Extracts from the Internet

Quantum Szilard engine with attractively interacting bosons

Szilard engine is the thought experiment proposed in 1929 and illustrating the connection between information and work: the knowledge of the gas molecule position in a vessel is converted to the work done by them through sliding partitions into the vessel. However, the influence of the quantum effects and the interactions between particles on the properties of the Szilard engine remained largely unexplored. In their theoretical work, J. Bengtsson (Lund University, Sweden) et al. showed that a Szilard engine containing particles — bosons with attractive pair interaction increases the efficiency of information-to-work conversion compared to the case of noninteracting particles. The ab initio calculations were performed in the full quantum-mechanical many-body problem. Considered was the working cycle of the Szilard engine for N=1-25 particles, and the average work output was found to increase with increasing N. The temperature dependence of the work had maximum which was absent in the case of noninteracting particles. The presence of the maximum is explained by the fact that strong interparticle correlations occur at low temperatures, which shows up efficiently as a decrease in N. For the relation between dynamics and information see the paper of B.B. Kadomtsev in Phys. Usp. 37 425 (1994). Source: Phys. Rev. Lett. 120 100601 (2018)

A test of fluctuation theorems

The generalized fluctuation-dissipative relations (theorems) describing the evolution and statistics of thermodynamic nonequilibrium systems were formulated in the general form for both open and closed systems by G.N. Bochkov and Yu.E. Kuzovlev in 1977-1984 (for the history of the question see Phys. Usp. 54 625 (2011) and Phys. Usp. 56 590 (2013)). Later, various modifications and versions of these general results, in particular, the “differential fluctuation theorem”, were reported in papers by K. Yarzhinskii, G. Kruks et al. A group of researchers from USA and China were the first to verify experimentally the differential fluctuation theorem. The quartz particles were used that were kept aweigh by focused laser light. Pulses of another laser exerted force action on these particles in forward and backward direction, and the instantaneous positions and velocities of particles were fixed during their Brownian roaming in the phase space. The forward and backward processes were compared in which the particle velocities in final positions have different signs. The experiment showed perfect agreement with the predictions of the differential fluctuation theorem. The generalized Jarzynski relation and the Hummer-Szabo relation were verified separately. An important applicability region of fluctuation theorems lies in microscopic molecular systems in biology, chemistry and physics. Source: Phys. Rev. Lett. 120 080602 (2018)

Terahertz Landau level transitions in graphene

An active research of nonlinear optic effects (see the review by L.N. Ovander in Phys. Usp. 8 337 (1965)) began soon after the appearance in 1960 of optical lasers which were created several years after masers, i.e., microwave-range quantum generators (see the paper by N.G. Basov and A.M. Prokhorov in UFN 57 485 (1955) (in Russian)). In recent years, the development of technology of obtaining intense ultrashort laser pulses has significantly extended this research area. An interesting case of nonlinear optics is transitions between Landau levels for electrons in a magnetic field. However, nonlinearity is then possible only if the levels are non-equidistant, which is the case, in particular, in graphene. In the experiment of G. Yumoto (the University of Tokyo, Japan) with colleagues, a graphene layer grown by epitaxy on a substrate was placed in a superconducting magnet and was cooled to a temperature of 5 K. The sample was exposed to a linearly polarized THz pump laser beam, and the Faraday rotation of polarization of the second probe beam was analyzed. The nonlinear suppression of Faraday rotation was revealed as associated with transitions between the Landau levels. For the discovery of graphene see the Nobel lectures of A.K. Geim in Phys. Usp. 54 (12)(2011) and K.S. Novoselov in Phys. Usp. 54 (12) (2011). Source: Phys. Rev. Lett. 120 107401 (2018)

Rotons in a quantum gas

The concept of quasi-particles — rotons existing at a minimum energy and a finite momentum was introduced by L.D. Landau in 1941 to explain the properties of superfluid 4He. It was considered earlier that in quantum gases, as distinct from liquid helium, phonon modes alone are excited, while rotons do not emerge because of weak interatomic interactions. In 2003, however, the possibility was predicted of roton mode excitation in a gas in two cases: in a field of nonresonance laser radiation and for magnetic dipole-dipole interatomic interactions in Bose – Einstein condensate. Rotons have already been observed in the first case, while the second version, which had been predicted in the paper by L. Santos, G.V. Shlyaplikov and M. Lewenstein, was realized for the first time in the new experiment by F. Ferlaino (the Institute of Experimental Physics and the Institute for Quantum Optics and Quantum Information, Innsbruck, Austria) and his colleagues. The condensate of erbium 166Er atoms was in an elongated cylindrical optical trap (for Bose-Einstein condensates in a laser radiation field see the review of L.P. Pitaevskii in Phys. Usp. 49 333 (2006)). In an external magnetic field, atomic spins drew up in one direction and the scattering length of atoms and the character of their interaction were controlled by the Feishbach method. Rotons occurred through a combination of attractive and repulsive interactions of atoms depending on their mutual position. The absorption method was used to measure the momentum distribution of scattering atoms after the trap potential was off. Peaks corresponding to the roton mode were seen in the spectrum. The system was numerically simulated and the generalized Gross – Pitaevskii equation was solved. The results were well consistent with experiment. Source: Nature Physics, online publication of March 5, 2018

Magnon transistor

Nearly 70 years have passed since the invention of the electron bipolar transistor (see the paper of W. Shockley in UFN 64 155 (1958) (in Russian)). Magnonics, along with electronics, is considered to be one of the promising trends in technology, for more details see Phys. Usp. 58 1002 (2015). In particular, the possibility of designing a full-fledged magnon analogue of transistor was discussed. In 2014, researchers from the Technical University Kaiserslautern (Germany) have already demonstrated a prototype of the magnon transistor for the case of coherent low-frequency magnons, but for compatibility with electron devices it is desirable to design analogous setups using high-frequency thermal magnons. L.J. Cornelissen (the University of Groningen, Netherland) et al. were a success and demonstrated the method of magnon flux modulation in the yttrium – iron garnet mineral YIG with three platinum contacts. Magnons are quasi-particles transferring excitations in a system of interacting spins. They were generated by the Hall spin effect on one side of the mineral and were registered on its other side. The injection of additional magnons induced by electric current through the control contact affected the magnon conductivity. The magnon flux modulation efficiency was 1.6 %/mA at T=250° K. The devices operating on some other principles, that can be called magnon valves, were designed independently by H. Wu with colleagues at the Institute of Physics of the Chinese Academy of Sciences and by a German-Japanese group (J. Cramer et al.). In their approaches they used plane layers of magnetic substances, and the magnon flux across the layers was shown to depend strongly on their mutual orientation. Source: Phys. Rev. Lett. 120 097702 (2018)

Phonon quadrupole metamaterial

The history of the study of metamaterials begins with the paper by V.G. Veselago in Sov. Phys. Usp. 10 509 (1968) considering half a century ago the electrodynamics of substances with simultaneously negative values of electric permittivity and magnetic permeability. In recent years, such substances have been created artificially as arrays of elements with definite electromagnetic properties. M. Serra-Garcia (Swiss Federal Institute of Technology, Zurich) with colleagues created a new type of mechanical metamaterial. Similarly to how the electric charges on the surface of a polarized insulator are connected with its dipole moment, the dipole moments on the edges and the charges in the corners are connected with the quadrupole moment. In the above-described experiment, a mechanical analogue of quadrupole insulator was created on the basis of an array of silicon plates with gaps. A weak mechanical constraint between the plates was realized by a set of silicon rods and the role of charges was played by the ultrasound-induced elastic deformations of one or another direction. The vibrational spectrum was measured by laser interferometer. The system had a nontrivial set of vibrational modes, including vibrations on the edges and in the corners, resembling in their properties a topological insulator. Source: Nature 555 342 (2018)

The first stars in the Universe and a 21-cm absorption line

The observation of absorption in the 21-cm neutral-hydrogen radio line displaced towards the red region due to the cosmological expansion provides valuable information on the physics of the early Universe in the so-called “dark-century” epoch which set in after hydrogen recombination till its repeated ionization. The first stars and galaxies were born in this epoch. The absorption by hydrogen of UV photons emitted by stars inhabits the upper energy levels of atoms, connecting the spin and kinetic gas temperatures and thus affecting the absorption in the 21-cm line. J.D. Bowman (the University of Arizona, USA) with colleagues performed new exact observations of 21-cm line with two radio telescopes located in Australia. The absorption line profile agrees on the whole with the theoretical calculations allowing for radiation of the early stars, but the signal amplitude is much larger than expected. Independent observations are needed, however, to estimate reliability this result. This discrepancy, if it does exist, implies that in that epoch the gas was at the minimum twice as cool as it must be in conditions of adiabatic cooling. One of the possible explanations may be a hypothetic interaction between baryons and dark matter particles inducing an additional gas cooling. The low-frequency front of the observed profile of the 21-cm line indicates that 180 million years after the Big Bang (the red shift z ≈ 15) stars already existed which created the background of Lyman-α photons. These may have been the first stars in the Universe (referred to as stars of population III). For the development of the notion of the internal star structure and the sources of their energy see, in particular, the papers by A.S. Eddington in UFN 4 11 (1924) (in Russian) and UFN 6 273 (1926) (in Russian), and also the papers by Ya.K. Syrkin in UFN 8 675 (1928) (in Russian), A. Unsold in UFN 65 499 (1958) (in Russian) and UFN 96 393 (1968) (in Russian). Source: Nature 555 67 (2018)

Heating of the solar chromosphere

S.D.T. Grant (Quin’s University Belfast, UK) with colleagues were the first to observe the effect of solar chromosphere heating upon Alfven wave dissipation. It had long before been predicted theoretically that Alfven waves must rise from the visible surface of the Sun (photosphere) to the upper layers and dissipate there releasing heat and heating the chromosphere and corona. Although there was evidence of the existence of Alfven waves on the Sun, the effect of their dissipation has not been confirmed. In the new observations with the high-resolution solar telescope in New Mexico (USA) and with the use of instruments of the cosmic Solar Dynamics Observatory (SDO), the magnetic field distribution in the sunspot was examined and on the basis of observation of Doppler shifts in the calcium spectrum the picture of plasma velocities was reconstructed and the shock waves due to Alfven wave transformation to acoustic waves were revealed. This process leads to energy dissipation and chromosphere heating. In 1942, H. Alfven predicted the existence of the waves in plasma, called after him. For plasma physics and the structure of the Sun see the Nobel lecture of H. Alfven in UFN 104 529 (1971) (in Russian) and the paper of A.S. Monin in ÓÔÍ 23 594 (1980). Source: Nature Physics, online publication of March 5, 2018

The discovery of a planet by microlensing

Several thousand planets (exoplanets) around other stars have been discovered by the present time. Most of them have been found with the Kepler telescope by the transit method or through measuring radial velocities, but 65 exoplanets were revealed from observations of the microlensing effect, i.e., gravitational focusing of a distant star light. A group of researchers from Italy, Slovakia, and Russia (Sternberg Astronomical Institute, MSU) observed a bright event of microlensing with a possible participation of an exoplanet belonging to the “super-Earth” type (planets with masses of 1 to 10 Earth masses). Since the event was observed from the direction to the anticenter of the Galaxy where the lens concentration is very low, it is rare and exclusive. The star, the source of light, is located at a distance of 700-800 pc from the Earth. Its variability showed signs indicative of the presence on the line of sight of a gravitational binary lens consisting of a star with a mass of 0,25M and a planet with a mass of 9,2 ± 6,6 Earth masses moving along the orbit with a radius of 0.5 a.e. This system lies at a distance of only 380 pc from us (the nearest microlensing event). For gravitational microlensing see the paper of A.F. Zakharov and M.V. Sazhin in ÓÔÍ 41 945 (1998). Note also that the first exoplanet in another galaxy (in Andromeda Nebula) was first discovered with the help of microlensing in 2009 (see ÓÔÍ 54 1077 (2011)). Source: Mon. Not. Roy. Astron. Soc. 476 296 (2018)

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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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