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| Issue 11, 2024 |
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Extracts from the Internet | ||
Direct observation of violation of T invariance in a system of B mesons1 January 2013Direct measurements of violation of T invariance (invariance of processes under time reversal) for K mesons were previously performed at CERN and at the Fermi National Accelerator Laboratory but the results obtained were not free of considerable uncertainty. Violation of T invariance in a system of B mesons was earlier established only indirectly, via violation of CP invariance. Now it proved possible to measure the effect of violation of T invariance for B mesons directly, without resorting to the CPT-theorem, by using a new method of data analysis in the BŕBŕă experiment conducted at the National Accelerator Laboratory (SLAC). In the BŕBŕă experiment, decays of the resonances Υ(4S) created pairs of B0-anti-B0 in quantum-entangled states. Entanglement made it possible to compare the rates of the processes corresponding to different ordering of decays of B0 and ŕíňč-B0 in time, and also under permutation of the final states (of decay products). As a result, the violation of T invariance has been established with high statistical significance — 14 σ. The measured parameters that characterize the violation of the T invariance correspond to values previously derived from the effect of violation of CP-invariance. Source: Phys. Rev. Lett. 109 211801 (2012) Superconductivity in La2-xSrxCuO41 January 2013The team led by Ivan Bozovic of Brookhaven National Laboratory continued to work on the experiments described earlier (e.g. see Phys. Usp. 51 170 (2008)) and discovered that under certain conditions, a drop in temperature, instead of resulting in superconductivity, suppresses superconductivity in the compound La2-xSrxCuO4. A layer of La2-xSrxCuO4 was grown on a substrate by an improved molecular-beam epitaxial technique which allows controlling doping level x. Near the superconducting transition temperature, Bozovic et al observed superconducting fluctuations which normally precede superconductivity. Unexpectedly, superconducting fluctuations were suppressed in specimens with x = 0.055 - 0.06, plus they were completely absent at sufficiently high magnetic fields. As temperature was further reduced, suppression was enhanced, and superconductivity would not emerge. One cause of it could lie in structural defects which at low temperatures hamper electron flow (the effect of electron localization). As doping level increased, the suppression effect disappeared. For example, a specimen with x = 0.07 became superconducting when cooled to Tc = (9 ± 1) K. Source: Nature Materials 12 47 (2013) Acoustic analog of the dynamic Casimir effect1 January 2013C.I. Westbrook and colleagues at the C. Fabry Laboratory (Universite Paris-Sud, France) implemented an acoustic analogue of the dynamic Casimir effect which was first observed in 2011. In the dynamic Casimir effect, virtual particles transform into real ones due to rapid non-adiabatic changes in boundary conditions. In the experiment of C.I. Westbrook et al., researchers varied the potential of the optical trap holding a Bose – Einstein condensate of helium atoms, which led to a change in the speed of sound and in the spectral composition of oscillations in the condensate. The potential was varied by changing the intensity of the laser beams that formed the trap. In version I of the experiment, the potential was changed sharply once, while in version II it was subjected to 10% sinusoidal modulation during 25 ms until the trap was turned off and the condensation cloud broke apart. As a result of these changes, the thermal fluctuations in the condensate transformed into pairs of elementary excitations — quasiparticles moving in opposite directions at momenta of identical magnitude and frequency equal to one half of the modulation frequency. Excitations corresponded to lateral components in the velocity distribution of gas particles in the expanding cloud. These excitations satisfied the Bogoliubov – de Gennes dispersion relation both in the phonon mode when excitation consisted of several correlated atoms, and in the single atoms mode. The researchers are looking forward to fabricating an acoustic analogue of the Hawking radiation, by analogy to how in 2009 an experiment by J. Steinhauer et al. generated an acoustic analog of the black hole horizon. Source: Phys. Rev. Lett. 109 220401 (2012) The effect of light on the conductivity of insulators1 January 2013F. Krausz (Institute for Quantum Optics, Max Planck Society, Germany) and colleagues were able to demonstrate a method of ultrafast control of dielectric conductivity using high-power femtosecond pulses of NIR, comprising a mere several oscillations of the light wave. The conductivity of amorphous silica SiO2 exposed to these pulses increased over ≈ 1 fs by about 18 orders of magnitude and dropped back over the same time. The wave field with intensity of several volts per Angstrom substantially altered the electronic structure but nevertheless, this transition occurred reversibly, without destroying the atomic structure of the specimen. Conductivity measurements were made by spectroscopic methods and by recording the current across the electrodes. The observed properties are well explained by the theoretical model developed by V. Apalkov and Ě. Stockman. Even though the conductivity of semiconductors is much simpler to control than that of insulators, the changes caused in conductivity are much slower. In principle, the new effect offers the possibility of ultrafast control of electrical signals in promising devices operating in the THz and even PHz ranges (1015 Hz). Sources: Nature 493 70 (2013), Nature 493 75 (2013) Gamma-ray bursts caused by lightning1 January 2013Sometimes lightning discharges generate gamma-ray flashes several thousandths of a second long, known as terrestrial gamma-ray flashes. A GBM detector on board the Fermi Gamma-ray Space Telescope is currently recording approximately two gamma-ray flashes due to lightnings per week with time resolution of about 2 µs. It was assumed in the past that powerful radio bursts which are also generated by lightning discharges are not directly traceable to generation of gamma-rays. However, as follows from the new data collected with the GBM detector, gamma-ray flashes and some broad peaks in radio bursts in fact occur simultaneously and have similar pulse shapes. Consequently, these gamma and radio signals appear to be of the same origin, being generated in one and the same area of the electrical discharge. It is highly probable that “runaway electrons” whose theory was developed by A.V. Gurevich and his colleagues (P N Lebedev Physical Institute) are responsible for high-energy phenomena in the lightnings. Source: www.nasa.gov |
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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