Extracts from the Internet


Measurement of 0.1-Hz optical resonance

W. Qu (Fundan University, China) with colleagues developed the method based on weak quantum measurements which they used to measure the resonance line less than 0.1 Hz wide [1]. Laser pulses were transmitted through the gas of 87Rb atoms in the magnetic field. Pulses of different circular polarization induced transparency with a phase shift with the result that transparency of the medium had the form of a narrow resonance in the region of pulse overlapping. The output pulses were registered by two detectors, and the correlation signal controlled the acousto-optical modulator at the input through the feedback loop. This allowed weak quantum measurements of the narrow resonance. This method was used to design a magnetometer with sensitivity of 7 fT Hz−1/2. Such magnetometers can be applied in various areas, including medicine. [1] Qu W et al., Nature Communications 11 1752 (2020)

Spectroscopy of molecules inside helium nanodroplets

Femtosecond time resolution spectroscopy allows investigation of the dynamics of fast electron transitions, including those in photovoltaic processes and in photosynthesis. One of the important directions is spectroscopy of molecules surrounded by other substances, e.g., in solutions. Transfer of energy generated in photoionization to the surrounding matter prevents the molecule from fragmentation, but at the same time the environment has a strong effect on the molecule hampering the measurement of its own properties. B. Thaler (Graz University of Technology, Austria) with co-authors revealed [2] that for In2 molecules inside superfluid helium nanodroplets this problem does not occur because the influence of helium is very weak. Nanodroplets were obtained through helium sputtering from the nozzle to a vacuum chamber and then the indium vapor transmission through the chamber, on the average two indium atoms got into each nanodroplet thus forming a molecule. Nanodroplets were illuminated by laser pulses, and then from the spectrum of ejected photoelectrons two types of dynamics were found, namely, In2 ejection from nanodroplets and In2 molecule vibration with a period of 0.42 pc inside nanodroplets. Vibrations lasted during tens of pc and recommenced each 145 pc with a smaller amplitude. The vibration damping was due to dephasing. The perturbations due to superfluid helium in the course of vibrations were 10 to 100 times lower compared to any other solvent. [2] Thaler B et al., Phys. Rev. Lett. 124 115301 (2020)

A new class of semiconductors

An ordinary silicon with a cubic crystal lattice is being widely applied, but it has an indirect forbidden band when the conduction band and the valence band are displaced relative to each other and photon emission is impossible. Hence, light sources for microelectronics cannot be designed on the basis of ordinary silicon. E.M.T. Fadaly (Eindhoven University of Technology) with co-authors revealed [3] that upon modification of the crystal lattice to hexagonal the silicon-germanium alloy hex-Si1-xGex becomes a direct-bandgap one for x > 0.65. This was shown in calculations by the density functional method and then was confirmed experimentally. The technologically complicated problem of obtaining an alloy with hexagonal structure and a low defect density was solved using a GaAs substrate with precipitated Si and Ge. Photoluminescent spectroscopy showed the presence of a narrow emission peak with temperature dependence and a recombination time of ≈ 1 ns typical of direct-bandgap semiconductors. Thus, this study demonstrated a new class of semiconductors that can underly the creation of light sources integrated directly into silicon chips. This paves the way for nanoelectronics, photonics and information technologies. For the application of semiconductors see [4, 5]. [3] Fadaly E M T et al., Nature 580 205 (2020) [4] Vavilov V S, Phys. Usp. 38 555 (1995) [Usp. Fiz. Nauk 165 591 (1995)] [5] Baranov P G et al., Phys. Usp. 62 795 (1995) [Usp. Fiz. Nauk 189 849 (2019)]

Anisotropy of the Universe?

The Standard Cosmological Model is based on the assumption that the expansion rate and other properties of the Universe are identical in all directions. Although the anisotropic cosmological models have already been long considered, the large-scale isotropy of the real Universe has hitherto almost never given rise to doubt. The strongest evidence of isotropy comes from observation of relic radiation. If the trivial dipole anisotropy due to the motion of the Solar system relative to relic radiation is excluded, then the Universe will look as practically isotropic except for some asymmetries that can be statistical fluctuations, or for Cold-spot type anomalies. For perturbations in relic radiation see the review by O.V. Verkhodanov [6]. K. Migkas (the University of Bonn, Germany) and his colleagues realized a new sensitive method of research that unexpectedly showed the presence of statistically significant large-scale anisotropy of the Universe [7]. The relation between X-ray luminosity and a hot-gas temperature in different directions in galactic clusters was examined. Together with the independent red shift determination this method gives model-independent results. The data of 313 galactic clusters from ROSAT All-Sky Survey and Chandra and XMM-Newton surveys were used. In different directions in the sky, the normalization of temperature-luminosity relation turned out to vary by 16 ± 3 %. With about ≈ 4σ confidence this implies that the University expands in different directions at different rates. And in combination with the additional data on 842 galactic clusters the confidence increases to about ≈ 5,5 σ. The additional anisotropy maximum is displaced by ≈ 50°-100° relative to the axis of the dipole associated with the motion of the Sun. The origin of the discovered anisotropy remains unclear. Absorption of X rays in gas clouds in a local Universe region is weak and cannot therefore serve as an explanation. Anisotropy is possibly due to inhomogeneity of the dark matter that occupies the Universe or to the large-scale matter flows. The proof of the Universe anisotropy would be of great scientific significance since it would change the conventional cosmological paradigm, and therefore additional verifications and research are needed. [6] Verkhodanov O V, Phys. Usp. 59 3 (2016) [Usp. Fiz. Nauk 186 3 (2016)] [7] Migkas K et al., Astronomy & Astrophysics 636 A15 (2020)

Physics of viruses

In modern microbiology, physical methods of research play an exclusively important role. For example, electron microscopes were used to obtain for the first time virus images, the X-ray pictures of crystalized viruses helped determining their structure, and scanning atomic-force microscopes allow a detailed examination of the form of the protein viral shell (capsids). Widely investigated in biophysics are virus self-assembly of RNA/DNA and proteins and the mechanical properties of virus particles [8]. All this may appear to be useful in elaboration of effective vaccines and drugs. And vice versa, the methods developed in microbiology find application in nanotechnology. Viruses and virus-like particles can be used in future, e.g., as a means for delivering drugs to cells and even as blocks in microelectronics. For these and some other purposes it is of importance to understand the physico-chemical properties of viruses. J. Shang (the University of Minnesota, USA) with colleagues [9] used in their new study the crystallization and X-ray diffraction method to construct a three-dimensional model of SARS-CoV-2 protein and to find receptors located at spike proteins responsible for virus entry into the cell. These results are important for both clarifying the evolutionary origin of SARS-CoV-2 and searching for the methods of treatment and prophylaxis. For the physical processes in microbiology and the physical methods of investigation of biological objects see also [10-12]. [8] Buzon P, Maity S, Roos W H; Physical virology: From virus self‐assembly to particle mechanics [9] Shang J et al., Nature, online-publication on March 30, 2020 [10] Vainshtein B K, Sov. Phys. Usp. 16 185 (1973) [Usp. Fiz. Nauk 109 455 (1973)]

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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.

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