JETP Letters最新文献

The laser writing of microstructures in the bulk of fused silica exposed to tightly focused laser pulses has been analyzed in detail. The self-consistent simulation of a single laser pulse exposure and the formation of electron–hole plasma together with the experimental data has made it possible to verify the mechanism of plasma self-organization and relate it to the mechanisms of defect accumulation in dense plasma regions under multipulse exposure. The dependence of the parameters of the induced microstructures on the wavelength, duration, energy, and exposure of the laser pulse has been investigated. The role of plasma-electron-induced defocusing effects, as well as the contribution of radiation self-focusing, has been analyzed. The heating of the material by the end of the laser pulse has been estimated.

The results of an experiment on the two-photon excitation of exciton states in krypton cryocrystals by the fifth harmonic of 209-nm laser radiation have been presented. It has been shown that the two-photon excitation mechanism allows reaching a density of 10¹⁶ cm⁻³ of free excitons with an excitation energy of ~8.4 eV at a focal radiation intensity of ( approx {{10}^{{11}}}{kern 1pt} ) W/cm². The observed broad photoluminescence line arising from the relaxation of excited exciton states covers the (3{text{/}}{{2}^{ + }}(8.4{kern 1pt} ;{text{eV}}) to 5{text{/}}{{2}^{ + }}(0.0)) low-energy nuclear isomeric transition in ²²⁹Th. Analytical estimates showing that two-photon laser generation of exciton states in noble gas crystals can be used as a new method for the excitation of the nuclear isomeric state of thorium at a pump intensity of ~10¹¹ W/cm² have been provided.

Ferrites with the garnet structure containing rare-earth ions have very diverse magnetic properties, in particular due to different Landé g-factors of rare-earth ions and to the splitting of their energy levels under the effect of crystal fields and/or the spin–orbit coupling. Thulium iron garnet Tm₃Fe₅O₁₂ has a low gyromagnetic ratio. It has been shown in this work that the effective gyromagnetic ratio in such materials can be significantly increased (by a factor of 3–5) by diluting iron with gallium ions. In this case, the gyromagnetic ratio depends on both the content of gallium ions and their distribution between the octahedral and tetrahedral sublattices of the iron garnet. The possibility of achieving a high gyromagnetic ratio in ferrimagnets, which have no magnetic and angular momentum compensation points, has been experimentally discovered and theoretically confirmed for the first time. The gyromagnetic ratio is a key parameter determining the speed of processes in a magnetic spin system, and the results obtained in this work are important for a significant increase in the operation speed of spintronic devices based on ferrimagnetic materials.

Sources of polarized protons and deuterons with a storage cell for polarized atoms in a charge-exchange plasma ionizer are considered. A new approach is based on the transverse injection of an atomic beam into a T-shaped storage ionizer cell. This scheme has a number of advantages over sources with the longitudinal injection of the atomic beam into the storage cell, which will help eliminate the limitations inherent in a source with longitudinal injection and significantly reduce the emittance of the polarized ion beam. This should increase the intensity and polarization of the proton and deuteron beams from the source, which is required for obtaining the design luminosity and polarization at the Nuclotron based ion collider facility (NICA, Joint Institute for Nuclear Research).

A new physical phenomenon—the dominance of the isotopic mass effect over the strain contribution in cubic crystals, which is manifested in a negative shift of a phonon mode—has been discovered. The universality of the relation ω ~ μ^–1/2 for symmetric lattices has been experimentally proven on the example of 3C-SiC. The contrast with a positive shift in anisotropic 4H/6H-SiC proves the key role of crystal symmetry in the competitive separation of the mass effect and strain contributions.

The possibility of conducting experiments with polarized protons at the Nuclotron of the NICA accelerator complex at JINR (Dubna) is under discussion. To preserve polarization during beam acceleration a partial siberian snake is supposed to be used based on dynamic superconducting solenoids developed at JINR. Design options for a solenoid snake in the Nuclotron, both with and without compensation of betatron coupling, have been proposed. Spin orientation control for experiments with an extracted proton beam in a continuous momentum range is achieved with a spin rotator based on longitudinal and transverse magnetic fields, located in the beam transport channel to the external target. At discrete energies, occurring approximately in 0.5 GeV steps, which correspond to integer spin resonances, the polarization direction at internal and external targets can be changed without a spin rotator, applying spin navigators based on weak magnetic fields, located inside the Nuclotron.

A theoretical model of the experimentally observed collimation of soft radiation from a capillary discharge with hollow electrodes without the use of collimating optics has been proposed. The effect of focusing and channeling of hard radiation with a small difference of the refractive index of the discharge plasma from the vacuum value is possible with its multiple refractions on density disturbances caused by the generation of small-scale waves due to two-stream plasma instabilities. The numerical solution of the eikonal equation shows the possibility of such focusing in a periodic plasma structure. The conditions for focusing of hard radiation in a turbulent plasma structure for almost paraxial rays have been obtained by averaging the equations over an ensemble of stochastic wave disturbances, and the effect of focusing and channeling of the inner part of the beam, similar to that observed in a capillary discharge has been numerically demonstrated. The quasiperiodic structure of rays during channeling with alternating antinodes and nodes has been shown. Due to the fundamental nature of the processes under consideration, it can be expected that the phenomenon of focusing and channeling of X rays by density waves should be observed not only in capillary discharges, but also in jets from accretion disks around stars and black holes, which has been qualitatively demonstrated by comparing the numerical tracing results with astronomical observations by the Chandra X-ray Observatory.

The numerical simulation of exact equations of motion (in conformal variables) for unsteady plane potential flows of an ideal fluid with a free surface over a strongly nonuniform bottom profile has revealed the nonlinear compression of a long wave packet undergoing Bragg reflection from a section with a smoothly increasing height of periodically located barriers. In this case, a short high packet of standing waves with sharp crests is formed and is then transformed into a backward wave. It is essential that the effect as a function of the frequency of the incident wave is insignificant in the middle of the barrier-induced spectral gap, but is maximal closer to its upper edge, when the forward wave penetrates deeply into the scattering region and forms, together with the appearing backward wave, a semblance of a Bragg soliton for some time interval.

Raman spectra have been measured in polycrystalline samples of La_0.5Sr_0.5FeO_3–δ and SrFeO_2.5 antiferromagnetic orthoferrites in the orthorhombic and brownmillerite phases, respectively, in the temperature range of 300–700 K. A significant decrease in the intensity of the 1300–1400 cm^–1 band when approaching the Néel temperature (T_N ~ 410 K in the substituted orthoferrite La_0.5Sr_0.5FeO_3–δ and T_N ~ 670 K in the brownmillerite SrFeO_2.5) indicates magnetic ordering in the crystals under normal conditions, and the band itself is due to two-magnon scattering, in contrast to the ~1100-cm^–1 biphonon scattering band, which retains its intensity over the entire temperature range.

The total blockade effect is an interference quantum effect reducing the two-particle correlation function of the total field, while the statistics for individual fields can be arbitrary. It has been shown that the total blockade effect can be observed in the correlated system of the polariton trimer, which is the bound state of the three polariton modes.