It has been shown that a one-dimensional quantum ring with an asymmetric (mutually perpendicular) contact arrangement exposed to a nonresonance terahertz pulse serves as an optical spin gate modulator operating in a zero magnetic field. The implementation of a dynamic (optically controlled) analogue of the Datta–Das spin field-effect transistor with ferromagnetic contacts, which ensures ultrafast contactless control of the spin channel conductivity, has been demonstrated using the Floquet–Magnus expansion. The proposed circuit opens new prospects for the creation of coherent terahertz spintronic logic devices.
Parametric studies of Cu–Ne and Cu–Ne–H2 lasers have been performed under significantly different conditions with an pump pulse front length of 1 ns. It has been shown that the average lasing power in the train mode in a gas-discharge tube 2 cm in diameter with a hydrogen pressure in the working mixture of 1.5 Torr increases linearly to 100 kHz, reaching a linear value of 270 W/m. The lasing efficiency in the Cu–Ne mixture exceeds 5
Magnetic configurations of finite-length chains of Mn atoms on a Pt(332) surface have been theoretically studied within a classical model taking into account the exchange Heisenberg interaction, biaxial magnetocrystalline anisotropy, the Dzyaloshinskii–Moriya interaction, the dipole–dipole interaction, and the Zeeman interaction of the magnetic moments of atoms with an external magnetic field. It has been shown that both nearly collinear and helical magnetic states can be stable. An applied magnetic field causes the rotation of the plane in which the helical magnetic structure lies. The stability of these states in the external magnetic field parallel to the easy axis has been examined. It has been found that the Mn/Pt(332) system is characterized by two critical fields, the lower of which is associated with the finite length of the atomic chain. The predicted effects are due to the Dzyaloshinskii–Moriya interaction and can be observed experimentally.
An approach based on a graph neural network has been developed to recognize phase states and to estimate the critical temperature of finite-size systems within the Ising model. The advantage of the new approach is the explicit specification of the lattice geometry and bond structure using a graph. This allows a unified transfer of equilibrium spin configurations of lattices of different spatial dimensions and geometries to the input of the neural network. This method has been applied to seven different lattices: two-dimensional square, triangular, hexagonal, and two-dimensional Apamea lattice, as well as three-dimensional simple cubic, bcc, and fcc lattices. It has been shown that the critical temperature can be extracted by crossing the output of the neural network with the 1/2 level. Feeding a small sample of equilibrium configurations to the input of a graph neural network allows achieving the mean absolute error 9.7 × 10–3 relative to the reference value T_c^ref . An additional scaling analysis has been performed for the Apamea lattice, which has shown that the neural network estimate of the critical temperature is consistent with the intersection point of the Binder cumulant curves for systems of different sizes.
The development of accelerator technology is limited by the resource-intensive nature of traditional methods for forming particle beams, especially at high energies. Current accelerators involve slow beam extraction and magnetic focusing, which are inefficient at energies above 1 TeV. A new method involving crystals for the focusing and deflection of beams has been developed in this work. At high energies, the scattering of secondary particles is reduced, allowing crystals to be effectively used instead of magnets. This technology is also promising for the U-70 accelerator (Protvino, Russia). The goal of this work is to create crystalline systems for forming various particle beams.
This work introduces a novel approach that makes it possible to take interaction into account in the problem of the evolution of a quantum field after an instantaneous local perturbation (quench). Using scalar theory gφ^4 as an example, we demonstrate a method for describing the influence of interaction on the field dynamics after a quench within the semiclassical approximation in the Keldysh technique. The proposed approach is nonperturbative in the coupling constant, valid for arbitrary initial conditions, and well suited for numerical computation. As an illustration, we show how the interaction drives thermalization toward a new stationary state of the system and leads to a loss of coherence, which manifests itself in the destruction of the interference pattern that is characteristic of the noninteracting case.
Analog computations based on spatial Fourier filtering in a 4f-system enable the approaches of mathematical operations on images entirely optically, providing their pre-digital processing at high speed. Conventional implementations based on refractive elements are bulky and challenging from the standpoint of scalability. Due to efficient wavefront control, optical metasurfaces make it possible to replace such elements with planar nanostructures, ensuring compactness and subwavelength resolution in controlling the amplitude and phase of light. In this paper a comparison is carried out between pixel-based and gradient-based design approaches for optical metasurfaces based on gallium arsenide nanorings for spatial differentiation. In the pixel-based algorithm, each region of the metasurface (pixel) is considered as a square periodic lattice of identical nanoresonators. The gradient-based algorithm enables topology optimization by smoothly varying the size of nanorings across the metasurface. Despite the assumption that the gradient approach minimizes the artifacts inherent in pixel-based structures and ensures a more accurate matching with the required mathematical operation, the pixel structure demonstrates a Pearson correlation coefficient with the theoretical dependences that is 6–8
A microscopic Monte Carlo analysis of the 169Tm NMR line shape in dilute LiY0.98Tm0.02F4 is presented. The random occupation of rare-earth lattice sites by Tm3+ ions is treated explicitly, and the distribution of dipolar local fields at the resonant 169Tm nuclei is calculated for 5 × 106 independent configurations. It is shown that, in the 2
We have shown that a composite boson, formed by two fermionic species on a lattice with a strong on-site attraction, can be bound at an one-site static “magnetic” impurity acting oppositely on different species. We have found a manifold of bound states and calculated their energies and critical values of the impurity strength determining the occurrence of these states.
Electromagnetic absorbance by most two-dimensional electron systems is typically well below unity, which hinders both practical applications in photodetection and fundamental studies of their optical properties. Here, we show that a periodic structure comprised of narrow two-dimensional sections connected with wide perfectly conducting metal sections enables large absorbance. It reaches 50 ≪ 1, large dielectric constants of the substrate, and grating periods comparable with λ0, the optimal distance to reflector tends to zero. Above the critical values of the grating geometrical parameters, the absorbance maximum ceases to exist. The critical behavior manifests as a large-amplitude resonance in “dirty” two-dimensional system with purely real conductivity, while enhancement of carrier momentum relaxation time lowers the resonant peak. Such resonance mimics the plasmonic one, but does not rely on high electron mobility.
The heating of 26–100-nm-thick gold films exposed to 515-nm femtosecond laser radiation has been studied in real time using short electron pulses. Analysis of diffraction kinetics within the Fermi gas model has made it possible to determine the electron–phonon coupling constants (1–3) × 1016 W/(K m3), which are in good agreement with theory at electron temperatures Te < 10 kK. The obtained results have made it possible to evaluate the lower limit of the melting time of Au (tm = 70 ps) at a threshold fluence corresponding to the energy required to heat the crystal and supply the latent heat of the phase transition.
The off-diagonal component of the correlation function of the phase gradients of a light wave passing through a stratified turbulent atmospheric path has been studied. In the weak scintillation regime, it has been shown that the dependence of the correlation function on the distance between points contains a characteristic maximum, the position of which can be used to determine the Rytov variance, as in the case of a homogeneous atmosphere [I. V. Kolokolov, V. V. Lebedev, and F. A. Starikov, J. Opt. Soc. Am. A 42, 1654 (2025)]. In the strong scintillation regime, with increasing Rytov variance, the calculated correlation function tends to the theoretical expression obtained under the assumption of a Gaussian envelope of the wave field, at r < r0, where r0 is the Fried parameter.
The characteristics of Smith–Purcell radiation are calculated for a hollow electron beam at the central axis of which a chain of sub-wavelength particles is located. We show that the spectral and angular characteristics of radiation depend in a complex way on the Lorentz factor of electrons. We show also that incoherent radiation depends on the transverse dimensions of the beam. Interestingly, the transverse component of the Coulomb field of the electron does not contribute to the intensity of coherent radiation. This leads to the fact that for high electron energies, coherent radiation becomes comparable to incoherent one, even for a short beam. The results will be useful for efficient radiation generation and diagnostics of hollow beams, including using incoherent radiation.
The OKA experiment has accumulated a significant statistics of K^ + -meson decays. This allowed us to select about 2300 events of the rare K^ + →π^ + π^ - π^ + γ decay with the photon energy in the kaon rest frame of 10 MeV < E_γ^* < 70 MeV and to measure the decay branching ratio for this region: BR(3πγ) = (6.36 ±0.14_stat±0.3_syst) ×10^ - 5 . The differential branching ratio as a function of the energy E_γ^* has also been obtained. The results have been compared with theoretical predictions derived within the framework of chiral perturbation theory (ChPT).
Coalescence and Rayleigh–Plateau instability of liquid jets and long cylinders are among the most studied areas of fluid dynamics. Two stages of the coalescence of isotropic droplets in a nematic environment in flat Hele–Shaw cells have been revealed. At the first stage, the formation of an isotropic bridge between droplets occurs only in the middle of the cell. Thin layers of nematic remain on the cell surface. In the second stage, the nematic layers on the surface become unstable and disappear with the formation of microdroplets. The second stage of transformation resembles the Rayleigh–Plateau instability with the formation of satellite droplets. The discovered two-stage coalescence has not yet been observed in either isotropic or liquid crystalline media and has not been predicted theoretically.
The influence of the initial microstructure of supercritical carbon dioxide (scCO2) in the steady state in the process of its clustering during rapid expansion into a vacuum has been demonstrated. The molecular dynamics simulation has shown the presence of aggregates in the initial medium (P0 = 80–200 bar, T0 = 310–330 K) with a size distribution described by a bimodal lognormal function. One of the distribution modes corresponds to “large” aggregates with a diameter of 15–30 nm, characteristic of the Widom delta (a region on the phase diagram near the critical point). Mie scattering diagnostic has been used to establish the relationship between the fluid microstructure in a supercritical reactor and clustering in the process of expansion of a CO2 jet into a vacuum. It has been experimentally shown that the cluster size distribution in the expanding jet is also described by a bimodal lognormal function with a mode of large clusters of 50–80 nm in size. It has been found that the concentration of large clusters in the jet changes linearly with the concentration of large aggregates for the Widom delta.
The possibility of forming periodic surface structures in amorphous GeTe and Sb 2 Te 3 thin films irradiated by 1030-nm femtosecond laser pulses has been studied. The inclusion of nonlinear absorption and photoinduced electron generation within the framework of the surface electromagnetic wave excitation theory allows the prediction of the formation or absence of surface gratings in these materials induced by ultrashort laser pulses. The simulation results are consistent with experimental data.
Recently, we have obtained a permanently existing Bose–Einstein condensate (BEC) of magnons in the self-generation mode. We have shown that magnon relaxation processes do not disturb the coherent state of the BEC when magnon relaxation is compensated by exciting new magnons under the effect of an amplified signal of magnon BEC emission applied to the sample. This is due to the boson enhancement effect, whereby only magnons corresponding to the quantum state of the BEC are excited in the presence of the BEC. In this work, we report the new studies, where a set of stable condensate states that are determined by the phase relationship of the radio-frequency signal emitted by the BEC and the amplified signal fed back have been obtained. The formation of stable regions of BEC generation corresponding to a phase jump multiple of 2π on the feedback line has been experimentally detected. A theoretical model has been constructed to describe the process of transition between BEC states in a self-generator. This model has much in common with the well-known washboard model for phase qubits.
