Narrow Results

The effect of 2MeV energy electrons with fluences from $0.5\times 10^{17}$ to $4.0\times 10^{17}$ electron/cm² on the crystal structure, surface morphology, absorption spectrum, band gap, Raman spectrum and microhardness of ZnS crystal was investigated. The crystal structure of ZnS is face-centered cubic with space group F-43m. Upon irradiation with a fluence of $4\times 10^{17}$ electron/cm², the unit cell parameter decreased by 0.0195Å, and the coordinates of the Zn${}^{+2}$ ions were changed. Irradiation with fluences ranging from $0.5\times 10^{17}$ to $4\times 10^{17}$ electron/cm² increased crystallite size from 20nm to 28nm. The study of the surface morphology of the ZnS single-crystal revealed that irradiation caused a reduction in both the width ($R_a$) and height ($R_z$) of the surface roughness. The band gap of the ZnS single-crystal decreased from 3.521 to 3.506eV when irradiated with fluence electrons from $0.5\times 10^{17}$ to $2.5\times 10^{17}$electron/cm². Raman spectrum observations showed an increase in the longitudinal optical (LO) mode peak (350cm${}^{-1}$) intensity following the irradiation of ZnS single-crystal with electrons. The microhardness of the ZnS single-crystal showed an exponential increase by 20% when irradiated with fluences from $0.5\times 10^{17}$ to $2.5\times 10^{17}$electron/cm².

The Ni_n (n =19, 20) +D₂ (v, j) collision systems have been studied to investigate the dependence of cluster reactivity on the cluster temperature and the initial rovibrational states of the molecule using quasiclassical molecular dynamics simulations. The clusters are described by an embedded atom potential, whereas the interaction between the molecule and the cluster is modeled by a LEPS (London–Eyring–Polani–Sato) potential energy function. Reaction (dissociative adsorption) cross-sections are computed as functions of the collision energy for different initial rovibrational states of the molecule and for different temperatures of the clusters. Rovibrational, temperature and size-dependent rate constants are also presented, and the results are compared with earlier studies. Initial vibrational excitation of the molecule increases the reaction cross-section more efficiently than the initial rotational excitation. The reaction cross-sections strongly depend on the collision energies below 0.1 eV.

We discuss thermodynamic stability of neutral real (quantum) matter from the point of view of a computer experiment at finite, nonzero, temperature. We perform (restricted) path integral Monte Carlo simulations of the two component plasma where the two species are both bosons, both fermions, and one boson and one fermion. We calculate the structure of the plasma and discuss about the formation of binded couples of oppositely charged particles. The purely bosonic case is thermodynamically unstable. In this case we find an undetermined size-dependent contact value unlike partial radial distribution function. For the purely fermionic case, we find a demixing transition with binding also of like species.

We study through a computer experiment, using the restricted path integral Monte Carlo method, a one-component fermion plasma on a sphere at finite, nonzero, temperature. We extract thermodynamic properties like the kinetic and internal energy per particle and structural properties like the radial distribution function. This study could be relevant for the characterization and better understanding of the electronic properties of hollow graphene spheres.

We investigate a binary Lennard–Jones mixture with molecular dynamics simulations. We consider first a system cooled linearly in time with the cooling rate γ. By varying γ over almost four decades we study the influence of the cooling rate on the glass transition and on the resulting glass. We find for all investigated quantities a cooling rate dependence; with decreasing cooling rate the system falls out of equilibrium at decreasing temperatures, reaches lower enthalpies and obtains increasing local order. Next we study the dynamics of the melting process by investigating the most immobile and most mobile particles in the glass. We find that their spatial distribution is heterogeneous and that the immobile/mobile particles are surrounded by denser/less dense cages than an average particle.

Using the modern equations of state derived from microscopic calculations, we have calculated the neutron star structure. For the neutron star, we have obtained a minimum mass about 0.1 M_⊙ which is nearly independent of the equation of state, and a maximum mass between 1.47 M_⊙ and 1.98 M_⊙ which is strongly dependent on the equation of state. It is shown that among the equations of state of neutron star matter which we have used, the stiffest one leads to higher maximum mass and radius and lower central density. It is seen that the given maximum mass for the Reid-93 equation of state shows a good consistency with the accurate observations of radio pulsars. We have indicated that the thickness of neutron star crust is very small compared to the predicted neutron star radius.

The equation of state of deconfined quark matter within the MIT bag model is calculated. This equation of state is used to compute the structure of a neutron star with quark core. It is found that the limiting mass of the neutron star is affected considerably by this modification of the equation of state. Calculations are carried out for different choices of the bag constant.

We present an exact solution that could describe compact star composed of color-flavor locked (CFL) phase. Einstein’s field equations were solved through CFL equation of state (EoS) along with a specific form of $g_{rr}$ metric potential. Further, to explore a generalized solution we have also included pressure anisotropy. The solution is then analyzed by varying the color superconducting gap $\delta$ and its effects on the physical parameters. The stability of the solution through various criteria is also analyzed. To show the physical validity of the obtained solution we have generated the $M-R$ curve and fitted three well-known compact stars. This work shows that the anisotropy of the pressure at the interior increases with the color superconducting gap leading to decrease in adiabatic index closer to the critical limit. Further, the fluctuating range of mass due to the density perturbation is larger for lower color superconducting gap leading to more stable configuration.

X-ray diffraction patterns of melt-spun Fe-Cu-Nb-Si-B (FINEMET-type) alloys reveal that crystallites of Fe₂Si and Fe₃B phases with average sizes of 15(5) and 20(2) nm are present in the surface layer of thickness ≈ 10 Å and these nanocrystallites occupy 5–10% of the total volume. The results of an elaborate analysis of the high-resolution electrical resistivity data taken in a temperature range from 13 K to 300 K and their discussion in the light of existing theories demonstrates that the enhanced electron–electron interaction (EEI), quantum interference (QI) effects, inelastic electron–phonon scattering, coherent electron–magnon (and/or electron-spin fluctuation) scattering are the main mechanisms that govern the temperature dependence of resistivity. Of all the inelastic scattering processes, inelastic electron–phonon scattering is the most effective mechanism to destroy phase coherence of electron wavefunctions. The physical quantities such as diffusion constant, density of states at the Fermi level and the phase-breaking time, determined for the first time for the alloys in question, exhibit a systematic variation with the copper concentration.

The crystal structure and magnetic properties of (Ru_1-xSn_x)Sr₂EuCeCu₂O_z and Ru(Sr_2-xLa_x)EuCeCu₂O_z(0≤x≤0.1) samples have been investigated to shed light on the doping-induced changes in the magnetic properties of Ru-1222 system. We show that La substitution for Sr leads to an increase in the temperature where the ferromagnetic component is observed and a moderate suppression of the ferromagnetic component whereas Sn substitution for Ru results in a significant decrease of the volume fraction of the ferromagnetic phase as well as a decrease in the magnetic ordering temperature. The experimental results are discussed in connection with the structural data studied by Rietveld refinement of the x-ray diffraction data.

The structure of xFe₂O₃·(100-x)[3B₂O₃·BaO] system with 0≤x≤50 mol% was studied by DTA, X-ray diffraction, density, optical microscopy and EPR measurements in vitreous and partial crystallized state, the samples being obtained by under cooling method. The data obtained show that, by melting the samples at T_e = 1200°C or T_e = 1250°C, glasses for x≤35 mol% were obtained, and the forming of crystalline microprecipitates of Fe₂O₃ in the sample with x = 50 mol%. It was also established that the thermal treatment at 565°C without and in the presence of magnetic field of 0.7 T is influencing the forming and the development of the Fe₂O₃ microcrystals in samples with x≥35 mol%. The samples melted at T_c = 1200°C and T_c = 1250°C were studied by magnetic susceptibility measurements which evidenced similar results with those obtained by EPR. Also, the magnetic measurements show that for the thermal untreated samples the iron ions participate at superexchange interactions for x≥5 mol% and for x≥10 mol%, respectively.

A molecular dynamics model was developed to search for stable copper clusters with up to 60 atoms by Gupts empirical potential based on the second-moment approximation to tight-binding model (TB-SMA). We found that isomers do not emerge until the clusters have more than 7 atoms, getting more for clusters with 30~52 atoms, and the magic number, 13, 19, 23, 26, 28, 32, 38, 43, 46, 49, and 55 have ground clusters with higher symmetry and have few isomers.

The geometries and energies of small silicon monohydride clusters (Si₂H–Si₁₀H) have been systematically investigated by density functional theory (DFT) scheme with DZP++ basis sets. Several possible geometric arrangements and electronic states have been considered for each cluster. The results on Si₂H–Si₄H are in good accordance with previous ab initio calculation. The geometry of ground state of Si₂H is found to be a bridged C_2v structure, and Si₃H to be a bridged C_2v, while Si₄H a non-bridged C_s symmetry with ²A′ state. The non-bridged geometries of ground state of Si₅H–Si₁₀H have been found to be corresponding to C_2v(²B₁), C_2v(²B₁), C_5v(²A₁), C_s(²A^′′) (have two types), C₁ (not symmetry), and C_s(²A′), respectively. The results on Si₅H, Si₆H, Si₈H and Si₉H are different from previous calculations. Compared silicon clusters (Si_n) with silicon monohydrides (Si_nH) clusters, the addition of a single hydrogen atom cannot cause great changes in the ground state geometries of Si₂, Si₃, Si₄, Si₇, Si₉, and Si₁₀ clusters, while in the ground state geometries of Si₅, Si₆ and Si₈ clusters the change is great. The dissociation energies calculated indicates that Si₄H, Si₇H, and Si₁₀H clusters are less stable than others.

The non-monotonic relationship of T_c with the number of Cu-O planes (n) per unit cell for compounds of Tl₂Ba₂Ca_n-1Cu_nO_2n+3 (n=1, 2, 3, 4 and 5) is investigated from the reaction between different structural blocks. The unit cell of the Tl superconductors is treated as two blocks: the perovskite block where the Cu-O planes are located and the rock salt block, which is considered as a charge-reservoir. A model was used to calculate the combinative energy of the two blocks. It is found that the combinative energy between the two blocks is closely related to the value of T_c. The relation demonstrates an interesting way to understand the nonlinear change of T_c with the number of Cu-O planes in the layered superconductors. This means that the interaction between the two blocks plays an important role in superconductivity. The results are somewhat different from that of another Tl-system superconductor, Tl₂Ba₂Ca_n-1Cu_nO_2n+4, with 4 superconductive compounds.

The structure and the dynamics of the gas-liquid phase interface of the three-dimensional Lennard-Jones (12-6) particle system are studied using nonequilibrium molecular dynamics simulation. Heat flux maintains the system into a gas-liquid coexisting state with a steady interface. In the steady state, the interface shows an asymmetric structure and this is well described by a free energy density model with an asymmetric double-well form. When the system approaches to the steady state, a gas of the temperature profile appears between each phase and the gap value is relaxed to that in the steady state following for large t. It is observed that heat resistance exists in gas-liquid interface in this scale.

Nanocrystalline Mn_0.6Zn_0.4Fe₂O₄ particles are synthesized by a phase transformation method. The crystal structure of these particles is that of spinel MnZn ferrite. The average particle size is about 50 nm and the grain size is about 11 nm. Magnetic measurements show that the saturation magnetization at 120 K is ~80% larger than that at 300 K, and imply that the majority of the nanoparticles are superparamagnetic from 65 to 300 K.

Silicon oxycarbide (SiCO) thin films were prepared by the RF reactive sputtering technique on n-type silicon substrates with the target of sintered silicon carbide (SiC), and high purity oxygen was used as the reactant gas. The as-deposited films were annealed at temperatures of 600^°C, 800^°C, and 1000^°C under nitrogen ambient, respectively. The films were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction and photoluminescence (PL) spectrophotometer. The results show that annealing temperature plays an important role in the structure and photoluminescence of the films. The temperature 600^°C is the most favorable annealing temperature for SiO₂ crystallization and the formation of 6H-SiC crystal phase in the SiCO films. The intense PL peaks located at 375 nm and 470 nm are observed at room temperature. The origin of the PL was discussed.

The structural characteristics of Bi_2-xPb_xSr₂CaCu₂O_8+y(Bi-2212) and Bi_2-xPb_xSr₂Ca₂Cu₃O_10+y(Bi-2223) with x changing from 0 to 0.8 were studied by X-ray diffraction and Rietveld refinement. By careful calculation of chemical bond lengths and angles, it is found that there exists a fixed triangle on the Cu–O planes in the two systems, and then makes the Cu–O planes stable. The fluctuations of this fixed triangle were investigated, and it is found that there is a close relationship between it and T_c's. In addition, we discussed the origin of this special local structure. It may be caused by the interaction between the perovskite block and the rock salt block in a unit cell, which may play an important role in the mechanism of high temperature superconductivity.

Polycrystalline samples of $({\mathrm{\text{Ru}}}_{0.9}{\mathrm{\text{Nb}}}_{0.1}){\mathrm{\text{Sr}}}_2({\mathrm{\text{Gd}}}_{1.34-x}{\mathrm{\text{Nd}}}_x{\mathrm{\text{Ce}}}_{0.66}){\mathrm{\text{Cu}}}_2{\mathrm{\text{O}}}_z$$(x=0-0.67)$ have been synthesized and characterized by X-ray diffraction (XRD), electrical resistivity, thermoelectric power and magnetization measurements. Resistivity measurements revealed that the onset transition temperature $(T_{\mathrm{\text{co}}})$ decreased slightly from $T_{\mathrm{\text{co}}}=40\mathrm{\text{K}}$ for $x=0$ to $T_{\mathrm{\text{co}}}=22\mathrm{\text{K}}$ for $x=0.67$. Magnetization measurements showed that the partial substitution of $\mathrm{\text{Gd}}$ by $\mathrm{\text{Nd}}$ leads to a significant decrease in the magnetic ordering temperature. A suppression of the weak ferromagnetic component of the field-cooled (FC) magnetization with $\mathrm{\text{Nd}}$-doping content was also observed for the samples with $x\ge 0.34$. The changes in the superconducting transition temperature, the magnetic ordering temperature and the weak ferromagnetic component of the FC magnetization are discussed in conjunction with the change in hole concentration and the local structural changes in the Ru sublattice induced by $\mathrm{\text{Nd}}$ doping, based on the Rietveld refinements of the XRD data.

Our present work is based on the density functional theory (DFT) studies of TiO₂ crystals doped with V impurities. Both rutile and anatase structures have been considered within the present research and different defect concentrations have been used as well. Our calculations reveal equilibrium geometry of the system showing atomic rearrangement around the point defect being mainly inward with respect to the impurity. Magnetism and electronic structure based on the density of states (DOS) patterns for both rutile and anatase crystals have been obtained and discussed in detail. It is shown that local magnetic moments arise mainly from the 3d states of the impurity atom with some admixture of 2p states from the vanadium-nearest O atoms.