Articles in "Physics and astronomy"

Summary: This scientific work is devoted to study the spatial structure and conformational possibilities of the tripeptide Phe1-Gly2-Gly3 and tetrapeptide Phe1-Gly2-Gly3-Phe4 molecules. The knowledge of the structural and functional properties of these peptide molecules is of great practical importance for medicine and pharmacology. This neuropeptide molecules are stable analogues of the nociceptin molecule. The calculations were carried out by the method of theoretical conformational analysis with regard to nonvalent, electrostatic and torsional interactions, energy of the hydrogen bonds and a special computer program. The low-energy conformations of these molecules and the values of the dihedral angles of the main chains and side chains are found and the energy of the intra- and inter-residue interactions were estimated. 15 low-energy conformations were found for tripeptide for four spatial structures and 12 low-energy conformations were found for tetrapeptide for six spatial structures. It is revealed that low energy conformations of these molecules have the half-folded and folded type of backbone. The side chains of the Phe1 and Phe4 amino acids in low-energy conformations carry out effective interactions and are conformationally labile amino acids. They bring together sections of the main chain and side chains of amino acids that are part of the tri- and tetrapeptide, which leads to important interactions.

Summary: The nature of the interaction in the ????????????????−???????????????? quasibinary section was studied, the solubility region of ???????????????? in ???????????????? was determined, ????????????????????1−???????????? solid solutions were obtained, complex physicochemical analyses were carried out and crystal parameters of the sample were determined. The specific electrical conductivity, Hall coefficient, and magnetic resistance of ????????????????????1−???????????? solid solutions were studied in the temperature range of 300-650 K, and the charge transport mechanisms were investigated. The dependence of magnetic resistance on composition, temperature, and magnetic field intensity was studied. The chemical bond and crystallization type structure in Tin monoselenide has also been investigated. Using a special X-ray diffractometer, the X-ray structural analysis was carried.

Summary: In this study, silver nanoparticles (AgNPs) were synthesized via chemical reduction. Then Ag-Ag2S core-shell structures were synthesized by simply mixing different concentrations of Na2S aqueous solution (5,10 and 15mM) and AgNPs. The structural and optical properties of these structures were analyzed. Changes in the structure of the samples were analyzed by X-ray diffraction (XRD). The optical properties and bandgap values were studied using ultraviolet-visible (UV-Vis) spectroscopy. The structural and optical properties of AgNPs revealed clear differences in their physical properties after sulfidation. From the study of optical properties, we determined that Ag-Ag2S core-shell structures have broadband absorption properties that can be controlled by changing the concentration of sulfur ions in the sulfidation process. Also, because of sulfidation, it was determined that the value of the bandgap of Ag-Ag2S structures changes.

Summary: Rubiscolin molecules belong to the class of opioids derived from food substances. To understand the various physiological functions they perform, to target them purposefully, and to synthesize artificial analogs that perform specific functions of the natural molecule, it is necessary to study their three-dimensional spatial structures. The spatial structures of rubiscolin molecules and their analogs were investigated using theoretical conformational analysis methods. The potential energy of the molecule was chosen as the sum of non-valent, electrostatic, torsion interaction energies and hydrogen bond energies. The spatial structures of [Ile3]- and [Phe3]-rubiskolin-5 molecules were studied in the context of the low-energy conformations of the natural rubiscolin molecule. The calculations revealed that the solid structure of both analogs is represented by eight lowenergy conformations, similar to those of the natural rubiskolin-5 molecule. It has been shown that the energy and geometric parameters of the molecules in both analogues are the same as in the natural rubiscolin-5 molecule, therefore it is not advisable to propose to synthesize both artificial analogues.

Summary: K0,945Rb0,055NO3 monocrystals were grown from aqueous solutions of KNO3 and RbNO3 by isothermal crystallization method and the morphology of crystal growth during III phase transformations in these crystals was studied by optical microscopy. It was determined that two polymorphic transformations occur in the studied crystal from room temperature to the melting temperature. The structural transformations in these crystals are enantiotropic and of the monocrystal-monocrystal type. The equilibrium temperature between the interconverting II and I modifications is T=4550.5K. In the investigated sample, a repeated III phase transformation was studied, and the growth of the process was observed in a mono-nucleated manner in the 001 crystallographic direction. This transformation was accompanied by the movement of the non-linear boundary separating the phases. The transformation rate is as 〖2−????[001]>1−????[001]. Experiments show that the rhombohedral modification existing between modifications I and II in potassium nitrate is not detected in the studied sample. Partial replacement of K+ ions with Rb+ ions increases the equilibrium temperature between modifica-tions II and I by about 55 K.

Summary: The dielectric properties and power dissipation of PtSi/n-Si Schottky diodes of small area (8x10-6 cm2) were studied when the bias voltage varied in the range -2V ÷ 4V at room temperature. The amplitude of the alternating signal (Vac) varied from 5 mV to 1x103 mV. Studies based on impedance measurements revealed the dependence of dielectric parameters (ε', ε″), ac-conductivity (σac) and power dissipation (P) only at Vac = 200 mV. The obtained result is explained by the inhomogeneity of polarization and surface states.

Summary: In this paper, the magnetization of diluted magnetic semiconductor superlattices with a magnetic impurity of manganese is studied. It was found that the magnetization of a quasi-two-dimensional electron gas, depending on the degree of filling of the miniband superlattices, the molar concentration of the impurity, the exchange interaction constant and the spin splitting factor, changes sign and in a strictly two-dimensional case becomes positive. In a magnetic field, magnetization oscillates, and in strong magnetic fields the oscillations weaken, and their amplitude and frequency decrease. The contribution of the impurity to the magnetization is calculated. In a relatively weak magnetic field the magnetization associated with the impurity increases linearly and at a certain condition to the depending on the magnitude of the exchange constant and the impurity concentration changes its sign.

Summary: In this work have been presented the result of investigation of K0,945Ag0,055NO3 and K0,945Cs0,055NO3 single crystals were grown from aqueous solution of KNO3, AgNO3 and CsNO3 using isothermal crystallization method. Then, structural and phase transformations in samples were studied by X-ray and optical microscopy methods. It has been determined that in K0,945Ag0,055NO3 monocrystal at T393K temperature, in K0,945Cs0,055NO3 monokrystal at T455K temperature the rhombic (II)  hexagonal (III) transformation occurs with the formation and growth of the III - modification crystal embryo within II – modification. The results obtained from the kinetic studies were determined that the temperature dependence of the conversion rate of II  III in K0,945Ag0,055NO3 and K0,945Cs0,055NO3 single crystals can be expressed by an empirical formula of υ=(aΔT+bΔT^2+cΔT^3)⋅10^(-2) cm/sec. ΔT=T_t-T_0 is temperature delay, T_t- is transformation temperature and is equilibrium temperature between interconverting modification crystals. Based on the results of velocity measurements, the activation energy of IIIII polymorphic transformations were calculated.

Summary: Doping effects on the electronic and magnetic properties of Zn1−x(Co,Cr)xO systems are investigated within Local Spin Density Approximation and Hubbard U methods. Based on Density Functional Theory the spin-polarization band structures, density of states for investigated systems are calculated. Systematic analysis of the electronic properties shows that TM-doped ZnO has generated new energy levels in the vicinity of Fermi energy level. From first-principle calculations we obtained Cr-ZnO and Co-ZnO systems are metallic and half-metallic ferromagnetic materials, respectively. The obtained results for Cr-doped ZnO 128- and 192-atom supercell systems show magnetic properties with higher Curie temperature than room temperature. There are large local moments, ∼2.9 and ∼4.2 for Co and Cr dopants, respectively. Magnetic moments are related with two electron defects in the supercell structure and unpaired electrons of transition metal. The ferromagnetic and antiferromagnetic phases and the total energy are obtained for x = 2.08%, 3.125%, 4.16%, 6.25%, 8.3%, 12.5%, and 25% impurity concentrations for doped ZnO.

Summary: The electrical and photoelectric properties of anisotype n-Si−p-GaSe heterojunctions obtained as a result of the deposition of a GaSe thin layer on a cold n-Si single crystal substrate by the thermal evaporation method were studied. It was determined that the height of the potential barrier in thermal annealing structures at T = 200°C during t = 3 hours occurs due to the decrease in the density of states of local levels located near the Fermi level in the amorphous layer. The mechanism of photosensitivity in an isotype heterostructures was analyzed and it was found that the photosensitivity of the heterojunction increases as a result of a decrease in the surface density of state at the contact boundary of the components, by thermal means. The spectral distribution of the quantum efficiency in the n-Si – p-GaSe heterojunction was studied and their perspective was determined.