Extracts from the Internet

Electron’s electric dipole moment

The Standard Model of elementary particles predicts the existence of an electric dipole moment de≈ 10−40e cm of an electron, but the effects beyond the Standard Model might strengthen it by many orders of magnitude. The ACME collaboration experiments on a spectroscopic study of neutral molecular beams gave the bound de < 9.4×10−29e cm. Researchers from the National Institute of Standards and Technology and the University of Colorado Boulder (USA) carried out a new experiment measuring de using molecular 180Hf19F+ ions in atrap with a rotating electric field. During ≈ 700 ms, while ions were in the trap, the spins of the external electrons in the molecules precessed and the precession angle might contain a contribution from the interaction of de with the internal electric field of the molecule. The precession angle was measured by molecule dissociation and registration of ions. At the current precision angle no nonzero de was revealed, and the bound de<1.3×10−28e cm was obtained. Although this bound is no stronger than that obtained earlier by ACME, the work is important for the fact that the ACME result has been confirmed using the new experimental method and another physical system. The existence of the electron’s dipole moment might be due to the asymmetry under time reversal and baryon asymmetry generation in the early Universe. Source: Phys. Rev. Lett. 119 153001 (2017)

Band gap control in an excitonicinsulator

As was shown theoretically by L.V. Keldysh and Yu.V. Kopaev in 1964, Bose-Einstein condensation of excitons (bound states of electrons and holes) leading to the formation of a stable phase of the “excitonic insulator” may take place in semiconductors with a narrow band gap. Later, this phase was actually revealed in some compounds, including the layered semiconductor Ta2NiSe5. In their new experiment, S. Mor (Fritz Haber Institute of the Max Planck Society, Germany) et al. investigated ultrafast nonequilibrium dynamics of the Ta2NiSe5 electronic structure exposed to laser pulses of the near IR range using the method of time-and angle-resolved photoelectron spectroscopy. It was established that with decreasing surface density FC of radiation below 0.2 mJ cm-2 the band gap sharply narrows, while with increasing FC it is enhanced. This behavior is opposite to that typical of conventional semiconductors. To clarify the mechanism of this phenomenon the theoretical Hartree-Fock calculations were carried out to show that the key role is played here by the increase of the order parameter (density) of the excitonic condensate. For the coherent states of the excitons see the article of L.V. Keldysh in the present issue of Physics Uspekhi. Source: Phys. Rev. Lett. 119 086401 (2017)

Ionization by phase-locked pulses

D.B. Foote (University of Maryland, USA) with colleagues investigated multiphoton ionization of Xe atoms by double phase-locked laser pulses. Double pulses were obtained from single pulses using liquid-crystal modulator or through pulse separation in an interferometer. Measurement of the number of Xe+ ions produced under the effect of double pulses allowed the establishment of the dependence of ionization efficiency on the shape and mutual position of pulses. In particular, even a small (about ≈ 10 %) time overlap of two pulses turned out to result in considerable changes in the ion yield due to optical interference that changes the composite-pulse intensity. A small contribution from quantum interference of wave functions of excited electrons is also present. Ionization in the field of a strong electromagnetic wave was considered in the theoretical paper by L.V. Keldysh in Soviet Physics JETP 20 1307 (1965). Source: Phys. Rev. A 96 023425 (2017)

Quantum-electrodynamic cascades with atom ionization

In the near future, lasers up to 10 PW are expected to appear in the radiation field of which quantum-electrodynamic cascades may develop (see Physics-Uspekhi 58 95 (2015)). In such cascades, an avalanche-like chain production of photons and electron-positron pairs takes place. High-intensity laser fields are needed for # production in vacuum. But cascades can develop in weaker fields when the sources of initial electrons are atoms under ionization. I.I. Artemenko and I.Yu. Kostyukov, researchers of the Institute of Applied Physics of RAS (Nizhny Novgorod) studied theoretically the quantum-electrodynamic cascades with heavy atom ionization in the field of two counterpropagating laser pulses. As compared to the previous simplified models, ionization of not only from external but also from internal electron levels was taken into account, and for the ionization rate a convenient formula was obtained describing both the weak intensity regime and the regime of extremely strong lasing intensity. The character of atom ionization for different values of ionization potential, the frequency and intensity of radiation is determined by the Keldysh parameter γK. The Monte Carlo simulation allowed calculation of the distribution and spectrum of cascade electrons and photons. An important factor in cascade development is that most electronswere expelled by ponderomotive forces from the region of the strongest lasing, but the small number of remaining electrons go on maintaining the cascade. The study of quantum-electrodynamic cascades is of importance for a number of promising laser technologies, including photonuclear reactions and laser-plasma particle acceleration. Source: Phys. Rev. A 96 032106 (2017)

Phase-free propagation in a waveguide

If the refractive index of a substance is n → 0, the electromagnetic wave in the samplehas a length λ → ∞ and its phase along the entire sample remains unchanged. The phase-free wave propagation has already been demonstrated in a number of systems. To develop this technology E. Mazur (Harvard University, USA) and his colleagues created a metamaterial-based silicon waveguide which supports the phase-free propagation, the new technology being compatible with conventional telecom devices. The waveguide is a plate with half-round cuts on the platform filled with a array of holes. Owing to the presence of simultaneously electric and magnetic dipole resonances the refractive index n goes through zero at λ=1625 nm. With n approaching zero, the wave preserves the finite group velocity and can transfer energy. The high wave frequency hampers a direct observation of its phase-free propagation, and therefore an IR camera observed beatings due to interference of two counterpropagating waves. The beatings were coherent through the entire waveguide. Source: ACS Photonics 4 2385 (2017)

Generation of terahertz radiation in liquid water

Electromagnetic terahertz (THz) radiation attracts much attention owing to the possibility to nondestructively radiograph many materials. One of the promising methods to generate terahertz radiation is an optico-terahertz transformation under the effect of laser light on a substance. Q. Jin (the University of Rochester (USA) and Huang Zhong University of Science and Technology (China)) et al. were the first to experimentally demonstrate the generation of broadband terahertz signals from liquid water under the action of femtosecond laser pulses with a repetition rate of 1 kHz which were focused by a parabolic mirror inside the water film ≈ 180 µm thick. The film moved at a velocity of 1.3 m s-1 and was held stationary between two aluminum wires owing to the water surface tension. The use of a thin film allows the radiation go outside without being absorbed. A strong dependence of the generated radiation was revealed on the lasing polarization direction relative to the film plane and on laser pulse duration. A probable mechanism of generation is multiphoton and cascade ionization of molecules and plasma oscillations. For other terahertz radiation sources see Physics-Uspekhi 54 837 (2011) and Physics-Uspekhi 59 595 (2016). For high-sensitivity terahertz radiation receivers see Physics-Uspekhi 49 955 (2006) and Physics-Uspekhi 57 959 (2014). Source: Appl. Phys. Lett. 111 071103 (2017)

Water motion under lasing

Since photons transfer pulses, the effect of light on gases and liquids may induce hydrodynamic flows, which has already been demonstrated in experiment. In particular, the flow was observed near the water surface because of its deformation. Y. Wang (The University of Electronic Science and Technology of China and the University of Houston (USA)) with colleagues discovered a new effect when pulsed laser radiation generates steady-state flows in a pure water volume. At first the volume was filled with aqueous suspension of gold nanoparticles and in several minutes a hydrodynamic flow appeared in the direction of the beam. The flow was observed by the reflection of light of another laser from polymeric microspheres in water that served as markers. The hydrophone registered the occurrence of flow under the effect of ultrasonic waves due to a sharp thermal expansion of gold nanoparticles heated by laser pulses. It is of importance that nanoparticles in a water volume were only needed for the emergence of the flow which remained after the suspension was replaced by pure water. The nanoparticles incorporated into the glass turned out to be responsible for the flow of pure water. On the inner surface of the vessel, microfunnels formed by the nanoparticles were found and examined by an electron microscope. The pure water flow was maintained for about an hour while the nanoparticles were being washed out of the funnels. Source: Science Advances 3 e1700555 (2017)

New LIGO/Vigro results

The gravitational-wave interferometers LIGO/Vigro registered for the first time the gravitational burst GW170814 by three detectors and the burst GW170814 associated with the gamma-ray burst GRB 170817A. Along with the observations of gravitational waves by two LIGO detectors located in USA the Virgo detector located in Italy has been used since August 1, 2017. On August 14, 2017 three detectors registered the gravitational-wave burst GW170814. Its characteristics correspond to the merge of two black holes with masses of 30.5M and 25.3M. The direction to the source is determined better by an order of magnitude by the data from three than two detectors. The gravitational wave polarization was also determined for the first time and the prediction of the General Relativity concerning the tensor character of polarization was confirmed, whereas the purely scalar and purely vector versions were excluded. On August 17, 2017 the LIGO/Vigro registered the burst GW170817 from whose localization region a short gamma-ray burst GRB 170817A was registered in 1.74 ± 0.05 s by the Fermi-GBM telescope. The masses of the merging objects range from 1.17M to 1.60M, which corresponds to the neutron star masses. Thus, the merge of neutron stars in a binary system was observed for the first time with the aid of gravitational waves and it was proved that such merges can induce short gamma-ray bursts. The velocity of gravitational-wave propagation coincided with the velocity of light with a relative precision of ≈ 10−15, which also confirms the General Relativity and limits the parameters of a number of models of cosmological dark energy. Several hours after the signal GW170817 several telescopes registered optical radiation from the same direction. The optical signal was also observed by the Russian global robotized net of telescopes MASTER designed under the guidance of V.M. Lipunov (Sternberg Astronomical Institute, MSU) (see Physics-Uspekhi 59 918 (2016)). The optical radiation source lies in galaxy NGC 4993 at a distance of 2 kpc from its center. The radiation was also registered in the X-ray, UV, and radiofrequency ranges. The properties of the signals are well consistent with the predictions of the “kilonova” model. In this model, the optical radiation is due to radioactive decays of heavy nuclei produced in the course of nucleosynthesis under the merge of neutron stars. For the calculation of the generation rate of gravitational signals from the merge of neutron stars see Physics-Uspekhi 44 1 (2001), for the history of development of gravitational wave detectors see Physics-Uspekhi 59 879 (2016), and on the importance of gravitational-wave generation see Physics-Uspekhi 57 342 (2014), Physics-Uspekhi 59 910 (2016), Physics-Uspekhi 59 918 (2016), and Physics-Uspekhi 60 823 (2017). Sources: Phys. Rev. Lett. 119 141101 (2017), Phys. Rev. Lett. 119 161101 (2017), Astrophys. J. Lett. 848 L12 (2017), arXiv:1710.05461 [astro-ph.HE]

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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.

Permanent editor is Yu.N. Eroshenko.

It is compiled from a multitude of Internet sources.

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