Journal of Experimental and Theoretical Physics最新文献

Abstract

A synthetic analog of rare secondary mineral namibite Cu(BiO)₂VO₄OH has been obtained by the hydrothermal method. The crystal structure of this compound contains isolated uniform chains of vertex-connected copper–oxygen octahedra. Magnetic susceptibility (χ) and magnetization (M) measurements have not indicated the long-range order in the temperature interval 2–300 K. Specific heat (C_p) measurements suggest the formation of a spin-liquid state at low temperatures. X-band electron paramagnetic resonance data recorded at low temperatures have demonstrated only a signal from impurities. First-principles calculations have estimated the exchange interaction in the chains as J = 555 K, whereas exchange interactions between the chains turn out to be one to two orders of magnitude smaller. Thus, namibite represents a rare example of an unordered half-integer spin chain.

Abstract

Spin pumping and angular momentum transfer, i.e., the emission of a spin current by a precessing magnetization and the reverse process of absorption, play an important role in coherent magnetic dynamics processes in multilayered structures. For ferromagnetic layers separated by a nonmagnetic interlayer these effects give rise to a dynamic coupling between the layers that is dissipative in nature and affects the damping of coherent magnetization precession. We have used micromagnetic simulations to analyze the influence of such a dynamic coupling on the propagation of a laser-induced surface magnetostatic wave (MSW) packet in a pseudo spin valve structure consisting of two ferromagnetic metallic layers separated by a nonmagnetic metallic interlayer. We have considered the MSW generation due to laser-induced heating, which leads to dynamic changes in magnetization and magnetic anisotropy, and added the dynamic coupling effect to the equations for our micromagnetic simulations. As a result, we have revealed that under certain conditions such a coupling leads to a decrease in the spatial damping of the wave packet that corresponds to the acoustic MSW mode forming in the structure considered.

Abstract

Films of metal-insulator nanogranular composites M_xD_{100 – x} with different composition and percentage of metal and dielectric phases (M = Fe, Co, CoFeB; D = Al₂O₃, SiO₂, LiNbO₃; x ≈ 15–70 at %) are investigated by magnetic resonance in a wide range of frequencies (f = 7–37 GHz) and temperatures (T = 4.2–360 K). In addition to the usual ferromagnetic resonance signal from an array of nanogranules, the experimental spectra contain an additional absorption peak, which we associate with the electron paramagnetic resonance (EPR) of Fe and Co ions dispersed in the insulating space between the granules. In contrast to the traditional EPR of Fe and Co ions in weakly doped non-magnetic matrices, the observed peak demonstrates a number of unusual properties, which we explain by the presence of magnetic interactions between ions and granules.

The influence of substitutional impurities on adhesion at the TiAl/Al₂O₃ interface with an oxygen termination has been studied by the projector augmented-wave method within the density functional theory. It has been shown that transition metals and a number of s,p-elements substituting for the interfacial titanium atom reduce adhesion, whereas Group VB and VIB elements enhance chemical bonding at the interface. The local densities of states, charge density distribution, overlap populations for interfacial atom bonding, and other electronic characteristics have been calculated that make it possible to reveal key factors influencing adhesion at the alloy–oxide interface. A correlation has been found between the influence of impurities on bonding energy at the inner and outer interfaces. A comparison of obtained data with those for the interface with Ti-enriched Ti₃Al alloy shows that the interface loses strength with decreasing Ti content in the alloy.

Time series of gamma-ray photons from natural radioactivity measured with the Large Volume Detector (LVD) at the Gran Sasso underground laboratory (Laboratori Nazionali del Gran Sasso, Italy) have been analyzed. The instrument is used to detect neutrinos from gravitational collapses of stellar cores in the Galaxy. The background of the experiment is due to neutrons and gamma-ray photons from the decays of daughter nuclei of uranium and thorium. An analysis of periodic variations of the number of gamma-ray photons caused by the radon concentration in the underground laboratory has shown the presence of diurnal, weekly, monthly, and annual modulations. Data collected from 2004 to 2021, including a period of low activity in the hall of the Gran Sasso laboratory during the COVID-19 pandemic, have been reported.

To date, the violation of T invariance (TI) has been experimentally established for the decays [J. H. Christenson et al., Phys. Rev. Lett. 13, 138 (1964)] and oscillations [A. Angelopoulos et al., Phys. Lett. B 444, 43 (1998)] of neutral kaons. Phenomenologically, TI violation in the system of neutral kaons is associated with the difference of the Kobayashi–Maskawa phase from zero (or from π) in the standard model of the electroweak interaction. For nucleon–nucleus interactions, this phase turns out to be very small [P. Herszeg, in Symmetries and Fundamental Interactions in Nuclei, World Scientific, Singapore (1995), p. 89)]. Estimates of the nucleon–nucleon matrix element corresponding to TI violation in various models are given in [V. Gudkov, in Fundamental Physics with Pulsed Neutron Beams, World Scientific, Singapore (2001), p. 117]. All these estimates are small and have a large spread. Therefore, the verification of TI in nuclear processes actually means a search for other mechanisms for its violation. Below is a description of the experimental methods for TI verification in the total interaction cross sections of low-energy resonance neutrons with unpolarized nuclei with the use of the polarization–asymmetry (PA) theorem.

The theory is developed and numerical simulation is performed for the coherent backscattering effect in a strongly inhomogeneous random medium with a finite spatial coherence length. It is shown using the Monte Carlo method that the limitation imposed on the number of scattering events corresponds to lowering of incident radiation coherence and leads to angular broadening of the backscattering peak, extending the possibility of using coherent backscattering in biomedical applications. Based on the diagrammatic technique, the modeling of coherent backscattering is developed for the first time beyond the frames of the ladder approximation.

A theoretical method for evaluating the single-electron capture (SEC) cross sections in collisions of fast ions with a ground-state H₂ molecule is presented. The scattering problem for ion-molecule collisions is formulated in the impact parameter representation using the relation between the quantum-mechanical amplitude and quasi-classical impact parameter one. The capture amplitudes and corresponding probabilities of capture to (nlm) states of an incident ion are derived within the framework of the Brinkman–Kramers approximation. The general expressions for the SEC probability amplitudes to n-states, summed over l and m quantum numbers, are deduced, from which the corresponding SEC probabilities can be then calculated using a procedure of multichannel normalization. The dependence of the differential cross sections, integrated over projectile impact parameters, on the molecular orientation for charge exchange in H⁺ + H₂ collisions is considered and compared with measurements and other calculations. Total SEC cross sections, integrated over the molecular orientations and summed over n-states for several bare and dressed ions, are calculated and compared with available experimental data and results of calculations by means of other theoretical methods.

The problem of finding the depolarization losses of neutrons is investigated in connection with the need to determine the systematic error for experiments with magnetic storage of ultracold neutrons in traps. In this paper, we consider three methods to estimate the neutrons depolarization probability: classical, quantum mechanical and approximate one. They are used to estimate the depolarization probability of ultracold neutrons in two magnetic traps: the trap developed in Los Alamos National Laboratory (LANL) and the one designed in the neutron physics laboratory of “NRC Kurchatov Institute”—PNPI. It is shown that all three methods are successfully used to estimate depolarization. This is of particular importance to compare of theoretical predictions with the results of measurement in experiments to determine the neutron lifetime.

The electrical hyperfine interactions of the ⁵⁷Fe probe nuclei stabilized in the structure of the BiMn₇O₁₂ manganite are studied by Mössbauer spectroscopy. Mössbauer spectra are measured in the para-electric temperature range, which includes the structural phase transitions I2/m ↔ Im(bar {3}) (T₁ ≈ 600 K) and Im ↔ I2/m (T₂ ≈ 450 K). The calculation of the parameters of the electric field gradient tensor with allowance for the dipole contributions of Bi³⁺ cations in the range of the first phase transition allowed us to confirm a random orientation of the dipole moments p_Bi in the cubic phase of the manganite (Im(bar {3})). Based on an analysis of the Mössbauer spectra recorded at T₂ < T < T₁, we considered various scenarios for the manifestation of the dynamic Jahn–Teller effect, which leads to the “melting” of the orbital order in the manganese sublattice, in terms of a two-level relaxation model.