Confocal Raman spectroscopy of island nuclei formed at the initial stage of quartz glass crystallization

Island nuclei formed on a polished quartz-glass surface upon heating to 1100°C have been investigated by confocal Raman spectroscopy. The structural and chemical composition of islands is shown to be a central nucleus, a shell around the nucleus, and a thin peripheral ring closing this shell. The formation and growth of individual regions of an island nucleus are found to occur in several stages. The shell around the nucleus is mainly formed by α-SiO₂ and α-cristobalite nanoparticles with a size ≥40 nm, whereas the α-SiO₂ nanoparticles in the nucleus and peripheral ring are 2–15 nm in size.     

Domain-wall mechanism of nucleation in a magnetic structural phase transition in La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The structure of domain walls and new-phase nucleation are investigated in a four-sublattice antiferromagnet (AFM) of the La₂CuO₄ type placed in a magnetic field which initiates an AFM-weak-ferromagnet (WFM) magnetic structural phase transition. The critical fields for nucleus growth are found in the case of two types of domain walls present. The magnetization curve is calculated and a two-step mechanism is proposed for the AFM-WFM phase transition observed in La₂CuO₄.     

Fusion-fission dynamics in the superheavy nucleus production

The fusion-fission reaction mechanism leading to the massive nucleus formation is studied. We investigate the superheavy nucleus formation in heavy-ion induced reactions by analysing the evaporation residue (ER) production in order to study the fusion dynamics and the decay properties of nuclei close to the stability island at Z=114. We consider the ⁶¹Ni+²⁰⁸Pb, ⁴⁸Ca+²³⁸U and ⁴⁸Ca+²⁴⁴Pu reactions that lead to the Z=110, 112 and 114 superheavy elements respectively. By using the dinuclear system (DNS) concept of the two interacting nuclei we calculate the quasifission-fusion competition in the entrance channel and the fission-evaporation competition along the de-excitation cascade of the compound nucleus. The dynamics of the entrance channel allows us to determine the beam energy window which is favorable to the fusion, while the dynamic evolution of the compound nucleus on the shell correction to the fission barrier and the dissipative effects influence the fission-evaporation competition in order to obtain the residue nuclei from the superheavy nucleus formation. We also calculate the τ _n /τ_tot ratio at each step of the de-excitation cascade of the compound nucleus and we present a systematics of τ _n /τ_tot (at first step of the cascade) for many reactions that lead to nuclei with Z=102–114.     

The formation mechanism and the binding energy the body—centred regular tetrahedral structure of He5^＋

We propose the formation mechanism of the body-centred regular tetrahedral structure of the He5^ cluster,The total energy curve for this structure has been calculated by using a modified arrangement channel quantum mechanics method.The result shows that a minimal energy of -13.9106 a.u.occurs at a separation of 1.14a0 between the nucleus at the centre and nuclei at the apexes.Therefore we obtain the binding energy of 0.5202a .u.for this structure.This means that the He5^ cluster may be stable with a high binding energy in a body-centred regular tetrahedral structure.     

Allowance for the shell structure of colliding nuclei in the fusion-fission process

The motion of two nuclei toward each other in fusion-fission reactions is considered. The state of the system of interacting nuclei is specified in terms of three collective coordinates (parameters). These are the distance between the centers of mass of the nuclei and the deformation parameter for each of them (the nose-to-nose orientation of the nuclei is assumed). The evolution of collective degrees of freedom of the system is described by Langevin equations. The energies of the Coulomb and nuclear (Gross-Kalinovsky potential) interactions of nuclei are taken into account in the potential energy of the system along with the deformation energy of each nucleus with allowance for shell effects. The motion of nuclei toward each other are calculated for two reaction types: reactions involving nuclei that are deformed (₄₂¹⁰⁰Mo + ₄₂¹⁰⁰Mo → ₈₄²⁰⁰Po) and those that are spherical (₈₂²⁰⁸Pb + ₈¹⁸O → ₉₀²²⁶Th) in the ground state. It is shown that the shell structure of interacting nuclei affects not only the fusion process as a whole (fusionbarrier height and initial-reaction-energy dependence of the probability that the nuclei involved touch each other) but also the processes occurring in each nucleus individually (shape of the nuclei and their excitation energies at the point of touching).     

Anomalies in orientation interaction of slow neutrons with nuclei and the problem of neutron molecule existence

Based on the Dirac equation, the features of long-range electromagnetic orientational interaction of slow neutrons with even-even and even-odd nuclei are considered. This interaction is controlled by a narrow potential barrier arranged beyond the nucleus. The barrier height is U _tot = 20–40 eV and depends on Z, A, and the nucleus magnetic moment μ_nucl. The barrier formation is associated with the ponderomotive nonlinear interaction of the anomalous neutron moment with the nucleus electric field. The barrier transparency for thermal neutrons is D(E) ≈ 0.8–0.95. For cold neutrons, the barrier transparency and their reaction cross sections with nuclei sharply decrease and, at E → 0, their cross sections tend toward zero. It was shown that the combined effect of the magnetic dipole-dipole and ponderomotive interaction of the neutron and even-odd nucleus results in the formation of removed symmetrically positioned potential wells for neutrons beyond the nucleus. The presence of these wells results in the possible existence of short-lived or virtual nucleus-neutron molecules and the “neutron halo” effect beyond the nucleus.     

Pulsed EPR spectroscopy of the V KA center in CaF2:Na

An EPR study of the self-trapped hole center (V _KA center) in Na-doped single-crystal CaF₂ is reported. A five-component hyperfine-interaction tensor was used for the first time to describe EPR spectra adequately. The parameters of the electron spin Hamiltonian of the V _KA center, the tensors of ligand hyperfine interaction with all nearest-neighbor nuclei, and constants of quadrupole interaction with the Na nucleus have been determined. The structure of the center has been unambiguously established from the results obtained, and the mechanisms of its formation are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1619–1622 (September 1998)     

Fragmentation of relativistic oxygen nuclei in interactions with a proton

The data on investigation of inelastic interactions of ¹⁶O nuclei in a hydrogen bubble chamber at an incident momentum of 3.25 A GeV/c are presented. Separate characteristics as fragments isotope composition and topological cross-sections of fragmentation channels are given. The processes of formation of light fragments and unstable nuclei, and the break-up of the ¹⁶O nucleus into multicharge fragments are investigated. A comparison between the experimental data and the calculations of the cascade fragmentation evaporation model (CFEM) is made. The observed singularities of the interactions point out the important role of the nucleus α-cluster structure in the formation of the final products. Received: 9 January 2001 / Accepted: 13 June 2001     

In situ transmission electron microscopy study of the nucleation and grain growth of Ge2Sb2Te5 thin films

The nucleation and grain growth of the Ge₂Sb₂Te₅ (GST) thin films were studied using high voltage electron microscope operated at 1250 kV. As a result, we have found that 2 nm-sized nucleus forms as a cluster which atoms are arranged regularly at the stage of nucleation prior to the formation of grains having crystal structure. The high-resolution transmission electron microscopy study and fast-Fourier transformations revealed that coexistence of face-centered-cubic (FCC) and hexagonal structure occurs, and formation of twin defect is found in the hexagonal structure during the grain growth as the annealing temperature is increased. GST grain having the hexagonal structure grow from the surface, and the growth proceeded perpendicular to the [0 0 0 1], namely the path parallel to the (0 0 0 1) plane. Consequently, grain growth to a large-scale result in a lengthened shape.     

Formation region of emitted α and heavier particles inside radioactive nuclei

We investigate the formation distance(R_0) from the center of the radioactive parent nucleus at which the emitted cluster is most probably formed. The calculations are performed microscopically starting with the solution to the time-independent Schr?dinger wave equation for the cluster-core system, using nuclear potentials based on the Skyrme-SLy4 nucleon-nucleon interactions and folding Coulomb potential, to determine the incident and transmitted wave functions of the system. Our results show that the emitted cluster is mostly formed in the pre-surface region of the nucleus, under the effect of Pauli blocking from the saturated core density. The deeper α-formation distance inside the nucleus allows less preformation probability and indicates a more stable nucleus for a longer half-life. Furthermore, the α-particle tends to be formed at a slightly deeper region inside the nuclei, with larger isospin asymmetry, and in the closed shell nuclei. Regarding the heavy clusters, we observed that the formation distance of the emitted clusters heavier than α-particle increased via increasing the isospin asymmetry of the formed cluster rather than by increasing its mass number. The partial half-life of a certain cluster-decay mode increased with increase of either the mass number or the isospin asymmetry of the emitted cluster.     

Study of the domain structure evolution in single crystals of relaxor ferroelectric Sr0.61Ba0.39Nb2O6:Ce1

Optical visualization with simultaneous recording of the switching current was used for studying of domain structure evolution in single crystals relaxor ferroelectric strontium barium niobate (Sr_0.61Ba_0.39Nb₂O₆) doped by cerium (0.22 mol %) (SBN61:Ce). It was shown that the maze domain structures can be observed during polarization reversal due to local change of refractive index induced by formation of microscale domains with charged domain walls. It was proposed, that optical inhomogeneities occur in the switching regions, while the optical uniform areas correspond to regions where the switching either has not started yet or has already completed. The original mathematical treatment of the obtained images and switching currents has been used for quantitative characterization of the polarization reversal process.     

Formation of 24Mg* in the spallation of 28Si nuclei by 1-GeV protons

The ²⁸Si(p, p′γ₀ X)²⁴Mg reaction has been studied at the ITEP accelerator by the hadron-gamma coincidence method for a proton energy of 1 GeV. Two reaction products are detected: a 1368.6-keV γ-ray photon accompanying the transition of the ²⁴Mg* nucleus from the first excited state to the ground state and a proton p′ whose momentum is measured in a magnetic spectrometer. The measured distribution in the energy lost by the proton in interaction is attributed to five processes: the direct knockout of a nuclear α cluster, the knockout of four nucleons with a total charge number of 2, the formation of the _ΔSi isobaric nucleus, the formation of the Δ isobar in the interaction of the incident proton with a nuclear nucleon, and the production of a π meson, which is at rest in the nuclear reference frame. The last process likely corresponds to the reaction of the formation of a deeply bound pion state in the ²⁸P nucleus. Such states were previously observed only on heavy nuclei. The cross sections for the listed processes have been estimated.     

Formation and growth of CuCl phase nuclei in glass

In glass, the CuCl phase starts to form a certain time after the onset of supersaturation. As the temperature is increased, the transient period (stage of formation of critical nuclei) shortens and the growth kinetics of the CuCl phase switches from the first to second stage. The observed pattern of the CuCl phase growth kinetics is fully consistent with the Zel’dovich-Frenkel classical theory of new-phase formation. The delay time is determined by the radius of the critical nucleus (CuCl nanomelt) and the diffusion coefficient of the limiting component, the Cu⁺ ions. The radius of the critical nucleus is about 1 nm and does not vary within a broad temperature range. The activation energy for the CuCl phase growth process does not change in the transition from the formation of critical nuclei to the first and, subsequently, second stage.     

Domain wall structure in BaTiO3 during switching

The increase in the complex permittivity during switching is calculated from a two-dimensional model of residual nuclei, assuming that domain walls possess a pyramidal step structure. The resulting formulae are compared with previous experimental data on BaTiO₃ and with corresponding formulae from the same model with smooth walls. It is shown that the real part of the permittivity during switching is very sensitive to the presence of steps on moving domain walls and that there is an indication that in BaTiO₃ such step structure actually exists.     

Synthesis of heavy and superheavy elements by reactions of massive nuclei

By comparing theoretical and experimental excitation functions of evaporation residues resulting from the same compound nucleus or heavy and superheavy nuclei, it is possible to understand the effect of the entrance channel and the shell structure of reacting nuclei on the fusion mechanism. The competition of complete fusion with the quasifission process is strongly related to the intrinsic fusion barrier B _fus ^* and the quasifission barrier B _qf as well as the size of the well in the nucleus-nucleus potential. In our calculations of the excitation functions for capture, fusion, and evaporation residues, we use the relevant variables such as mass asymmetry of nuclei in the entrance channel, potential energy surface, driving potential, spin distribution, and surviving probability of compound nucleus that are responsible for the mechanism of the fusion-fission process. As a result, we obtain a beam energy window for the capture of the nuclei before the system fuses and the Γ_n/Γ_f ratio at each step along the deexcitation cascade of the compound nucleus. Calculations performed in the framework of the model taking into account the nuclear shell effect and shape of colliding nuclei allow us to reach useful conclusions about the mechanism of the fusion-fission process and the production of the evaporation residues. We analyze the ⁴⁰Ar + ¹⁷⁶Hf, ⁸⁶Kr + ¹³⁰Xe, and ¹²⁴Sn + ⁹²Zr reactions leading to ²¹⁶Th*; the ³²S + ¹⁸²W and ⁶⁰Ni + ¹⁵⁴Sm reactions leading to ²¹⁴Th*; the ⁴⁸Ca + ²⁴⁸Cm reaction leading to the ²⁹⁶116 compound nucleus; and the ⁴⁸Ca + ²⁴⁹Cf reaction leading to the ²⁹⁷118 compound nucleus.     

On the possibility of detecting superheavy quasimolecules

The formation of superheavy electronic quasimolecules leads to measurable deviations from the Rutherford cross section in heavy ion collisions. For the system ²³⁸₉₂U-²³⁵₉₂U with E_c.m. = 800 MeV we find an average correction of 1.5% for the scattering cross section in forward directions. Additional background contributions like electronic shielding of the nucleus, vacuum polarization, nuclear Coulomb excitation and static deformations of the nuclei are taken into account.     

Evidence for a nuclear phase transition in target nuclei after relativistic nuclear interactions

The degree of excitation of the emulsion target nuclei due to nuclear interactions of oxygen and sulfur projectiles at 200 GeV/nucleon incident energy has been investigated. Using the plausible assumption that the numberN _b of slow particles emitted from the struck target nucleus can be interpreted as a measure of the temperatureT of the residual nucleus, we have found that there exists a critical temperatureT _c of the excited target nucleus. For Ag and Br target nuclei this temperature corresponds to <N _b>≌12 and it is attained when the impact parameters are less than about 4 fm.     

E.S.R. spectra of PF4 radicals produced in a single crystal of PF3

E.S.R. spectra of PF₄ radicals were investigated with single crystals of PF₃ made at a low temperature and irradiated with γ-rays. The angular dependence of the spectral lines was satisfactorily analysed in terms of the second-order equations for large couplings of one ³¹P nucleus and two magnetically equivalent ¹⁹F nuclei and of the first-order equation for two ¹⁹F nuclei with small couplings. We conclude that the two magnetically equivalent fluorine nuclei occupy the axial positions of the trigonal bipyramidal structure of the PF₄ radical and the other two fluorine nuclei are in the equatorial positions. Furthermore, the unpaired electron of the PF₄ radical was shown to occupy an orbital consisting mainly of the 2p orbital of each of the two axial fluorine atoms along the F-P-F axis and the 3s and 3p orbitals, directed towards the vacant equatorial position of the central phosphorus atom. The orbital of the unpaired electron can be reasonably represented as a Rundle three-centre non-bonding orbital. The results obtained for the PF₄ radicals not only strongly support the results given for POCl₃ ^- radicals, but also allow us to discuss the electronic structure of phosphoranyl radicals in more detail.     

Growth, structure, and morphological stability of nuclei growing from eutectic melts

Kinetics of self-consistent growth of new-phase nuclei from eutectic melts have been studied. The growth kinetics of nuclei of eutectic composition are shown to depend on the sum of supersaturations over all components. It has been established that nuclei in a eutectic melt reach a common critical radius determined by supersaturation for both components. The concept of a diffusion dipole is introduced: a two-phase object in a eutectic melt or solid solution, in which two nuclei of different composition are related through a common diffusion field. The morphological stability of such dipoles is studied. It is found that a nucleus of eutectic composition is more stable with respect to small fluctuations in its shape than the corresponding one-component nucleus. It is shown that perturbations resulting in dipole-shape distortions develop perpendicular to the axis connecting the centers of the nuclei of different compositions (i.e. the dipole axis). This is consistent with the well-known experimental observation of the layered structure of eutectic compositions. Fiz. Tverd. Tela (St. Petersburg) 39, 1464–1469 (August 1997)     

Bi系超导体高Tc相的形成机制

本文研究Bi系超导体高T_c(2223)相的形成机制。提出Bi系氧化物超导体高T_c相的形成过程是:首先Sr-Ca-Cu氧化物(Ca_0.85Sr_0.15CuO₂)与低T_c(2212)相反应形成高T_c相的核,然后样品内2223相核通过象结晶学中晶体生长那样的方式长大成2223相晶粒。2223相成核过程中Sr-Ca-Cu氧化物的形成途径有两条:一条是通过固相反应 关键词：