Glass-transition temperature of water: a simulation study

We report a computer simulation study of the glass transition for water using the extended simple point charge potential. To mimic the difference between standard and hyperquenched glass, we generate glassy configurations with different cooling rates, and we calculate the temperature dependence of the specific heat on heating. The absence of crystallization phenomena allows us, for properly annealed samples, to detect in the specific heat the simultaneous presence of a weak prepeak ("shadow transition") and an intense glass transition peak at higher temperature. Our results support the view-point that the glass transition temperature is higher than the conventionally accepted value 136 K. We also compare our simulation results with the Tool-Narayanaswamy-Moynihan phenomenological model.     

A Two Dimensional Fermi Liquid. Part 2: Convergence

Using results established in other papers in our series, we prove the existence of the infinite volume, temperature zero, thermodynamic Greens functions of a two dimensional, weakly coupled fermion gas with an asymmetric Fermi curve and short range interactions. This is done by showing that our sequence of renormalization group maps converges.Research supported in part by the Natural Sciences and Engineering Research Council of Canada and the Forschunginstitut für Mathematik, ETH Zürich.     

The Effect of Pea Chloroplast Alignment and Variation of Excitation Wavelength on the Circularly Polarized Chlorophyll Luminescence

Circularly polarized luminescence (CPL) is a powerful technique to study the macroorganization of photosynthetic light-harvesting apparatus in vivo and in vitro. It is particularly useful for monitoring environmental stress induced molecular re-organization of thylakoid membranes in green leaves. The current study focuses on two questions which are important to perform and interpret such experiments: how does CPL depend on the excitation wavelength and how on the orientation of the granal thylakoids. CPL and circular dichroism (CD) of pea chloroplasts were complementarily applied when chloroplasts were either in suspension or trapped in a polyacrylamide gel (PAAG) after alignment in a magnetic field. In contrast to the CD spectrum, the CPL signal was found to be independent of the excitation wavelength in both the Soret and the Qy absorption region for chloroplasts in both suspension and PAAG. The improved resolution of luminescence measurements revealed a relatively small negative CPL band in addition to the previously described large positive band. No effect of photoselection upon excitation on the CPL spectra was detected. The CPL intensity at 690 nm at the edge of the granal thylakoids was found to be higher than at the face of the grana suggesting the CPL anisotropy.     

Computer-aided quantification of focal cartilage lesions of osteoarthritic knee using MRI

Noninvasive assessment of articular cartilage using magnetic resonance imaging (MRI) has gained popularity in the diagnosis of osteoarthritis (OA), a condition that affects 20 million Americans. Focal cartilage lesions, a defect found in roughly 19% of the OA population, currently can only be evaluated with confidence using minimally invasive arthroscopy. This article presents a computer-aided procedure using MRI to quantify focal cartilage lesions and aims to support clinical practices of diagnosis and monitoring of lesion progress. Upon a local minima search for identifying focal lesions, the proposed gradient peak method outlines lesion boundaries and then generates morphological properties, such as lesion volume and lesion area. The procedure was evaluated using simulated and in vivo data. First, a simulated lesion was created and analyzed, and the results were compared with the exact solutions. Second, an in vivo evaluation was carried out on seven human knees in which nine focal lesions were identified and quantified. Three of the subjects had follow-up analyses, at either 1 or 2 years. Finally, in an attempt to characterize local biochemical changes underlying focal lesions, MR-derived T2 values of defective cartilage within the lesion boundaries were examined and compared with the values of adjacent cartilage compartments.     

Ray helicity: geometric invariant for multidimensional resonant wave conversion

For a multicomponent wave field propagating into a multidimensional conversion region, the rays are shown to be helical, in general. For a ray-based quantity to have a fundamental physical meaning, it must be invariant under the following two groups of transformations, which are used to construct solutions: congruence transformations (which involve linear combinations of components of the multicomponent wave field) and canonical transformations (which act on the ray phase space). It is shown that for conversion between two waves there is a new invariant not previously discussed: the intrinsic helicity of the ray.     

Comparative tests to improve the gas chromatographic analysis of chlorobornanes in fish samples

A comparison was made between electron capture negative ionization quadrupole mass spectrometry (ECNI-MS) and electron capture detection (ECD) with regard to repeatability and reproducibility for the gas chromatographic (GC) analysis of toxaphene congeners [chlorobornanes (CHBs)]. The tests, including standard solutions and several cleaned fish extracts, showed larger relative standard deviations (RSDs) for the repeatability of ECNI-MS but no differences in the reproducibility of the 2 techniques. The sensitivity of the GC-ECNI-MS was considerably better than that of GC/ECD. Four stepwise-designed comparative tests were also conducted on GC analysis, cleanup, and the complete method. The results showed that, according to the current state-of-the-art, coefficients of variation for the between-laboratory performance were not < 20% and were usually between 20 and 30%. In spite of separation problems, e.g., for CHB 26, which cannot be separated into a single-component peak, a 95% methyl 5% phenyl polysiloxane (CP Sil 8) column was preferred to more polar columns for the analysis of CHBs 26, 40, 41, 44, 50, and 62. CHB 62 was more difficult to determine than CHB 26 and 50. Addition of the CHBs 40, 41, and 44 to the standard set of 3 chlorobornanes (26, 50, and 62) resulted in more separation problems. A 3-step cleanup method was recommended.     

Amphicatalytic polymerization: synthesis of stereomultiblock poly(methyl methacrylate) with diastereospecific ion pairs

Diastereospecific ion pairs consisting of isospecific transition-metal cations and syndiospecific main-group anions promote what is termed amphicatalytic polymerization of MMA, producing PMMA with isotactic-b-syndiotactic stereomultiblock microstructures.     

Ergodicity of mapping class group actions on representation varieties,II. Surfaces with boundary

The mapping class group of a compact oriented surface of genus greater than one with boundary acts ergodically on connected components of the representation moduli corresponding to a connected compact Lie group, for every choice of conjugacy class boundary condition.     

From 1/f noise to multifractal cascades in heartbeat dynamics

We explore the degree to which concepts developed in statistical physics can be usefully applied to physiological signals. We illustrate the problems related to physiologic signal analysis with representative examples of human heartbeat dynamics under healthy and pathologic conditions. We first review recent progress based on two analysis methods, power spectrum and detrended fluctuation analysis, used to quantify long-range power-law correlations in noisy heartbeat fluctuations. The finding of power-law correlations indicates presence of scale-invariant, fractal structures in the human heartbeat. These fractal structures are represented by self-affine cascades of beat-to-beat fluctuations revealed by wavelet decomposition at different time scales. We then describe very recent work that quantifies multifractal features in these cascades, and the discovery that the multifractal structure of healthy dynamics is lost with congestive heart failure. The analytic tools we discuss may be used on a wide range of physiologic signals. (c) 2001 American Institute of Physics.     

Parametric Transducers for the Advanced Cryogenic Resonant-Mass Gravitational Wave Detectors

It is shown that a large improvement in liquid helium and ultra-cryogenically cooled resonant-mass gravitational wave detectors can be achieved through improved parametric transducers using either low loss sapphire dielectric resonators or niobium re-entrant cavities. Performance is analysed in relation to the existing resonant bar antenna Niobe and to massive spherical detectors. Applied to Niobe, a millisecond burst sensitivity of order 10^–20 can be achieved, corresponding to a spectral strain sensitivity of 2 × 10^-22 Hz with a 50 Hz bandwidth. Applied to an ultra-cryogenic 117 tonne spherical detector made from CuAl, a spectral strain sensitivity of better than 2 ×10^-23/ Hz with a bandwidth of order 100 Hz can be achieved, which is close to the quantum limit. This sensitivity is comparable to that of advanced interferometer detectors at this frequency.