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| Issue 11, 2024 |
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Extracts from the Internet | ||
Direct measurement of the magnetic moment of the proton1 July 2014A. Mooser and his colleagues (University of Mainz, Germany) measured the magnetic moment μp of a proton to a record precision for today. To this time, the most precise measurements were taken by an indirect method, i.e., by analyzing the hydrogen maser spectra in a magnetic field. In the new experiment, a double Penning trap was used. The spin state of the proton was measured in a region with magnetic inhomogeneity, and then the proton was moved to the part of the trap with a homogeneous magnetic field, where the cyclotron frequency of proton rotation round the trap axis and the Larmor frequency of precession under the effect of the radio-frequency field were measured. The ratio of these frequencies contains information about μp. A similar method was already used earlier to measure the magnetic moments of electrons and positrons. The measured μp value makes up μp = 2.792847350(9)μN, where μN = eh/(4πmp) is a nuclear magneton. The precision of this result is 760 times higher than the precision of the earlier direct measurements in double Penning traps and thrice as high as that in the indirect method. When such high-precision measurements are performed with antiprotons, it will become possible to check another time the CPT theorem which implies equality of the magnetic moments of particles and their antiparticles. Source: Nature 509 596 (2014) Verification of the equivalence principle using atomic interferometer1 July 2014D. Schlippert (Wilhelm Leibniz University, Germany) and his colleagues were the first to compare the free-fall time of atoms of two different elements, 39K and 87Rb, in an atomic interferometer and to confirm the equivalence principle (universality of gravitational acceleration) to an accuracy of ≈ 10-7. Earlier, such measurements were only taken for different isotopes of one element, but for verification of the Einstein equivalence principle the difference in the composition of nuclei is of paramount importance. In the new experiment, the beams of 39K and 87Rb atoms began simultaneously falling in a magneto-optical trap. Upon laser light scattering, each of the beams split into two interfering beams. From the interference pattern observed by fluorescent radiation of atoms, a gravitational phase shift was obtained which depends on the free-fall accelerations gK and gRb of 39K and 87Rb atoms, respectively. The Eötvös parameter η = 2(gRb-gK)/(gRb+gK) is restricted to the level η = (0.3 ± 5.4) × 10-7. The measurement data serve for the verification of some theories which predict violation of Einstein’s equivalence principle. In atomic interferometers, quantum effects are examined, and hence in spite of a lower accuracy such experiments are an important supplement to the classical measurements based on torsion pendulums. Source: Phys. Rev. Lett. 112 203002 (2014) Superradiation in Bose – Einstein condensate1 July 2014P. Engels (Washington State University, USA) with colleagues demonstrated in the experiment with Bose – Einstein condensate an analogue of the superradiation effect predicted by R. Dicke in 1954. In the original form this effect consists in the fact that an ensemble of atoms having two energy levels and interacting with each other by means of the radiation field can undergo collective spontaneous transitions and radiate coherently (see the review in Sov. Phys. Usp. 23 493 (1980)). The superradiation effect has recently been observed in an experiment with Bose – Einstein condensate in the optical cavity. C. Zhang predicted theoretically that a similar superradiation effect takes place in the case of spin-orbital interaction in an external potential. P. Engels with colleagues realized this version in the Bose – Einstein condensate of 87Rb atoms. In their experiment, the relation between the two spin states and the states of atomic motion in a trap was established with the help of Raman lasers by the “Raman dressing” method. A cloud of atoms was observed by its light absorption at the stage of free expansion after magnetic spin separation (like in the Stern-Gerlach experiment). By measuring different cloud characteristics, including its quadrupole oscillations, it was confirmed that on a change in the value of Raman coupling the condensate actually experienced phase transition to the state of superradiation in accordance with the R. Dicke theory. The behavior of the atomic cloud is described by the Gross – Pitaevskii equation, and comparison of the experimental data with calculations showed excellent agreement. Source: Nature Communications 5 4023 (2014) Plasmon control in graphene1 July 2014P. Alonso-Gonzalez (the joint Center for Research in Nanoscience, Spain) et al. demonstrated a new method of directed plasmon generation and control in graphene using dipole resonance antennas nearly 3µm long. Plasmons in graphene (surface plasmon-polaritons) are bound states of photons and charges. The antennas, which were made of gold and had contact with graphene, absorbed photons of polarized light and generated a near optical field under the effect of which plasmons were born in graphene with a wavelength several times smaller than the antenna size. The use of such antennas for plasmon excitation is much simpler than the near-field microscope employed earlier for the same purpose. Refraction of plasmons upon their passage through a conduction inhomogeneity was investigated. This effect promises the possibility in principle to control plasmons. In future optoelectronics, plasmons may become a connecting link between optical and electron signals. For optical nanoantennas see Phys. Usp. 56 539 (2013). Source: Science 344 1369 (2014) Magnetic field near supermassive black holes1 July 2014On the basis of systematic radio observations of 76 galaxies by VLBA radio telescope, researchers from the Berkeley National Laboratory (USA) and the Max Planck Institute for Astronomy (Germany) confirmed that the magnetic fields near supermassive black holes in galactic nuclei are so strong that the magnetic field strength acting on plasma is comparable to the gravitational attraction of black holes. Thus, the magnetic field can be an important or even decisive factor of accreting plasma dynamics in galactic nuclei. To all appearances, the magnetic fields are responsible for the formation of relativistic jets emanating from the centers of active galaxies, and therefore the magnetic field studies can proceed from the character of jet radio emission. In the described work, the calculations of the magnetic field strength were based on the data on the position of features in the map of jet radio emission at different frequencies. The results obtained agree well with the numerical accretion models worked out by A. Tchekhovskoy (Berkeley Lab) and his colleagues. Source: Berkeley Lab News Center |
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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