MODERN TRENDS IN PHYSICS RESEARCH: Second International Conference on Modern Trends in Physics Research MTPR-06
Experimental Observation of the Triplet States of 39K2 by Infrared‐Infrared Double Resonance Spectroscopy888(2007); http://dx.doi.org/10.1063/1.2711083View Description Hide Description
Perturbation facilitated Infrared‐Infrared double resonance spectroscopy has been used to study the triplet states of K2. The 13Δg, , and 23Πg states have been observed. Resolved fluorescence spectra into the and b3Πu states are recorded. Perturbations between the 23Πg and states have been observed.
888(2007); http://dx.doi.org/10.1063/1.2711084View Description Hide Description
We present a first‐principles calculation of the density of states(DOS), electronic band structure and magnetic moment of SmCo4B. The magnetic properties of SmCo4B using density functional theory(DFT) calculations are treated beyond the local density approximation (LDA) within the LDA+U scheme as implemented in the wien2k package. The calculated magnetic moment is in fair agreement with available experimental values. The features of both DOS and band structure depend on the scheme used in the calculation. The results are discussed and compared to those of SmCo5.
888(2007); http://dx.doi.org/10.1063/1.2711085View Description Hide Description
We studied slow dissociative collisions between He and diatomic molecular ions using Cold Target Recoil Momentum Spectroscopy (COLTRIMS) technique in combination with fragment imaging technique. All final state momentum components, as well as the masses of the molecular fragments were determined. As the complete information on the kinematics is available, we are able to calculate the final state binding energy as well as the kinetic energy of the molecular fragments in the molecular center of mass system (kinetic energy release). The results show that the probability of the reaction channels is strongly dependent on the relative geometric orientation of the participants.
888(2007); http://dx.doi.org/10.1063/1.2711086View Description Hide Description
In most cases, electron‐ion (e/I) collisions proceed through the capture of free electrons by positive ions. The mode of radiationless capture which leads to the formation of intermediate doubly‐excited (d) states is known as resonance capture (RC) process. When d‐states stabilize by emission of radiation (x‐rays), dielectronic recombination (DR) is completed. The DR is an interesting process which is responsible for self‐cooling of thermal astrophysical and laboratory plasma.
In the present work, the DR rates with cascade effect are computed for O‐like Al5+, Cl9+, Ti14+ and Zn 22+ ions with 1s‐excitation. It is found that, the group of resonant states of the form 1s2s22p5nl (l = 0, 1, 2) has the highest contribution to the total DR rates for each of the previous ions. The peak values are found 1.01, 2.09, 3.12 and 3.46×10−14 cm3/sec at KT = 70, 120, 210 and 400 Ry for Al5+, Cl9+, Ti14+ and Zn 22+ respectively. This indicates that the total DR rates increase as the effective charge of the ions increases, in the same isoelectronic sequence.
888(2007); http://dx.doi.org/10.1063/1.2711087View Description Hide Description
The solution of the energy problems of our universe is based on the use of the ultimate source of energy, THE SUN, as the main source of useable energy. The trials to obtain solar cells of appropriate efficiency and suitable price represent one of the main tasks of different research groups over the whole world. In this respect silicon represent the main absorber of sun light that could be converted to electricity, photovoltaic cells, or to high energy chemical products, photoelectrochemical cells. Photovoltaic and photoelectrochemical systems were prepared by the formation of a thin porous film on silicon. The porous silicon layer was formed on the top of a clean oxide free silicon wafer surface by anodic etching in HF/H2O/C2H5OH mixture (2:1:1). The silicon was then covered by an oxide film (tin oxide, ITO or titanium oxide. The oxide films were prepared by the spray/pyrolysis technique which enables the incorporation of foreign atoms like In, Ru or Sb in the oxide film matrix during the spray process/. The incorporation of foreign atoms improves the surface characteristics of the oxide film which leads to the improvement of the fill factor and higher solar conversion efficiency. The prepared solar cells are stable against environmental attack due to the presence of the stable oxide film. It gives relatively high short circuit currents (Isc) compared to our improved silicon single crystal solar cells /6/, due to the presence of the porous silicon layer, which leads to the recorded high conversion efficiency. Although the open‐circuit potential (Voc) and fill factor (FF) were not affected by the thickness of the porous silicon film, the short circuit current was found to be sensitive to this thickness. An optimum thickness of the porous film and also the oxide layer is required to optimize the solar cell efficiency. The results represent a promising system for the application of porous silicon layers in solar energy converters. The use of porous silicon instead of silicon single crystals in solar cell fabrication and the optimization of the solar conversion efficiency will lead to the reduction of the cost as an important factor and also the increase of the solar cell efficiency making use of the large area of the porous structures.
888(2007); http://dx.doi.org/10.1063/1.2711088View Description Hide Description
Glass system of the composition (Na 2B4O7)100−x (V2O5)x, with x=0, 5, 7.5, 10, 12.5, 15 and 20 mol% has been prepared by conventional melt‐quenching technique. The optical absorption and A.c conductivity have been measured. The optical absorption reveals that the electronic transition is indirect. The exponential dependence of the absorption coefficient α (α ≈ 103–104 cm−1) as a function of the incident photon energy (hυ) suggests that the urbach role is obeyed. A.C conductivity measurements are performed in the frequency range (0.1–100 KHz), over the temperature range (300–600K).Analysis of data suggests that correlated barrier hopping (C.B.H) is the dominant conduction mechanism. The dependence of dielectric constant (έ) and dielectric loss (tanδ) on both temperature and frequency are also discussed.
888(2007); http://dx.doi.org/10.1063/1.2711089View Description Hide Description
We present first principles study of structural properties of Zinc‐blende and Wurtzite phases of Aluminum Nitride, Gallium Nitride and Indium Nitride binary compounds. The study has been done using Full‐Potential Linearized Augmented Plane Wave plus local orbitals method, within the framework of density functional theory. Results for lattice constant, bulk modulus, its pressure derivative and cohesive energy of these compounds are also compared with experimental results.
888(2007); http://dx.doi.org/10.1063/1.2711090View Description Hide Description
We present a review on using a statistical mechanics‐based model for calculating magnetic properties of hexagonal, cubic and mixed‐anisotropy systems. Although our simple model is not as elaborate as, for example, monte carlo simulation it demonstrates a success in producing all the major features of magnetization curves of the ferromagnetic systems studied. Examples of these features are sequence of easy‐axes of magnetization, spin reorientation and superparamagnetism in small particles. The magnetization curves, magnetic susceptibility and orientation probability of the magnetization vector are calculated in a wide range of temperature, particle size and magnetic field. Examples of the systems we report are unaxial ferromagnets e.g. Gd, PrCo5 and Nd2Fe14B; cubic materials e.g. Fe and Ni and mixed‐anisotropy systems, e.g. Fe on stepped Ag (001) substrates, in which a jump in the magnetization is found to depend on the ratio of the uniaxial to biaxial anisotropy present in these systems.
888(2007); http://dx.doi.org/10.1063/1.2711091View Description Hide Description
Fine structure levels, oscillator strengths and radiative transition probabilities have been calculated for Ni‐like argon. Configuration interactions and relativistic effects have been included in the calculations. The calculated energies and the transition probabilities of the ion are in good agreement with the available calculated values. The results are expected to be useful in the analysis of X‐ray and EUV spectra obtained from astrophysical sources.
888(2007); http://dx.doi.org/10.1063/1.2711092View Description Hide Description
Oscillator strengths (f‐values) are calculated for five members of the F‐isoelectronic sequence, namely, P6+, Ar9+, Sc12+, Cr15+ and Co18+. The Single Configuration Hartree‐Fock program (SCHF) is used to generate the wave functions needed in the calculation of radiative transition probabilities and oscillator strengths (f‐values) for singly excited states in the studied ions. Specifically, the work includes the radiative transitions of the form n1 l 1 → n2 l 2. The transitions are classified into sets according to the values of l 1 and l 2, with n1 and n2 = 9, 8, …, 3. Both l 1 and l 2 are limited (here) to be less than 4. The angular momentum average scheme (AMA) is utilized for all the calculations of the radiative rates Ar’s, and f‐values. The trends of oscillator strengths with the effective charge (Zeff) and principal quantum numbers n1 and n2 of the upper and lower states are investigated. It is found that, the f‐values decrease as (1/n1)4.5 for dominant transitions in low‐ionized ions (P6+).
Electron Impact Excitation and Ionization Rate Coefficients and Electron Densities Calculations for Lithium‐like Ion Si XII and S XIV888(2007); http://dx.doi.org/10.1063/1.2711093View Description Hide Description
Absolute excitation and ionization rate coefficients have been evaluated for arbitrary excited states at certain electron temperatures kTe and their corresponding electron densities Ne of the Lithium‐like ions Si XII and S XIV. The populations of the chosen excited levels are calculated namely, for the doublet state of the Li‐like ions. The calculations have been carried out by using the coupled rate simultaneous equations in which, the monopole and quadruple transitions have been introduced in the calculations in addition to the dipole transitions.
A theoretical population model has been developed to study the influence of the different processes that might contribute to the population of the different levels at the plasma parameters. The population densities of these different levels were then derived from these rate coefficients. For the most levels the theoretical excitation rate coefficients were found to be in good agreement with the available theoretical and experimental data. The other calculations were found to be in fair agreement with available ones in literature.
A Mean‐Field Study of the Magnetic and Magneto‐Thermal Properties of Selected Amorphous Rx‐Fe1−x and Crystalline RFe10V2 Systems888(2007); http://dx.doi.org/10.1063/1.2711094View Description Hide Description
We present a mean‐field theory study on the magnetization, magnetic specific heat and entropy for a selected amorphous R‐Fe and crystalline RFe10V2 alloys. The systems we report are a‐GdxFe1−x and a‐ Erx Fe1−x where the atomic concentration x lies in the range 0.42 to 0.60 and 0.085 to 0.20 for these two systems respectively. These systems allow for a systematic study on the dependence of the magnetic and magneto‐thermal properties on the rare‐earth concentration within a given series. On the other hand the fixed‐composition RFe10V2 system, where R=Y, Lu, Nd, Gd, Tb, Ho, Er and Tm, enables a detailed study of the dependence of the aforementioned properties on the R species. A two‐sublattice magnetic system is devised for each of the studied alloys where inter‐ and intra‐ sublattice exchange interactions are taken into account. The temperature dependencies of the sublattice magnetizations and their temperature derivatives are used in calculating the magnetic specific heats and the associated entropies either in zero field or in the presence of an external field. In particular, both of the specific heat and its associated entropy, at constant field, consist of four terms: one for each magnetic sublattice and two for inter‐sublattice contributions. For the RFe10V2 system, where R is magnetic, we found that the Fe‐Fe contribution to the zero‐field specific heat and entropy, although not the same for all R, is the largest compared to the R‐R and R‐Fe contributions. We calculated the largest zero‐field Smax for Ho and Dy (∼ 1.2 J/K.cm3) and the smallest for Y and Lu (∼ 0.9 J/K.cm3). Furthermore we found that heavy rare earths have higher Smax compared to the light ones for example ∼1.1 J/K.cm3 for R=Nd. In all cases our calculation fairly agree with the equation: Smax=NkB ln (2J+1). Our calculations showed that the specific heat anomaly and Smax of the amorphous alloys increase with increasing the Fe content. For the Gd system they lay in the range 0.15–0.22 and 0.12–0.14 J/g. K respectively. For the Er‐system the corresponding ranges are 0.19–0.30 and 0.18–0.20 J/g. K respectively. The effect of the external field on the specific heat is to reduce it below Tc and increase it above Tc relative to its zero‐field value. Its effect on the entropy, however, is to reduce it at temperatures close to and above Tc relative to its zero‐field value as expected.
888(2007); http://dx.doi.org/10.1063/1.2711095View Description Hide Description
Ionization waves, analogous to positive column of gaseous plasma, are shown to occur in ZnS. This is demonstrated with a model in which a single type of carrier (electron) impact ionizes a deep level trap in ZnS. The no‐local nature of impact ionization is described using lucky drift theory.
Ionization waves were discovered by using our numerical simulation model and were seen to arise as a consequence of the non‐local nature of the impact ionization process. This is contrast to the prediction of the local‐impact ionization theory. Under certain conditions space‐charge striations are produced and a NDR occurs for some film thicknesses.
888(2007); http://dx.doi.org/10.1063/1.2711096View Description Hide Description
We have studied the various k.p models including Kane’s model, the 8×8 matrix model and 14×14 matrix models. We include the nonparabolic terms in the energy formula using each of these models where we took into account the fourth and sixth order terms in the energy‐wavevector expansion. We then calculate the electron impact ionization threshold energy in terms of these nonparabolic parameters for some of zinc‐blende‐type semiconductor. Using the impact ionization energy, we extract the lucky drift ionization parameters. Our calculation shows that the 14×14 matrix model gives the closest result to that of the ab initio calculation of impact ionization threshold energy and that of Monte Carlo empirical pesudpotintial calculation. The advantage of our work is that by using simple derivation and computation, we manage to get a closer result to that obtained by heavy computational resources.
888(2007); http://dx.doi.org/10.1063/1.2711097View Description Hide Description
Monte Carlo simulation of electron transport in Ga0.53In0.47 As has been performed for three valley conduction band model. Scattering Sources include polar optical phonons, non‐polar intervalley phonons, non‐polar acoustic phonons, charged impurity and random potential alloy. A negative differential mobility is observed at field of about 5.0 × 105 v/m. Ga0.53In0.47 As exhibits higher low‐field drift mobility and a higher peak electron drift velocity than both GaAs and InP.
Preparation and Characterization of the Porous (TiO2) Oxide Films of Nanostructure for Biological and Medical Applications888(2007); http://dx.doi.org/10.1063/1.2711098View Description Hide Description
In this paper, galvanostatically and potentiostatically formed surface oxide film on titanium in H2O2 free and H2O2 containing H2SO4 solutions were investigated. Conventional electrochemical techniques and electrochemical impedance spectroscopy (EIS) measurements beside the scanning electron microscope (SEM) were used. In absence of H2O2, the impedance response indicated a stable thin oxide film which depends on the mode of anodization of the metal. However, the introduction of H2O2 into the solution resulted in significant changes in the film characteristics, which were reflected in the EIS results. The film characteristics were found to be affected by the mode of oxide film growth and polarization time. The H2O2 addition to the solution has led to a significant decrease in the corrosion resistance of the passive film. The electrochemical and the use of equivalent circuit models have led to the understanding of the film characteristics under different conditions.
888(2007); http://dx.doi.org/10.1063/1.2711099View Description Hide Description
The microstructure and magnetic properties of Ti substituted CoCd ferrites of the general formula CdxCo1−x+tTitFe2−2tO4, x = 0.20, 0.00⩽t⩽0.25 have been reported. The ferrite samples were prepared by standard double sintering ceramic technique and structural analysis was carried out using X‐ray diffraction. The spinel structure is confirmed for all concentration. Some physical properties (such as lattice parameter, density and porosity) have been also calculated. Temperature and magnetic field dependence of susceptibility is illustrated for all Ti contents. Both experimental and theoretical values of the effective magnetic moment were increased with increasing Ti content. The Curie temperature increases with the addition of Ti up to t = 0.15 after which it decreases. The effect of mechanical pressure on the dc resistivity at room temperature enhances the use of the samples in some applications.
888(2007); http://dx.doi.org/10.1063/1.2711100View Description Hide Description
Mixed ferrite system of com position Ni x Fe3−x O4 Where (x= 1,1.25,1.5,2) (inverse spinel) has been prepared in nano form. Ni and Fe nitrates are mixed and shacked thoughrly, then NaOH has been used as a precipitating agent. Obtained samples were washed by the ionized water and ethanol. Finally dried at 70°C for one day the ingots have been annealed at different temperatures. XRD and ME spectra have been measured. It was found that the particle size increases by increasing annealing temperature. All samples annealed at T ⩽ 650°C show superparamagnetic effects. Samples annealed at 900,1000°C exhibit magnetic hyperfine interaction reflecting high magnetic moments.
888(2007); http://dx.doi.org/10.1063/1.2711101View Description Hide Description
Glassy system: xNa2O‐(100‐x‐y)B2O3‐yCo3O4 has been prepared by conventional melt quenching technique. Optical absorption spectra have been obtained in the range 300 – 2500 nm at room temperature. An absorption edge was observed in the near UV range, the analysis of which reveals that indirect transition is the dominant absorption mechanism. All prepared samples exhibit blue color, indicating that the Co ions are acted upon by tetrahedral ligand field. Obtained spectra were used to estimate some ligand field parameters.