JETP Letters最新文献

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.

We study the interplay of electron–phonon and electron–impurity scattering in graphene and its manifestations in thermopower S. Electron scattering by acoustic phonons dominating in clean samples results in energy-independent carrier diffusivity, which translates into zero value of S. Inclusion of charged impurities plays a twofold role. At low impurity densities, the diffusivity becomes energy-dependent, which elevates S. In largely disordered samples, the carrier density becomes inhomogeneous, which results in self-averaging of thermopower. The latter effect is tackled here with effective medium theory, and predicts a slow drop in (S) with increasing the impurity density. The competition of these effects results in thermopower maximization at an optimal density of impurities ({{n}_{{{text{imp}}}}}). The latter is estimated as (2 times {{10}^{{11}}}) cm^–2 at room temperature, which corresponds to the highest-quality chemical vapor deposited graphene.

Differential measurements of the fundamental constant (mu = {{m}_{e}}{text{/}}{{m}_{p}}) (the electron-to-proton mass ratio) for two sources near the Galactic Center—the Sgr B2(N) and B2(M) molecular clouds—suggest that (mu ) is lower in these clouds than its laboratory value. Based on observations of methanol (CH₃OH) emission lines in the 80–112 GHz range (data from the IRAM 30-m telescope), a weighted mean value (langle Delta mu {text{/}}mu rangle ) ( equiv ) (langle ({{mu }_{{{text{obs}}}}} - {{mu }_{{{text{lab}}}}}){text{/}}{{mu }_{{{text{lab}}}}}rangle = ( - 2.1 pm 0.6) times {{10}^{{ - 7}}}) ((1sigma )) was obtained for Sgr B2(N) at the sample size (n = 9). This value of (Delta mu {text{/}}mu ) has the same sign as the result of recent measurements of methanol lines in the higher frequency range of 542–543 GHz (data from the Herschel space telescope) for Sgr B2(N): (langle Delta mu {text{/}}mu rangle = ( - 4.2 pm 0.7) times {{10}^{{ - 7}}}) (sample size (n = 2)).

We present experimental study of the iron-free palladium based analogue of the 122 iron arsenide. Single crystals of BaPd₂As₂ with sharp superconducting transition were studied by two distinct techniques: local magnetization and Break–Junction. Local magnetization was used to obtain temperature dependence of the first critical field (({{H}_{{{text{c1}}}}})) from ({{T}_{{text{c}}}}) down to 10 mK. Extracted data was fitted with various models to obtain order parameter value and draw a conclusion about its potential symmetry. The ({{H}_{{{text{c1}}}}}(T)) data fitting with theoretical models suggested either a single (s)-wave gap with high anisotropy factor or two s-wave gaps. Multiple Andreev Reflections spectroscopy was used to determine the quantity of condensates and their order parameter amplitudes. The experimental results of Multiple Andreev Reflections showed the presence of two gap-like features. Studying Andreev spectra temperature evolution from 1.6 K up to ({{T}_{{text{c}}}}) we’ve extracted the gap temperature dependencies. Further fitting proved the presence of two superconducting condensates. The obtained evidence for two gap superconductivity in iron-free analogue of 122 iron arsenide family is reported for the first time. Despite the two gap superconducting state, the characteristic ratio of the large gap value is almost equal to that typical for Bardeen–Cooper–Schrieffer theory; this result shows that BaPd₂As₂ is a conventional superconductor unlike BaFe₂As₂.

Generation of photocurrent via photon drag effect enables very fast light detection with response time limited by momentum relaxation. At the same time, photon drag in bulk uniform samples is small by the virtue of small photon momentum. We show that the edge of metal gate placed above a two-dimensional electron system provides highly non-uniform electromagnetic field that enhances the drag effect. We study the drag photovoltage using an exact solution of diffraction problem for two-dimensional electron system with semi-infinite metal gate. We show that the only non-trivial dimensionless parameters governing the drag responsivity are the two-dimensional electron system conductivity scaled by the free-space impedance (eta ) and gate-two-dimensional electron system separation scaled by the incident wavelength (d{text{/}}{{lambda }_{0}}). For radiation with electric field polarized orthogonal to the gate edge, the responsivity is maximized for inductive two-dimensional conductivity with ({text{Im}}eta sim 1) and ({text{Re}}eta ll 1), and becomes very small for the capacitive two-dimensional conductivity. The electromagnetic ponderomotive force pushes the charge carriers under the gate at arbitrary two-dimensional conductivity, and the force direction is opposite to that at metal-two-dimensional system lateral contact. These patterns are explained by the dominant role of gated two-dimensional plasmons in the formation of photon drag photovoltage.

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.