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| Issue 12, 2024 |
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Extracts from the Internet | ||
Acoustic surface plasmons1 August 2007Physicists at the University of New Hampshire announced first experimental observations of acoustic surface plasmons (ASP) - low-energy collective electron excitations on the surface of a metal that have linear disperse relation. Even though ASP were predicted theoretically rather a long time ago, all previous attempts to detect them failed; doubts were even expressed that they may not be observable due to the strong screening of APS by electrons in the bulk of the metal. B.Diaconescu and co-workers succeeded in building extremely precise “electron gun” which shoots a beam of slow electrons onto the surface of a beryllium crystal, carefully polished and cleaned of oxygen atoms. The experiment was run in ultra-high vacuum chamber at room temperature. ASP generated by the beam on the surface were identified through the spectra of reflected electrons. Energy losses of electrons were exactly equal to the energy required to excite APS. According to theoretical calculations, ASP propagate along the surface to a distance of several nanometers and live for several femtoseconds. ASP may play an important role in some very fast chemical reactions that occur on metal surfaces. Hypotheses were also advanced that ASP assist in the formation of Cooper pairs of electrons in high-temperature superconductors. Sources: Nature 448 57 (2007), http://www.unh.edu/news/cj_nr/2007/july/bp05electron.cfm Electron p-n-junction in graphene1 August 2007N.Marcus and colleagues at Harvard University created for the first time a p-n junction in graphene which is a planar sheet of carbon only one atom thick. The electric field of contacts generated ? and n regions with deficiency and excess of electric charge. The method of producing a p-n junction in graphene was discussed earlier but its implementation was thwarted most of all by the difficulty of mounting contacts to brittle graphene layer. Harvard scientists succeeded in doing this by turning to the atomic layer deposition technique typically used in production of carbon nanotubes. First a thin insulating layer was evaporated onto graphene, after which titanium-gold electrodes were evaporated onto it. The graphene layer itself was placed on top of a silicon oxide layer sitting on silicon substrate that served as the second electrode. An analysis of the electric properties confirmed that a p-n junction was indeed formed in the specimen. Furthermore the quantum Hall effect was observed in magnetic field, which pointed to a planar structure of the obtained specimen. As a two-dimensional specimen of graphene has no gap in its electron spectrum, the resulting p-n junction cannot be used in a straightforward manner as part of diode or transistor; in the future, however, it is considered feasible to produce such junctions in very narrow graphene ribbons that have a gap in the spectrum. When these attempts prove successful, devices based on graphene may evolve into efficient replacements of conventional semiconductor elements in microelectronics. Sources: http://www.sciencemag.org/cgi/content/abstract/1144672 Entangled states of photons1 August 2007O.Wilk and her colleagues in Germany and Great Britain developed a technique for preparing entangled quantum states of two photons using a single atom. A rubidium atom was placed into an optical trap. A short laser pulse triggered the atom into emitting a photon; as a result the atom and the emitted photon became entangled. Approximately a microsecond later the next laser pulse caused the emission of the second photon which was entangled with the first. The quantum state thus transfers from the atom to the second photon, resulting in two entangled photons. The probability of obtaining a quantum-correlated entangled state was 1.3% but further improvement of this technique may increase its efficiency. This approach may be useful in designing quantum computers because it offers both a source of photons in entangled states and a method of transferring quantum information between photons and atoms. Sources: Science 317 488 (2007) Interference on the nanoscale1 August 2007R.Zia and I.Brongersma (Stanford University, USA) carried out an analog of a quantum-mechanical two-slit experiment on the nanometer scale. The surface electromagnetic waves - the plasmon polaritons - were excited by the light wave on the surface of gold film and propagated along a microscopic waveguide arranged as two parallel gold strips 2μm wide, spaced by another 2μm. The waveguide with this geometry allowed Zia and Brongersma to conduct measurements beyond the diffraction limit, that is, to work on a scale smaller than the wavelength of the electromagnetic wave in vacuum. The polaritons were observed on the entire length of the waveguide using a photon scanning tunneling microscope; the scattered electromagnetic waves were measured by a photodetector. As anticipated, the observed pattern revealed wave interference similar to that in the two-slit experiment. This study is important for designing nanometer-scale devices operating on electromagnetic waves in the subwavelength range. Sources: Nature Nanotechnology 2 426 (2007) Galactic clusters in collision1 August 2007Astronomers of Michigan University using orbiting x-ray telescopes XMM-Newton and Chandra discovered that the galaxy cluster Abel576 is composed of two galaxy subclusters undergoing head-on collision. This conclusion follows from the data of chemical composition and type of motion of the gas that was found to consist of two components. The cores of the two clusters are almost exactly on the line of sight, so that it was difficult to differentiate between them using only optical images. A collision of galaxy clusters is a very rare event; only about 0.1% of large clusters betray signs of collision. However, the most unexpected feature of the observations of Abel576 was a fairly high relative velocity of the colliding clusters - 3300km/s. A large gradient of gas velocities in Abel576 was revealed by the observations from the orbiting ASCA. The available theoretical models will have to be improved to provide explanation of such a high collision velocity. Sources: http://arxiv.org/abs/0706.1073 |
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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