Excitons in van der Waals heterostructures based on atomically thin transition metal dichalcogenides are considered as potential candidates for the formation of a superfluid state in two-dimensional systems. A number of studies reported observations of ultrafast nondiffusive propagation of excitons in van der Waals heterostructures, which was considered by their authors as possible evidence of collective effects in excitonic systems. In this paper, after a brief analysis of exciton propagation regimes in two-dimensional semiconductors, an alternative model of ultrafast exciton transport is proposed, based on the formation of waveguide modes in van der Waals heterostructures and the radiation transfer by these modes.

The optical properties of semiconductors and nanoheterostructures based on them are determined near the fundamental absorption edge by electron–hole complexes such as excitons and charged three-particle complexes, aka trions. We present the results of theoretical studies of the structure and binding energies of localized excitons and trions in nanosystems within the variational approach. This approach is applicable to a wide range of semiconducting systems, from quantum wells, wires, and dots based on classical group III–V and II–VI semiconductors to van der Waals heterostructures made of monolayers of transition-metal dichalcogenides. We also discuss many-particle effects in structures containing resident charge carriers. Our treatment of theoretical approaches is accompanied by a discussion of extensive experimental results available in the literature.

Олег Николаевич Крохин (к 90-летию со дня рождения), Багаев С.Н., Гаранин С.Г., Колачевский Н.Н., Конов В.И., Кульчин Ю.Н., Панченко В.Я., Попов Ю.М., Рыкованов Г.Н., Сергеев А.М., Сурис Р.А., Шалагин А.М., Щербаков И.А.

corresponding member of the Russian Academy of Sciences (RAS), and chief researcher at the A F Ioffe Institute of Physics and Technology (IPT) RAS, passed away on December 19, 2021. Vadim L'vovich was born on June 4, 1934 in Leningrad. He was the son of the well-known physicist Lev Emmanuilovich Gurevich. In 1956, V L Gurevich graduated from the Physical Department of Leningrad State University and began working at the Institute of Semiconductors of the USSR Academy of Sciences (ISAS). In 1960, he defended his candidate thesis and, in 1965, his doctoral thesis; in 1971, he was conferred the rank of professor. The same year, Vadim L'vovich organized the Sector of Physical Kinetics at ISAS. After the ISAS and the A F Ioffe Physical Technical Institute united in 1972, this sector moved over to PTI, where V L Gurevich had been working till the end of his life. His most well-known achievements in theoretical physics (together with Yu A Firsov) include the prediction in 1961 of magnetophonon resonanceÐa new type of oscillation of a conductor's kinetic coefficients in a strong magnetic field due to resonant energy absorption upon the transition of charge carriers between Landau levels with the participation of longitudinal optical phonons. As a result, specific magnetophonon oscillations of magnetic resonance, periodic in magnitude, inverse magnetic field, were observed in some semiconductors. This important phenomenon was found in a whole number of materials and underlay the method of magnetophonon spectroscopy of electrons and phonons. Studies of magnetophonon resonance in semiconductors, carried out together with experimentalists, were reported by A V Gurevich in 1964±1965 at academician P L Kapitza's seminars at the Institute of Physical Problems, at FIAN, and at a section of the Division of General and Applied Physics of the USSR Academy of Sciences. In recent years, studies of magnetophonon oscillations have been concentrated on twodimensional nanostructures and other low-dimensional systems, e.g., graphene. Together with V G Skobov and Yu A Firsov, Vadim L'vovich predicted giant ultrasound absorption oscillations in metals in a magnetic field and developed a theory of this effect. This interesting and at first glance unexpected effect turned out also to be important for a certain type of electromagnetic waveÐso-called helicons. Well known are studies by Vadim L'vovich and his coauthors on the theory of ultrasound amplification by the drift of charge carriers in semiconductors. In these works, he formulated a consistent linear and nonlinear theory of the amplification effect in different regimes. For a series of studies on the development of theoretical bases of acoustoelectronics, V L Gurevich (along with I A Viktorov, Yu V Gulyaev, and V I Pustovoit) was awarded the 1974 State Prize of the USSR. The abovementioned studies do not at all exhaust the scientific achievements of V L Gurevich. Studies on theory fluctuations, the theory of dielectric relaxation, the theory of low-temperature properties of dielectric and metal glasses, superconductor kinetics, and others should also be mentioned. In a series of studies, V L Gurevich and his disciples S V Gantsevich and R V Katilyus constructed a consistent theory of fluctuations near a nonequilibrium but stationary state. A specific correlation of fluctuations due to electronelectron interaction was examined. An important role was played by V L Gurevich's works conducted together with his disciple A K Tagantsev on the dielectric relaxation theory, developing the theory of highfrequency losses in dielectric (and ferroelectric) crystals. A systematic analysis for crystals of different symmetries is presented in the frequently cited review in the journal Advances in Physics and also in V L Gurevich's monograph, Uspekhi Fizicheskikh Nauk 192 (2) 229 ± 230 (2022) Translated by M V Tsaplina PERSONALIA PACS number: 01.60.+q

Вадим Вениаминович Бражкин (к 60-летию со дня рождения), Арсеев П.И., Виноградов Е.А., Кведер В.В., Литасов К.Д., Муртазаев А.К., Пудалов В.М., Рыжов В.Н., Садовский М.В., Стрельцов С.В., Сурис Р.А., Суровцев Н.В., Щербаков И.А.

Памяти Николая Николаевича Сибельдина, Арсеев П.И., Горбацевич А.А., Демихов Е.И., Кведер В.В., Колачевский Н.Н., Красильник З.Ф., Крохин О.Н., Кукушкин И.В., Месяц Г.А., Сурис Р.А., Тимофеев В.Б., Щербаков И.А.

physical-mathematical sciences, professor, corresponding member of the Russian Academy of Sciences (RAS), Nikolay Nikolaevich Rosanov, an outstanding world-renowned scientist in the field of laser physics and nonlinear optics. N N Rosanov was born on December 26, 1940 into the family of hydraulic engineers Nikolay Semenovich and Marianna Vladimirovna in Leningrad, where they stayed throughout the blockade. In 1948, Nikolay went to School No. 181 (before the revolution, it was gymnasium No. 3, to which S Ya Marshak, D S Merezhkovskii, D I Pisarev, and I I Sollertinskii would go). In his latter school days, Nikolay took part in a math club at the Palace of Pioneers on Nevskii Prospect. In his school days, he found the area of an ellipse (without using integrals) by comparing the areas of two cross sections of a cylinder cut obliquely to the axis and orthogonally. He distinguished himself at the city's physics & math olympics. In 1958, NNRosanov entered the Physics Department of Leningrad State University with a specialization in the Department of Theoretical Physics. He was lucky to have good teachersÐhe attended academician V A Fock's lectures on quantum mechanics and V I Smirnov's on mathematics, and seminars run by O A Ladyzhenskaya, later an academician at the USSR Academy of Sciences. In 1963, N N Rosanov was employed at the S I Vavilov State Optical Institute (SOI), where the first Russian laser had been put into operation some time before, in June 1961. Since 2019, he has been working at A F Ioffe Physical-Technical Institute of RAS. N N Rosanov did his first scientific work under the guidance of V A Fock's disciple A V Tulub, who suggested that he explain the experiment by American authors on the Zeeman effect in a gas laser. The results were submitted by academician AALebedev for publication in the journal Doklady Akademii Nauk SSSR (Soviet Physics ë Doklady in English translation that time) in 1965. Studies of the theory of gas lasers continued. N N Rosanov managed to énd an original solution to the important problem of frequency locking in laser gyroscopes, which ruled out the possibility of a traditional determination of angular velocity: it was proposed that the dependence of phase difference of counter waves on the angular velocity of gyroscope rotation be used for this purpose in the entrapment region. In coauthorship with G N Vinokurov, he developed a theory of transverse modes interaction in a gas laser. N N Rosanov is one of the pioneers of the theory of nonlinear laser dynamics. In the 1970s, long before the frequently cited paper by Lang and Kobayashi appeared, N N Rosanov derived the dynamic equations of a solid-state laser with frequency dispersion, including a laser setup with an additional mirror to show that the delay of optical feedback is one of the laser pulsation mechanisms. Another important problem solved by N N Rosanov together with V A Smirnov was a theory of small-scale self-focusing in multielement laser systems. N N Rosanov supervised the theoretical part of the research, performed at SOI, on a number of applied projects related, in particular, to the development of high-power gas lasers and to the propagation of their radiation through the atmosphere, including the use of adaptive optics. Among the more `academic' results N N Rosanov obtained were new relativistic optical effects in a medium with a nonuniform velocity distribution, as well as a rigorous proof of the impossibility of ideal invisibility even in the case of monochromatic radiation. A theory of particles, waves, and solitons in dynamic resonators with oscillating walls was also formulated. N N Rosanov carried out pioneering work on nonlinear optical effects in an electron-positron vacuumÐa subject that has recently become exceedingly topical owing to achievements in laser physics and technology. An important field initiated and developed by N N Rosanov concerns bistability in spatially distributed systems. For Uspekhi Fizicheskikh Nauk 191 (4) 445 ± 446 (2021) Translated by M V Tsaplina PERSONALIA PACS number: 01.60.+q

A review of many-body effects in exciton ensembles in semiconductors is given with the emphasis on two-dimensional systems: structures with single and double quantum wells and with quantum microcavities. The Bose–Einstein condensation effect, an accumulation of a macroscopic number of excitons in the ground state of the system, is discussed. The known prohibition on condensation in low-dimensional systems can be lifted due to the disorder resulting from the chaotic potential. Manifestations of the finite exciton lifetime and, correspondingly, of the nonequilibrium of the excitonic system caused by processes of excitons entering and leaving the condensate state are analyzed. Other collective phases of excitons, namely, two-dimensional crystals of dipolar excitons and an electron–hole liquid, formed as a result of interparticle interactions, are discussed.

Александр Александрович Каплянский (к 90-летию со дня рождения), Агранович В.М., Александров Е.Б., Багаев С.Н., Грехов И.В., Забродский А.Г., Иванов С.В., Ивченко Е.Л., Кведер В.В., Новиков Б.В., Сурис Р.А., Тимофеев В.Б., Щербаков И.А.

the Russian Academy of Sciences (RAS), Dmitrii Aleksandrovich Varshalovich, well known for his studies in the field of quantum theory, radiation theory, physics of interstellar matter, quasar physics and cosmology, passed away on April 21 at the age of 86. Varshalovich was born in 1934 in Leningrad into the family of a prominent entomologist, Aleksandr Aleksandrovich Varshalovich, and a biologist, Vera Yakovlevna Skurikhina. In 1941, when a child, he was evacuated from Leningrad, where he returned with his mother in 1944. In 1952, he finished school with a gold medal and entered the physical faculty of Leningrad State University. He graduated from the University in 1957 as a specialist in nuclear spectroscopy and joined the laboratory of nuclear isomerism of the Leningrad Physical-Technical Institute (LPTI, now Ioffe PTI). Within several years, he carried out a series of studies in nuclear isomerism and nuclear spectroscopy, collaborating with L I Rusinov and L K Peker. Those same years, together with I V Kurcharov and Rusinov, he wrote a book on nuclear isomerism. The book was unfortunately not published because of the untimely death of Kurcharov and Rusinov in 1960. In 1961, Varshalovich was transferred to the Department of Theoretical Physics at LPTI, where he continued working in nuclear spectroscopy and interferential methods of studying gamma-ray emission of nuclei and at the same time became engaged in astrophysics. In 1963, he joined the Sector of theoretical research in astrophysics (now the Sector of theoretical astrophysics) just then founded on the initiative of the LPTI director B P Konstantinov (the first head of the Sector was A Z Dolginov). Varshalovich worked in this Sector for all of his life. In December of 1966, he defended his candidate (PhD) thesis ``Dynamic orientation of atoms in the cosmic medium.'' The paper appeared to be so significant that the official opponents Ya B Zel'dovich and I S Shklovskii, as well as Ya A Smorodinskii, who took part in the work of the committee, suggested that Varshalovich be given the degree of Doctor of Science in physics and mathematics. The chair of the dissertation committee, Konstantinov, and the members of the committee unanimously supported the suggestion (the decision of the committee was approved by the Higher Attestation Commission in May of 1968). From 1986 to 2010, Varshalovich was head of the Sector of theoretical astrophysics and managed to maintain the activity and operability of the Sector even in the period of deep crisis in the 1990s. Varshalovich had profound fundamental knowledge, great keenness and competence, scientific intuition, and the ability to put forward and implement original ideas. He made an invaluable contribution to the development of a number of areas in astrophysics. In the 1960s±1970s, he published a series of studies for the first time exploring the dynamic alignment of atomic and molecular spins in a rarefied cosmic medium caused by resonance scattering of anisotropic radiation fluxes. The effect proved to be important for the investigation of the physical parameters and chemical composition of interstellar gas clouds, the shells of stars, nebulae, comets, and other objects. At the same time, in the early 1960s, Varshalovich proposed the hypothesis (published in 1966) that maser pumping mechanisms in quantum transitions in hydrogen atoms can be realized in thermodynamically nonequilibrium interstellar gas clouds. The discovery in 1965 by radio astronomers of interstellar masers on transitions between rotational levels of OH molecules piqued his interest in cosmic masers. Uspekhi Fizicheskikh Nauk 190 (7) 783 ± 784 (2020) Translated by M V Tsaplina PERSONALIA PACS number: 01.60.+q