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| Issue 12, 2024 |
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Extracts from the Internet | ||
Superconductivity of H2S at a temperature of 203 K under pressure1 October 2015M.I. Fremets (Max Planck Institute of Chemistry, Germany) with colleagues reported the discovery of conventional superconductivity (described by the Bardeen-Cooper-Schrieffer – BCS – theory) of hydrogen sulfide H2S at a record superconducting transition temperature Tc=203 K at a pressure of 150 GPa. A record temperature Tc=164 K had been observed before for cuprate under high pressure, while for conventional superconductors (MgB2) Tc had not exceeded 39 K. The BCS theory does not limit Tc and suggests the way of its increase: a high phonon oscillation frequency, a strong electron-phonon coupling and a high density of electron states are needed. These factors must be best of all pronounced in metallic hydrogen or hydrogen compounds. In this experiment, an H2S sample was first examined at P>100 GPa. It was compressed in a diamond anvil where the pressure was controlled by the Raman spectrum. Superconductivity was registered both by the electric resistance fall and on the basis of the Meissner effect in a magnetic field. The observed effect of isotopic Tc shift in D2S is indicative of the BCS mechanism of superconductivity. The authors of the experiment believe that under pressure H2S is decomposedand is partially transformed into H3S which is the carrier of superconductivity. The temperature Tc=203 K (-70°C) already exceeds the natural on-Earth temperatures, and there is hope to discover room-temperature superconductivity in future (for more details see V.L. Ginzburg in Phys. Usp. 43 573 (2000) and Phys. Usp. 48 173 (2005)). Source: Nature 525 73 (2015) Zero quantum fluctuations of a mechanical resonator1 October 2015The experiment conducted under the guidance of K.C. Schwab (California Institute of Technology, USA) demonstrated the method of “squeezing” zero quantum fluctuations of a mechanical system when the magnitude of fluctuations of one variable X1 describing the system is lowered with increasing fluctuations of the second conjugate variable X2 (in the Wigner diagram this looks like squeezing a circle to an ellipse). The root-mean-square fluctuations of two noncommuting variables cannot be diminished simultaneously because of the quantum-mechanical uncertainty principle. A micrometer aluminum plate had a resonance frequency νm=3.6 MHz of mechanical oscillations and was one of the capacitor plates of the oscillatory circuit with resonance frequency νc=6.23 GHz, which gave rise to parametric resonance. Quantum variables were the coefficients in the plate coordinate decomposition x=X1cos(ωm t)+X2sin(ωm t). Quantum fluctuations were squeezed by the action of an additional electromagnetic field on the plate at frequencies νc ± νm, the action being stronger at the lower frequency. The amplitude-frequency characteristic of the circuit near the resonance was analyzed. The measurements showed that the fluctuations were squeezed by about 9%. This method may appear to be useful in the design of ultrasensitive sensors for gravitational wave detectors for which the quantum restrictions following from the uncertainty principle are of paramount importance. Source: Science 349 952 (2015) Quantum squeezing under resonance fluorescence1 October 2015A squeezed quantum state of light is most often obtained in nonlinear crystals under highly intense laser radiation. However, as far back as 1981 D.F. Walls and P. Zoller proposed another way to obtain a squeezed state making use of resonance scattering of photons by a two-level system. At levels in real atoms, this method could not yet be realized by reason of the low value of fluorescence yield. M. Atature (Cambridge University, Great Britain) with colleagues were the first to demonstrate this method of squeezing making use of not real but “artificial” atoms represented by the energy levels of electrons at a semiconductor quantum dot. Owing to the high intensity of dipole transitions the photon detection rate was increased by two orders of magnitude compared to the case of real atoms. The quantum dot was illuminated by laser, and the fluorescent light was gathered by a lens and transmitted through splitters and an interferometer which assist in separation of photon of the original and fluorescent radiation. As a result the correlation function of photons that had passed through the interferometer arms was measured and the squeeze of quantum fluctuations was recorded: one of the conjugate variables describing the electromagnetic field of a reemitted wave had dispersion lower by 3.1 ± 1 % than the level of quantum noise along of the large uncertainty of the other variable. Source: Nature 525 222 (2015) Radon and thoron as predecessors of earthquakes1 October 2015Heightening of radon isotope 222Rn concentration in the air is sometimes associated with approaching earthquakes, but this correlation is however not rigorous: radon ejections and earthquakes occur most often independently, and therefore it is typically impossible to forecast earthquakes proceeding from 222Rn. Researchers from the Seoul National University (republic of Korea) Y.H. Oh and G. Kim showed that a more clear dependence can be established by a simultaneous measuring of 222Rn and thoron 220Rn concentration. 222Rn and 220Rn were monitored in a cave in South Korea for 13 months with the help of silicon detectors of α-particles. In February of 2011 a strong simultaneous ejection of 222Rn and 220Rn was detected which cannot be explained by ordinary weather or season variations. This ejection preceded the 9.0-magnitude earthquake in Japan on March 11, 2011 at a distance of 1200 km from the detector – such a large distance can be explained by an overall shift of the tectonic plate. Strong ejections of 222Rn, but without 220Rn were also detected in the Summer of 2010, however no earthquake then followed. In the course of diffusion through rock microcracks 222Rn, whose half-life T1/2=3,82 days can emerge on the surface. On the contrary, because of the small half-life T1/2=55,6 s 220Rn has not enough time to reach the detector through diffusion, but is rather transported by advection air flows. In researchers’ opinion this is the reason for a lower sensitivity of 220Rn concentration to meteorological conditions and a higher sensitivity to earthquake-preceding geological events. Thus, detection of a pair of radon-thoron isotopes may provide a good tool for earthquake prognosis provided the corresponding network of underground detectors is arranged. Source: Scientific Reports 5 13084 (2015) A bright supernova and an ultra-long gamma-ray burst1 October 2015A class of bursts lasting longer than 104 s is distinguished among the cosmic gamma-ray bursts. It was assumed that some of them may be due to supernova explosions, but such supernovae have not been observed before. J. Greiner (Max Planck Institute for Extraterrestrial Physics and Technical University of Munich, Germany) with colleagues revealed for the first time a rather convincing relation between supernova 2011kl and the ultra-long burst GRB 111209A which occurred at the red shift z=0.677. After the burst GRB 111209A a high-power afterglow related to supernova 2011kl was observed during approximately 43 days. Its luminosity cannot be due to the 56Ni decay because too large a mass of ejected 56Ni would be needed. This can be explained by the model in which extra energy is transferred by the magneto-rotational mechanism from a strongly magnetized neutron star–a magnetar produced in an explosion. Magnetar models have already been discussed earlier, but the most comprehensive and self-consistent picture was obtained in the case of GRB 111209A and 2011kl. Source: Nature 523 189 (2015) |
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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