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| Issue 12, 2024 |
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Extracts from the Internet | ||
Metallic hydrogen1 February 2017In the paper “What problems of physics and astrophysics seem now to be especially important and interesting” (see Phys. Usp. 42 353 (1999)), V.L. Ginzburg ascribed the possibility of creating metallic hydrogen to most important and interesting problems. The hydrogen transition to the metal phase under high pressure was predicted by E. Wigner and B. Huntington in 1935. The observation of metallic properties of hydrogen was reported earlier but these results have not yet been confirmed. R.P. Dias and I.F. Silvera (Harvard University) carried out a new experiment with a diamond anvil and reported the observation of Wigner-Huntington phase transition corresponding to the hydrogen transition to the metal phase. The transition was evidenced by the growth of sample reflectance in the optical region up to 0.91. The pressure increase up to 595 GPa was achieved through removal of defects from the diamond surface in the anvil and also through placing the sample into an aluminum oxide envelope obstructing the hydrogen diffusion. The transition to the state of metallic hydrogen occurred presumably in the range of 565-595 GPa. It is not yet clear whether the obtained metal phase is solid as follows from the theory or is liquid. The electron concentration determined by the Drude theory corresponds to atomic hydrogen, that is, the molecular hydrogen in the experiment dissociated to atoms. In two separate experiments (at temperatures of 83 K and 5.5 K) the sample was compressed and in both cases the state of metallic hydrogen was achieved. Another important possibility of obtaining the metal phase is plasma compression at high temperatures (see, e.g., V.E. Fortov et al., Phys. Rev. Lett. 99 185001 (2007)). In the other experiment, M.I. Eremets, I.A. Troyan, and A.P. Drozdov also obtained in 2016 the evidence of the formation of metallic hydrogen under the pressure of 360 GPa (arXiv:1601.04479 [cond-mat.mtrl-sci]). To verify the described results independent experiments are needed. In 1968 N.W. Ashcroft put forward theoretical arguments suggesting that metallic hydrogen may exhibit superconducting properties even at room temperature (see also the paper by M.I. Eremets and A.P. Drozdov in Phys. Usp. 59 1154 (2016)) and in 1972 E.G. Brovman, Yu. Kagan, and A. Kholas (JETP 34 1300 (1972)) pointed out that room-temperature metallic hydrogen can remain metastable even after the high pressure is released. These properties, if confirmed, can be of great practical importance. In nature, according to the calculations, metallic hydrogen presents a considerable part of Jupiter’s and other giant planets’ interior. For the metallic hydrogen problem see the review by E.G. Maksimov and Yu.I. Shilov in Phys. Usp. 42 1121 (1999). Source: Science 355 715 (2017) Precision source of weak current1 February 2017F. Hohls (National Metrology Institute of Germany) with colleagues elaborated a new way to count single electrons whose beam generates weak electric current. The device in the field transistor configuration was arranged on the basis of quantum dot in a semiconductor. An electric potential varying with a frequency of 0.5 GHz is applied to the electrode above the dot. In each period, the quantum dot pumps in and then out single electrons. This can be used to control the number of passing electrons. The current generated by them is amplified and measured with a relative accuracy of 1.6×10-7, which is close to the accuracy of the modern standard of current in the SI system. Measurements of weak currents can be used, for example, to determine the level of radioactivity in ionization chambers or to count aerosol particles in the air. Source: physicsworld.com Quantum measurements1 February 2017At the Institut Laue-Langevin (France) a new effective method of quantum state tomography was used on the basis of combination of weak and strong quantum measurements. In 2016, the researchers of the University of Padua (Italy) G. Vallone and D. Dequal showed theoretically that the so-called weak quantum quantities can be determined not only by weak but also by strong measurements with a high value of the impact on the system. S. Sponar (The Institute of Nuclear Physics, the University of Vienna, Austria) and his colleagues realized this approach for the first time in their experiment. A neutron beam was split into two beams that interfered after the flight along different paths. The trajectory-dependent neutron spin rotation was realized using the magnetic field, which related the degrees of freedom of the trajectory and the spin. Thus, the trajectories were determined by the value of spin rotation. An additional spin rotation was realized with the help of the magnetic field, the turns through 15° and 90° represented respectively the weak and strong measurement. As predicted, the employment of strong measurements made the process of quantum tomography faster and more effective, which is particularly important for small signal extraction from the background. Source: Phys. Rev. Lett. 118 010402 (2017) Hydrogen atom positions in a nanocrystal1 February 2017L. Palatinus (the Institute of Physics of the CAS, Czech Republic) et al. developed the method allowing the use of electron diffraction to localize hydrogen atoms in a micro- and nanometer crystal lattices where the X-ray and neutron diffraction methods are inapplicable. The interaction of electrons with charged particles induces multiple declinations of electrons in the crystal, which smears the diffraction pattern and requires improvement of the data processing method. L. Palatinus with colleagues performed a structural 3D analysis of crystals using the dynamic theory of diffraction allowing for multiple electron scatterings as distinct from the kinetic theory of diffraction describing single scatterings. Accordingly, the data processing followed more complicated mathematical algorithms. The new method was applied to determine the hydrogen atom positions in both organic (paracetamol) and inorganic (cobalt aluminophosphate) materials. Electron diffraction was already employed to determine the electron atom positions by B.K. Vainshtein, B.B. Zvyagin, and A.S. Avilov in 1992, but in the case of powder consisting of macroscopic crystals. The new method proposed by L. Palatinus et al. can be applied to clarify the morphology of organic molecules constituting micro-sized crystals and to provide insight into functioning of active components of some drugs. Source: Science 355 166 (2017) Localization of a source of fast radio bursts1 February 2017The nature of the recently revealed fast radio bursts remains unclear in spite of a lot of the models proposed. Their distinctive feature is a high value of the dispersion measure testifying to the fact that the radio burst sources are located at cosmological distances. The observations with VLA and 300-meter telescope in Arecibo made it possible to establish that one of the sources of fast radio bursts, FRB 121102, is co-located with the source of persistent radio emission, the probability of an accidental projection being estimated at the level of ≈10-5. The origin of the persistent <1 pc radio source is not yet known either. The position of FRB 121102 and of the persistent radio source coincides, in turn, with the optical object which is likely to be a small galaxy with a low star-formation rate at the red shift z=0.2. Perhaps, FRB 121102 is related with the nucleus of this galaxy. The source FRB 121102 is the only one from which several (17 to date) fast radio bursts were registered. Source: Nature 541 58 (2017) |
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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