Extracts from the Internet

Polyatomic molecules in an ultracold gas

1 March 2024

Polyatomic molecules in an ultracold gas are of interest for application in quantum information devices, in investigations into nonequilibrium dynamics, and as supersensitive sensors. However, some difficulties arise on the way of obtaining such molecules. For example, the use of Feshbach resonance [1] allowed creating weakly bound triatomic NaK₂ molecules at a temperature of 100 nK, but for larger molecules collisional losses destroy resonance. X-Y Chen (Max Planck Institute of Quantum Optics and Munich Center for Quantum Science and Technology, Germany) and their co-authors demonstrated a new method for obtaining tetratomic (NaK)₂ molecules in an ultracold gas through electro-association of smaller polar NaK molecules [2]. Microwave-induced scattering resonance was used to bind NaK pairs. The bond between the components in (NaK)₂ is much weaker than ordinary chemical bonds, but it acts over distances hundreds of times greater. About 10³ molecules of (NaK)₂ were obtained at a temperature of 134 nK, which is 3000 times lower than the temperature at which tetratomic molecules were obtained previously. And the lifetime of the molecules was ≈ 8 ms both in the free state and in the optical trap, which demonstrated their resistance to collisions. The measurements performed during dissociation of molecules showed anisotropy of their wave function, coincident with the expected anisotropy for the p-wave structure of molecules. [1] Pitaevskii L P Phys. Usp. 49 333 (2006); UFN 176 345 (2006) [2] Chen X-Y et al. Nature 626 283 (2024)

The second sound in a Fermi gas

1 March 2024

While in ordinary substances heat is transported by diffusion, in a superfluid liquid heat can propagate as waves in the form of a “second sound” generated by an out-of-phase motion of the normal and superfluid components. Although the second sound has already been recorded in experiments, its wavelike motion has not been directly observed. Z Yan and their co-authors (Massachusetts Institute of Technology, USA) demonstrated a new method of local temperature measurement in the Fermi gas and observed for the first time the second sound propagation in space [3]. The method is based on temperature sensitivity of the spectral response of molecules. Depending on the temperature of degenerate gas, the spectral maximum in the radio frequency range has different positions corresponding to different ratios of the number of fermion pairs to unpaired atoms. This effect allowed measuring local temperature and directly observing the wavelike heat propagation typical of the second sound. For the second sound in superfluid helium, see [4]. [3] Yan Z et al. Science 383 629 (2024) [4] Efimov V B Phys. Usp. 61 929 (2018); UFN 188 1025 (2018)

Magnetic reconnection and plasmoids

1 March 2024

Reconnection of magnetic field lines is an important factor in cosmic plasma and in laboratory experiments. Under reconnection, the field changes its topology and releases magnetic energy. In the contemporary theoretical description of this phenomenon, the classical Sweet-Parker model is supplemented with the formation of magnetic islands, called plasmoids. This causes enhancement of the rate of reconnection and energy release. J A Pearcy (Massachusetts Institute of Technology, USA) and their co-authors performed an experiment [5] to observe for the first time plasmoids in laser-driven plasma with a large thermal-to-magnetic pressure ratio β ≈ 10. A plastic foil was exposed to laser pulses, and when it evaporated, two intersecting plasma bubbles appeared. In the bubbles, a magnetic field was generated by the Biermann battery mechanism (due to shifted temperature and density gradients). In this magnetic field, magnetic reconnection occurred and was observed using proton radiography. Plasmoids predicted by the theoretical model were seen in the plasma structure. For magnetic reconnection, see [6, 7]. [5] Pearcy J A et al. Phys. Rev. Lett. 132 035101 (2024) [6] Ledentsov L S, Somov B V Phys. Usp. 58 107 (2015); UFN 185 113 (2015) [7] Zelenyi L M, Malova H V, Grigorenko E E, Popov V Yu Phys. Usp. 59 1057 (2016); UFN 186 1153 (2016)

Primordial black holes as dark matter

1 March 2024

13 microlensing events in the direction of Large Magellanic Cloud – a satellite of our Galaxy, have already been detected earlier by MACHO and EROS (EROS-2) collaborations. Microlensing is a gravitational focusing and star light amplification by objects on the line of sight. One of the hypotheses suggests that the discovered objects are primordial black holes (PBH), whose possible birth in the early Universe was grounded by Ya B Zel’dovich and I D Novikov [8] in 1966. However, calculations based on a flat curve of galactic rotation showed that the observed microlensing objects can amount to no more than ≈ 10 % of the total dark matter (DM) mass. Recently, observations of millions of stars with the Gaia telescope presented new data [9] that will possibly entail revision of the previous notions of the Galactic structure. According to these data, the total DM halo mass is several times less than previously thought, and the rotation curve is not flat, but bends downward at distances of more than 20 kpc from the center of the Galaxy. Based on the data obtained, J Garcia-Bellido (Autonomous University of Madrid, Spain) and M Hawkins (University of Edinburg, United Kingdom) constructed a 4-component model of the Galaxy including a bulge, a stellar disc, a gaseous and dark halo, and reconsidered the previous constraints on PBH [10]. The constraints in the new Galaxy model turned out to be significantly weakened, and the PBH, if responsible for microlensing, can even constitute all the DM in the Galaxy. An exception is the region of their masses near ≈ 0.01 M_☉, where PBH may be no more than ≈ 20 % or ≈ 12 % according to MACHO or EROS-2, respectively. If this conclusion is correct, then the status of PBH as a possible candidate for the role of DM is significantly strengthened. [8] Zel'dovich Ya B, Novikov I D Soviet Astronomy 10 602 (1967); Astronomicheskii Zhurnal 43 758 (1966) [9] Ou X et al. Mon. Not. Roy. Astron. Soc. 528 693 (2024) [10] Garcia-Bellido J, Hawkins M, arXiv:2402.00212 [astro-ph.GA]

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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.