Bulletin of the Lebedev Physics Institute最新文献

We report a study of the intensity distribution of spiral light beams in the form of a closed curve, constructed using a holographic method and a spatial light modulator. The limitations imposed on the spatial spectrum of the beams formed by a liquid crystal phase spatial light modulator are determined. Along with a qualitative assessment of the experimentally obtained intensity distributions of the studied beams, the values of the mean square error, peak signal-to-noise ratio, and structural similarity coefficient are presented.

We report a study of the vibrational spectra of ortho-xylene in pure form and in solutions with chloroform at room temperature and atmospheric pressure. Raman spectra of binary mixtures of ortho-xylene and chloroform with concentrations of 0.9, 0.7, 0.5, 0.3 and 0.1 mole fractions are analyzed. It is found that the 733 cm^‒1 band of symmetrical stretching vibrations of CH₃ groups in pure ortho-xylene shifts toward lower frequencies (728 cm^‒1) when chloroform is added to the solution. The main reason for the shift is the interaction of the π-electrons of the aromatic ring with the hydrogen of the CH-group of chloroform (CH…π interaction), leading to a decrease in the vibration frequency. Furthermore, a noticeable shift of the band associated with C–H stretching vibrations is observed in the region of 3011 cm^–1, which also indicates a change in the nature of the intermolecular interactions between the components of the solution. Furthermore, the spectrum reveals an additional shift in the region of 3044 cm^–1, confirming the influence of solvation effects: the C–H dipole of chloroform interacts with the π-electrons of ortho-xylene, changing the C–H bond length and, consequently, shifting the vibrational frequency toward higher frequencies. The exchange of charges between atoms during the formation of the complex is analyzed. The structural parameters of molecules, molecular electrostatic potential (MEP), and Mulliken distribution of atomic charges are studied using the density functional theory (DFT) method.

It is shown that many anomalies observed in underdoped cuprates, including anomalous spectral weight transfer and a large pseudogap, appear to have a common nature due to both the cluster structure of the underdoped phase and the specific mechanism of superconducting pairing. The combined action of these factors leads to the fact that at a temperature T lying in a certain temperature range T_c < T < T*, the crystal contains small isolated clusters that can exist both in superconducting and normal states, randomly switching between them. In this case, below T_c with a very high probability the cluster is in a superconducting state, and above T* it is in a normal state, and the interval T_c < T < T* is the region of the so-called pseudogap phase. At a given T in the same temperature range, the time sequence of randomly arising superfluid density pulses from each cluster can be represented as a random process. The effective width Δω_eff of the spectrum of such a random process will be determined by a correlation time, i.e. the characteristic time between successive on/off superconductivity in two different clusters. This time, according to the estimate, is ~10^–15 s, which corresponds to Δω_eff ~ 1 eV and explains the effect of spectral weight transfer to the high-frequency region. This approach also makes it possible to explain other anomalies observed in the vicinity of T_c: the reversibility of magnetization curves in a certain temperature range below T_c, the anomalous Nernst effect and anomalous diamagnetism above T_c.

The effect of the duration of ultrashort laser pulses with a wavelength of 1030 nm on the process of their nonlinear absorption in polymethylmethacrylate is studied. It is found that the dependence of absorption on pulse duration is nonlinear, with an increase in the range from 250 to 450 fs, a decrease from 450 to 1500 fs, and insignificant changes after 1500 fs.

The paper examines the evolution of weak perturbations excited at the boundary of a “warm” coronal loop in a strong magnetic field. The primary focus is on the effect of thermal misbalance caused by heating and radiative cooling processes on the dispersion properties and energy distribution between slow magnetoacoustic and entropy modes. An exact analytical solution to the boundary value problem is obtained using the Fourier method and Duhamel’s principle, allowing one to analyze the dependence of the energy distribution on the characteristic times of the thermal misbalance. It is shown that local heating and cooling parameters determine not only the phase velocities and wave decrements but also the initial energy contribution of each mode. The obtained results can be used to interpret observational data and diagnose plasma parameters in the solar corona.

Integral cross sections and mean energy loss of slow protons and electrons in water are calculated in the framework of the photo-absorption ionization model. The calculation results are compared with experimental data. Practical applications of the model are discussed.

The paper presents a method for numerically simulating the synchrotron radiation of relativistic electrons in a turbulent, inhomogeneous galactic magnetic field. Using the developed software, maps of the synchrotron background distribution on the celestial sphere are constructed. Particular attention is paid to a multipole analysis of the obtained maps, which make it possible to investigate the influence of the distributed source generation depth on the formation of the angular structure of radiation. It is shown that at shallow generation depths, low-frequency harmonics dominate the multipole decomposition spectrum. A pronounced anisotropy is observed, due to the limited spatial scales. With increasing depth, the multipole decomposition spectra become smoother and closer to isotropic, reflecting the averaged nature of the turbulent medium. It is found that for deep generation of the spatial structure of distributed sources, changes in the turbulence spectrum slope significantly affect the shape of the angular power distribution, leading to a redistribution of energy between large-scale and small-scale components. The results highlight the importance of scaling parameters in simulating the structure of the synchrotron background and may be useful for interpreting radio astronomical observations.

In this work, the dynamic behavior of polyvinylpyrrolidone (PVP) hydrogel (2%) over time from the moment of dilution was studied using the dynamic light scattering (DLS) method. The stability of the dynamic characteristics, the persistent presence of a power-law component in the autocorrelation function (ACF) of scattered light intensity, and the appearance of artifactual relaxation times when using an exponential expansion program for the ACF were demonstrated. The fractal dimension of the PVP gel was determined to be D_f = 1.81.

The Monte Carlo method is used to analyze a vortex system of magnesium diboride MgB₂ with a triangular lattice of point defects. Magnetization reversal curves and current‒voltage characteristics are calculated for samples with different pinning center densities. Matching peaks (peak effects) are observed during magnetization reversal, which persist with increasing temperature. The dependences of the critical current on the external magnetic field at a temperature of 30 K are obtained, which are consistent with published data for MgB₂ films grown by hybrid physical-chemical vapor deposition (HPCVD).

The paper proposes and investigates a method for increasing the sensitivity of laser-induced breakdown spectroscopy by directed freezing of particle suspensions. When certain freezing conditions are met, particles are displaced by the phase-interface front and are concentrated on the surface, thus increasing the intensity of emission lines. This method implemented two freezing configurations wherein one results in the formation of a densely packed flat layer of particles and the other concentrates particles in a small volume on the sample surface. In the first case, with unidirectional front movement, the signal is amplified to a maximum of 15 to 20 times compared to a liquid sample. The concentration in a small volume made it possible to increase the intensity by almost an order of magnitude compared to unidirectional freezing with low particle concentrations. This method can be used both for studying the elemental analysis of the particles themselves and the substances adsorbed on their surface.