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| Issue 12, 2024 |
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Extracts from the Internet | ||
High-energy neutrinos1 August 2013The neutrino detector IceCube has registered two neutrinos with a record-high energy of about 1 PeV = 1015 eV at confidence 2.8 σ. IceCube is composed of 5160 Cherenkov detectors in a volume of about ≈ 1 km3 deep into Antarctic ice at a depth of about 1450-2450 m. These detected events which created cascades of particles in the ice, can be explained in terms of the reaction of neutrinos νe,μ,τ (or antineutrinos) with nuclei via weak neutral currents, and by by the interaction between νe (anti-νe) via the charged currents. The task of recording high-energy neutrinos is facilitated by the fact that cosmic rays create very few background events in the atmosphere at energies ≥1 PeV. In addition, muon events from the upper hemisphere were excluded and only events caused by neutrinos that have passed through the Earth were taken into account. In fact, thereforore, the probability of having two or more background events in the data obtained was only 2.9 × 10-3. If the two recorded events were actually caused by neutrinos of astrophysical origin, their sources could, for example, be cosmic gamma-ray bursts or processes in active galactic nuclei. Source: Phys. Rev. Lett. 111 021103 (2013) The van der Waals forces between Rydberg atoms1 August 2013L. Begum (Charles Fabry Laboratory of the Institute of Optics, France) and others were the first to measure the van der Waals interaction between two atoms occupying highly excited Rydberg states. In comparison with the atoms in the ground state, the dipole-dipole interactions between Rydberg atoms (via van der Waals forces) are much stronger. In the past, van der Waals forces were measured by indirect methods in a large variety of experiments. For example, people measured attraction of macroscopic bodies, or measured frequency shift of the atomic vibrations in diatomic molecules with van der Waals binding, or worked on the interaction of Rydberg atoms with the surface of the conductor. In the new experiment, the two atoms of rubidium were captured into traps created by focused laser beams at a controlled distance (from 3 to 20 microns). A focusing aspherical lens at the same time collected photons emitted by the atoms. Resonant laser pulses caused oscillations between the ground and Rydberg states of the two-atom quantum system, and the Rabi frequency of these oscillations were perturbed by interactions between atoms. The experiment was carried out in a mode of partial Rydberg blockade, when the energy of the van der Waals interaction was comparable to the energy of oscillations UvdW. The probability of finding the atoms in the ground and Rydberg states depending on the distance between atoms and the laser pulse length was measured by recording the photons emitted by the atoms. The curve UvdW∝ r-6 obtained from this data fits quite well the theoretical calculations. The experiment is also of interest due to the fact that it proved possible to sustain the coherence of a pair of Rydberg atoms. This promises the creation of quantum logic gates based on such pairs in the future. Source: Phys. Rev. Lett. 110 263201 (2013) The structure of the energy gap in CeCoIn51 August 2013J.C. Davis (Brookhaven National Laboratory and Cornell University, USA) and his colleagues investigated the structure of the energy gap in heavy-fermion superconductor CeCoIn5. It is assumed that the mechanism of Cooper pairing in these compounds is dictated by spin fluctuations, although further research is needed for fully clarifying the mechanism of superconductivity. The gap structure Δ(k) in momentum space in the experiment was studied in terms of the interference of quasiparticles (i.e. of the Bogoliubov quasiparticle interference imaging). The electrons of the split Cooper pairs went through the interference on the impurities; the standing electron waves arising from this were recorded using the scanning tunneling microscope. This technique made it possible to identify very small variations of gap energy as a function of direction. It was found that the gap is oriented in crystal direction along the Ñå-Ñå bonds and possesses the dx2-y2 symmetry. The shape of the Fermi surface and the structure of energy levels were also measured, including the splitting at low effective mass to the hybridized levels of heavy fermions. Source: Nature Physics 9 468 (2013) Photocatalyst TiO21 August 2013We know that rutile and anatase, the polymorphic crystalline modifications of titanium dioxide TiO2, are good photocatalysts: when illuminated, they efficiently split water molecules (i.e. perform photolysis) on their surface. In 1972 it was discovered that that specimens with a mixture of two modifications possess significantly better catalyst properties than pure rutile and pure anatase. It was assumed that this behavior is caused by the relative shift of the valence bands in rutile and anatase, even though there was no unequivocal experimental confirmation. D.O. Scanlon (University College Kathleen Lonsdale Materials Chemistry, United Kingdom) and colleagues performed a new experiment and carried out computer modeling of the structure of TiO2 which confirmed quite reliably the differences in energy levels. The heterostructure comprised of rutile and anatase layers was studied using an x-ray photoemission technique. It turned out that the anatase's valence band of is lower by 0.4 eV than the rutile valence band. This observations point to an electron flux from anatase to rutile under photoexcitation. It may be that this arrangement of valence bands results in a considerable improvement of the photo-catalytic properties of the mixed crystal. Source: Nature Materials 12 798 (2013) Extragalactic radio bursts1 August 2013Observers at the 64-meters Parkes radio telescope (Australia) discovered a new population of cosmological radio emission bursts lasting ≈ 1 ms and reaching spectral flux density 0.4-1.3 Jy. A similar powerful burst has already been recorded in 2007 (the Kerry Lorimer splash) whose origin remains unknown. Four more splashes from the directions ≥ 40° over the galactic disk were now recorded. The strong dispersion of the signal could only develop if they arrived from distances of 1.7-3.2 Gpc (from sources at z = 0.45-0.96) as a result of interaction between radio waves and cosmic plasma, since dispersion in the rarefied plasma of the Galaxy at high galactic latitudes is not insufficient for the formation of the observed spectra. In connection with this, a conclusion of the cosmological origin of bursts is made. The number of such radio bursts per day from all directions is predicted as ≈ 104 if the result is extrapolated to the entire celestial sphere. No repeated or overlapping events were found yet in other wavebands. Although the origin of these radio bursts remains unclear, high-energy processes close to black holes or neutron stars were suggested as possible sources. Source: Science 341 53 (2013) |
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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