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| Issue 12, 2024 |
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Extracts from the Internet | ||
Conductivity of graphene1 March 2009In the experiments carried out so far the conductivity of graphene (one-atom thick carbon layer) was measured on a substrate and no data was available on graphene conductivity in vacuum, that is, when the structure of graphene is not disturbed by the influence of substrate charges (see e.g. Phys. Usp. 51 744 (2008) ). Theoretical investigation of this characteristic is not easy since the effective electrodynamic coupling constant αg in graphene is approximately 300 times greater than the fine structure constant α = 1/137, so perturbations theory cannot be used. J.E. Drut and Ò.À. Lahde developed a new theoretical approach for calculating electron properties of graphene. They applied a numerical “lattice” computation technique resembling methods used in quantum chromodynamics where the coupling constant is also large. Computations revealed how the energy gap in the graphene electron spectrum changes depending on where graphene is placed. It was shown that conductivity is affected by the coupling constant αg. For graphene on SiO2 substrate the authors obtained αg ≈ 0.79 so graphene is conducting. As αg increases beyond the critical value αgcr = 1.11 ± 0.06, graphene turns into insulator. For instance, in vacuum αg ≈ 2.16 and graphene is insulator. The validity of the formulated theoretical predictions can be tested in future experiments. Source: Phys. Rev. Lett. 102 026802 (2009) Type-1.5 superconductors1 March 2009V.V. Moshchalkov and his colleagues in Belgium and Switzerland confirmed experimentally that the two-component superconductor MgB2 possesses superconducting properties of both type-1 and type-2 and consequently proposed to refer to it as superconductor of intermediate type-1.5. They studied the spatial distribution of superconducting vortices in a single crystal of MgB2 as it was cooled and went into superconducting state in external magnetic field perpendicular to the specimen surface. It was found that vortices are not uniformly spread but form denser patterns of spider web or ribbon-like shape. This implies that at small distances vortices repel one another like they do in type-2 superconductors but attract one another at larger distances like they do in type-1 superconductors. This phenomenon is theoretically explained in terms of two weakly coupled order parameters in superconducting MgB2 so that MgB2 manifests a combination of the properties of two superconductors with different ratios ξ/λ (coherence length to penetration length) typical of type-1 and type-2 superconductors. Measurement results are in good agreement with the predictions of numerical modeling of vortex dynamics and calculations in terms of the Ginzburg – Landau two-component theory. Source: http://arxiv.org/abs/0902.0997 Entangled state of mechanical oscillators1 March 2009J.D. Jost and his colleagues at the US National Institute of Standards and Technology (NIST) for the first time entangled two mechanical systems — two oscillators consisting of pairs of ions 9Be+—24Mg+ in a potential well. First all four ions were trapped into a potential well in which the internal degrees of freedom of two 9Be+ ions got entangled. Then two laser beams were used to modify the shape of the potential well and the ions were separated into two pairs of 9Be+—24Mg+ of size ≈4 µm the separation between the pairs was 0.24 mm. These pairs of ions resemble macroscopic springs with weights at their ends. Laser pulses forced the internal state of entanglement in each pair to transfer to the mechanical oscillations of ions in the pair relative to one another. This resulted in the formation of two pairs of ions entangled over mechanical oscillational degrees of freedom. Source: http://arxiv.org/abs/0901.4779v1 Powerful gamma-ray burst1 March 2009NASA's Fermi Gamma-ray Space Telescope recorded a space gamma-ray burst of extraordinary power, designated GRB 080916Ñ. The optical afterglow of the burst was observed with the GROND detector of the 2.2 m telescope, as well as with other telescopes. The source of the burst was probably a supernova explosion in a remote galaxy with red shift of z = 4.35 ± 0.15; consequently, GRB 080916Ñ belongs to the 5% of the remotest recorded gamma-ray bursts. It is assumed that violent energy releases resulting in gamma-ray bursts take the form of narrow jets. It was established from the variability of gamma emission that the Lorentz factor of a burst exceeds 1090. If we assume for the sake of comparison that the emission is isotropic, the energy released in the explosion comes to 6.5 × 1054 erg, or roughly 9000 times the energy of ordinary supernovas. This allows classifying the gamma-ray burst GRB 080916Ñ to have record-high energy. As observed with some other bursts, high energy photons arrive with certain time delay relative to photons from the low energy part of the spectrum. This may indicate that emission from different parts of the spectrum is generated in different conditions at different stages of the explosion, or in different regions of the jet-carried ejecta. Source: http://arxiv.org/abs/0902.0761 Coherence of electrons in photosynthesis1 March 2009It was earlier assumed that the energy transfer between electrons of different molecules in protein complexes responsible for photosynthesis proceeds classically (via Coulomb collisions). However, data were obtained in 2007 that electrons in protein molecules are quantum-coherent, so energy is transferred by a wave mechanism. The 2007 experiment required prolonged irradiation of molecules by laser light which resulted in their degradation and destruction; furthermore, the spectra could be obtained only at select points of protein complexes. I. Mercer (of University College Dublin, Ireland) and his colleagues in Great Britain worked out a new and perfected technique which made it possible to clarify details of energy transfer in photosynthesis. By using irradiation with a train of short laser pulses with different wavelengths Mercer et al obtained a two-dimensional spectrum of protein complexes and were able to build a spatio-temporal picture of the processes. Thus the new experiment has confirmed that electrons transfer energy in a coherent manner. Sources: Phys. Rev. Lett. 102 057402 (2009) ; http://focus.aps.org/story/v23/st5 |
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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