Vol 165, No 2 (2024)

The most likely candidate for the role of a nuclear optical standard is the isomer of the nuclear isotope 229mTh with an energy of 8.338 eV. The possibility to specify the value of this energy by resonant optical pumping through an electron bridge was discussed. Proper use of the natural atomic linewidths, which are many orders of magnitude larger than the natural nuclear isomeric linewidth, is critical. Recent studies have shown that broadening due to internal conversion in neutral thorium atoms leads to a gain in scan time of nine orders of magnitude, facilitating the search for electron-nuclear resonance to a feasible level. The reverse resonance conversion method proposed in this article is applicable to ionized thorium atoms. It has the potential to improve experimental efficiency by orders of magnitude. The implementation of this method requires simultaneous excitation of the nucleus and electron shell in the final state. The causal connection between this principle and the solution to the thorium riddle is shown.

Femtosecond lasers are widely used in scientific research and modern technologies. When applied to metals, ultrashort optical laser radiation produces a pronounced two-temperature state with hot electrons: Te >> Ti, where Te and Ti are the temperatures of the electron and lattice subsystems. Our experimental measurements were carried out using phase-sensitive (lock-in) detection technique on bulk and film (100 nm thick) gold targets. Due to the fact that in our experiments the repetition rate of heating (pump) pulses was reduced to 31 Hz, we were able to reach lattice temperatures near the melting point of gold. This occurs at the exit of the two-temperature stage in bulk targets. As we know, at the end of this stage, the temperatures converge, Te ≈ Ti. In bulk targets, at the highest fluences we achieved, the peak electron temperature increases to values around 20 kK. Theoretical calculations available in the literature give certain dependences for the electron-phonon coupling parameter a and the electron thermal conductivity coefficient k; they are the key parameters that characterize the two-temperature state. Our experiments showed that in the range of fluences with peak temperatures Te above 10 kK and up to 20 kK, the measured values of a and k are significantly lower than than the values given by theories. Below this range of fluences, i.e., when the peak Te is less than 10 kK, our measured values are in agreement with previous data. This is the first result of the paper. In addition, it is shown that at one-temperature stage, when the thermal energy stored in the electrons is very small, there is a significant influence of the fundamentally two-temperature coefficient a on heat transfer from the skin layer. This is due to the relatively small thickness of the heated layer, which is of the order of 200-300 nm in gold.

Increasing the efficiency and stability of femtosecond pulses generation in solid-state lasers has great significance for technological processes. At the same time, there is a need for research into a number of physical issues. The variable action of the instantaneous Kerr nonlinearity in the crystal, necessary for passive mode locking of the resonator, and the dispersion of the prisms, providing generation of ultrashort pulses, inevitably leads to regular perturbation of the shape of the generated pulses. In our work, we study the loss transformation regimes in a passive mode locking femtosecond laser pulses oscillator on a Mg₂SiO₄:Cr⁴⁺ crystal (chromium-forsterite) when the intracavity peak field power of the order of 2 MW, close to the critical self-focusing power. Analysis of the spectra and pulse durations in various parts of the cavity shows that the quasi-soliton pulse generation regime with the maximum peak power for the laser is supported by removing excess energy from the cavity through the generation of spectral Kelly sidebands and broadening the pulse spectrum beyond the gain of the active medium. A strong broadening of the pulse spectrum in the crystal upsets the balance of dispersive and nonlinear phase shifts and leads to deformation of the generated pulse shape. Additional passive losses arising due to nonlinear transformation in the crystal significantly reduce the efficiency of laser generation and limit peak power of the pulses.

Based on a critical analysis of the traditional theory of homogeneous nucleation of incoherent precipitates of a new phase in solid solutions, it is shown that the elastic energy associated with a difference in the atomic volumes of two phases does not contribute to the nucleation barrier due to the absorption of thermal point defects at the particle-matrix interface (in contrast to the traditional approach). Correspondingly, a new kinetic model is developed for the rate of nucleation of incoherent precipitates in a supersaturated solid solution of alloying atoms, which has also been generalized to take into account excess vacancies formed under non-equilibrium conditions of quenching tests.

Localization of electrons in 1D disordered systems is usually described in the random phase approximation, when distributions of phases j and q, entering the transfer matrix, are considered as uniform. In the general case, the random phase approximation is violated, and the evolution equations (when the system length L is increased) contain three independent variables, i.e. the Landauer resistance r and the combined phases y = q j and c = q + j. The phase c does not affect the evolution of r and was not considered in previous papers. The distribution of the phase y is found to exhibit an unusual phase transition at the point Ɛ₀ when changing the electron energy Ɛ, which manifests itself in the appearance of the imaginary part of y. The resistance distribution P(r) has no singularity at the point Ɛ₀, and the transition looks unobservable in the electron disordered systems. However, the theory of 1D localization is immediately applicable to propagation of waves in single-mode optical waveguides. The optical methods are more efficient and provide possibility to measure phases y and c. On the one hand, it makes observable the phase transition in the distribution P(y), which can be considered as a “trace” of the mobility edge remaining in 1D systems. On the other hand, observability of the phase c makes actual derivation of its evolution equation, which is presented below. Relaxation of the distribution P(c) to the limiting distribution P_∞ (c) at L → ∞ is described by two exponents, whose exponentials have jumps of the second derivative, when the energy Ɛ is changed.

The evolution of the magnetic hysteresis loops of the granular high-temperature superconductor YBa₂Cu₃O_7−δ with varying the maximum external applied field Hmax has been experimentally studied. In the range of weak fields (up to ∼10 Oe at a temperature of 78 K), the small hysteresis loop is observed, associated with diamagnetism and the penetration of Josephson vortices into the subsystem of intergranular boundaries, which is a Josephson medium. With further growth of Hmax, the larger magnetization hysteresis loop appears, associated with the penetration of Abrikosov vortices into superconducting granules. When analyzing the experimental data, a non-trivial fact was discovered: the magnetic response from the subsystem of intergranular boundaries becomes less noticeable with increasing Hmax, and at a certain value of Hmax this response disappears. This occurs even though the small hysteresis loop at small values of Hmax is comparable to the magnetic response of superconducting granules. The described evolution of magnetic hysteresis is explained using the concept of an effective field in an intergranular medium. The total magnetic field in the subsystem of intergranular boundaries is determined not only by the external field, but also by closing fields from the magnetic moments of superconducting granules. In other words, the interaction between the superconducting subsystems of granules and intergranular boundaries leads to the small hysteresis loop in sufficiently small fields, and to its complete disappearance with increasing magnetization modulus of superconducting granules.

Screened electrostatic and van der Waals interactions of nano- and micron-sized particles in dusty plasma were considered. The electrostatic interaction is considered on the basis of the linearized Poisson-Boltzmann equation for particles both with fixed charges uniformly distributed over their surfaces and with fixed surface electric potentials. The found solution of the problem makes it possible to study the interaction of both particles of comparable radius and particles of very different sizes. The interaction force takes into account the osmotic component, which in the case of constant charges leads to the restoration of the equality of the forces acting on the first and second particles. For the van der Waals interaction, the screening of static fluctuations and the retardation of electromagnetic fields for the dispersive part of the interaction were taken into account. Based on the analysis of various expressions for the geometric factor, taking into account the retardation of the electromagnetic field, a numerically stable method for calculating this factor was proposed. The total energy of interaction of two charged dust particles is calculated for plasma parameters characteristic of dusty plasma: the electron and ion number densities from 10⁸ to 10¹² cm^-3, the particle radius from 10 nm to 1 μm and the particle charges from 10 to 10³ elementary charges per micron of particle radius.