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| Issue 12, 2024 |
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Extracts from the Internet | ||
Test of the equivalence principle for spin-polarized atoms1 August 2016Z.-K. Hu (Huazhong University of Science and Technology, China) with colleagues performed an experiment comparing the free fall acceleration of 87Rb atoms having opposite total spin directions. Some theories predict the spin dependence of free fall acceleration. A possible reason is spin interaction with the acceleration vector or with the torsion tensor. The equivalence principle has already been checked for large spin-polarized ensembles of atoms, but if the number of atoms was small, the precision of measurements was low. In the new experiment the precision was heightened by the method of double differential measurements which excludes the effect of magnetic field inhomogeneities. Atoms in the states mF=+1 or mF=-1 of hyperfine splitting of level 52S1/2 in which spins have opposite directions fell in the atomic Mach-Zehnder interferometer. At the attained level of accuracy no differences in the free fall accelerations were revealed, and for their relative difference the restriction ηS=(0.2±1.2)×107 was obtained which in turn gives the restriction <5.4×10-6 m-2 on the possible valur of the torsion field gradient. Source: Phys. Rev. Lett. 117 023001 (2016) T+He3 reaction cross section1 August 2016A.B. Zylstra (Massachusetts Institute of Technology, USA) with colleagues were the first to measure the cross section of the nuclear reaction T(He3,γ)6Li at energies inherent in the epoch of primordial nucleosynthesis in the early Universe. Although the theory of primordial nucleosynthesis is in the large well consistent with the observed chemical composition of the Universe, according to some data the 6Li content in some stars is much larger than predicted. One of the assumptions is that the T(He3,γ)6Li reaction cross section has a higher value than that used in the calculations. To check this hypothesis the capsule with the mixture of T and He3 was exposed in the new experiment to high-power laser pulses. Upon evaporation of the external shell of the capsule a convergent shock wave was formed, a temperature of 2.3×108 K was reached in the capsule center upon compression, and in the dense plasma the T(He3,γ)6Li nuclear reactions proceeded during ≈100 pc. A gas Cherenkov detector registered # photons, and the reaction cross section was found by their spectrum. The cross section proved to be close to the theoretically calculated one and it also corresponds to extrapolation of the data of other experiments performed earlier at higher energies. Thus, the hypothesis of a heightened T(He3,γ)6Li reaction cross section was not confirmed, and the reaction rate turns out to be too low to solve the 6Li problem. Possibly, if the 6Li content is actually anomalous, it might have been produced directly in the stars or alternatively its synthesis in the early Universe was affected by supersymmetry or by some other now unknown processes. Source: Phys. Rev. Lett. 117 035002 (2016) Spin wave emitter1 August 2016S. Wintz (Helmholtz-Center Dresden-Rossendorf, Germany) with colleagues designed a new antenna for emission of spin waves with a wavelength of ≈100 nm. The spin wave is a collective electron spin excitation transferred through magnetic and exchange interactions. Spin waves are typically excited by small metallic antennas but in the nanometer region such antennas are ineffective. The new emitter is a Co/Ru/Ni81Fe19 heterostructure consisting of two thin ferromagnetic discs separated by a non-magnetic layer. Under the action of a magnetic field generated by alternating current in a nearby conductor, a pair of magnetic vortices appears in the disc center and emits spin waves. The propagation of spin waves generated by this device was observed by the method of transmission X-ray microscopy. Source: Nature Nanotechnology, online publication of July 18, 2016 Refraction and reflection of spin waves1 August 2016C. Back (the University of Regensburg, Germany) and his colleagues investigated the laws of spin wave refraction and reflection at the interface between two ferromagnetic film (iron-nickel alloy) regions of different thickness (30 nm and 60 nm). The refraction occurs because the spin wave velocity depends on the film thickness. The spin waves were observed through registration of laser pulses reflected from the film, the rotation of whose polarization vector depends on magnetization at one or another point. The second method, the interference of the reflected and reference beams, provided information about the phase of the spin waves and the direction of their propagation. The obtained refraction law at small angles of incidence corresponds to Snell’s law, but at >25° a deviation was observed due to anisotropy of the medium that occurred owing to magnetization in the external field. The authors of the experiment deduced a generalized Snell’s law for spin waves with allowance for anisotropy and showed good agreement with the results of their measurements. The refraction of spin waves makes it possible to change their direction, which is important for practical applications. Spin waves are attractive in that they have a much smaller wavelength than that of electromagnetic waves of the same frequency. Furthermore, under spin wave propagation the charges remain motionless, and therefore the Joule energy loss is absent. Source: Phys. Rev. Lett. 117 037204 (2016) IceCube search for cosmogeneous neutrinos1 August 2016New restrictions on the properties of the sources of ultrahigh-energy cosmic rays (UHECR), i.e., charged particles with energies above 109 GeV whose origin is nor yet known were obtained using the neutrino telescope IceCube located in the Antarctica ice. The interaction of UHECR with cosmic background radiation must lead to generation of neutrinos with energies Eν≥107 GeV called cosmogenic neutrinos. During seven years of observations two neutrinos with Eν≈106 GeV were registered that are most likely to be of astrophysical origin, however cosmogenic neutrinos with Eν≥107 GeV were not observed. Their flux appeared to be much smaller than is predicted by ordinary UHECR models. This implies that the evolution of UHECR sources cannot go faster than the evolution of star formation rate in galaxies. This restriction presents certain problems for the models of UHECR origin in galactic nuclei, in sources of gamma-ray bursts or in young pulsars provided that protons dominate in UHECR composition. Source: arXiv:1607.05886 [astro-ph.HE] |
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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