Precursor film controlled wetting of Pb on Cu

Wetting in a system where the kinetics of drop spreading are controlled by the rate of formation of a precursor film is modeled for the first time at the atomistic scale. Molecular dynamics simulations of Pb(l) wetting Cu(111) and Cu(100) show that a precursor film of atomic thickness evolves and spreads diffusively. This precursor film spreads significantly faster on Cu(111) than on Cu(100). For Cu(100), the kinetics of drop spreading are dramatically decreased by slow advancement of the precursor film. Slow precursor film kinetics on Cu(100) are partly due to the formation of a surface alloy at the solid-liquid interface which does not occur on Cu(111).     

基于平面声源实施结构声辐射有源控制的理论研究

摘要研究了利用分布式平面声源对结构声辐射进行有源控制的问题。首先建立了系统的数学模型,然后推导了有源控制条件下次级声源的强度和声功率降低的计算公式。在实际应用中,次级声源参数(面积大小、安放位置、个数等)对控制效果有重要影响,本文基于有源控制的物理机理和数值仿真研究这些问题。结果表明: -般情况下,次级声源板的振动模态分布与初级结构振动模态分布不相同,因此,在低频范围内,需要至少4个分布式次级声源,方能有效地控制初级结构声辐射。     

Characterization of Standard Reference Material 2941, uranyl-ion-doped glass, spectral correction standard for fluorescence

Standard Reference Material^® (SRM^®) 2941 is a cuvette-shaped, uranyl-ion-doped glass, recommended for use for relative spectral correction of emission and day-to-day performance validation of fluorescence spectrometers. Properties of this standard that influence its effective use or contribute to the uncertainty in its certified emission spectrum have been explored here. These properties include its photostability, absorbance, dissolution rate in water, anisotropy, temperature coefficient of fluorescence intensity, and fluorescence lifetimes. The expanded uncertainties in the certified spectrum are about 4% around the peak maximum at 526 nm, using an excitation wavelength of 427 nm. The SRM also exhibits a strong resistance to photodegradation, with no measurable decrease in fluorescence intensity even after 8 h of laser irradiation.     

Site Preference Model for Hyperfine Impurities in Compounds

A thermodynamic model for site preferences of solute atoms in ordered binary phases is applied in this paper for substitutional and interstitial sites of the Cu₃Au, or L1₂, structure. Site preferences are found to depend on formation energies of combinations of elementary point defects and on energies for transfer of solutes among different sites. The composition dependence in compounds having a wide phase field is examined in detail. A phenomenology of site preference behavior is outlined for regular lattice sites as well as for non-lattice sites such as grain boundaries. This revised version was published online in September 2006 with corrections to the Cover Date.     

Electric-Field Gradients at Group-III Sites on GaAs and InAs (111)B Surfaces

We report a comparison of electric-field gradients (EFGs) measured at the group-III sites on epitaxially-grown surfaces of 2×2-reconstructed GaAs and InAs crystals. For this purpose, we used ¹¹¹In→¹¹¹Cd perturbed-angular-correlation (PAC) spectroscopy. Sharp spectral lines characterize the perturbation functions corresponding to both surfaces. On GaAs surface, we observe only one well-defined nuclear electric-quadrupole interaction (NQI); and on the InAs surface, we observe a primary and a secondary NQI. Very similar but experimentally-distinguishable values of the EFG-parameters characterize the primary interactions corresponding to the GaAs and InAs surfaces. Specifically, for the GaAs and InAs surfaces, ω _Q =28.0±0.2 Mrad s⁻¹ and η=0.43±0.02 and ω _Q =28.8±0.2 Mrad s⁻¹ and η=0.39±0.02, and the angles between EFG z-axis and the (111) direction are 65°±3° and β=53°±3°, respectively. These unexpected results indicate that the so-called lattice contribution to the EFG is not significant. Moreover, for the primary NQIs on both surfaces, the similar parameter values demonstrate that chemical differences between the ¹¹¹In probe and the indigenous Ga surface atoms cause no large quantitative effects. This information indicates that impurity probes and PAC spectroscopy can be used effectively to investigate III–V surfaces. This revised version was published online in September 2006 with corrections to the Cover Date.     

Time-oscillating Lyapunov modes and the momentum autocorrelation function

The Lyapunov vectors corresponding to the steps of Lyapunov spectra for many-particle systems show time-oscillating behavior in two types of Lyapunov modes, one associated with time-translational invariance and the other with spatial translational invariance. Our result is that, for each coordinate direction, the longest period of the Lyapunov modes is twice as long as the period of the momentum autocorrelation function. A simple explanation for this relation is proposed and we argue that this result is generally true for many-particle systems. This gives the first quantitative connection between the Lyapunov modes and an experimentally accessible quantity.     

Enhancing the acquisition efficiency of fast magnetic resonance imaging via broadband encoding of signal content

Current efficient magnetic resonance imaging (MRI) methods such as parallel-imaging and k-t methods encode MR signals using a set of effective encoding functions other than the Fourier basis. This work revisits the proposition of directly manipulating the set of effective encoding functions at the radiofrequency excitation step in order to increase MRI efficiency. This approach, often termed "broadband encoding," enables the application of algebraic matrix factorization technologies to extract efficiency by representing and encoding MR signal content in a compacted form. Broadband imaging equivalents of fast multiecho, parallel and k-t MRI are developed and analyzed. The potential of these techniques to increase the time efficiency of data acquisition is experimentally verified on a commercial MRI scanner using simple spin-echo imaging. A three-dimensional gradient-echo dynamic imaging application that demonstrates the potential benefits of this approach compared to the present state of the art for certain applications is also presented.     

Comparison of time-dependent effects of (+)-methamphetamine or forced swim on monoamines,corticosterone, glucose,creatine, and creatinine in rats

Background  

Methamphetamine (MA) use is a worldwide problem. Abusers can have cognitive deficits, monoamine reductions, and altered magnetic resonance spectroscopy findings. Animal models have been used to investigate some of these effects, however many of these experiments have not examined the impact of MA on the stress response. For example, numerous studies have demonstrated (+)-MA-induced neurotoxicity and monoamine reductions, however the effects of MA on other markers that may play a role in neurotoxicity or cell energetics such as glucose, corticosterone, and/or creatine have received less attention. In this experiment, the effects of a neurotoxic regimen of (+)-MA (4 doses at 2 h intervals) on brain monoamines, neostriatal GFAP, plasma corticosterone, creatinine, and glucose, and brain and muscle creatine were evaluated 1, 7, 24, and 72 h after the first dose. In order to compare MA's effects with stress, animals were subjected to a forced swim test in a temporal pattern similar to MA administration [i.e., (30 min/session) 4 times at 2 h intervals].