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W Götze

Publications and source records attributed to W Götze.

At least 19 recordsLinked to original sources
PubMed2004

Nearly logarithmic decay of correlations in glass-forming liquids.

Nearly logarithmic decay of correlations, which was observed for several supercooled liquids in optical-Kerr-effect experiments [Phys. Rev. Lett. 84, 2437 (2000)]; Phys. Rev. Lett. 90, 197401 (2003)]], is explained within the mode-coupling theory for ideal glass transitions as a manifestation of the beta-peak phenomenon. A schematic model, which describes the dynamics by only two correlators, one referring to density fluctuations and the other to the reorientational fluctuations of the molecules, yields for strong rotation-translation coupling response functions in agreement with those measured for benzophenone and Salol for the time interval extending from 2 ps to about 20 and 200 ns, respectively.

Journal Article↗
PubMed2004

alpha-Relaxation processes in binary hard-sphere mixtures.

Molecular-dynamics simulations are presented for two correlation functions formed with the partial density fluctuations of binary hard-sphere mixtures in order to explore the effects of mixing on the evolution of glassy dynamics upon compressing the liquid into high-density states. Partial-density-fluctuation correlation functions for the two species are reported. Results for the alpha-relaxation process are quantified by parameters for the strength, stretching, and time scale, where the latter varies over almost four orders of magnitude upon compression. The parameters exhibit an appreciable dependence on the wave vector, and this dependence is different for the correlation function referring to the smaller and that for the larger species. These features are shown to be in semiquantitative agreement with those calculated within the mode-coupling theory for ideal liquid-glass transitions.

Journal Article↗
PubMed2003

Mixing effects for the structural relaxation in binary hard-sphere liquids.

We report extensive molecular-dynamics-simulation results for binary mixtures of hard spheres for different size disparities and different mixing percentages, for packing fractions up to 0.605, and over a characteristic time interval spanning up to 5 orders in magnitude. We explore the changes in the evolution of glassy dynamics due to mixing and discover two opposite scenarios: For large size disparity, increasing the mixing percentage of small particles leads to a speed up of long-time dynamics, while small disparity leads to a slowing down. These results agree with predictions based on the mode-coupling theory for ideal-glass transitions.

Journal Article↗
PubMed2003

Effect of composition changes on the structural relaxation of a binary mixture.

Within the mode-coupling theory for idealized glass transitions, we study the evolution of structural relaxation in binary mixtures of hard spheres with size ratios delta of the two components varying between 0.5 and 1.0. We find two scenarios for the glassy dynamics. For small size disparity, the mixing yields a slight extension of the glass regime. For larger size disparity, a plasticization effect is obtained, leading to stabilization of the liquid due to mixing. For all delta, a decrease of the elastic moduli at the transition due to mixing is predicted. A stiffening of the glass structure is found as is reflected by the increase of the Debye-Waller factors at the transition points. The critical amplitudes for density fluctuations at small and intermediate wave vectors decrease upon mixing, and thus the universal formulas for the relaxation near the plateau values describe a slowing down of the dynamics upon mixing for the first step of the two-step relaxation scenario. The results explain the qualitative features of mixing effects reported by Williams and van Megen [Phys. Rev. E 64, 041502 (2001)] for dynamical light-scattering measurements on binary mixtures of hard-sphere-like colloids with size ratio delta=0.6.

Journal Article↗
PubMed2002

Logarithmic relaxation in glass-forming systems.

Within the mode-coupling theory for ideal glass transitions, an analysis of the correlation functions of glass-forming systems for states near higher-order glass-transition singularities is presented. It is shown that the solutions of the equations of motion can be asymptotically expanded in polynomials of the logarithm of time t. In leading order, a ln(t) law is obtained, and the leading corrections are given by a fourth-order polynomial. The correlators interpolate between three scenarios. First, there are surfaces in parameter space where the dominant corrections to the ln(t) law vanish, so that the logarithmic decay governs the structural relaxation process. Second, the dynamics due to the higher-order singularity can describe the initial and intermediate part of the alpha process thereby reducing the range of validity of von Schweidler's law and leading to strong alpha relaxation stretching. Third, the ln(t) law can replace the critical decay law of the beta process, leading to a particularly large crossover interval between the end of the transient and the beginning of the alpha process. This may lead to susceptibility spectra below the band of microscopic excitations exhibiting two peaks. Typical results of the theory are demonstrated for models dealing with one and two correlation functions.

Journal Article↗
PubMed2002

Structural relaxation in a system of dumbbell molecules.

The interaction-site-density-fluctuation correlators, the dipole-relaxation functions, and the mean-squared displacements of a system of symmetric dumbbells of fused hard spheres are calculated for two representative elongations of the molecules within the mode-coupling theory for the evolution of glassy dynamics. For large elongations, universal relaxation laws for states near the glass transition are valid for parameters and time intervals similar to the ones found for the hard-sphere system. Rotation-translation coupling leads to an enlarged crossover interval for the mean-squared displacement of the constituent atoms between the end of the von Schweidler regime and the beginning of the diffusion process. For small elongations, the superposition principle for the reorientational alpha process is violated for parameters and time intervals of interest for data analysis, and there is a strong breaking of the coupling of the alpha-relaxation scale for the diffusion process with that for representative density fluctuations and for dipole reorientations.

Journal Article↗
PubMed2002

Idealized glass transitions for a system of dumbbell molecules.

The mode-coupling theory for ideal glass transitions in simple systems is generalized to a theory for the glassy dynamics of molecular liquids using the density fluctuations of the sites of the molecule's constituent atoms as the basic structure variables. The theory is applied to calculate the liquid-glass phase diagram and the form factors for the arrested structure of a system of symmetric dumbbells of fused hard spheres. The static structure factors, which enter the equations of motion as input, are calculated as function of the packing fraction phi and the molecule's elongation zeta within the reference-interaction-site-model and Percus-Yevick theories. The critical packing fraction phi(c) for the glass transition is obtained as nonmonotone function of zeta with a maximum near zeta=0.43. A transition line is calculated separating a small-zeta-glass phase with ergodic dipole motion from a large-zeta-glass phase where also the reorientational motion is arrested. The Debye-Waller factors at the transition are found to be somewhat larger for sufficiently elongated systems than those for the simple hard-sphere system, but the wave-number dependence of the glass-form factors is quite similar. The dipole reorientations for zeta> or =0.6 are arrested as strongly as density fluctuations with wave vectors at the position of the first sharp diffraction peak.

Journal Article↗
PubMed2001

Mean-squared displacement of a molecule moving in a glassy system.

The mean-squared displacement (MSD) of a hard sphere and of a dumbbell molecule consisting of two fused hard spheres immersed in a dense hard-sphere system is calculated within the mode-coupling theory for ideal liquid-glass transitions. It is proven that the velocity correlator, which is the second time derivative of the MSD, is the negative of a completely monotone function for times within the structural-relaxation regime. The MSD is found to exhibit a large time interval for structural relaxation prior to the onset of the alpha process, which cannot be described by the asymptotic formulas for the mode-coupling-theory-bifurcation dynamics. The alpha process for molecules with a large elongation is shown to exhibit an anomalously wide crossover interval between the end of the von Schweidler decay and the beginning of normal diffusion. The diffusivity of the molecule is predicted to vary nonmonotonically as a function of its elongation.

Journal Article↗
PubMed2000

Mode-coupling theory for the glassy dynamics of a diatomic probe molecule immersed in a simple liquid.

Generalizing the mode-coupling theory for ideal liquid-glass transitions, equations of motion are derived for the correlation functions describing the glassy dynamics of a diatomic probe molecule immersed in a simple glass-forming system. The molecule is described in the interaction-site representation and the equations are solved for a dumbbell molecule consisting of two fused hard spheres in a hard-sphere system. The results for the molecule's arrested position in the glass state and the reorientational correlators for angular-momentum index l=1 and l=2 near the glass transition are compared with those obtained previously within a theory based on a tensor-density description of the molecule in order to demonstrate that the two approaches yield equivalent results. For strongly hindered reorientational motion, the dipole-relaxation spectra for the alpha process can be mapped on the dielectric-loss spectra of glycerol if a rescaling is performed according to a suggestion by Dixon et al. [Phys. Rev. Lett. 65, 1108 (1990)]. It is demonstrated that the glassy dynamics is independent of the molecule's inertia parameters.

Journal Article↗
PubMed2000

Higher-order glass-transition singularities in colloidal systems with attractive interactions.

The transition from a liquid to a glass in colloidal suspensions of particles interacting through a hard core plus an attractive square-well potential is studied within the mode-coupling-theory framework. When the width of the attractive potential is much shorter than the hard-core diameter, a reentrant behavior of the liquid-glass line and a glass-glass-transition line are found in the temperature-density plane of the model. For small well-width values, the glass-glass-transition line terminates in a third-order bifurcation point, i.e., in a A3 (cusp) singularity. On increasing the square-well width, the glass-glass line disappears, giving rise to a fourth-order A4 (swallow-tail) singularity at a critical well width. Close to the A3 and A4 singularities the decay of the density correlators shows stretching of huge dynamical windows, in particular logarithmic time dependence.

Journal Article↗
PubMed2000

Reorientational relaxation of a linear probe molecule in a simple glassy liquid.

Within the mode-coupling theory (MCT) for the evolution of structural relaxation in glass-forming liquids, correlation functions and susceptibility spectra are calculated characterizing the rotational dynamics of a top-down symmetric dumbbell molecule, consisting of two fused hard spheres immersed in a hard-sphere system. It is found that for sufficiently large dumbbell elongations, the dynamics of the probe molecule follows the same universal glass-transition scenario as known from the MCT results of simple liquids. The alpha-relaxation process of the angular-index j=1 response is stronger, slower, and less stretched than the one for j=2, in qualitative agreement with results found by dielectric-loss and depolarized-light-scattering spectroscopy for some supercooled liquids. For sufficiently small elongations, the reorientational relaxation occurs via large-angle flips, and the standard scenario for the glass-transition dynamics is modified for odd-j responses due to precursor phenomena of a nearby type-A MCT transition. In this case, a major part of the relaxation outside the transient regime is described qualitatively by the beta-relaxation scaling laws, while the alpha-relaxation scaling law is strongly disturbed.

Journal Article↗
PubMed1991

Parapulpal pins and their effects on the fracture resistance of pin-retained cores.

The bending characteristics of various parapulpal pins, the retention of a parapulpal pin in different core buildup materials, and the fracture resistance of pin-retained cores were investigated in an in-vitro study. In amalgam and composite cores, the use of parapulpal pins produced an increase in fracture resistance. The highest fracture resistance values were observed in amalgam cores, and the lowest fracture strength was exhibited by cores of glass ionomer cements (GICs) containing metallic particles. The different parapulpal pins and their flexural rigidity had little effect on the fracture resistance of the pin-retained cores. For extensive pin-retained cores it appears advisable to use GICs with restraint and, particularly in the case of large-scale bridge reconstructions, only after critical examination of the loading conditions.

Alloys↗
PubMed1991

[Examination of shear strength of repaired composite specimen].

The test of the shearstrength of two composite-inlay-materials and two posterior composites exhibited a statistically significant less bond-strength of repaired specimen compaired with unrepaired control-groups, whereas each material showed different results. The indirect-inlay-system proved to have the lowest bondstrength after repairing. The combination of the two posterior composites as well as the two inlay-systems with each other showed that none of these materials was convincing as a repairing material.

Acrylic Resins↗
PubMed1990

[Detection of human papillomavirus (HVP) 16/18 infections of the vaginal uterine cervix in females before and following conisation].

280 women have been tested by FISH to detect an HPV 16/18-infection of the portio uteri. In a group of 133 women treated by conisation (all with a history of abnormal smear and colposcopy) 67 (50%) were positive for HPV 16/18 DNA before the operation. Follow up examinations of 20 cases with positive HPV 16/18 test before conisation, showed 6 and 12 months after operation only in 3 cases a persistence of HPV 16/18 infection. Only 5 (6.4%) of 78 women with a history of conisation and diagnosis CIN III some years ago were positive for HPV 16/18. In another group of 47 elderly women, treated by abrasio and with normal cervical histology, only 5 (10.6%) had a positive result in the HPV 16/18 FISH-test. 2 (10.6%) of 22 women with normal smear and colposcopy (control group) were positive for HPV 16/18.

Adult↗
PubMed1990

[Prognostic significance of the length of the uterus, depth of infiltration and grading of endometrium cancer].

84 patients who underwent operation for adenocarcinoma of the endometrium stage I have been investigated with regard to morphometric and histologic prognostic factors. Measuring the maximal depth of infiltration of the carcinoma in relation to the thickness of the uterine wall, tumor grading and carcinomatous lymphangiosis are of dominant value. Measuring the length of the uterine cavity is without significance. Patients with a short length had more often recurrences than patients with a longer one.

Adenocarcinoma↗