JETP Letters最新文献

In the studied crystals of FeSe_0.7Te_0.3, a structural phase transition occurs in two stages. At higher temperatures, the electronic subsystem undergoes a rearrangement, leading to a significant increase in elastoresistance. ⁷⁷Se nuclear magnetic resonance data show an abrupt change in the relaxation rate during this transition. The subsequent transition occurs at a temperature several degrees below and is also accompanied by anomalies in the electronic properties. Thus, for the phase diagram of Fe(Se,Te), as well as for the phase diagrams of FeSe under pressure, there is a region where structural transitions occur in two consecutive stages. We explain this similarity between the corresponding phase diagrams by the same deformation of the iron coordination environment in Fe(Se,Te) compounds and in FeSe under pressure. Our findings provide new and significant information on the phase diagram of Fe(Se,Te) compounds, and in particular, suggest the existence of specific orbital or structural instability in the basic structural element of iron-based superconductors.

The nonlinear interaction of 250-fs laser pulses at a wavelength of 1030 nm with polymethyl methacrylate is investigated using an optimized z-scan technique. It is shown that the mechanism of pulse attenuation for peak intensities below 0.65 TW/cm² can be described as four-photon absorption with the coefficient β₄ = (35 ± 5) cm⁵/TW³. The nonlinear refractive index n₂ = (7.6 ± 0.4) × 10⁻⁴ cm²/TW and the critical power for the onset of self-focusing P_cr = (1.42 ± 0.08) MW are determined and these results are used to demonstrate the correspondence between the calculated shift of the nonlinear focal point and the location of the laser-modified regions.

Using magnetospheric multiscale (MMS) mission observations in the Earth’s magnetotail, it has been shown that numerous secondary sources of electron acceleration are formed in a hot collisionless plasma, which is turbulized by a bursty bulk flow propagating from the primary reconnection region. Such sources may be regions of secondary microreconnections occurring at electron kinetic scales. The observed electrostatic solitons, which serve as markers of “secondary magnetic separatrices,” along which electron beams accelerated to superthermal energies propagate, similar to solitons near primary reconnection separatrices have been studied in this work.

Data on the efficiency of the volume capture of high-energy particles into channeling in a weakly bent crystal are presented. The volume capture of 50-GeV protons in a weakly bent silicon crystal with a bending radius of 5 m (60 critical radii) has been investigated at the U-70 accelerator (Protvino, Russia). The fractions of the deflected beam have been measured to be ((3.6 pm 1){kern 1pt} % ) and ((1.1 pm 0.3){kern 1pt} % ), depending on the particle deflection angle and the path in the crystal. The experimental data obtained are in good agreement with the theoretical calculations. The method for deflecting the beam using volume capture is promising for steering secondary particle beams with high angular divergence.

The DANSS detector is located near the power reactor at the Kalinin Nuclear Power Plant (at distances of 10.9–12.9 m) and is able to detect up to 5000 antineutrino events per day. The results of the search for Large Extra Dimensions in the simplest case of a single dominant large extra dimension are discussed. The particle oscillations to a finite-size extra dimension are assumed in this theory, and its predictions depend on the difference between the squared masses, as well as on the absolute scale of the neutrino masses. The sensitivity of the experiment to Large Extra Dimensions for various values of the model parameters such as the size of the large extra dimension a and the lightest-neutrino mass ({{m}_{0}}) can be determined by its Monte Carlo simulation. No statistically significant indications of the existence of Large Extra Dimensions have been obtained from the analysis of almost 5.8 million antineutrino events (the statistical confidence of the best point is only 2.0σ (1.8σ) for the normal (inverted) ordering of neutrino masses). The bounds have been determined for the size of the extra dimension and the lightest neutrino mass. These bounds for a number of regions are the best in the world. These bounds exclude a large fraction of the parameters preferable for explaining the gallium anomaly and the reactor antineutrino anomaly in this model, including the corresponding best points.

Magnetic hysteresis loops of two representative samples of synthetic ferrihydrite nanoparticles with the same sizes (the average size of ( approx )2.7 nm) and various interparticle distances have been studied under cooling conditions in the presence and absence of an external field. One initial sample is characterized by the aggregation of nanoparticles, and a shift in the magnetic hysteresis loop along the abscissa axis is observed after cooling from a temperature exceeding the superparamagnetic blocking temperature in the external field. The particles in another sample are spatially separated by coating their surface with an arabinogalactan layer, and the shift of the hysteresis loop after cooling in the external field is not observed in this sample. This experimental fact indicates that one of the important factors determining the shift of the hysteresis loop of nanoparticle systems is a pronounced subsystem of surface spins formed during the close contact of particles, which can be considered as a kind of the surface effect. Because of the exchange coupling between the subsystem of surface spins (common for a conglomerate of particles) and uncompensated moments of particles, an additional source of the unidirectional magnetic anisotropy arises during cooling in the external field, which is the origin of the observed exchange bias of the magnetic hysteresis loop.

The effects of the Stark interaction in the collective radiation of an ensemble of atoms in the field of a broadband electromagnetic field in a squeezed quantum state have been considered. It has been shown that the inclusion of the conditions for the formation of the squeezed field allows one to describe the dynamics of atoms in terms of random quantum creating, annihilating, and counting processes. The latter process is the Stark interaction of atoms and the field and manifests itself in the form of a trigger superradiance pulse generated by an ensemble of identical atoms excited by a quantized resonant squeezed electromagnetic field. The description is based on the kinetic equation derived for an open localized quantum system in a broadband squeezed quantized electromagnetic field. The resulting equation differs from the known ones, but agrees with them in particular and limiting cases.

The error arising in the description of the magnetic field is evaluated when the constraint is imposed that the external three-dimensional (3D) perturbation ({mathbf{b}}) does not penetrate into the tokamak plasma. Such a three-dimensional approach has been used in the plasma equilibrium evolutionary problems solved by the CarMa code [F. Villone, L. Barbato, S. Mastrostefano, and S. Ventre, Plasma Phys. Control. Fusion 55, 095008 (2013)], where the plasma is treated as a two-dimensional (2D) object, while the vacuum vessel wall is three-dimensional (3D). The toroidal surface separating the 2D and 3D regions is called the coupling surface (CS). This surface acts as a virtual casing, but with the additional condition ({mathbf{b}} = 0) imposed within the torus CS. Here, attention is attracted to the fact that, in a normal situation, the field ({mathbf{b}}) in the plasma-wall gap must depend on the plasma response. However, the prescription ({mathbf{b}} = 0) inside the torus CS eliminates this ambiguity. As a consequence, a discontinuity in the tangential component of ({mathbf{b}}) inevitably arises at the CS, necessitating the presence of a current on this surface. The magnitude of this fictitious current and its contribution to the magnetic perturbation ({mathbf{b}}) are estimated. It is shown that this current significantly influences both the magnitude and the distribution of the field ({mathbf{b}}).

We perform transport and noise measurements for device consisting of a thin NbSe₂ flake laid onto the predefined gold electrodes and covered with a thin hBN flake. In the shot noise of a NbSe₂/Au tunnel junction, we identify Andreev reflection regime by demonstrating the effective charge doubling. Further, by creating temperature gradient across the tunnel junction and measuring its delta-(T) noise in the normal state, we extract electron–phonon scattering length in NbSe₂ and its T-dependence. The results of delta-(T) noise measurements in the absence of a magnetic field when the flake is superconducting are in qualitative agreement with expectations. The introduced approach is promising for the study of nonequilibrium configurations in superconductors.

Generalized Rosenfeld–Tarazona scaling predicts the power-law dependence of the excess heat capacity of simple liquids on temperature. The two-phase model treats a liquid as a superposition of gas- and solid-like components whose relative abundance is quantified by a liquid rigidity parameter. We demonstrate here that the generalized Rosenfeld–Tarazona scaling emerges naturally in the two-phase model from the scale invariance of the liquid rigidity parameter.