Noninvasive measurement of scattering anisotropy in turbid materials by nonnormal incident illumination

Many existing methods for the recovery of optical parameters from turbid materials rely on the diffusion approximation, which does not permit the recovery of the degree of anisotropy in the scattering phase function. These methods also make the explicit assumption that light is normally incident at the top surface of the material. We demonstrate a steady-state imaging technique that uses nonnormally incident light to determine anisotropy parameter g by fitting Monte Carlo simulation results to high dynamic range images of the intensity profiles of samples. The proposed method is simpler than existing methods and does not rely on thin samples to produce reasonable results.     

Synthesis of crystalline Ge nanoclusters in PE-CVD-deposited SiO2 films

The synthesis of evenly distributed Ge nanoclusters in plasma-enhanced chemical-vapour-deposited (PE-CVD) SiO₂ thin films containing 8 at. % Ge is reported. This is of importance for the application of nanoclusters in semiconductor technology. The average diameter of the Ge nanoclusters can be controlled in the range of 3–6 nm by variation of the annealing parameters. Using a combination of transmission electron microscopy and Raman-scattering spectroscopy, the nanoclusters were shown to be crystalline. However, photoluminescence measurements showed no light emission that could be definitively correlated to the presence of the nanoclusters. PACS 61.82.Rx; 78.67.Bf; 81.07.Bc     

Theoretical study of the temperature dependence of the width of magnetic domain walls

The temperature dependence of magnetic domain walls in ferromagnetic systems with strong exchange coupling and weak lattice anisotropy is studied assuming that the thermal influence results mainly from the temperature dependence of the magnetization. We obtain that in lattices with an uniaxial symmetry like Co the wall width increases with temperature, but stays finite up to the Curie temperature T_c. In contrary, for a cubic lattice like Fe the wall width diverges for T → T_c, if only the lattice anisotropy is taken into account. The shape of the domain walls is not conserved, since at T_c the wall is determined only by the lowest order of anisotropy. In addition, the temperature dependence of a domain wall width for a thin magnetic film is determined. Using a special symmetry, we obtain a diverging wall width at a temperature markedly lower than T_c. However, the consideration of additional domain wall modes should modify this result.     

Shadow effects in simulated ultrasound images derived from computed tomography images using a focused beam tracing model

Simulation of ultrasound images based on computed tomography (CT) data has previously been performed with different approaches. Shadow effects are normally pronounced in ultrasound images, so they should be included in the simulation. In this study, a method to capture the shadow effects has been developed, which makes the simulated ultrasound images appear more realistic. The method using a focused beam tracing model gives diffuse shadows that are similar to the ones observed in measurements on real objects. Ultrasound images of a cod (Gadus morhua) were obtained with a BK Medical 2202 ProFocus ultrasound scanner (BK Medical, Herlev, Denmark) equipped with a dedicated research interface giving access to beamformed radio frequency data. CT images were obtained with an Aquilion ONE Toshiba CT scanner (Toshiba Medical Systems Corp., Tochigi, Japan). CT data were mapped from Hounsfield units to backscatter strength, attenuation coefficients, and characteristic acoustic impedance. The focused beam tracing model was used to create maps of the transmission coefficient and scattering strength maps. Field II was then used to simulate an ultrasound image of 38.9 × 55.3 × 4.5 mm, using 10(6) point scatterers. As there is no quantitative method to assess quality of a simulated ultrasound image compared to a measured one, visual inspection was used for evaluation.     

Fourier Transform Microwave Spectrum and ab Initio Study of Dimethyl Methylphosphonate

The rotational spectrum of dimethyl methylphosphonate (DMMP) has been studied using a pulsed molecular beam Fourier transform microwave spectrometer. The spectrum is complicated by the internal rotation motions of the three methyl tops in the molecule as well as an interconversion motion of the two methoxy groups. Here, we present the microwave spectrum, the ab initio calculations, and the assignment of the rigid-rotor A-symmetry state of the molecule. The rotational constants for this state are A=2828.753(2) MHz, B=1972.360(3) MHz, and C=1614.267(2) MHz. In the following paper, a group theoretical analysis is developed for DMMP. The observed conformation of the molecule has no point-group symmetry and all three electric-dipole selection rules are active, with c-type transitions being the most intense. Ab initio calculations were carried out at both the Hartree-Fock 6-31G* and MP2/6-311G* levels of theory. These calculations indicate that two low-energy conformations are possible separated by energies of less than 170 cm⁻¹. Furthermore, the calculated lowest energy conformer is in agreement with the one observed experimentally. The relative energies of the two low-energy conformers rise from 34 cm⁻¹ at the HF level to 170 cm⁻¹ at the MP2 level.     

Towards Treating Correlations in Bose Condensates

 We use the adiabatic hyperspheric expansion and the Faddeev decomposition of the wave function with only s-waves. We derive for a fixed hyperradius an integro-differential equation for the angular eigenvalue and wave function. The correlations lower the interaction energy for N = 20 by about a factor of 5. Received October 22, 2001; accepted for publication November 5, 2001     

Experimental and theoretical N.M.R. studies of the coupling constants in seven isotopic ethyl fluorides

The N.M.R. spin-spin coupling constants and chemical shifts are reported for seven isotopically substituted ethyl fluorides. ¹H, ²H and ¹⁹F spectra have been observed and a consistent set of data obtained. Isotope effects are reported in ¹H and ¹⁹F spectra due to replacement of ¹H by ²H and ¹²C by ¹³C.

Using an accurate microwave structure of ethyl fluoride, CNDO and INDO calculations have been carried out, including dipolar and orbital terms. As independent options, configuration interaction between all single-excited states, variable radial electron distribution <r ^-3> and variable electron density at the site of nuclei, <s(0)²>, are included. Calculated values for all combinations of the options are given and a discussion of the results presented.     

Structure of low-lying <Superscript>12</Superscript>C resonances

The hyperspherical adiabatic expansion is combined with complex scaling and used to calculate low-lying nuclear resonances of ¹²C in the 3α model. We use Ali-Bodmer potentials and compare results for other potentials α-α with similar ⁸Be properties. A three-body potential is used to adjust the ¹²C resonance positions to desired values extending the applicability of the method to many-body systems decaying into three α-particles. For natural choices of three-body potentials we find 14 resonances below the proton separation threshold, i.e. two 0⁺, three 2⁺, two 4⁺, one of each of 1^±, 2^-, 3^±, 4^-, and 6⁺. The partial-wave decomposition of each resonance is calculated as a function of the hyperradius. Strong variation is found from small to large distance. The connection to previous experimental and theoretical results is discussed and agreements as well as disagreements are emphasized.     

Entanglement generated by dissipation and steady state entanglement of two macroscopic objects

Entanglement is a striking feature of quantum mechanics and an essential ingredient in most applications in quantum information. Typically, coupling of a system to an environment inhibits entanglement, particularly in macroscopic systems. Here we report on an experiment where dissipation continuously generates entanglement between two macroscopic objects. This is achieved by engineering the dissipation using laser and magnetic fields, and leads to robust event-ready entanglement maintained for 0.04 s at room temperature. Our system consists of two ensembles containing about 10(12) atoms and separated by 0.5 m coupled to the environment composed of the vacuum modes of the electromagnetic field. By combining the dissipative mechanism with a continuous measurement, steady state entanglement is continuously generated and observed for up to 1 h.     

A computationally efficient finite element model with perfectly matched layers applied to scattering from axially symmetric objects

A frequency-domain finite-element (FE) technique for computing the radiation and scattering from axially symmetric fluid-loaded structures subject to a nonsymmetric forcing field is presented. The Berenger perfectly matched layer (PML), applied directly at the fluid-structure interface, makes it possible to emulate the Sommerfeld radiation condition using FE meshes of minimal size. For those cases where the acoustic field is computed over a band of frequencies, the meshing process is simplified by the use of a wavelength-dependent rescaling of the PML coordinates. Quantitative geometry discretization guidelines are obtained from a priori estimates of small-scale structural wavelengths, which dominate the acoustic field at low to mid frequencies. One particularly useful feature of the PML is that it can be applied across the interface between different fluids. This makes it possible to use the present tool to solve problems where the radiating or scattering objects are located inside a layered fluid medium. The proposed technique is verified by comparison with analytical solutions and with validated numerical models. The solutions presented show close agreement for a set of test problems ranging from scattering to underwater propagation.