Vol 166, No 2 (2024)

The study of interaction features between unipolar subcycle pulses and matter has shown the necessity of both revising standard theories of light-matter interaction and introducing new concepts in optics, such as pulse area interference. In this work, based on numerical solution of the time-dependent Schrödinger equation, we study the features of nonlinear interference of pulse areas during particle ex- citation in a rectangular potential well driven by a pair of half-cycle attosecond pulses. It is shown that when changing the delay between pulses, the population dependence of bound states on delay exhibits characteristic beating pattern, unlike the simple harmonic dependence obtained in the case of small field amplitude. The conducted research directly demonstrates the possibility of controlling quantum systems using sequences of half-cycle pulses, particularly the possibility of increasing ionization probability or its complete suppression and the possibility of inducing population difference gratings in multilevel media.

In the present study, we consider the effects of vacuum birefringence and dichroism in strong electromagnetic fields. According to quantum electrodynamics, the vacuum state exhibits different refractive properties depending on the probe photon polarization and one also obtains different probabilities of the photon decay via production of electron-positron pairs. Here we investigate these two phenomena by means of several different approaches to computing the polarization operator. The external field is assumed to be a linearly polarized plane electromagnetic wave of arbitrary amplitude and frequency. Varying the probe-photon energy and the field parameters, we thoroughly examine the validity of the locally-constant field approximation (LCFA) and techniques involving perturbative expansions in terms of the external-field amplitude. Within the latter approach, we develop a numerical method based on a direct evaluation of the weak-field Feynman diagrams, which can be employed for investigating more complex external backgrounds. The polarization operator depends on two parameters: classical nonlinearity parameter ξ and the product η = ωq0/m2 of the laser field frequency ω and the photon energy q0 (m is the electron mass). The domains of validity of the approximate techniques in the ξη plane are explicitly identified.

The process of photon splitting γ → γγ in a strong magnetic field considering the contribution of positronium to photon dispersion has been examined. It is shown that such conditions lead to the opening of a new reaction channel and changes in the selection rules for photon polarizations. The corresponding partial probabilities for allowed channels have been calculated. An estimate of the efficiency of the process under consideration has been obtained.

Using density functional theory calculations, the atomic mechanism of the influence of compressive strains formed on the Ge(111) – 7 × 7 surface of epitaxial layers , grown on Si(111) substrate, on the diffusion of Ge adatoms was investigated. It was found that the energy barrier limiting the migration of Ge adatoms over long distances is located near corner vacancies of the 7 × 7 structure and is caused by the formation of a covalent bond between the Ge adatom and a dimer atom within the 7 × 7 structure. It is shown that the barrier increase on the elastically compressed surface occurs due to strengthening of the dimer bond during surface compression, which leads to weakening of the bond between the Ge adatom and the dimer atom.

For a series of iron-doped polycrystalline high-temperature superconductors Y_1–xFe_xBa₂Cu₃O_y (0 ≤ x ≤ 0.05), synthesized using the nitrate-citrate variant of the sol-gel method, studies of structural (by X-ray and electron microscopy methods) and magnetic (in alternating and constant magnetic fields) properties were conducted. For these samples, the dependencies of crystallographic parameters, crystallite sizes, superconducting transition temperatures on the doping level were determined, as well as the type and magnitude of magnetization hysteresis in fields up to 6 T. Field dependencies of intracrystalline critical current density Jc were calculated. It was shown that uniform distribution of the dopant throughout the crystallite volume due to the application of the sol-gel method leads to significant improvement in functional parameters compared to samples obtained by solid-state method. The microstructure improves, which is manifested in increased sizes and more distinct crystallite faceting, as well as narrowing of the temperature interval of transition to the superconducting state, increase in the magnitude of magnetic field hysteresis of magnetization and critical current. As a result, in sol-gel samples with iron doping level x ≈ 0.03, an increase effect Jc exceeding an order of magnitude is achieved.

We put forward a generalized procedure which allows to restore the bulk-like electron and hole wave functions localized in certain valleys from the wave functions of quantum confined electron/hole states obtained in atomistic calculations of nanostructures. The procedure is applied to the lead chalcogenide quantum dots to accurately extract the intravalley velocity matrix elements and the constants of the effective intravalley Hamiltonian of the exchange interaction for the ground exciton state in PbS and PbSe quantum dots. Our results suggest that intravalley parameters in PbS quantum dots are much more anisotropic than the ones in PbSe. Renormalization of the velocity matrix elements, forbidden band gap, valley and exchange splittings of exciton and exciton binding energy are also calculated.