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| Issue 12, 2024 |
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Extracts from the Internet | ||
Superconductor in contact with ferromagnet1 May 2008Electric current destroys superconductivity because of the dissipation of energy by moving magnetic vortices. The critical current increases in the presence of factors, such as defects in crystalline structure, that constrain the vortices. Theory predicted that the critical current can also be increased by bringing the superconductor in contact with a layer of ferromagnet whereby local magnetic fields hold the vortices within the boundaries of magnetic domains at the surface of the ferromagnet. Some evidence of this mechanism have already been observed in experiments. The new experiment by Vitalii Vlasko Âëàñêî – Vlasov (Argonne National Laboratory) and coworkers studied this effect in more detail and a method was found for controlling the critical current through changing the characteristics of magnetization of the ferromagnet. A 0,8 µm thick film of ferromagnetic Ni80Fe20 was deposited on a 20 µm thick superconducting crystal of NbSe2. Magnetic field parallel to the film was used to create in the ferromagnet an array of elongated magnetic domains with alternating directions of magnetization. This additional magnetic field generated magnetic vortices in the superconductor. The structure of magnetic domains in Ni80Fe20 was studied with a superconducting magnetometer and a magnetic force microscope while the pattern of magnetic vortices was observed through changes in light polarization. It was found that pinning vortices within magnetic domains increases the critical current flowing parallel to the film surface and across the magnetic domains by a factor of about three, as compared to the critical current in the direction perpendicular to the film surface. Sources: Phys. Rev. B (in press); arXiv:0705.0555 Graphene transistor1 May 2008At the moment the industry of silicon-based electronic components reached the scale of ≈ 45 nm which is not far from the limiting scale of ≈ 10 nm at which silicon is not capable any more of forming stable structures. The promise of further miniaturization in electronics is associated with using carbon in the form of nanotubes or graphene, and experimental transistors have already been created. A.K. Geim, K.S. Novoselov (Manchester University) and their colleagues created a graphene transistor of record small size — its width is a mere 10 carbon atoms. Furthermore, in contrast to graphene transistors created earlier which operated at ultralow temperatures, the new transistor can function at room temperature. The transistor is a quantum dot consisting of five carbon rings, with potential well depth of about 0.5 eV. The efficiency of electron capture by the quantum dot and correspondingly the current flowing through the transistor are controlled by magnetic field. Source: Science 320 356 (2008) Protons in supercooled water1 May 2008A. Pietropaolo and his colleagues in Italy used the method of deep inelastic neutron scattering to study the energy distribution of protons (hydrogen nuclei) in molecules of supercooled water. Compared to water in its ordinary state, significant excess of high-energy protons was observed, with the measured values not following a simple linear extrapolated dependence on temperature. The discovered effect may originate with the effect of quantum delocalization of protons between oxygen atoms of two neighboring water molecules. The distance between neighboring oxygen atoms in supercooled water is less than in water in its ordinary state which results in changed interaction potential of protons and surrounding atoms. Source: Phys. Rev. Lett. 100 127802 (2008) Single-photon ultrashort light pulse1 May 2008P.J. Mosley (Oxford University) and his colleagures obtained a record-short light pulse consisting of a single photon. Photons in pure quantum states were obtained by parametric conversion, i.e. by splitting photons in a nonlinear birefringent crystal into pairs of photons at doubled wavelength. The high state purity (more than ≈ 95%) in pairs of photons was achieved by a special choice of dispersion properties of the crystal, angle of incidence of the beam and light wavelength, so that the group velocity of the initial photon was equal to the group velocity of the photons produced by splitting the initial photon. It was therefore possible to eliminate quantum correlations between photons of the pair. The wave corresponding to the photons obtained was 65 fs long which is shorter by a factor of 15 than record-short photons generated previously. In fact even shorter pulses were reported earlier (see Physics Uspekhi 48 254 (2005)) but only in wave packets consisting of many photons. Source: Phys. Rev. Lett. 100 133601 (2008) Testing of general relativity1 May 2008The quasar OJ287 is known to generate two powerful optical bursts approximately every 12 years. This quasiperiodic activity is a consequence of the presence of a pair of supermassive black holes in the quasar core. A black hole with a mass of 1.8 × 108 solar masses revolves on an elongated orbit around a 200 times more massive black hole with a period of ≈ 12 years. The bursts occur when the smaller black hole travels through the gas accretion disk surrounding the larger black hole. In view of the considerable mass and compact size of the binary system, effects of general relativity theory (GRT) must be clearly manifested in the motion of the black holes, namely, rapid precession of the orbit and emission of gravitational waves. Theoretical calculations carried out under the guidance of Ì. Valtonen (Finland) and the observation of bursts on 13 September 2007 made it possible to conduct a new successful test of GRT predictions for strong gravitational fields. With precession and gravitational radiation taken into account, it was predicted that bursts should occur on the date above, give or take a day or two. Observations of the quasar OJ287 by several telescopes were started by that time and as expected, two bursts were indeed recorded on 13 September 2007. This has confirmed that OJ287 does indeed contain a system of two supermassive black holes. GRT predictions for gravitational radiation proved correct to within 10%. Without gravitational waves taken into account, the date of bursts would shift by approximately 20 days. The rate of precession of black hole orbits is 39° per period. When these black holes approach, this binary system is the most powerful emitter of gravitational radiation among the sources known in the Universe. There is a prospect therefore to observe gravitational waves from the system OJ287 in 12 years time, using the space laser inteferometer LISA to be launched before the event. Source: Nature 452 851 (2008) Search for dark matter particles1 May 2008Possible detection was reported in 2000 of dark matter particles (hidden mass) in the DAMA experiment (Gran Sasso National Laboratory, Italy). The detector allegedly revealed seasonal variations in recoil nuclei which could be interpreted as a periodic variation in the velocity of the Earth relative to the Galactic halo (for details see Physics Uspekhi 43 414 (2000)). However, this result was not confirmed in indepenendent experiments and it was suggested that it was caused by systematic experimental errors due to, for instance, seasonal temperature variations. In the last several years the DAMA experiment was significantly modified. The updated DAMA/LIBRA experiment uses about 250 kg of Nal(Tl) as a working substance highly purified of radioactive inclusions that would generate background noise. Exposure of about 530 kg × yr has already been achieved in the measurements. The first DAMA/LIBRA results have complitely confirmed the data of the preceding DAMA experiment. Seasonal variations of recoil nuclei were observed, unexplainable by any other known source of systematic errors. It is claimed that taking into account the data of the preceding experiment, the convidence level of registration of dark matter particles is 8,2 σ. At the present moment it is necessary to clarify why other experiments gave negative results for the same range of parameters of dark matter particles and to study in detail all other possible causes of the observed seasonal variations. The conclusion that the signature of dark matter particles have been successfully recorded can only be made when confirmation is provided by independent experiments. Source: http://arxiv.org/abs/0804.2741 “Echos” of x-ray flares1 May 2008The data obtained with x-ray telescopes Suzaku, XMM-Newton, Chandra and ASCA indicate that the x-ray luminosity of the Fe K-α line of iron in the large gas cloud Sgr B2 located 300 light years from the supermassive black hole at the center of our Galaxy was decreasing in recent years. The factor causing this x-ray luminance was most probably a flare that occurred close to the central black hole when a gas blob fell into it. The radiation of the flare reached Earth 300 years ago and the central region of the Galaxy in the x-ray spectrum was brighter for several years then now, by a factor of 106. Then 300 years later the light of the flare reached the object Sgr Â2 and excited atoms of iron which reradiated the x-ray “echos” we observe now. The x-ray flare indicates that the quiet state of the central black hole is sometimes replaced by an active phase involving accretion of matter. A similar but less powerful flare from the Galactic center was observed in 2001 (see Physics Uspekhi 44 1088 (2001)). The “echos” of flares is observed in other galaxies too. S. Komossa (Max Planck Institute for Extraterrestrial Physics, Germany) and his colleagures carried out detailed observations of the effect exerted on matter at the center of the galaxy SDSSJ0952 + 2143. The flare occurred as a result of a tidal destruction of a star near a black hole and the accretion of the generated gas cloud (stellar residue) onto the black hole. In this case as well, the X-ray and UV radiation caused ionization of the gas and the subsequent “echos”. The structure of the molecular torus surrounding the black hole and the accretion disk was studied in detail by observing the emission lines of iron, the degree of ionization and the distribution of gas velocities. The emmission in hydrogen lines emerging from the accretion disk was also observed. Sources: arXiv:0803.1528; Astrophys. J. Letters 678 13 (2008) |
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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