JETP Letters最新文献

The frequency dependence of the complex refractive index of an InAs/GaSb superlattice where plastic relaxation is suppressed by interface compensation of strains and the fundamental absorption edge falls in the far-infrared range is experimentally determined for the first time at temperatures of 77–300 K. The energy structure of the superlattice minibands and its variation with temperature are discussed.

Shubnikov–de Haas oscillations and magneto-intersubband oscillations of the magnetoresistance in structures with single HgTe quantum wells with a width of 10−18 nm have been experimentally investigated. The conduction band spectrum in these arrays is split by the spin–orbit interaction. This leads to beats in the Shubnikov−de Haas oscillations and gives rise to low-frequency magneto-intersubband oscillations. The mutual positions of the antinodes of the Shubnikov−de Haas oscillations and the peaks in the magneto-intersubband oscillations are unusual: at low magnetic fields, they are opposite to theoretical predictions. The measurements in high magnetic fields, at which the relative amplitude of Shubnikov–de Haas oscillations exceeds 0.2–0.3, show a change in the mutual positions of the antinodes of the Shubnikov–de Haas oscillations and the peaks in low-frequency oscillations. Numerical calculations and additional measurements at different temperatures suggest that the observed effects are due to magnetic-field-induced oscillations of the Fermi level.

The spectra of anticrossing of spin sublevels have been recorded and spin-3/2 color centers have been identified for the first time in commercially available 4H-SiC Schottky diodes irradiated with 0.9-MeV electrons or 15-MeV protons. The effect of the irradiation density on the defect formation has been shown. It has been demonstrated that the increase in the temperature at which proton irradiation is carried out acts as a short-term annealing, leading to a decrease in the concentration of point defects.

Direct interband and intragap photoexcitation by intense mid-infrared (4.0 and 4.7 μm) femtosecond (fs) laser pulses was explored in ultrapure chemical-vapor deposited (CVD) diamond via acquisition of characteristic ultraviolet photoluminescence of free excitons and A-band photoluminescence of electrons anchored at deep donor–acceptor or dislocation-related traps, respectively. At lower laser intensities (<10 TW/cm²) the excitonic photoluminescence yields exhibit highly nonlinear dependences with Iλ²-scaling and power slopes N ≈ 17 (4.0 μm) and 14 (4.7 μm), still insufficient to cross over the direct bandgap (≥6.5 eV) by ≈1.2 and 2.8 eV, respectively. Similarly high slope of ≈9 (4.7 μm) for intragap (≥3.5 eV) photo-population of donor–acceptor traps is still insufficient for their direct excitation by ≈1 eV. At the intermediate Iλ²-dependent values of the Keldysh parameter γ ∼ 1 such incomplete multiphoton excitation anticipates the hybrid total “multiphoton + tunneling” photoexcitation generally predicted by the Keldysh theory, but never unambiguously experimentally demonstrated. At higher laser intensities (>10 TW/cm²) both the excitonic and A-band photoluminescence yields exhibit (sub)linear slopes, apparently, indicating formation of more strongly absorbing electron–hole plasma. These findings shed light on the hybrid multiphoton + tunneling character of Keldysh photoexcitation at intermediate values γ and pave the way to defect/impurity band engineering of intragap nonlinear optical properties in bulk dielectrics for their precise fs-laser nanomodification.

A new approach has been proposed to study the dynamics of crystal structure deformations under external influences using the X-ray rocking curve imaging technique. Using this approach, the spatial distribution, type, and magnitude of deformations of the crystal structure of a langasite crystal, which is promising for industrial application, exposed to the ultrasonic irradiation (excitation of a standing ultrasonic wave) have been studied.

A search for the ({{K}^{ + }} to {{pi }^{0}}{{pi }^{0}}{{pi }^{0}}{{e}^{ + }}nu ) decay is performed by the OKA collaboration. The search is based on (3.65 times {{10}^{9}}) K⁺ decays. No signal is observed. The upper limit set is BR(({{K}^{ + }} to {{pi }^{0}}{{pi }^{0}}{{pi }^{0}}{{e}^{ + }}nu )) < 5.4 × 10^–8 90% CL, 65 times lower than the one currently listed by PDG.

Obtaining system parameters and reconstructing the full flow state from limited velocity observations using conventional fluid dynamics solvers can be prohibitively expensive. Here we employ machine learning algorithms to overcome the challenge. As an example, we consider a moderately turbulent fluid flow, excited by a stationary force and described by a two-dimensional Navier–Stokes equation with linear bottom friction. Using dense in time, spatially sparse and probably noisy velocity data, we reconstruct the spatially dense velocity field, infer the pressure and driving force up to a harmonic function and its gradient, respectively, and determine the unknown fluid viscosity and friction coefficient. Both the root-mean-square errors of the reconstructions and their energy spectra are addressed. We study the dependence of these metrics on the degree of sparsity and noise in the velocity measurements. Our approach involves training a physics-informed neural network by minimizing the loss function, which penalizes deviations from the provided data and violations of the governing equations. The suggested technique extracts additional information from velocity measurements, potentially enhancing the capabilities of particle image/tracking velocimetry.

The diffraction of a light wave by fluctuations in the refractive index in a turbulent medium leads to its distortions. Their analysis allows one to extract the main parameters of turbulence. In this paper, we propose a new method based on measurements of the correlation function of the gradients of the light wave phase, which allows us to independently find the Fried parameter ({{r}_{0}}) and, then, the outer scale of turbulence L₀. The method has been successfully tested on measurement data obtained during the passage of laser radiation through a gaseous medium with artificially created turbulence.7

The problem of analytical estimation of the Lyapunov exponents and Lyapunov timescales of the motion in multiplets of interacting nonlinear resonances is considered. To this end, we elaborate a unified framework, based on the separatrix map theory, which incorporates both an earlier approach for the first fundamental model of perturbed resonance (given by the perturbed pendulum Hamiltonian) and a new one for its second fundamental model (given by the perturbed Andoyer Hamiltonian). Within this framework, new accurate estimates for the Lyapunov timescales of the inner and outer subsystems of the Solar planetary system are presented and discussed.

A regular method has been proposed to quantitatively describe the fluorescence of molecules randomly moving in a nanowell. Correlation functions of the fluorescence of single molecules depending on the geometry of the nanowell and the penetration depth of an exciting field into it have been determined. The obtained results can be used to quantitatively analyze the properties of molecules and to extract information on the parameters of the nanowell.