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| Issue 11, 2024 |
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Extracts from the Internet | ||
Quantum model of “Zitterbewegung”1 February 2010Ñ. Roos and his group at the Institute for Quantum Optics and Quantum Information (Innsbruck, Austria) carried out an experiment with 40Ca+ ions in which they recorded the “quivering motion” effect (Zitterbewegung) predicted for electrons by E. Schrodinger in 1930 using the Dirac equation. Rapid spatial oscillations of a particle arise owing to a superposition of states with positive and negative energies. The experiment with ions is a quantum model of the “Zitterbewegung” of electrons; in other words, the joint evolution of the vibrational degrees of freedom and the internal spin state of the ion is described by the same equation as the spatial trajectory of free electrons. Laser pulses transferred ions into a prescribed initial state and a short time later their fluorescent radiation was observed. The properties of this radiation yielded the characteristics of the “Zitterbewegung”. The “Zitterbewegung” can be experimentally observed with a quantum model in which it is linked to the spin state of the ion. At the moment it is quite unfeasible to detect this effect with real electrons since its amplitude is only ≈ 10-10 cm and its frequency is ≈ 1021 Hz. The idea of quantum models was suggested by R. Feynman in 1982 for those cases in which the complex behavior is beyond any hope of direct or numerical computer experiments. Source: Nature 463 68 (2010) The E8 group in crystals1 February 2010R. Coldea (Oxford University) and his colleagues in Germany and Great Britain discovered in neutron scattering experiments that the distribution of spins in quasi-one-dimensional ferromagnetic crystal of cobalt niobate CoNb2O6 at low temperature and in high magnetic field obeys the symmetry group E8. In 1989 A.B. Zamolodchikov (The L.D. Landau Institute of Theoretical Physics) showed that in certain cases this Lie group describes the spectrum of excitations for the Ising model. In recent years the E8 group was also discussed in elementary particles theory but has never been observed in real physical systems. The phase transition in quasi-one-dimensional Ising model occurs when the magnetic field directed along the atomic chains increases above a certain critical value which separates the magnetically ordered and the paramagnetic phases. The experiment studied the spin excitations in the two above phases and also investigated the properties of crystals in the immediate vicinity of the phase transition point. The measured ratio of the two lower resonance frequencies (two meson states) of atomic chains close to the critical magnetic field of ≈ 5 T was found to lie close to the golden ratio 1.618, in agreement with the prediction of A.B. Zamolodchikov's theory. Source: Science 327 177 (2010) Goos – Hanchen effect for neutrons1 February 2010Researchers at the Delft University of Technology (The Netherlands) and the Rutherford Appleton Laboratory (Great Britain) were able for the first time to detect the Goos – Hanchen effect in the reflection of spin-polarized neutrons by a potential barrier. This effect which was already predicted by Isaac Newton for light, produces a gap between the points of incidence and reflection of the beam. In a manner of speech, we can say that the beam penetrates into the sample and is reflected at a certain depth under the reflecting surface. The effect was first measured for light by F. Goos and H. Hanchen in 1947. In the case of neutrons, reflection imparts a certain phase shift to the wave function of a particle, which may be represented by a spatial translation along the reflecting surface. The experiment above made it possible to measure the Goos – Hanchen effect owing to the fact that the height of the relecting potential barrier in magnetic field depends on neutron spin direction (and on the orienttion of the neutron magnetic moment) so that neutrons of different degree of spin polarization are reflected at a different efficiency. The neutron beam was reflected by a layer of permalloy Fe0.2Ni0.8, 3µm thick. The choice of this material with high magnetic permeability was predicated on the need to create magnetic field under the reflecting surface and thus produce phase-shifted wave function. Reflected neutrons were recorded by a neutron reflectometer OffSpec. A small change in the polarization of the reflected beam was an indication of nonzero Goos – Hanchen effect. The Goos – Hanchen effect may be used for practical applications, such as generation of coherent neutron beams, and for creating neutron “waveguides”. Source: Phys. Rev. Lett. 104 010401 (2010) Electron “liquid crystal” in iron-based superconductor1 February 2010J.C. Davis (Brookhaven National Laboratory), Ð.Ñ. Canfield (Ames Laboratory) and co-workers conducted a scanning tunneling spectroscopy study of the compound Ca(Fe1-xCox)2As2 (x is the fraction of dopant atoms) and discovered a static spatially ordered state of electron wave functions whose structure resembles that of liquid crystals. This compound is a parent of one class of superconductors — layered iron-based high-temperature superconductors (see review papers in Phys. Usp. 51 1201, 1229, 1261 (2008)). Electron wave functions of elongated shape were observed on the specimen surface, stretching along one of the crystal axes. Their longitudianl size was approximately eight time the distance between two neighboring iron atoms, and they were in spatial correlation with the dopant atoms. It was conjectured that the “liquid crystal” state of electrons is critically important for the high-temperature superconductivity to emerge. Similar ordered electron states have already been reported in cuprate superconductors. This may be an indication that the superconductivity mechanism in iron-based superconductors is closer to the currently unknown superconductivity mechanism in cuprates than to the Bardeen – Cooper – Schrieffer mechanism in low-temperature superconductors. Source: Science 327 181 (2010) Gamma-radiation background at intermediate galactic latitudes1 February 2010The results have been published of observing cosmic gamma radiation at intermediate galactic latitudes 10° ≤ |b| ≤ 20° in the energy range 100 MeV to 10 GeV, obtained by the Large Area Telescope (LAT) on board the cosmic Fermi gamma observatory during the first five months after launch. New background measurements do not confirm some earlier observations conducted by the EGRET gamma telescope on the orbital Compton Observatory in 1991 – 2000. According to the standard model of the origin of the diffuse galactic gamma radiation, it is generated when charged particles of cosmic rays interact with interstellar gas and radiation. This model is calibrated using cosmic rays data and provides sufficiently specific predictions for the gamma radiation background. The EGRET has earlier detected an excess of gamma radiation flux in comparison with the standard model outlined above. This information gave rise to conjectures on additional contributions to gamma background from particles of dark matter and other hypothetical sources. The LAT is more sensitive than the EGRET by approximately an order of magnitude but the LAT data reveal no excess in the gamma background and thus the measured spectrum supports with high accuracy the predictions of the standard model of background generation. Source: Phys. Rev. Lett. 103 251101 (2009) |
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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