مجله پژوهش فیزیک ایران
سال یکم شماره 4 (تابستان 1377)

The origin the concept of LZ compexity is in information science. Here we use this notion to characterize chaotic dynamical systems. We make contact with the usual characteristics of chaos, such as Lyapunov exponent and K-entropy. It is shown that for a two-dimensional system LZ complexity is as powerful as other characteristics. We also apply LZ complexity to the study of the quasiperiodic Fibonacci sequence. We prove a theorem about its LZ complexity and based upon it conclude its long range order.

In a systematic approach, we have investigated the effect of the presence of Pb and Sb in the Bi site in a BSCCO (2223 phase) superconductor. There are some contradictory reports in substitution of Sb in the Bi site. Some researchers report an increase in the Tc of these materials. So, we have made an accurate stoichiometry of these superconductors and selected extra pure starting materials with appropriate ratios of Pb+Sb. The susceptility of these samples have been measured and the structures of the systems have been studied by SEM and XRD. The results of this investigation show that, Although the presence of Pb is essential for formation of (2223) phase, but addition of small amount of Sb helps to stabilize and enhance the ratio of higher phase. Our results show that, presence of Sb would raise the critical current density, but would not affect the Tc of these superconductors

The third order term in the density functional is required for study of an inhomogeneous fluid. This term can be chosen such that, the thermodynamic potential gives the correct bulk pressure. The density porofile of a hard sphere fluid near hard wall which is an inhomogeneous fluid is obtained and it is shown that, if the third order term is ignored, although an exact bulk direct correlation function which gives the correct bulk pressure is used, the thermodynamic potential does not yield the correct density profile.

Monte Carlo simulation is widely used in calculations involing transport of photons through different materials of different shapes. The method consists of randomly generating a finite set of photon histories over which the quantities of interest are averaged. In photon transport calculations, sampling the photon scattering angle from the Klein-Nishina probability distribution is of special importance. Various methods of sampling the Klein-Nishina distribution exist in the literature which are mainly based either on approximate inverse sampling or non-uniform rejection sampling methods. A direct sampling method also exists which can only be used if the incident photon energy is greater than 1.4 MeV. In this work a weighting method for considering the Klein-Nishina distribution for the scanttering angle is presented, which is more accurate and faster than all other existing methods and is applicable for all incident photon energies. In this method an angle θ (0≤ θ ≤ Π) is randomly generated at each scattering point and a weight W, which is proportional to the Klein-Nishina function at the generated, is calculated, and each event is weighted by the amount W. Events with multiple interactions are weghted by multiplication of the weights obtained at each scattering point. Using this method, the photon absorbed fraction, which is simply the fraction of the emitted photon energy that is absorbed in the region of interest, was calculated for central point sources in water spheres of different dimensions, and the results were compared with the results obtained by other methods. The consistency of the results shows that the weighting method presented here can efficiently be used in photon transport calculations.

Single-valence electron atoms are an important class of atoms. Their oscillator strengths are their important properties. Knowing the oscillator strengths one can easity calculate the transition probabilities of the spectral lines and hence the lifetimes of energy levels of most atoms. The oscillator strengths of the spectral lines of most atoms are not knoen with sufficient accuracy due to the experimental difficulties. The results of most measurements are subject to large inaccuracies due to uncertainties in vapor pressure data. A quick and simple theoretical method for calculation of atomic oscillator strength seems to be the Coulomb approximation of Bates and Damagaard. This method reveals some interesting properties that are generally confirmed by experimental results. In this paper, we have studied oscillator strengths and line strengths of the different allowed transitions in AgI and AuI using the Coulomb approximation. The log (λfg) curves(λ, f and g are the wavelength of transition, oscillator strength and statistical weight of upper level, respectively) versus the reciprocal of the principal quantum number of upper level, 1/n, show a linear behavior only for large values of the principal quantum number of lower level. The effect of change of total angular momentum,Δ J, in the curvature and slope of the plotted curves has been also investigated. The deviation of the curves from straight lines, which indicates failure of the Coulomb approximation is due to the exchange forces. In addition, the n3fg curves (n, the effective total quantum number of upper level) have been plotted versus n for different allowed transitions in AgL and AuI. It has been found that f is proportional to 1/n and this proportionality is linear for large values of n. For some transitions, however, there is a significant deviation from the linear dependence for large values of n, which can be attributed to the signature of total angular momentum quantum numbers of the initial and final states of jumping electron