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| Issue 4, 2024 |
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Extracts from the Internet | ||
Pressure-induced quenching of a superconductor1 August 2021Room-temperature superconductivity discovered recently in hydrides only exists under a giant pressure of 267 GPa. In this connection, a search for a possibility of pressure lowering with retention of superconductivity has become a topical problem. L. Deng (University of Houston, USA) and their co-authors carried out an experiment showing, on an example of another type superconductors – iron selenide (FeSe) single crystals, that the problem can be solved through pressure-induced quenching, i.e., an abrupt pressure removal at low temperature [1]. The superconducting transition temperature of FeSe is Tc≈9 K at atmospheric pressure and Tc=37 K at a pressure of several GPa. FeSe specimens were pressed in a diamond anvil to 4.15 GPa, cooled to 4.2 K, and then the pressure was abruptly removed. After such a quenching, the specimens retained Tc=37 K at atmospheric pressure for 7 days. The transition between orthorhombic and tetragonal structures of crystal lattice, when FeSe remains in the metastable phase after quenching, is likely to explain the quenching effect on Tc. The results obtained for FeSehold promise that pressure-induced quenching may also stabilize hydride superconductors. For high-temperature superconductors, see [2]. The laureate of 2003 Nobel Prize in physics V.L. Ginzburg referred creation of room-temperature superconductors to the most topical problems in physics [3]. [1] Deng L et al. PNAS 118 e2108938118 (2021) [2] Eremets M I, Drozdov A P Phys. Usp. 59 1154 (2016); UFN 186 1257 (2016) [3] Ginzburg V L Phys. Usp. 45 205 (2002); UFN 172 213 (2002) Townes solitons in two-dimensional systems1 August 2021Calculations show that in two-dimensional system solitons are only stable for a certain relation between the particle number and the value of the interaction constant. Such solitons, referred to as “Townes solitons”, have been actively studied in nonlinear optics. B. Bakkali-Hassani (Sorbonne University, France) and their co-authors have obtained and investigated Townes solitons in a 2D mixture of Bose-Einstein condensates [4]. Within a small region, nearly 10% of 87Rb condensate atoms in the state |F=1,mF=0⟩ was transferred, using laser, to the state |F=2,mF=0⟩. The time variation of the obtained soliton profile was traced. For the number 790 ± 40 of atoms, the soliton was most stable and scale invariant. In another experiment, C.-A. Chen and C.-L. Hung obtained a set of separate Townes solitons in cesium atomic gas in a 2D trap [5]. The normalized density profile coincided with the universal Townes soliton profile in a wide range of parameters, which confirmed the predicted scale invariance. For solitons in ultracold gases, see [6, 7]. [4] Bakkali-Hassani B et al. Phys. Rev. Lett. 127 023603 (2021) [5] Chen C-A and Hung C-L Phys. Rev. Lett. 127 023604 (2021) [6] Pitaevskii L P Phys. Usp. 59 1028 (2016); UFN 186 1127 (2016) [7] Kamchatnov A M Phys. Usp. 64 48 (2021); UFN 191 52 (2021) Doublons in quantum metamaterial1 August 2021I.E. Besedin (NUST MISiS and Russian Quantum Center at Skolkovo) and his co-authors investigated bound photon pairs (quasiparticles “doublons”) in quantum metamaterial, which was a one-dimensional array of 11 superconducting qubits [8]. Topological edge states of doublons were observed experimentally for the first time. The array was made of aluminum Josephson junctions on a silicon substrate via electron beam evaporation. The coupling between qubits was in turn weak and strong, which resulted in the appearance of two bands and topological edge states. With increasing pumping signal, first single-photon and then two-photon modes were excited in qubits. In both cases, photons were localized at the chain edges, which corresponds to topological edge states. For metamaterials, see [9-11]. [8] Besedin I S et al. Phys. Rev. B 103 224520 (2021) [9] Rybin M V, Limonov M F Phys. Usp. 62 823 (2019); UFN 189 881 (2019) [10] Davidovich M V Phys. Usp. 62 1173 (2019); UFN 189 1249 (2019) [11] Remnev M A, Klimov V V Phys. Usp. 61 157 (2018); UFN 188 169 (2018) Collective modes in an excitonic insulator1 August 2021The possibility of the existence of excitonic insulators was predicted in 1968 by L.V. Keldysh and A.N. Kozlov [12, 13], and also independently by two foreign groups of researchers. For excitons (bound states of electrons and holes), see [14, 15]. Some evidence of the occurrence of the state of an excitonic isolator, when an exciton condensate appears in a substance, has already been obtained in experiment, but these results have been ambiguous. A. Rao (Cavendish Laboratory, University of Cambridge, Great Britain) and their colleagues examined the compound Ta2NiSe5 and used the new method to show that an exciton condensate is likely to occur in it at room temperature [16]. The method of femtosecond spectroscopy with resolution of 10 fs and 10 nm was used. A laser pumping pulse illuminated a 400-nm spot on a Ta2NiSe5 plane. Wider test beams illuminated the region around the spot, and the transmitted radiation was registered by a CCD-camera. This made it possible to observe propagation of coherent oscillating waves, excited by a pumping pulse, to the region beyond the spot to a distance up to µm at a velocity of 1.5×105 m s−1. These waves are most likely to be the result of hybridization of phonon modes of crystal lattice and collective modes of exciton condensate, for the other known mechanisms fail to explain the observed picture. [12] Keldysh L V, Kozlov A N Sov. Phys. JETP 27 521 (1968) [13] Keldysh L V Phys. Usp. 60 1180 (2017); UFN 187 1273 (2017) [14] Durnev M V, Glazov M M Phys. Usp. 61 825 (2018); UFN 188 913 (2018) [15] Glazov M M, Suris R A Phys. Usp. 63 1051 (2020); UFN 190 1121 (2020) [16] Bretscher H M et al. Science Advances 7 eabd6147 (2021) PeV gamma-ray sources in the Galactic disc1 August 2021LHAASO observatory in China at an altitude of 4.4 km above sea level records extensive air showers (EAS) induced by cosmic ray particles and gamma-ray photons. Within less than a year of observations by the array of KM2A scintillation detectors in the composition of LHAASO, 530 photons with energy from 100 TeV to 1.4 PeV were detected. From clusterization of these photons, 12 gamma-ray sources in the Galactic disc plane [17] with angular dimensions up to 1° were revealed with ≥7σ significance. One of the sources coincides with the well-known Crab Nebula. The gamma-ray photon generation mechanism has not yet been found, but it implies the presence of high-power particle accelerators (“PeVatrons”). In the neighborhood of each source, several possible candidates exist, namely, pulsars, pulsar wind nebulae, supernova remnants, and young clusters of massive stars. However, the sources have not been identified, except for Crab Nebula. New theoretical models of sources may possibly be neeeded to explain the high photon energy. Russian researchers from INR RAS and MIPT are taking part in LHAASO collaboration. [17] Cao Z et al. Nature 594 33 (2021) [18] Lidvansky A S Phys. Usp. 61 921 (2018); UFN 188 1019 (2018) [19] Spiering Ch Phys. Usp., accepted |
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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