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Andreas Heuer

Publications and source records attributed to Andreas Heuer.

96 records · Page 6Linked to original sources
arXiv2000

Local Reorientation Dynamics of Semiflexible Polymers in the Melt

The reorientation dynamics of local tangent vectors of chains in isotropic amorphous melts containing semiflexible model polymers was studied by molecular dynamics simulations. The reorientation is strongly influenced both by the local chain stiffness and by the overall chain length. It takes place by two different subsequent processes: A short-time non-exponential decay and a long-time exponential reorientation arising from the relaxation of medium-size chain segments. Both processes depend on stiffness and chain length. The strong influence of the chain length on the chain dynamics is in marked contrast to its negligible effect on the static structure of the melt. The local structure shows only a small dependence on the stiffness, and is independent of chain length. Calculated correlation functions related to double-quantum NMR experiments are in qualitative agreement with experiments on entangled melts. A plateau is observed in the dependence of segment reorientation on the mean-squared displacement of the corresponding chain segments. This plateau confirms, on one hand, the existence of reptation dynamics. On the other hand, it shows how the reptation picture has to be adapted if, instead of fully flexible chains, semirigid chains are considered.

cond-mat.soft↗
arXiv1999

Cooperativity and Spatial Correlations near the Glass Transition: Computer Simulation Results for Hard Spheres and Discs

We examine the dynamics of hard spheres and discs at high packing fractions in two and three dimensions, modeling the simplest systems exhibiting a glass transition. As it is well known, cooperativity and dynamic heterogeneity arise as central features when approaching the glass transition from the liquid phase, so an understanding of their underlying physics is of great interest. Cooperativity implies a reduction of the effective degrees of freedom, and we demonstrate a simple way of quantification in terms of the strength and the length scale of dynamic correlations among different particles. These correlations are obtained for different dynamical quantities $X_i(t)$ that are constructed from single-particle displacements during some observation time $t$. Of particular interest is the dependence on $t$. Interestingly, for appropriately chosen $X_i(t)$ we obtain finite cooperativity in the limit $t \to \infty$.

cond-mat.soft↗
arXiv1999

The potential energy landscape of a model glass former: thermodynamics, anharmonicities, and finite size effects

It is possible to formulate the thermodynamics of a glass forming system in terms of the properties of inherent structures, which correspond to the minima of the potential energy and build up the potential energy landscape in the high-dimensional configuration space. In this work we quantitatively apply this general approach to a simulated model glass-forming system. We systematically vary the system size between N=20 and N=160. This analysis enables us to determine for which temperature range the properties of the glass former are governed by the regions of the configuration space, close to the inherent structures. Furthermore, we obtain detailed information about the nature of anharmonic contributions. Moreover, we can explain the presence of finite size effects in terms of specific properties of the energy landscape. Finally, determination of the total number of inherent structures for very small systems enables us to estimate the Kauzmann temperature.

cond-mat.soft↗
arXiv1999

Metastable states as a key to the dynamics of supercooled liquids

Computer simulations of a model glass-forming system are presented, which are particularly sensitive to the correlation between the dynamics and the topography of the potential energy landscape. This analysis clearly reveals that in the supercooled regime the dynamics is strongly influenced by the presence of deep valleys in the energy landscape, corresponding to long-lived metastable amorphous states. We explicitly relate non-exponential relaxation effects and dynamic heterogeneities to these metastable states and thus to the specific topography of the energy landscape.

cond-mat.soft↗
arXiv1997

Tunneling dynamics of side chains and defects in proteins, polymer glasses, and OH-doped network glasses

Simulations on a Lennard-Jones computer glass are performed to study effects arising from defects in glasses at low temperatures. The numerical analysis reveals that already a low concentration of defects may dramatically change the low temperature properties by giving rise to extrinsic double-well potentials (DWP's). The main characteristics of these extrinsic DWP's are (i) high barrier heights, (ii) high probability that a defect is indeed connected with an extrinsic DWP, (iii) highly localized dynamics around this defect, and (iv) smaller deformation potential coupling to phonons. Designing an extension of the Standard Tunneling Model (STM) which parametrizes this picture and comparing with ultrasound experiments on the wet network glass $a$-B$_2$O$_3$ shows that effects of OH-impurities are accurately accounted for. This model is then applied to organic polymer glasses and proteins. It is suggested that side groups may act similarly like doped impurities inasmuch as extrinsic DWP's are induced, which possess a distribution of barriers peaked around a high barrier height. This compares with the structurlessly distributed barrier heights of the intrinsic DWP's, which are associated with the backbone dynamics. It is shown that this picture is consistent with elastic measurements on polymers, and can explain anomalous nonlogarithmic line broadening recently observed in hole burning experiments in PMMA.

cond-mat.stat-mech↗