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| Issue 11, 2024 |
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Extracts from the Internet | ||
Tetraneutron1 January 2022T. Faestermanna (Technical University of Munich, Germany) and their co-authors performed an experiment [1], where the bound state of four neutrons – a tetraneutron 4n was observed, perhaps, for the first time. It has been sought (together with 2n and 3n) since the 1960s, and some indications of the occurrence of events – candidates for 4n have been obtained. These results remained, however, ambiguous. In the new experiment, a beam of 7Li- ions obtained at the accelerator in Garching (Germany) was directed to the 7Li2O oxide target, sputtered onto the lithium foil, and the reactions 7Li(7Li,10C)4n have been analyzed. The energy spectrum of 10C nuclei escaping at angles of 6-9.5° was investigated using a wire proportional counter and an array of silicon detectors. With 3σ significance, a peak was revealed, which can be interpreted as a 10C nucleus in the first excited state and 4n with the binding energy of 0.42(16) MeV and the half-time of 450 s. An alternative hypothesis that the four escaping electrons are not bound cannot yet be excluded, but the authors of this study believe that this possibility is less probable because the observed peak in the spectrum is much narrower than it should be in this case. The reaction 7Li(7Li,10C)4n has already been examined earlier in Kurchatov Institute [2], but at higher energies, for which reason the presence of the bound state of 4n could not be found. [1] Faestermanna T et al. Phys. Lett. B 824 136799 (2022) [2] Aleksandrov D V et al. Pis’ma ZhETF 81 49 (2005); Pis’ma ZhETF 81 49 (2005) Quantum spin liquid in an artificial crystal1 January 2022The state of quantum spin liquid (QSL) predicted theoretically by F. Anderson in 1973 has already been observed in ordinary substances. G. Semeghini (Harvard University, USA) and their co-authors created an artificial two-dimensional crystal of 219 87Rb atoms held in an optical lattice and demonstrated the presence of QSL in it [3]. The lattice consisted of combination of triangles and hexagons, and the interaction of atoms could be controlled under Rydberg blockade. Thus, this system was a programmable quantum simulator, different versions of which have already been exploited to analyze quantum effects. The fluorescence visualization method was used to observe chains along which interactions between neighboring atoms occurred - dimer bonds appeared. A QSL state appeared as soon as the number of dimers became four times larger than that of monomers. This effect was predicted by R. Verresen, M.D. Lukin, and A. Vishwanath in 2020 [4]. QSL may turn out to be useful for creation of topological qubits. [3] Semeghini G et al. Science 374 1242 (2021) [4] Verresen R, Lukin M D, Vishwanath A Phys. Rev. X 11 031005 (2021) The study of FeSe/SrTiO31 January 2022The three atoms thick monolayer FeSe on a SrTiO3 substrate possesses some interesting properties: its superconducting transition temperature is five times higher than that of bulk samples and it has a record (among iron-based superconductors) superconducting gap width [5]. Replica bands in the photoemission spectrum, that can be due to superconductivity, are a not yet quite clear feature of the FeSe/SrTiO3 spectrum. Two models of replica bands have been proposed, namely, a forward scattering of 3-d iron electrons by phonons in SrTiO3 and the energy loss by photoelectrons in their interaction with surface phonons. C. Liu and their co-authors carried out new studies of SrTiO3 by the method of photoelectron emission with angular resolution and a polarized photon beam from a synchrotron source [6]. The choice of polarization direction allows one to single out and examine certain regions of electron bands of the substance and lessen the background effect from other regions. Higher-order replica bands were observed, and their relative amplitude was measured. The amplitude has a higher value than that assumed earlier and depends on orbital directions. The two above-mentioned models separately cannot fully explain these data. The replica bands possibly result from a combination of two or more mechanisms. [5] Liu C et al. Nature Comm. 12 4573 (2021) [6] Sadovskii M V Phys. Usp. 59 947 (2016); UFN 186 1035 (2016) Exciton propagation in thin semiconductor layers1 January 2022Semiconductor layers several atoms thick are important for application in nanoelectronics [7]. Their optoelectronic properties are largely determined by the behavior of excitons - bound systems of electrons and holes. But excitons are electrically neutral and cannot therefore be controlled by an electric field. The method for control over the exciton motion by forming mechanically deformed regions in the substance has recently begun to develop. A change in the exciton propagation under deformation typically causes their motion from low-strain to high-strain regions. R. Rosati (University of Marburg, Germany) and his co-authors examined excitons in WS2 and WSe2 monolayers using spatiotemporal photoluminescence observation [8]. Unexpectedly, it has turned out that the excitons observed in WS2 and WSe2 move in the direction opposite to expected, their velocity reaching a record value of 1µm/0.8 ns. The authors performed a detailed theoretical analysis to conclude that this is due to the presence in monolayers of a counterflow of “dark excitons” not observed directly but affecting the properties of bright excitons. The interaction between bright and dark excitons leads to a counter shift of their energy in a mechanically strained semiconductor, which changes the propagation direction. The control experiment with MoSe2 has confirmed this explanation: in MoSe2, bright excitons move towards the “correct” side because the energy levels of dark excitons in MoSe2 lie higher than the levels of bright excitons, and the interaction is weak. [7] Ratnikov P V, Silin A P Phys. Usp. 61 1139 (2018); UFN 188 1249 (2018) [8] Rosati R et al. Nature Comm. 12 7221 (2021) Non-Gaussian optomechanics1 January 2022The progress in experimental technology has recently allowed a study of quantum properties of optomechanical systems at the level of individual photons and phonons. In the new experiment with an optomechanical microcavity, G. Enzian (Empire College London, University of Oxford, and University of Copenhagen) and their co-authors observed non-Gaussian nonclassical distributions in the cavity phase space occurring with a change in the occupation numbers by one or several phonons [9]. The cavity was a BaF2 crystal with the resonant mechanical frequency equal to the difference of two neighboring optical frequencies. A pumping laser excited the resonator at the lower optical frequency at room temperature. Then, in an anti-Stokes process, excitation was transferred to higher frequencies and to sound waves. Heterodyne detection was used to characterize finite quantum states of the system. It has been found that with variation of the phonon occupation numbers the thermal distribution becomes non-Gaussian. Non-Gaussian processes are of importance in optomechanics for functioning supersensitive sensors [10]. [9] Enzian G et al. Phys. Rev. Lett. 127 243601 (2021) [10] Sukachev D D Phys. Usp. 64 1021 (2021); UFN 191 1077 (2021) Objective wave function collapse theories1 January 2022In quantum mechanics, a problem of measurement exists - the question of how the deterministic Schrodinger equation agrees with a random outcome of quantum state measurement (see, e.g., [11]). In Born and some other quantum mechanics interpretations, quantum randomness in measurements is thought to be one of the fundamental postulates. However, attempts are still being made to construct alternative theories in which the quantum measurement outcome is explained by some dynamic processes. These theories are referred to as objective collapse theories of a wave function. Such mechanisms typically need corrections to the Schrodinger equation, which makes verification of these theories possible in principle. Researchers from the University of Amsterdam (Netherlands) and the Institute of Theoretical Solid State Physics (Dresden, Germany) investigated the properties of the objective collapse theories to conclude that the quantum system evolution admitted by them must necessarily be nonlinear [12]. L. Mertens and their co-authors demonstrated this result on an example of a two-level quantum system. In particular, they formulated a minimal nonlinear theory, which reproduces the Born rules for quantum probability amplitudes. For some basic elements of quantum mechanics, see [13, 14]. [11] Kadomtsev B B Phys. Usp. 37 425 (1994); UFN 164 449 (1994); Kadomtsev B B, Kadomtsev M B Phys. Usp. 39 609 (1996); ; UFN 166 651 (1996); Kadomtsev B B Phys. Usp. 46 1183 (2003); UFN 173 1221 (2003) [12] Mertens L et al. Phys. Rev. A 104 052224 (2021) [13] Barley K et al. Phys. Usp. 65 (1) (2022); UFN 192 100 (2022) [14] Belinsky A V Phys. Usp. 63 1256 (2020); UFN 190 1335 (2020) Quantum time arrow1 January 2022Most of the fundamental physical laws remain unchanged under time reversal. In this connection, the question arises of how the nature chooses a correct direction of physical evolution [17]. One of the popular approaches is the thermodynamic time arrow, in which the direction of time is determined by the direction of entropy growth (an increase in disorder). However, for small systems with quantum properties, superposition of states is possible with increasing and decreasing entropy, which smears the concept of thermodynamic time arrow. G. Rubino, G. Manzano, and C. Brukner have shown [15] that despite the above-mentioned superposition, the concept of thermodynamic time arrow can be introduced at the quantum level provided that additionally measured is the dissipative work Wdiss=W-ΔF (usual work minus the difference of free energies) associated with a summary entropy production. Depending on the relation between this quantity and the temperature T, one or the other direction corresponding to the classical concept of the time arrow is chosen. One direction is chosen for Wdiss/(kBT)>>1 and the other direction for Wdiss/(kBT)<<-1. However, if Wdiss/(kBT)≈1, then the two directions interfere. In this case, no classical (not quantum) analogue of W fluctuations exists. [15] Rubino G, Manzano G, Brukner C Communications Physics 4 251 (2021) Quantum disagreement theorem1 January 2022In 1976, in the framework of the classical probability theory, R. Aumann proved a theorem (Aumann disagreement theorem) stating that under certain conditions two subjects cannot agree to disagree with each other. P. Contreras-Tejada (Institute of Mathematical Sciences, Spain) and their co-authors extended this theorem to the case of quantum events and proved it in two forms, namely, in the formulation resembling that of the classical theorem and assigning a sequence of estimations by subjects of common beliefs [16]. Thus, the Aumann disagreement theorem may be thought of a general physical principle valid in both the classical and quantum cases. This principle is of importance, in particular, in that it helps exclude quickly some theories aimed at generalizing quantum mechanics. [16] Contreras-Tejada P et al. Nature Comm. 12 7021 (2021) Galaxy without dark matter1 January 2022Ultradiffuse galaxies are relatively large galaxies with a low surface brightness. Observations have shown that at least some of them may have a small amount of dark matter - much less than the ordinary galaxies with the same summary stellar mass. Using a K. Jansky Very Large Antenna (VLA) array of radio telescopes, P.E. Mancera Pina (University of Groningen and the Netherlands Institute for Radio Astronomy) and their co-authors investigated the kinematics of another similar galaxy AGC 114905 [17] with a spatial resolution nearly 2.5 times higher than before. It has been established that the character of gas disc motion in the galaxy can be described by the gravitational field of only ordinary baryon matter without a necessary presence of dark matter in the halo. The origin of galaxies with a deficit or the absence of dark matter is still an enigma. Possibly, a collision of galaxies and a tidal gravitational stripping of layers had taken place [18]. In the central part of the galaxy that survived after the stripping, baryon matter may prevail. The tidal heating leading to a stellar orbit expansion has also been considered. [17] Mancera Pina P E, arXiv:2112.00017 [astro-ph.GA] [18] Ogiya G, arXiv:2111.12104 [astro-ph.GA] Black hole in a dwarf galaxy1 January 2022M.J. Bustamante-Rosell (University of Texas at Austin, USA) and her colleagues measured the black hole (BH) mass in the dwarf spheroidal galaxy Leo I in the local group of galaxies at a distance of 820 thousand light years from the Sun [19]. Both the earlier data and the results of new observations obtained at VIRUS-W spectrograph from the 2.7-meter telescope of the McDonald Observatory were used. The star velocity dispersion within the angular distance of 75” from the center makes up 11.76 ± 0.66 km s−1. This is indicative of the presence of a massive (3.3 ± 2) × 106M☉ BH in the galactic center, while the hypothesized absence of a BH is excluded at over 95 % significance. The BH mass exceeds the one expected for such a galaxy from extrapolation of the empiric relation for BH mass and galaxies by about two orders of magnitude. Why such a massive black hole resides in a dwarf galaxy is not yet clear. For supermassive black holes, see [20]. [19] Bustamante-Rosell M J et al. Astrophys. J. 921 107 (2021) [20] Cherepashchuk A M Phys. Usp. 59 702 (2016); UFN 186 778 (2016) |
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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