Lithuanian Journal of Physics

Title

Lietuvos mokslų akademija — 2025-03-24

Nonlinear exciton equation factorization for non-perturbative absorption modelling

Vytautas Bubilaitis — 2025-03-24

Various types of optical spectra of molecular systems are often analyzed via perturbative series expansion in the powers of optical field. The simplest absorption is related to the linear optical response. However, observed spectral features can be mislabelled if higher orders are not vanishing. High-intensity excitation field breaks the established assumption of quickly converging perturbative regime. Non-perturbative quantum methods can solve these problems. However, they lead to endless hierarchies of equations that, in general, cannot be solved analytically. Dropping terms at a specific order or factorizing (expressing high-order terms as products of several lower-order terms) can be used to close the hierarchy. We propagate the nonlinear exciton equations (NEE) with exciton–exciton annihilation (EEA) non-perturbatively in a high-excitation regime and calculate absorption spectra of a molecular aggregate using various factorization schemes. The results demonstrate that the solution is weakly sensitive to the factorization method when EEA is included.

Expression of special stretched 9j coefficients in terms of 5F4 hypergeometric series

Jean-Cristophe Pain — 2025-03-24

The Clebsch–Gordan coeﬀicients or Wigner 3j symbols are known to be proportional to a 3F2(1) hypergeometric series, and Racah 6j coeﬀicients to 4F3(1). In general, however, non-trivial 9j symbols cannot be expressed as 5F4. In this paper, we show, using the Dougall–Ramanujan identity, that special stretched 9j symbols can be reformulated as 5F4(1) hypergeometric series.

Electron-impact ionization for Kr atom

Aušra Kynienė — 2025-03-24

Electron-impact ionization cross sections for the ground level of the Kr atom are studied using the scaled distorted-wave (DW) approximation. It is demonstrated that the DW cross sections calculated in the potential of the ionizing ion overestimate the experimental data at low and medium energies of the impacting electron. The scaled DW results, that include in calculations the value of the ionization threshold provided by National Institute of Standards and Technology, lead to good agreement with the measurements. A negligible contribution from the indirect process of the ionization to the total ionization cross sections is obtained in the final results. The study demonstrates that the higher ionization stages appear as a result of the ejection of additional electrons from the atomic system by the sequential ionization.

Second-order Rayleigh–Schrodinger perturbation theory for the GRASP2018 package: Valence–valence correlations

Gediminas Gaigalas — 2025-03-24

The accurate description of electron correlations remains a major challenge in atomic calculations. In order to perform accurate calculations, it is necessary to consider various types of electron correlations and this often leads to extensive configuration state function (CSF) expansions. This work presents further development of the method based on the second-order perturbation theory to identify the most significant CSFs that have the greatest influence on core–valence, core, core–core and valence–valence correlations. This method is based on a combination of the relativistic configuration interaction method and the stationary second-order Rayleigh–Schrödinger many-body perturbation theory in an irreducible tensorial form [G. Gaigalas, P. Rynkun, and L. Kitovienė, Second-order Rayleigh–Schrödinger perturbation theory for the Grasp2018 package: core–valence correlations, Lith. J. Phys. 64(1), 20–39 (2024),
https://doi.org/10.3952/physics.2024.64.1.3, G. Gaigalas, P. Rynkun, and L. Kitovienė, Second-order Rayleigh–Schrödinger perturbation theory for the Grasp2018 package: core correlations, Lit. J. Phys. 64(2), 73–81 (2024), https://doi.org/10.3952/physics.2024.64.2.1, and G. Gaigalas, P. Rynkun, and L. Kitovienė, Second-order Rayleigh–Schrödinger perturbation theory for the Grasp2018 package: core–core correlations, Lith. J. Phys. 64(3), 139–161 (2024), https://doi.org/10.3952/physics.2024.64.3.1]. The method is extended to include additionally valence–valence electron correlations. It can be applied for an atom or ion with any number of valence electrons for the calculation of energy spectra and other properties. Meanwhile, the correlations which cannot be included according to perturbation theory are taken into account in a regular way. The use of the developed method allows a significant reduction of CSFs especially for complex atoms and ions. As an example of its application, the atomic calculations of the energy structure for Se III ion are presented.

High-frequency conductivity-based terahertz gain models in quantum semiconductor superlattices: A comparative study

Lukas Stakėla — 2025-03-24

The development of high-power, stable and portable terahertz (THz) sources that can operate at room temperature remains one of the biggest challenges in THz and solid-state physics. Despite modern semiconductor devices such as resonant tunnelling diodes and quantum cascade lasers demonstrating a significant progress, they still face several limitations related to a low power output, temperature sensitivity and the lack of frequency tunability. In this respect, semiconductor superlattices operating in the miniband transport regime continue to represent promising quantum materials for the realization of the desirable THz gain. In this study, we briefly overview basic semiclassical models describing the high-frequency conductivity of superlattices. We cover the popular model of Ktitorov et al. and the lesser-known and more advanced model of Ignatov and Shashkin, and also make their comparative analysis with reference to the classical quasistatic model of gain in devices with the negative differential conductivity. This work aims to offer a simple introduction to these models and their practical relevance to THz device design and development.