Extracts from the Internet


Asymmetry in the neutron capture by hydrogen

Electroweak parity-violating forces make a certain contribution to the quark interaction inside protons and neutrons, which causes some small correlation between nucleon spins and momenta. NPDGamma collaboration registered this contribution for the first time in the experiment with a neutron source in Oak Ridge National Laboratory, USA. Measured was the cross section of polarized neutron capture by liquid parahydrogen molecules. The gamma-ray emission under these captures is asymmetric – photons emerge more often in the neutron-spin than in the opposite direction. The asymmetry parameter Anpγ = [-3.0 ± 1.4(stat.) ± 0.2(syst.)] × 10−8 was measured in the experiment. Since the calculations of this effect encounter technical difficulties, the obtained data will be useful in verifying the applied theoretical models. Source: Phys. Rev. Lett. 121 242002 (2018)

The Casimir torque

In 1961, in the paper of I.E. Dzyaloshinsky, E.M. Lifshitz and L.P. Pitaevsky published in UFN 73 381 (1961) (see also the translation of this paper in Sov. Phys. Usp. 4 153 (1961)) it was shown that under certain conditions the Casimir effect can induce the torque. The quantitative theory of this effect was formulated in the paper by E.I. Kats in ZhETF 60 1172 (1971) [Kats E.I. Sov. Phys. JETP 33 634 (1971) and then the theory of this phenomenon was developed by V.A. Parsegian and G.H. Weiss in J. Adhes. 3 259 (1972), Yu.S. Barash and V.L. Ginzburg in UFN 116 5 (1975) [Sov. Phys. Usp. 18 305 (1975)], Yu.S. Barash in Radiophys. Quantum Electron. 21 1138 (1978) and others. The torque occurs when the nearby bodies possess optically anisotropic properties. The spectral composition of quantum fluctuations of the electromagnetic field in the space between the bodies depends on mutual orientation of the principal optical axes of these bodies, and therefore the bodies try to rotateto a position of the lowest total energy. This theoretical prediction was first confirmed in the paper by D.A.T. Somers (the University of Maryland, USA) et al. They measured the Casimir torque between a solid birefringent crystal and a liquid 5CB class crystal. A polarized light beam was transmitted through the crystals and the variation of its brightness depending on the angle between the principal axes of the crystals was investigated. The turn of the liquid crystal molecules under the effect of the Casimir torque influenced the light transmission. Thus, a liquid crystal was also used to induce the torque and for measurements. The surface density of the torque was measured to be ≈ 3 × 10−9 N m m−2. The experiment showed perfect agreement with the theoretical calculations for both the sign and the strength of the Casimir torque. (See also the recent review written by E.I. Kats to the centenary of E.M. Lifshitz and published in UFN 185 964 (2015) [Phys. Usp. 58 892 (2015)] Source: Nature 564 386 (2018)

High-pressure superconductivity of LaH10

Two teams of researchers independently reported their observation of asignature of superconductivity in lanthanum hydride under high pressure at a record high temperature. This substance belongs to conventional superconductors described by Bardeen – Cooper – Schrieffer and Migdal – Eliashberg theories. The recent discovery of superconductivity in H2S at a temperature of 203 K (see the recent review by Eremets M.I. and Drozdov A.P. ``Vysokotemperaturnye obychnye sverkhprovodniki'' UFN 186 1257 (2016) [Eremets M.I., Drozdov A.P. “High-temperature conventional superconductivity” Phys. Usp. 59 1154 (2016)] published in the special UFN issue devoted to the centenary of V.L. Ginsburg) aroused new interest in this type of superconductors since theoretically they encounter no obstacle to their superconductivity even at room temperature. M.I. Eremets (Max Planck Institute for Chemistry, Germany) and his colleagues revealed that in # the pressure dependence of the critical temperature has a maximum in the region Tc=250-252 K at a pressure of 170 GPa. The sample was compressed in a diamond anvil. Superconductivity was noticed through the drop of electric resistance and the isotopic effect upon a replacement of conventional hydrogen by deuterium. Moreover, # depended on an external magnetic field because of its effect on the Cooper pairs. The other team guided by R.J. Hemley (G. Washington University, USA) performed a similar experiment with a diamond anvil to observe a drop of resistance in a LaH10 sample upon cooling to 280 K (7°C) at a pressure of ≈ 196 GPa. These results give hope to obtain room-temperature superconductivity in the near future. Sources: arXiv:1812.01561 [cond-mat.supr-con], Phys. Rev. Lett. 122 027001 (2019)

Coherence time of graphene-based qubit

W.D. Oliver (Massachusetts Institute of Technology, USA) and his colleagues were the first to measure the coherence time (55 ns) of a superconducting qubit in which the Josephson junction is made using graphene. Graphene lowers dissipation, which improves coherence. A graphene layer was placed between two layers of hexagonal boron nitride h-BN on an aluminum base. The obtained heterostructure belongs to so-called layered van der Waals materials. The energy spectrum of the created transmon qubit is analogous to the spectrum of ballistically travelling massless Dirac fermions. Layered van der Waals materials have already been obtained earlier, but the properties of their quantum coherence have not been examined. The new experiment demonstrated coherence and the possibility to control the state of the created qubit using electric voltage. (For the use of graphene in nanoelectronics see also the recent review in UFN: Ratnikov P.V., Silin A.P. “Dvumernaya grafenovaya elektronika: sovremennoe sostoyanie i perspektivy” (Two-dimensional graphene electronics: “current status and prospects” UFN 188 1249 (2018) [Ratnikov P.V., Silin A.P. Phys. Usp. 61 1139 (2018)]). Source: Nature Nanotechnology, online publication of 31 December 2018

Intermediate-mass black hole in the Galaxy

S. Takekawa (National Astronomical Observatory of Japan) with co-authors discovered evidence for the existence near the center of Galaxy of an intermediate-mass black hole (BH) – between the masses of stellar-origin BH and the masses of supermassive BH. In the gas cloud HCN-0.009-0.044, molecular spectral lines were observed using the complex of ALMA radio telescopes. High-resolution observations showed that the cloud consists of a bulk structure and a narrow stream that experience a fast Kepler motion around the dynamical center with a mass of (3.2 ± 0.6) × 104M, which is about a hundred times less than the mass of the central BH in the Galaxy. The small size of the massive object (< 0.07 pc) and the absence of visible stars implies that it is most probably a BH. This BH must have been formed in the center of a globular star cluster destroyed by tidal forces near the center of Galaxy. Then the BH entrapped the cloud HCN-0.009-0.044 which flew by it. The massive object inside HCN-0.009-0.044 is already the third such candidate for an intermediate-mass BH near the center of Galaxy. Source: ApJL 871 L1 (2019)

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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.

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