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| Issue 12, 2024 |
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Extracts from the Internet | ||
Proton size1 March 2013The charge radius rE and the Zemach radius of the proton were measured in atoms of the muonic hydrogen pμ- at the Paul Scherrer Institute (Switzerland). The nuclear size affects the energy levels of muonic atoms to a higher extent than that of ordinary ones as the muon is on average 186 times closer to the nucleus than the electron. The pμ- atoms in excited states were formed in collisions of the μ- beam with the H2 gas as a result of substitution of electrons with muons. The measured quantities were the Lamb shift and the hyperfine splitting of the levels. To achieve this, the laser pulses were applied to cause 2S-2Ð-resonance transitions of the 2Ð-1S x-ray photons were recorded. The rE radius was found to be rE = 0,84087(39) fm while the Zemach radius, defined in terms both of the electric charge distribution and the magnetic moment, was 1.082(37) fm. The new higher-accuracy experiment confirmed the discrepancy discovered in 2010 by the same collaboration in the measurements of rE of the ordinary and muonic hydrogen. The deviation from the CODATA values (Committee of Data for Science and Technology) from those consolidated over a number of experiments (based on ordinary hydrogen and e-p-scattering) comes to ≈ 7 σ. The cause of the discrepancy remains unclear. The effects beyond the Standard model are not excluded, provided the interactions between the proton and e- and μ- differ in some unknown respect. Source: Science 339 417 (2013) Analog of Josephson junction in the Bose – Einstein condensate1 March 2013Researchers of the National Institute of Standards and Technologies (NIST) and the Maryland University observed the effect of quantum phase slip in a ring of the Bose – Einstein condensate with revolving barrier. The Bose – Einstein condensate of ≈ 6 × 105 23Na atoms was kept in a toroidal optical trap. The condensate was initially at rest, but a barrier — a region of reduced density of condensate (a spot of weak link) — was rotating along the ring at a frequency of up to 3 GHz. The barrier was created by a laser beam driven by an acousto-optical deflector. The flux of atoms relative to the barrier was an analogue of the electron current in the superconducting ring of the SQUID. The measurements of state of the condensate were conducted at the stage of free fly-out of the condensate cloud after the trap potential has been turned off. Up to three consecutive phase slips were observed as the frequency of barrier rotation increased, that is, of transitions in the condensate between the quantum states of unequal angular momenta. Vortices due to the presence of the radial velocity gradient emerged in the condensate after the first such jump. This technique may lead to creating exceptionally sensitive gyroscopes with characteristics far above those based on superfluid helium. Source: Phys. Rev. Lett. 110 025302 (2013) Topological insulator SmB61 March 2013Three groups of researchers have established that cooling samarium hexaboride SmB6 to below 4 K converts it into a 3D topological insulator which in its bulk is a dielectric and on its surface a conductor. The role of topological effects in Kondo insulators (to which SmB6 belongs) was predicted by V. Galitsky and his colleagues in 2010. These effects take place in SmB6 by virtue of interaction between localized (pinned) and free electrons. The conducting properties of specimens were investigated in the experiment by S. Wolgast et al (Michigan University, USA) using eight electric contacts attached to it, four contacts placed on the opposite sides. This arrangement allowed S. Wolgast et al. to reliably establish that the specimen conductivity is of surface nature. The same conclusion was made in the experiment of the Californian University where the same conclusion was made by X. Zhang et al. (University of Maryland) on the basis of measuring the Hall effect resistivity, namely, where it was established that resistance is independent of specimen thickness. X. Zhang et al. applied the method of microcontact spectroscopy, and additionally confirmed that SmB6 is a topological insulator. Sources: arXiv:1211.5104 [cond-mat.str-el], arXiv:1211.6769 [cond-mat.str-el], arXiv:1211.5532 [cond-mat.str-el] NMR technique on nanoscale1 March 2013Two independent groups of researchers implemented the method of nuclear magnetic resonance for studying nanometer-scale specimens. NV-defects in a diamond crystal at room temperature were used as detectors. The NV defect is an atomic pair formed of the nitrogen atom which replaced the carbon atom, and the atomic vacancy, i.e. an empty crystal lattice site. Ò. Staudacher (Schtuttgardt University, Germany) et al. placed specimens of different compositions directly onto surface of diamond, with the NV-defect placed underneath it at a depth of ≈ 7 nm. The specimen placed in a magnetic field was irradiated by microwaves, and the variable magnetic fields generated in it induced transitions between spin states in the NV-defect. The fluorescent photons studied immediately after this were recorded by a photodiode. The NV-defect thereby responded to spin precession of ≈ 104 protons in a volume of ≈ 5 nm3. A similar method was used in an experiment by H.J. Mamin (The IBM research Center at San-Jose, USA) et al. but the organic specimen was now at some distance from diamond surface and the NV-defect recorded a signal from a larger volume. The researchers hope that in the future this method will yield a 3D image of nanoparticles; this would be important for progress in nanotechnologies. In some ways the NMR method is simpler than using the force resonance microscope, since the NMR does not require cooling to cryogenic temperatures. Sources: Science 339 557 (2013), Science 339 561 (2013) Mass ejection from supernova1 March 2013The Palomar Transient Factory Wide-Field Survey discovered that 37 days prior to the supernova explosion SN 2010mc, the pre-supernova star outburst took place, radiating energy of 6 × 1047 erg. The width of spectral lines pointed to the speed of the emitting gas on the order of 2000 km s-1. A possible explanation of this event is the loss of mass of ≈ 0.01M☉. There probably was causal connection between the mass-loss outburst event and the supernova explosion as the probability of accidental coincidence of such events over the length of 37 days evaluates to only 0.1%. Such outbursts preceding the main explosion were predicted by the theory of stellar evolution. The photometric and spectroscopic data of the observed outbursts agree well with the wave model. In this model, powerful convective motions developed shortly before the collapse reaching the surface of the star and creating matter loss due to energy dissipation; it is not possible to explain the high outburst velocity and high mass loss using alternative models (Thorne – Zytkow objects or pair-instability supernovas). Source: Nature 494 65 (2013) |
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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