In the static Schwarzschild metric, the quantum equivalence principle turned out not to be violated. However, in the variable field of a gravitational wave, the Fischer information depends on the particle mass. This means violation of the weak equivalence principle for a quantum particle. Gravitational waves are emitted, for instance, under the merge of two black holes. For single particles, it is now impossible to reveal the violation experimentally. J.Q. Quach emphasized that the effect can be stronger for Cooper pair condensate in superconductors. [7] Quach J Q The European Physical Journal C 80 987 (2020); https://doi.org/10.1140/epjc/s10052-020-08530-6

Metalens

Metalenses consisting of a periodic array of elements on a flat base can in many cases substitute successfully for ordinary convex lenses. B. Xu (Nanjing University, China) and co-authors assembled a metalens straight on the radiation sensor (on the light sensitive CMOS matrix in this case) to show that such a construction possesses a number of unique properties [8]. It can exceed substantially other analogs in resolution, signal-to-noise ratio and width of the field of vision. The resolution of ≈ 1,74 µm was attained for acentimeter lens. A two-phase matrix of metalenses with an extensive field of view was also investigated. Since the metalens was made of silicon, it can be readily integrated with CMOS matrices. The new method of exploiting metalenses can be used, in particular, in broad-field microscopy. For optical metamaterials see [9, 10, 11]. [8] Xu B et al Advanced Photonics 2 066004 (2020) [9] Kildishev A V, Shalaev V M Phys. Usp. 54 53 (2011); UFN 181 59 (2011) [10] Davidovich M V Phys. Usp. 62 1173 (2019); UFN 189 1249 (2019) [11] Remnev M A, Klimov V V Phys. Usp. 61 157 (2018); UFN 188 169 (2018)

Gas in cosmic filaments

On the scales of galaxy superclusters, matter is distributed in the form of walls, filaments and nodes separated by extensive empty space regions. These structures are thought to contain of a considerable part of hot gas in the Universe. This gas has already been registered by the thermal Zel’dovich-Sunyaev effect, but the data obtained did not allow its density and temperature to be independently determined because of degeneracy in the observed parameters. Some evidence of the presence of gas has been also obtained from X-ray observations. H. Tanimura (Paris-Saclay University, France) with colleagues registered gas in filaments [12] for the first time with high confidence by its X-ray emission. The statistical sampling from the SDSS optical survey includes 15165 30-100-Mpc filaments at redshifts 0.2 < z < 0.6. They investigated the correlation of these filaments with the ROSAT X-ray observations. X-ray point sources and massive galactic clusters were excluded. This was how gas emission in the range of 0.56-1.21 keV was discovered with 4.2 statistical significance. The gas density at filament cores exceeds 30 ± 15 times the average cosmological gas density, which is consistent with the data on Zel’dovich – Sunyaev thermal effect. However, the measured gas temperature of 0.9-0.6+1.0 keV is several times higher. Further examination is needed to reveal the cause of this difference. For the intergalactic gas see [13]. [12] Tanimura H et al Astron. Astrophys. 643 L2 (2020) [13] Shchekinov Yu A, Lukash V N, Mikheeva E V, Pilipenko S V Phys. Usp. 60 961 (2017); UFN 187 1033 (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.

It is compiled from a multitude of Internet sources.

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