The experimentally obtained luminescence characteristics of a praseodymium (Pr$^{3+})$ doped chalcogenide glass fiber are studied numerically using a rate equation approach. The numerical model includes both the radiative and non-radiative transition paths whilst it neglects the up-conversion processes. Photoluminescence spectra at mid-infrared wavelengths ranging from 3.5 to 6 ${\upmu }$m were obtained by using two pump wavelengths: 1.55 and 1.94 ${\upmu }$m. A good agreement between the experiment and theory is obtained for the photoluminescence decay profiles. 相似文献
To assess for associations between hippocampal atrophy and measures of cognitive function, hippocampal magnetization transfer ratio (MTR), and diffusion measures of the fornix, the largest efferent white matter tract from the hippocampus, in patients with multiple sclerosis (MS) and controls.
Materials and Methods
A total of 53 patients with MS and 20 age- and sex-matched healthy controls participated in cognitive testing and scanning including high spatial-resolution diffusion imaging and a T1-MPRAGE scan. Hippocampal volume and fornicial thickness measures were calculated and compared to mean values of fornicial transverse diffusivity, mean diffusivity, longitudinal diffusivity, fractional anisotropy, mean hippocampal MTR, and scores on measures of episodic memory, processing speed, and working memory tasks.
Results
In patients with MS, hippocampal volume was significantly related to fornicial diffusion measures (P < 7 × 10− 4) and to measures of verbal (P = 0.030) and visual spatial (P = 0.004) episodic memory and a measure of information processing speed (P < 0.037).
Discussion
These results highlight the role of the hippocampus in cognitive dysfunction in patients with MS and suggest that measures of hippocampal atrophy could be used to capture aspects of disease progression. 相似文献
A synthesis of two structurally related macrodiolides representing the aglycone of natural products elaiophylin and halichoblelide is described. The key transformation for both is a Ti(II)-mediated (silyloxy)enyne cyclization, generating a new methyl stereocenter and providing a diene that can be selectively cross metathesized with crotonic acid. 相似文献
Thermodynamical considerations have largely been avoided in the modelling of complex fluids by invoking the assumption of incompressibility. This approximation allows pressure to be defined as a Lagrange multiplier, and therefore its natural connection with other thermodynamic variables such as density and temperature is irretrievably lost. Relaxing this condition to allow more realistic modelling involves much more than prescribing an equation of state. Even for a simple isothermal viscoelastic model, as explored in this paper, the transition to a compressible model is non-trivial. This paper shows that pressure enters the governing equations in a non-intuitive way. Furthermore, a fluid volume element, which is no longer constant, radically changes the way the basic element of the constitutive equations is viewed—stress is no longer the fundamental constitutive link between the momentum equations and velocity. The importance of geometry in fluid modelling is emphasised through the use of the Lie derivative, which is of a more fundamental character than the “upper” and “lower” convected derivatives prevalent in the literature and which are found to be almost redundant for a compressible fluid. There is now a strong body of non-equilibrium thermodynamics theory for flowing systems, which proves indispensible for this development. These fundamental principles are described herein using methodology and examples, that are sometimes conflicting, from the literature. The main conflict arises from the relationship between thermodynamic pressure and the trace of Cauchy stress, where the current preferred choice is (up to a constant) to set them equal—this is shown to be incorrect. Other issues such as the dependence of viscosity on density, bulk viscosity, integral modelling, the principle of objectivity and convected derivatives, are also clarified and resolved. 相似文献
Within the weak-coupling regime the spin current through a quantum dot system is calculated using a quantum master equation
approach which includes a sum over Matsubara terms. To be able to efficiently calculate, also at low temperatures, the time
evolution of the reduced density matrix a high-temperature approximation was derived which proves to be rather accurate in
comparison to the exact results. In the present model it is assumed that the energy levels of the dot are split by a constant
magnetic field. An additional external (laser) field is used to control the currents of the two spin polarizations. This is
either done using the phenomenon of coherent destruction of tunneling or optimal control theory. Scenarios are studied in
which the spin current is reversed while the charge current is kept constant. 相似文献
Sensitive surveying : Fluorescence correlation spectroscopy (FCS) was used to study receptor density and to monitor ligand–receptor interactions on live cell membranes by the introduction of fluorescently marked aptamers, which specifically bind to certain cell‐surface receptors (see schematic representation; FITC: fluorescein isothiocyanate).