Georgi Nadjakov Institute of Solid State Physics

Abstract We report on a new series of polycrystalline perovskite-related compounds from the A n B n O 3n-2 homologous series with general formula PbBaFe 2-x Co x O 5 (x = 0, 0.25, 0.5, 0.75, 1). The compounds were successfully synthesized by employing a modified solution-combustion technique. An additional experimental information was obtained by neutron diffraction, confirming that the studied compounds crystallize in the orthorhombic space group Pnma and are isostructural with the end member PbBaFe 2 O 5 . At 300 K, an antiferromagnetic (AFM) structure with a propagation vector k = (0, ½, ½) was determined to develop in both undoped PbBaFe 2 O 5 and low-doped PbBaFe 1.75 Co 0.25 O 5 . For the low doped composition, the saturated effective magnetic (Fe,Co) moment at T=10 K was refined to M s =4.05±0.02 μ B and the antiferromagnetic transition temperature was estimated as T N =561±10 K.

Femtosecond laser processing has emerged as a promising post-deposition method for tailoring the properties of dielectric thin films, offering localized modification without thermal damage. This study investigates the effect of sub-ablative femtosecond laser irradiation on the nonlinear optical response of a TiO2 single-layer coating deposited on soda-lime glass by electron-beam evaporation. The coating was modified using 35 fs pulses at 800 nm delivered at a repetition rate of 1 kHz and a fluence of 0.083 J/cm2 while varying the number of pulses per spot. The effective nonlinear refractive index (n2,eff) and effective nonlinear absorption coefficient (βeff) were measured using the z-scan technique with femtosecond excitation. The as-deposited TiO2 coating exhibited a negative effective nonlinear refractive index, signifying a self-defocusing nonlinear response, while femtosecond laser irradiation leads to pronounced changes in the effective nonlinear parameters. An increase in the magnitude of both effective nonlinear coefficients and a reversal of the sign of the effective nonlinear refractive index are experimentally observed after irradiation with higher pulse numbers. These findings provide experimental evidence that sub-ablative femtosecond laser processing can be used as a post-deposition tool to control the nonlinear optical response of TiO2 thin films.

Many biomedical applications require a detailed understanding of the action of antimicrobial peptides on biological membranes. The cationic hemolytic peptide melittin, a major component of European honey bee (Apis mellifera) venom, is considered a model for elucidating lipid-protein interactions that are important for the function of biological systems. Here, we address the surface properties of human erythrocytes and rat liver mitochondrial membranes under in vitro melittin treatment. These membranes are negatively charged at neutral pH and represent primary targets of melittin's effects in the onset of inflammatory diseases. The correlation between the functional activity of membrane systems and their surface electrical charge was assessed using microelectrophoresis, hemolysis assays, membrane transport measurements, lipid peroxidation analysis, and fluorescence microscopy. A mechanistic hypothesis for the divergent effects of sub-lytic, pre-pore doses of melittin on erythrocytes and mitochondria is discussed. At low concentrations, melittin interacts electrostatically with erythrocyte membranes, resulting in altered proton transport through the Band 3 protein. Melittin also induces changes in erythrocyte morphology and malondialdehyde content, as well as aggregation of mitochondrial vesicles. The electrokinetic mechanism of melittin action, associated with membrane stability, provides a novel perspective on its potential relevance to biomedical applications.

Dual-mode 1060 nm VCSELs based on oxide-confined elliptical apertures are presented here, showing an orthogonal polarization suppression ratio (OPSR) of 11.3 dB and a high 3 dB bandwidth of 33.6 GHz. 100 Gbps (PRBS15) PAM-4 signal transmission with a transmitter and dispersion eye closure quaternary (TDECQ) of only 0.98 dB and an energy to data radio (EDR) as low as 99.6 fJ/bit is reported. At the 106 Gbps IEEE standard, the TDECQ is 2.35 dB with an EDR of 125.15 fJ/bit. For 67 Gbps NRZ, the signal-to-noise ratio (SNR) is still 6.22, the extinction ratio (ER) is 4.75 dB, and the EDR is 198 fJ/bit. These VCSELs are highly promising for large bit rates, and more energy-efficient optical interconnects.

Agreement is found between the experimentally and theoretically determined branch point level for ZnO located (2.7 ± 0.1) eV above the valence band maximum. This result is obtained on the basis of a combined study where its experimental position is extracted from temperature‐dependent capacitance versus voltage measurements on AZO/n‐Zn(1−x)MgxO:Al/SiOx/p‐Si heterostructures with Mg content equal to ≈0.07 and ≈0.23 combined with transmission electron microscopy and ellipsometry measurements to assess the SiOx interlayer contribution. This analysis confirms the values previously published in Schifano et al. J. Appl. Phys. 136, 245304 (2024). In contrast, density‐functional theory calculations are used for obtaining the theoretical estimates. A similar comparison, in the MgO case, reveals a difference of ≈0.6 eV between the theoretically predicted and experimentally obtained values placed (4.3 ± 0.1) and (3.7 ± 0.4) eV above the valence band maximum, respectively. In this case, the assumption of a linear variation of the ZnMgO branch‐point level between the two binary compounds, ZnO and MgO, used for the extraction of the experimental value, is among the probable causes of the residual difference found. Overall, the presented results indicate that the whole set of p(O) related valence bands should be included for a precise determination of the branch point energies if ab‐initio methods are used.

This paper is devoted to studies of the conversion mechanism of carbon-containing fuels on multicomponent catalyst with the composition of 1.9 wt. % Pt–Ce 0.75 Zr 0.25 O 2 (Pt/CZ) using in-situ Raman spectroscopy (RS) and catalytic activity measurements. It was found that during heating and transition from inert atmosphere to reducing conditions, oxygen escapes from the crystal lattice of the samples. This leads to formation of oxygen vacancies in the near-surface layer of the catalyst and increasing Ce 3+ concentration by more than 20 % already at a temperature of 400 ◦ C. Meanwhile, when returning to the initial external conditions (temperature, gas mixture), a reversible process occurs. The use of Pt/CZ catalysts allows to obtain up to 35 vol % H 2 and about 25 vol % CO in syngas during dimethyl ether and dimethoxymethane partial oxidation. Thus, Pt/CZ catalysts are of particular interest as efficient catalysts for the conversion of hydrocarbons to synthesis gas, suitable for feeding SOFC stacks.

This work presents new results and conclusions on nanomembrane filtration and reverse osmosis of Mavrud red wine, produced in Bulgaria. The experiments were focused on lowering the alcohol content while preserving the valuable substances in the wine. Commercially available nanomembranes were used (Alfa Laval NF99HF, Alfa Laval RO99, NADIR NP030P). Two modes of nanofiltration (concentration mode and diafiltration mode, including constant volume diafiltration and two-step diafiltration) and reverse osmosis were employed for this study. The nanofiltration membranes (Alfa Laval NF99HF, NADIR NP030P) used for wine dealcoholization showed high separation effectiveness. Several wine components were recognized as indicators to be monitored during the process: carboxylic acids (citric, tartaric, malic, succinic, acetic); monosaccharides (glucose, fructose); alcohol (ethanol). The monitoring of the named compounds was performed with an HPLC-RID system on an H-charged ion exclusion analytical column. Based on the analysis of the collected samples, it could be stated that the alcohol content in the wine was lowered from 11.8% to 4.3 vol% of ethanol, when the sequential diafiltration mode of operation is used. Content change depends on the type of molecule; for example, in most cases the citric acid is strongly retained (Rej > 90%) by the membrane, whereas the acetic acid could permeate significantly (Rej < 20%). The obtained results present valuable information about the changes in the composition of the Mavrud wine which will aid in the preservation of the chemical composition and valuable substances in the event of future full or partial dealcoholization of this wine variety.

Since the early days of silicon manufacturing, hydrogen gas treatment has been used to control the defect concentrations. Its beneficial effect can be enhanced using hydrogen plasma as a source of active atomic hydrogen. Hydrogen plasma modification of c-Si surface can be challenging because the plasma can induce precursors of defect centers that can persist at the interface and/or grown oxide after subsequent thermal oxidation. In the present study, we investigate nanoscale silicon dioxides with thicknesses in the range of 6–22 nm grown at low temperature (850 °C) in dry oxygen on radio frequency (RF) hydrogen plasma-treated silicon surface. The properties of these oxides are compared to oxides grown following standard Radio Corporation of America (RCA) Si technology. Electroreflectance measurements reveal better interface quality with enhanced electron mobility and lowered oxidation-induced stress levels when the oxides are grown on H-plasma modified c-Si substrates. These results are in good accordance with the reduced defect concentration established from the analysis of the current–voltage (I-V) and multifrequency capacitance–voltage (C-V) characteristics of metal-oxide-semiconductor (MOS) capacitors incorporating the Si-SiO2 structures. The study proves the potential of hydrogen plasma treatment of Si prior to oxidation for various Si-based applications.