Extracts from the Internet


Antiproton-to-electron mass ratio

Ì. Hori (Max Planck Institute of Quantum Optics, Germany and the University of Tokyo, Japan) and his colleagues have measured with record accuracy the ratio of masses of the antiproton anti-p and the electron e- in a metastable antiprotonic helium atom anti-pHe+ which is the nucleus of ordinary helium with an electron on the ground orbit and the antiproton anti-p on the high Rydberg orbit. The lifetime of anti-p within anti-pHe+ is large enough for precise spectroscopic measurements, since the wave function of anti-p has almost no overlap with the nucleus while e- shields anti-p from destructive interactions with other atoms. The experiment was performed at CERN where the beams of anti-p to be trapped into anti-pHe+ were produced in the «Antiproton Decelerator». Hori et al studied two-photon transitions (n,l) → (n-2,l-2) as the gas of anti-pHe+ was irradiated by oppositely directed UV laser beams. The two-photon spectroscopy has allowed Hori to achieve high precision (2.3-5) × 10-9 in the measurement of spectral lines due to partial compensation of Doppler broadening. To calculate the ratio màíòè-p/me using the spectral data, theoretical QED calculations of the levels anti-pHe+ were used. The obtained result màíòè-p/me = 1836.1526736(23) agrees very well, within experimental errors, with the ratio of proton and electron masses which at present is known at a comparable accuracy. According to the CPT theorem, these ratios must be exactly equal. Source: Nature 475 484 (2011)

Quantum decoherence in photodetectors

Decoherence of the quantum states of different systems has already been studied in a number of experiments. V. D'Auria and her colleagues in the Laboratoire Kastler Brossel, Universite Pierre et Marie Curie (Paris, France) performed a new original experiment in which they investigated the decoherence not in a quantum state but in a detector through which the observation was conducted. Light was sent into a detector (avalanche photodiode) as an attenuated laser beam in which only a few photons were left in each pulse. External noise which caused decoherence was simulated by a second continuously working laser. Statistics of detector counts made it possible to find the evolution of the Wigner function which characterizes the distribution of quantum probabilities. Negative values of the Wigner function at low noise level were an indication of the quantum nature of the detector. When the noise level reached approximately half of the quantum efficiency of the detector, the Wigner function grew positive everywhere which corresponded to the decoherence of the detector and its transition to the semi-classical state. This study is important for designing devices processing quantum information as the decoherence of the detector may cause an undesirable decoherence of quantum states at the subsequent stages of information processing. Source: Phys. Rev. Lett. 107 050504 (2011)

Violation of the Wiedemann – Franz law in one-dimensional conductor

N.E. Hussey (University of Bristol, UK) and his colleagues found that the ratio of the Hall (transverse) thermal conductivity coefficient to that of the Hall electric conductivity in the metallic phase of the compound Li0.9Mo6O17 (characterized by quasi-one-dimensional crystal structure) increases with decreasing temperature; at 25 K the ratio κxyxy exceeds the value typical of conventional metals by a factor of 105. Such behavior differs greatly from the Wiedemann – Franz law which states that κ/σT≈ const; indeed, the heat and charge in the ordinary 3D-metals are transferred by the same quasiparticles. The inapplicability of the Wiedemann – Franz law to 1D-systems occurs, according to the Tomonaga – Luttinger theory, because heat is transferred in them by collective excitations both of spin (spinons) and of charge (holons) while charge is transferred by holons only. The separation of fluxes of quasiparticles and more efficient heat transfer are caused by much stronger scattering of holons by impurities in comparison with spinons. Owing to this factor the transfer of holons in 1D-systems is hindered. Source: Nature Communications 2 396 (2011)

Mosaic distribution of static charges

It is usually assumed that the electrification by friction of two different dielectrics causes their surfaces to acquire approximately uniform distribution of charges of opposite signs. Í.Ò. Baytekin (Northwestern University, USA) and his colleagues studied charge distribution on the surface of polymeric dielectrics (polycarbonates etc) and were able to show that in reality a mosaic pattern forms on the surfaces of these materials in which oppositely charged areas alternate in a random fashion. Measurements with an atomic force microscope in the surface potential mode (the Kelvin method) showed that statistically the mosaic distribution can be described by two random fields with mean fluctuations scales of 0.45 µm and 0.044 µm. In the past the static charge was understood to be only the charge with mean surface density ≈ 0.2 nC cm-2 averaged over the scale > 0.45 µm. This averaging results in compensation of charges of opposite signs; however, much larger alternating charges ≈ ± 1 µC cm-2 remain on the smaller scale. Experimenters noticed that such factors as duration of friction, or pressure applied during friction, or method of friction application, or inhomogeneity of the surfaces of dielectrics had no significant effect on the appearance of the mosaic. No mosaic distribution of opposite charges was found on the surface of samples of simple substances — silicon and aluminum — subjected to similar electrification. The mechanism of electrification of dielectrics is not yet completely understood. Í.Ò. Baytekin et al used confocal Raman and X-ray photoelectron spectroscopies to investigate the chemical properties of polymer surfaces after electrification and found that friction changed the configuration of the chemical bonds Ñ=O in molecules at the surface of the samples and that there was an exchange of matter between chemically dissimilar friction surfaces. These phenomena may be connected in some manner with the mosaic distribution of charges. Sources: Science 333 308 (2011), elementy.ru

Antiprotons in the Earth's radiation belts

Detailed theoretical calculations have shown that anti-p born in high-energy particle collisions of cosmic rays with the atmosphere must be captured by the Earth's magnetic field and form a wide antiproton radiation belt at an altitude of several hundred kilometers above the Earth surface. The basic process of production of anti-p are decays of anti-n generated by ðp collisions. The lifetime and the number of anti-p in the belt are constrained by their annihilation and scattering by other particles. The predicted belt of anti-p with energies of 60-750 MeV was first detected by the PAMELA detector placed on board the Russian satellite Resurs-DK1. At its orbit, PAMELA can observe the belt of atmospheric anti-p only when crossing the South Atlantic Magnetic Anomaly in which radiation belts dip close to the Earth. On the whole, 28 atmospheric anti-p were recorded during the observation period, which is three orders of magnitude higher than the amount that could be produced by the flux of galactic anti-p. The spectrum of galactic anti-p created outside the solar system in the collisions of cosmic rays with interstellar matter was also measured earlier by the PAMELA detector. Source: Astrophys. J. Lett. 737 L29 (2011)

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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.

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