JETP Letters最新文献

The development of the two-photon laser lithography technique for the fabrication of optical elements with characteristic dimensions of a few microns is an important goal. Here, two-photon laser lithography is used to produce micro-optical waveguides from OrmoComp® hybrid photoresist. The waveguides are optically isolated from a substrate and are connected to total internal reflection prism adapters for coupling optical radiation in and out of them. The transmission spectra of the entire input adapter–waveguide–output adapter structure are calculated and measured and it is shown that the transmission coefficient in the few-mode regime is 20–40% in the spectral range of 700–1650 nm. According to calculations, the main mechanism of losses in such a structure is determined by strong scattering in the region of joint between the conical part of the adapter and the waveguide caused by the complex structure of the optical field, as well as by the violation of the total internal reflection regime in the prisms due to the large angular aperture of the focused radiation beam. It is shown that the Goos–Hänchen effect has to be taken into account in the design of the coupling elements.

The simulation of the motion of ultracold neutrons is important for estimating their losses, accurately measuring their lifetimes, and describing other experiments. In material traps, it is necessary to take into account not only the specular reflection, but also the diffuse elastic reflection of ultracold neutrons from the walls of a trap. The angular distribution of diffusely reflected neutrons is usually described by Lambert’s cosine law, which does not have a strict theoretical justification and is often violated. In this work, an experiment has been proposed to measure the deviation of the angular distribution of diffusely reflected ultracold neutrons from Lambert’s cosine law. This deviation can be determined by the difference in the number of neutrons leaving a long, narrow trap for ultracold neutrons through its central and end windows. The performed Monte Carlo calculations corresponding to the proposed experiment have shown a significant effect for different shapes of the trap.

The superconducting order parameter of the RbCa₂Fe₄As₄F₂ compound belonging to the new 12442 family of iron-based superconductors with a critical temperature of T_c ~ 32 K has been studied. Two superconducting condensates with the order parameters Δ_L ~ 6.3 meV and Δ_S ~ 2.8 meV have been detected for the first time by multiple Andreev reflection spectroscopy. The temperature dependence of the superconducting critical current density J_c(T) in the intrinsic field has been measured. The approximation of the dependence J_c(T) demonstrates the correspondence of the experimental data to the double-gap model with the s-wave order parameter and the gaps of Δ_L ~ 6 meV and Δ_S ~ 2 meV. The superconducting order parameters obtained by two different methods are in good agreement with each other.

A theory has been developed to describe the magnetization curves of an ensemble of single-domain particles. This theory includes their temperature-induced excitations and is in fact a generalization of the two-level Stoner–Wohlfarth model and its relaxation version, as well as a multilevel relaxation model for describing Mössbauer spectra. A basically new aspect of this theory is the solution of a system of differential equations for nonequilibrium populations of stochastic states. The resultant model includes the physical mechanisms for the formation of magnetization curves of nanoparticles in real situations and describes self-consistently the qualitative features of the transformation of these curves under the variation of the temperature and the strength and direction of an external field.

The electric and magnetic frequency rearrangement of a Bragg resonance in the spectrum of spin waves in a magnonic crystal in the form of 100-nm-thick yttrium iron garnet film with attached 10-nm-thick platinum strips is reported. The effect of the spin current on the position of the Bragg band gap depends on the polarity of the voltage applied to the platinum strips. The positive voltage does not affect the band gap position, while the applied negative voltage lowers the band gap by about of 5 MHz. At frequencies outside the band gap, depending on the polarity of the voltage applied to platinum, either the enhancement or suppression of the spin wave is observed.

A realistic numerical model of a photonic tensor core based on the crossbar architecture with absorbing GeSbTe chalcogenide glass films as weight elements of a photonic matrix has been developed. The performance of the model for the matrix–vector multiplication has been demonstrated. The possibility of using the tensor core based on the implemented architecture in convolutional neural networks for image recognition tasks has been shown. Numerical simulations have been used for the first time to estimate the potential performance and energy efficiency of a photonic hardware accelerator, taking into account the modern experimental element base.

In the article the effect of twisting of Bessel radiation on two-photon ionization of single atoms localized on the axis of the incident beam is studied. The matrix element of two-photon ionization of this type is obtained for arbitrary polarization and multipolarity of the incident radiation. The differential and integral probability of ionization of an atom over the photoemission angle is analyzed. Illustrative calculations are performed for helium and neon atoms in the simplest case of a circularly polarized field in the electric dipole approximation in the single-active-electron model.

The superconducting sigma neuron is a single-junction interferometer, with a part of the circuit shunted by an additional inductance, which is also used to generate the output signal. It was previously predicted that the transfer function of this device would be close to the sigmoidal one at a certain relation between the values of inductance of its parts. This interferometer can be fabricated as a multilayer thin-film structure over a superconducting screen, which allows measuring the output magnetic flux in a single element. An analysis of the experimental data showed that the use of a superconducting screen did not ensure complete independence of the sigma neuron elements, as was assumed in the theoretical model. This paper presents a generalized model of the stationary state of a sigma neuron that takes into account the interaction between all its parts, including the input and readout elements.

The generation of two electron–hole pairs (a biexciton) in a nanocrystal of a direct-gap semiconductor with a nondegenerate conduction band upon the absorption of a single photon with the energy equal to twice the band gap of the nanocrystal is considered. It is shown that the process can take place only in the presence of the interaction of electrons both with each other and with the crystal-lattice field. The rate of this process is calculated and found to depend strongly on the nanocrystal size.

The study of transport phenomena in conductive media of different dimensions often involves impedance diagnostics. The desire to exclude the influence of contact phenomena accompanying dc measurements on the current–voltage characteristic is a general reason for the application of complicated ac measurements instead of the quite methodologically simple dc regime. Relaxation phenomena in electrolytes with electrohydrodynamics linear in the density of the dopant n_d have been analyzed in detail in this work. It has been shown that the well-known Debye–Hückel–Onsager theory of the electrolyte conductivity cannot ensure the linearity of electrohydrodynamics of dilute solutions in the density n_d. Its linear alternative based on the theory of transport in finely dispersed two-phase systems called Maxwell formalism has been proposed. It has been shown that this allows one to interpret the observed relaxation time in the form ({{tau }_{c}} simeq RC), where R is the resistance of the bulk portion of a cell with an electrolyte in terms of the Maxwell formalism and C is the electrolytic capacitance of the metal–electrolyte transition regions appearing on its control electrodes. Examples of the successful use of RC-matched ac diagnostics have been discussed.