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| Issue 11, 2024 |
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Extracts from the Internet | ||
The Josephson effect in ultracold gas1 November 2007Typically the stationary (d.c.) and nonstationary (a.c.) Josephson effects are implemented at the tunneling contact between two superconductors, while the exclusively a.c. Josephson effect was also observed in a vessel with superfluid 3He divided in two parts by a membrane. J. Steinhauer and colleagues at the Israel Institute of Technology observed the Josephson effect in essentially different conditions, namely in the Bose – Einstein condensate of rubidium atoms in a magnetic trap. The potential barrier between the two parts of the condensate was created using a laser beam: a nonspherical lens stretched the beam to planar shape and separated the cylindrical atomic trap into two parts parallel to its axis. Additional magnetic field produced non-symmetrical distortion of the trap potential, resulting both in a difference between chemical potentials of the two halves of the trap and in oscillatory motion of atoms across the trap in the direct and reverse directions: the a.c. Josephson effect. When the trap potential was moved at low velocity relative to the barrier, tunneling of atoms in one direction only was observed: the d.c. Josephson effect. Source: Nature 449 579 (2007) Electronic properties of graphene1 November 2007C.N. Lau and colleagues of the University of California at Riverside studied the low-temperature electric conduction of one- and two-layer microscopic sheets of graphene as a function of applied voltage and of the ratio of side lengths of the specimen. It was found that the theoretically lowest value of conductance 4e2/πh is only reached when the width and length of a graphene sheet differ by a factor of at least four. If the side ratio is less than 4, the conductance is several times larger. At low temperatures, the waves of electrons and holes move virtually freely through a graphene sheet but undergo multiple reflections at specimen edges; this results in quantum interference of charge wave functions and periodic oscillations of conductance depending on voltage applied. Systems with such reflections and interferences are known as “quantum billiards”. Furthermore C.N. Lau and colleagues also studied graphene conductance with superconducting electrodes connected to the specimen. They observed both induced increase in conductance at low electron energies and reduction of conductance at energies much higher than the superconducting band gap. Source: Science 317 1530 (2007) Quantum spin Hall effect1 November 2007Quantum spin Hall effect was first observed experimentally by D. Awschalom (University of California) and colleagues in thin semiconductor films. The effect manifests itself as discrete conduction and results in spin-polarized conduction electrons appearing at lateral faces of the specimen even in the absence of magnetic field. In 2006 B.A. Barnevig, T.L. Hughes and S.C. Zhang predicted theoretically that quantum spin Hall effect may be observed in quantum walls — semiconductor structures that are macroscopic in one dimension while being only several atoms thick in the transverse direction. This prediction was confirmed in a new experiment by M. Konig and coworkers in Germany and the USA who studied the compound HgTe/Hg0,3Cd0,7Te. For wall thickness exceeding 6.3 nm, residual conductance plateau was observed; it was independent of specimen thickness as it is determined by electron properties only at specimen boundaries. It was also noticed that the residual conductance is destroyed by low magnetic field which is an indication that polarization was produced by the quantum spin Hall effect mechanism. The spin Hall effect is important for future practical applications in that it opens a way to controlling spin currents in magnetic-field-free spintron devices using only electric fields. Source: http://arxiv.org/abs/0710.0582 Noise compensation in laser interferometer1 November 2007The sensitivity of optical interferometers expected to be used in gravitational wave detectors is limited by a number of effects (see V.B. Braginsky “Gravitational-wave astronomy: new methods of measurements” Physics Uspekhi 170 743 (2000) ), including the quantum noise acting via back-action of light pressure on the instrument mirrors. A method was found that goes a long way in compensating for the back-action. The resonance mechanical frequencies of two mirrors of the detector are always slightly different and compensation occurs when mechanical responses of mirrors to fluctuations of light pressure at intermediate frequencies are not in phase. T. Caniard and his colleagues in France carried out an experiment that demonstrated this mechanism. They used high-finesse mirrors placed in vacuum and forming a resonator for a stabilized laser beam. Quantum fluctuations of radiation pressure were simulated by an additional acoustically modulated beam. The noise spectrum revealed an effect of back-action compensation; this compensation increased the sensitivity of the detector by a factor of 25. The experimental setup may be regarded as a precursor of a detector implementing the mechanism of compensation of the actual quantum noise. Source: Phys. Rev. Lett. 99 110801 (2007) Cosmic radio burst1 November 2007Astronomers analyzed archived observations data collected by the 64-m Parkes radiotelescope (Australia) at frequencies in the vicinity of 1.4 GHz and discovered an unusual burst of radio emission recorded on 24 August 2001. This burst was not detected earlier because the principal aim of the radio survey was a search for repetitive radio emissions, such as periodic emission from pulsars. A singular burst at least 5 ms long arrived from an area of the sky 3° to the south of the center of the Small Magellanic Cloud — a satellite of our Galaxy. The spectral density of burst emission was approximately 30 Jy, which is greater by a factor of 100 than the signal recording threshold of the Parkes telescope. Even though the burst is located on the celestial sphere near the Small Magellanic Cloud, the source of the burst was probably much further away - perhaps at hundreds of Mpc. This conclusion was made on the basis of the characteristic dispersion of the signal (the quadratic dependence of delay time on frequency) which corresponds to cosmic plasma interacting with radio waves. This form of dispersion practically excludes a terrestrial origin of the burst. No simultaneous events (supernova flairs, gamma bursts and so forth) were recorded at other telescopes in other wavelength ranges in the same area of celestial sphere. Furthermore, no repeated bursts were found in subsequent observations with the Parkes telescope. It appears that the detected burst belongs to a new class of cosmic radio signals whose origin is as yet unknown. According to estimates of the probability of recording this signal again, more than 200 such bursts should arrive every day; however, they were never recorded because no dedicated searches were ever conducted. Source: http://arxiv.org/abs/0709.4301 |
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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