Physics-Uspekhi

Certain one-dimensional Fermi systems have an energy gap in the bulk spectrum while boundary states are described by one Majorana operator per boundary point. A finite system of length L possesses two ground states with an energy difference proportional to exp(-L/l₀) and different fermionic parities. Such systems can be used as qubits since they are intrinsically immune to decoherence. The property of a system to have boundary Majorana fermions is expressed as a condition on the bulk electron spectrum. The condition is satisfied in the presence of an arbitrary small energy gap induced by proximity of a three-dimensional p-wave superconductor, provided that the normal spectrum has an odd number of Fermi points in each half of the Brillouin zone (each spin component counts separately).

The development of X-ray diffractometry at the turn of the 21st century is presented. The review covers instrumentation development for structural studies based on the use of both standard continuously radiating X-ray generators and state-of-the-art sources of ultrashort and ultra-bright X-ray pulses. The latter technique enables investigation of the structural dynamics of condensed matter in a 4D space–time continuum with a resolution reaching a tenth of a femtosecond. New engineering approaches to enhancing the sensitivity, accuracy, and efficiency of X-ray diffraction experiments are discussed, including new and promising X-rays sources, reflective collimating and focusing X-ray optical devices, and fast low-noise and radiation-resistant position-sensitive X-ray detectors, as well as a new generation of X-ray diffractometers developed based on these elements. The presentation is focused on modern engineering solutions that enable academic and applied-research laboratories to perform X-ray diffraction studies on-site, which earlier were only feasible using synchrotron radiation sources at international resource sharing centers.

A review of physical and mathematical methods for reconstructing the functional networks of the brain based on recorded brain activity is presented. Various methods are considered, as are their advantages and disadvantages and limitations of the application. Problems applying the theory of complex networks to reconstructed functional networks of the brain to explain the effects of dynamic integration in the brain and their influence on the diverse functionality of the brain and consciousness, as well as processes leading to the pathological activity of the central nervous system, are examined. Questions concerning the application of these approaches are considered both to describe the functioning of the brain in various cognitive and pathological processes and to create new brain–computer interfaces based on the detection of changes in functional connections in the brain.

The methods of laser cooling of atoms have long been applied to obtain cold and ultracold atomic gases, including degenerate states and the atomic Bose–Einstein condensate in particular. Until recently the application of laser cooling methods to molecules was assumed to be impossible because of the complex structure of molecular levels and the absence of closed cooling cycles for transitions between the electron levels of molecules in the general case. However, it has recently become clear that laser cooling can be performed for a large class of molecules, including not only the simplest diatomic molecules but also polyatomic molecules. We here present the general principles for identifying suitable molecules and discuss current studies on and further developments in the laser cooling of molecules.

The bulk photovoltaic effect (BPVE) — the generation of electric currents by light in noncentrosymmetric materials in the absence of electric fields and gradients — was intensively investigated at the end of the last century. The outcomes, including all main aspects of this phenomenon, were summarized in reviews and books. A new upsurge of interest in the BPVE occurred recently, resulting in a flood of misleading theoretical and experimental publications centered around the so-called shift current. Numerous top-rated recent publications ignore the basic principles of charge-transport phenomena and the previous results of joint experimental-theoretical studies. Specifically, leading (or substantial) contributions to currents caused by asymmetry of the momentum distributions of electrons and holes are missed. The wide-spread basic relation for the shift current ignores the kinetic processes of relaxation and recombination of photo-excited electrons and leads to nonvanishing shift currents in thermal equilibrium. The goals of this methodological note is to specify and substantiate the benchmarks of the BPVE theory and return studies to the right track in the interest of developing photovoltaic devices.

Disorder — impurities and defects violating the ideal long-range order — is always present in solids. It can result in interesting and sometimes unexpected effects in multiband superconductors, especially if the superconductivity is unconventional, thus having symmetry other than the usual s-wave. This paper uses the examples of iron-based pnictides and chalcogenides to examine how both nonmagnetic and magnetic impurities affect superconducting states with s_± and s₊₊ order parameters. We show that disorder causes the transition between s_± and s₊₊ states and examine what observable effects this transition can produce.

Quantum entanglement is a powerful resource that revolutionizes information science, opens new horizons in communication technologies, and pushes the frontiers of sensing and imaging. Whether or not the methods of quantum entanglement can be extended to life-science imaging is far from clear. Live biological systems are eluding quantum-optical probes, proving, time and time again, too lossy, too noisy, too warm, and too wet to be meaningfully studied by quantum states of light. The central difficulty that puts the main roadblock on the path toward entanglement-enhanced nonlinear bioimaging is that the two-photon absorption (TPA) of entangled photons can exceed the TPA of uncorrelated photons only at the level of incident photon flux densities as low as one photon per entanglement area per entanglement time. This fundamental limitation has long been believed to rule out even a thinnest chance for a success of bioimaging with entangled photons. Here, we show that new approaches in nonlinear and quantum optics, combined with the latest achievements in biotechnologies, open the routes toward efficient photon-entanglement-based strategies in TPA microscopy that can help confront long-standing challenges in life-science imaging. Unleashing the full potential of this approach will require, however, high throughputs of virus-construct delivery, high expression efficiencies of genetically encodable fluorescent markers, high-brightness sources of entangled photons, as well as a thoughtful entanglement engineering in time, space, pulse, and polarization modes. We demonstrate that suitably tailored nonlinear optical fibers can deliver entangled photon pairs confined to entanglement volumes many orders of magnitude smaller than the entanglement volumes attainable through spontaneous parametric down-conversion. These ultracompact modes of entangled photons are shown to enable a radical enhancement of the TPA of entangled photons, opening new avenues for quantum entanglement in life-science imaging.

State-of-the-art studies of dielectric magnonics and magnon spintronics are reviewed. Theoretical and experimental approaches to exploring physical processes in and calculations of the parameters of magnonic micro- and nanostructures are described. We discuss the basic concepts of magnon spintronics, the underlying physical phenomena, and the prospects for applying magnon spintronics for data processing, transmission, and reception. Special attention is paid to the feasibility of boosting the operating frequencies of magnonic devices from the gigahertz to terahertz frequency range. We also discuss specific implementations of the component base of magnonics and ways to further develop it.

Calculated and experimental data on the AC Stark shift of atomic levels in an external, subatomic-strength variable field are considered. Theoretical predictions concerning the disturbance of atomic spectra by fields of atomic and superatomic strength are discussed. The limiting value of the atomic AC Stark shift in a light-frequency radiation field is estimated.

The Sachdev–Ye–Kitaev model and two-dimensional dilaton gravity have recently been attracting increasing attention of the high-energy and condensed-matter physics communities. The success of these models is due to their remarkable properties. Following the original papers, we broadly discuss the properties of these models, including the diagram technique in the limit of a large number of degrees of freedom, the emergence of conformal symmetry in the infrared limit, effective action, four-point functions, and chaos. We also briefly discuss some recent results in this field. On the one hand, we attempt to be maximally rigorous, which means considering all the details and gaps in the argument; on the other hand, we believe that this review can be suitable for those who are not familiar with the relevant models.

This review is devoted to studies of quantum optics effects for quantum emitters (QEs) in the near field of nanoparticles (NPs). In the simple model of a two-level QE located near a plasmon NP, we analyze the mechanisms for modifying the radiative and nonradiative decay rates and discuss the distribution of the near-field intensity and polarization around the NP. This distribution has a complex structure, being significantly dependent on the polarization of the external radiation field and on the parameters of NP plasmon resonances. The quantum optics effects in the system (NP + QE + external laser field) are analyzed, including the near-field modification of the resonance fluorescence spectrum of a QE, the bunching/antibunching effects and photon quantum statistics effects in the spectrum, the formation of squeezed light states, and quantum entangled states in such systems.

We review the most significant results obtained in the framework of the microscopic approach to a systematic study of magnetic dynamics in two-dimensional ferromagnetic and antiferromagnetic materials with a strong Rashba spin-orbit coupling. For model systems, we discuss the microscopic derivation of the Gilbert damping tensor, spin-orbit and spin-transfer torques, and symmetric and antisymmetric exchange interactions. It is shown that in both antiferromagnetic and ferromagnetic systems, the presence of a sufficiently strong spin-orbit coupling leads to an anisotropy of spin torques and Gilbert damping. We focus on an analysis of spin-orbit torques in a two-dimensional Rashba antiferromagnet. We also address the possibility of switching the antiferromagnetic order parameter via short current pulses in the plane of the sample.

This review describes the current state of research on the formation of a nanocrystalline structure in amorphous alloys under thermal and deformation effects. The processes of formation of nanocrystals in homogeneous and heterogeneous amorphous structures (nanoglass) are considered. Changes in the magnetic and mechanical properties during the formation of a composite amorphous-nanocrystalline structure with different structural parameters are analyzed. The possibility of amorphous phase rejuvenation from a partially crystalline structure under cryogenic thermocycling treatment is shown.

The main issues and areas of application of photothermal and optoacoustic spectroscopy are reviewed. Progress in innovative techniques in the most actively developing areas is presented, including microspectroscopy, multispectral techniques, the measurements of single particles and objects with a resolution better than the diffraction limit (nanoscopy) by both optical and probe-based methods. Possible applications of photothermal and optoacoustic spectroscopy for determining the properties of materials, studying photochemistry and fluorescence, chemical reactions, and analytical and applied chemistry, and solving biomedical problems is discussed. Some prospects for the development of these methods are presented.

This review is devoted to studies of quantum optics effects for quantum emitters (QEs) in the near field of nanoparticles (NPs). In the simple model of a two-level QE located near a plasmon NP, we analyze the mechanisms for modifying the radiative and nonradiative decay rates and discuss the distribution of the near-field intensity and polarization around the NP. This distribution has a complex structure, being significantly dependent on the polarization of the external radiation field and on the parameters of NP plasmon resonances. The quantum optics effects in the system (NP + QE + external laser field) are analyzed, including the near-field modification of the resonance fluorescence spectrum of a QE, the bunching/antibunching effects and photon quantum statistics effects in the spectrum, the formation of squeezed light states, and quantum entangled states in such systems.

We review the most significant results obtained in the framework of the microscopic approach to a systematic study of magnetic dynamics in two-dimensional ferromagnetic and antiferromagnetic materials with a strong Rashba spin-orbit coupling. For model systems, we discuss the microscopic derivation of the Gilbert damping tensor, spin-orbit and spin-transfer torques, and symmetric and antisymmetric exchange interactions. It is shown that in both antiferromagnetic and ferromagnetic systems, the presence of a sufficiently strong spin-orbit coupling leads to an anisotropy of spin torques and Gilbert damping. We focus on an analysis of spin-orbit torques in a two-dimensional Rashba antiferromagnet. We also address the possibility of switching the antiferromagnetic order parameter via short current pulses in the plane of the sample.

This review describes the current state of research on the formation of a nanocrystalline structure in amorphous alloys under thermal and deformation effects. The processes of formation of nanocrystals in homogeneous and heterogeneous amorphous structures (nanoglass) are considered. Changes in the magnetic and mechanical properties during the formation of a composite amorphous-nanocrystalline structure with different structural parameters are analyzed. The possibility of amorphous phase rejuvenation from a partially crystalline structure under cryogenic thermocycling treatment is shown.

This review is devoted to a discussion of new (and often unexpected) aspects of the old problem of elastic light scattering by small metal particles, whose size is comparable to or smaller than the thickness of the skin layer. The main focus is on elucidating the physical grounds for these new aspects. It is shown that, in many practically important cases, the scattering of light by such particles, despite their smallness, may have almost nothing in common with the Rayleigh scattering. So-called anomalous scattering and absorption, as well as Fano resonances, including unconventional (associated with the excitation of longitudinal electromagnetic oscillations) and directional Fano resonances, observed only at a small solid angle, are discussed in detail. The review contains a Mathematical Supplement, which includes a summary of the main results of the Mie theory and a discussion of some general properties of scattering coefficients. In addition to being of purely academic interest, the phenomena considered in this review can find wide applications in biology, medicine, pharmacology, genetic engineering, imaging of ultra-small objects, ultra-high-resolution spectroscopy, information transmission, recording, and processing, as well as many other applications and technologies.

We discuss the properties of topologically nontrivial superconducting phases and the conditions for their realization in condensed matter, the criteria for the appearance of elementary Majorana-type excitations in solids, and the corresponding principles and experimental methods for identifying Majorana bound states (MBSs). Along with the well-known Kitaev chain and superconducting nanowire (SW) models with spin–orbit coupling in an external magnetic field, we discuss models of quasi-two-dimensional materials in which MBSs are realized in the presence of noncollinear spin ordering. For finite-length SWs, we demonstrate a cascade of quantum transitions occurring with a change in the magnetic field, accompanied by a change in the fermion parity of the ground state. The corresponding anomalous behavior of the magnetocaloric effect can be used as a tool for identifying MBSs. We devote considerable attention to the analysis of the transport characteristics of devices that contain topologically nontrivial materials. The results of studying the conductance of an Aharonov–Bohm ring whose arms are connected by an SW are discussed in detail. An important feature of this device is the appearance of Fano resonances in the dependence of conductance on the magnetic field when the SW is in a topologically nontrivial phase. We establish a relation between the characteristics of such resonances and the spatial structure of the lowest-energy SW state. The conditions for the occurrence of an MBS in the phase of the coexistence of chiral d + id superconductivity and 120-degree spin ordering are determined in the framework of the t – J – V model on a triangular lattice. We take electron–electron interactions into account in discussing the topological invariants of low-dimensional superconducting materials with noncollinear spin ordering. The formation of Majorana modes in regions with an odd value of a topological invariant is demonstrated. The spatial structure of these excitations in the Hubbard fermion ensemble is determined.