arXiv subjects

Michael Vogel

Publications and source records attributed to Michael Vogel.

35 records · Page 2Linked to original sources

arXiv2016

Fragile-to-strong transition in liquid silica

We investigate anomalies in liquid silica with molecular dynamics simulations and present evidence for a fragile-to-strong transition at around 3100 K-3300 K. To this purpose, we studied the structure and dynamical properties of silica over a wide temperature range, finding four indicators of a fragile-to-strong transition. First, there is a density minimum at around 3000 K and a density maximum at 4700 K. The turning point is at 3400 K. Second, the local structure characterized by the tetrahedral order parameter changes dramatically around 3000 K from a higher-ordered, lower-density phase to a less ordered, higher-density phase. Third, the correlation time τ changes from an Arrhenius behavior below 3300 K to a Vogel-Fulcher-Tammann behavior at higher temperatures. Fourth, the Stokes-Einstein relation holds for temperatures below 3000 K, but is replaced by a fractional relation above this temperature. Furthermore, our data indicate that dynamics become again simple above 5000 K, with Arrhenius behavior and a classical Stokes-Einstein relation.

cond-mat.soft↗

arXiv2015

The Slowdowns of Water Dynamics when Approaching a Glass Transition or a Solid Interface: A Common Rationale

Performing molecular dynamics simulations, we investigate the enormous slowdowns of water dynamics when approaching a glass transition or a solid interface. We show that both effects can be described on common grounds within a theoretical framework, which was recently proposed by Schweizer et al. and considers coupled local hopping and elastic distortion. For confined water, we correctly describe the variation of the alpha-relaxation time tau_alpha as a function of both temperature and position with respect to the interface. Exploiting our knowledge of a cooperative length scale xi(T) from the confinement studies, we quantitatively rationalize the glassy slowdown, tau_alpha(T), and the Stokes-Einstein breakdown of bulk water. For both confined and bulk liquid, variations of the alpha-relaxation time are intimately related to changes of the cage-rattling amplitude.

cond-mat.soft↗

arXiv2015

Electric field unbinding of solid-supported lipid multilayers

We studied by X-ray reflectivity the behaviour of fully hydrated solid-supported lipid multilayers under the influence of a transverse electric field, under conditions routinely used in the electroformation process. The kinetics of sample loss (unbinding) was measured as a function of the amplitude and frequency of the applied field by monitoring the integrated intensity of the Bragg peaks. We also performed a time-resolved analysis of the intensity of the first Bragg peak and characterized the final state of the sample.

cond-mat.soft↗

arXiv2014

Water Dynamics at Rough Interfaces

We use molecular dynamics computer simulations and nuclear magnetic resonance experiments to investigate the dynamics of water at interfaces of molecular roughness and low mobility. We find that, when approaching such interfaces, the structural relaxation of water, i.e., the $α$ process, slows down even when specific attractive interactions are absent. This prominent effect is accompanied by a smooth transition from Vogel to Arrhenius temperature dependence and by a growing importance of jump events. Consistently, at protein surfaces, deviations from Arrhenius behavior are weak when free water does not exist. Furthermore, in nanoporous silica, a dynamic crossover of liquid water occurs when a fraction of solid water forms near 225 K and, hence, the liquid dynamics changes from bulk-like to interface-dominated. At sufficiently low temperatures, water exhibits a quasi-universal $β$ process, which is characterized by an activation energy of $E_a\!=\!0.5$ eV and involves anisotropic reorientation about large angles. As a consequence of its large amplitude, the faster $β$ process destroys essentially all orientational correlation, rendering observation of a possible slower $α$ process difficult in standard experiments. Nevertheless, we find indications for the existence of structural relaxation down to a glass transition of interfacial water near 185 K. Hydrated proteins show a highly restricted backbone motion with an amplitude, which decreases upon cooling and vanishes at comparable temperatures, providing evidence for a high relevance of water rearrangements in the hydration shell for secondary protein relaxations.

cond-mat.soft↗

arXiv2014

NMR Studies on the Temperature-Dependent Dynamics of Confined Water

We use $^2$H NMR to study the rotational motion of supercooled water in silica pores of various diameters, specifically, in the MCM-41 materials C10, C12, and C14. Combination of spin-lattice relaxation, line-shape, and stimulated-echo analyses allows us to determine correlation times in very broad time and temperature ranges. For the studied pore diameters, 2.1-2.9 nm, we find two crossovers in the temperature-dependent correlation times of liquid water upon cooling. At 220-230 K, a first kink in the temperature dependence is accompanied by a solidification of a fraction of the confined water, implying that the observed crossover is due to a change from bulk-like to interface-dominated water dynamics, rather than to a liquid-liquid phase transition. Moreover, the results provide evidence that $α$ process-like dynamics is probed above the crossover temperature, whereas $β$ process-like dynamics is observed below. At 180-190 K, we find a second change of the temperature dependence, which resembles that reported for the $β$ process of supercooled liquids during the glass transition, suggesting a value of $T_g\!\approx\!185$ K for interface-affected liquid water. In the high-temperature range, $T\!>\!225$ K, the temperature dependence of water reorientation is weaker in the smaller C10 pores than in the larger C12 and C14 pores, where it is more bulk-like, indicating a significant effect of the silica confinement on the $α$ process of water in the former 2.1 nm confinement. By contrast, the temperature dependence of water reorientation is largely independent of the confinement size and described by an Arrhenius law with an activation energy of $E_a\!\approx\!0.5\ $eV in the low-temperature range, $T\!<\!180 $K, revealing that the confinement size plays a minor role for the $β$ process of water.

cond-mat.soft↗

arXiv2010

NMR and dielectric studies of hydrated collagen and elastin: Evidence for a delocalized secondary relaxation

Using a combination of dielectric spectroscopy and solid-state deuteron NMR, the hydration water dynamics of connective tissue proteins is studied at sub-ambient temperatures. In this range, the water dynamics follows an Arrhenius law. A scaling analysis of dielectric losses, 'two-phase' NMR spectra, and spin-lattice relaxation times consistently yield evidence for a Gaussian distribution of energy barriers. With the dielectric data as input, random-walk simulations of a large-angle, quasi-isotropic water reorientation provide an approximate description of stimulated-echo data on hydrated elastin. This secondary process takes place in an essentially rigid energy landscape, but in contrast to typical β-relaxations it is quasi-isotropic and delocalized. The delocalization is inferred from previous NMR diffusometry experiments. To emphasize the distinction from conventional β-processes, for aqueous systems such a matrix-decoupled relaxation was termed a ν-process. It is emphasized that the phenomenology of this time-honored, 'new' process is shared by many non-aqueous binary glasses in which the constituent components exhibit a sufficient dynamical contrast.

cond-mat.soft↗

arXiv2009

Temperature-dependent mechanisms for the dynamics of protein-hydration waters: a molecular dynamics simulation study

Molecular dynamics simulations are performed to study the temperature-dependent dynamics and structures of the hydration shells of elastin-like and collagen-like peptides. For both model peptides, it is consistently observed that, upon cooling, the mechanisms for water dynamics continuously change from small-step diffusive motion to large-step jump motion, the temperature dependence of water dynamics shows a weak crossover from fragile behavior to strong behavior, and the order of the hydrogen-bond network increases. The temperature of the weak crossover from fragile to strong behavior is found to coincide with the temperature at which maximum possible order of the hydrogen-bond network is reached so that the structure becomes temperature independent. In the strong regime, the temperature dependence of water translation and rotational dynamics is characterized by an activation energy of ca. E_a=0.43 eV, consistent with results from previous dielectric spectroscopy and nuclear magnetic resonance studies on protein hydration waters. At these temperatures, a distorted pi-flip motion about the twofold molecular symmetry axes, i.e., a water-specific beta process is an important aspect of water dynamics, at least at the water-peptide interfaces. In addition, it is shown that the hydration waters exhibit pronounced dynamical heterogeneities, which can be traced back to a strong slowdown of water motion in the immediate vicinity of peptide molecules due to formation of water-peptide hydrogen bonds.

cond-mat.soft↗

arXiv2008

On the origins of apparent fragile-to-strong transition of protein hydration waters

2H NMR is used to study the mechanisms for the reorientation of protein hydration water. In the past, crossovers in temperature-dependent correlation times were reported at T_x1=225K (X1) and T_x2=200K (X2). We show that neither X1 nor X2 are related to a fragile-to-strong transition. Our results rule out an existence of X1. Also, they indicate that water performs thermally activated and distorted tetrahedral jumps at T<T_x2, implying that X2 originates in an onset of this motion, which may be related to a universal defect diffusion in materials with defined hydrogen-bond networks.

cond-mat.soft↗

arXiv2007

Conformational and Structural Relaxations of Poly(ethylene oxide) and Poly(propylene oxide) Melts: Molecular Dynamics Study of Spatial Heterogeneity, Cooperativity, and Correlated Forward-Backward Motion

Performing molecular dynamics simulations for all-atom models, we characterize the conformational and structural relaxations of poly(ethylene oxide) and poly(propylene oxide) melts. The temperature dependence of these relaxation processes deviates from an Arrhenius law for both polymers. We demonstrate that mode-coupling theory captures some aspects of the glassy slowdown, but it does not enable a complete explanation of the dynamical behavior. When the temperature is decreased, spatially heterogeneous and cooperative translational dynamics are found to become more important for the structural relaxation. Moreover, the transitions between the conformational states cease to obey Poisson statistics. In particular, we show that, at sufficiently low temperatures, correlated forward-backward motion is an important aspect of the conformational relaxation, leading to strongly nonexponential distributions for the waiting times of the dihedrals in the various conformational states

cond-mat.soft↗

arXiv2006

Ion and polymer dynamics in polymer electrolytes PPO-LiClO4: II. 2H and 7Li NMR stimulated-echo experiment

We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation functions characterizing the polymer segmental motion in polymer electrolytes PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of the salt on the polymer dynamics, we compare results for different ether oxygen to lithium ratios, namely, 6:1, 15:1, 30:1 and infinity. For all compositions, we find nonexponential correlation functions, which can be described by a Kohlrausch function. The mean correlation times show quantitatively that an increase of the salt concentration results in a strong slowing down of the segmental motion. Consistently, for the high 6:1 salt concentration, a high apparent activation energy E_a=4.1eV characterizes the temperature dependence of the mean correlation times at Tg < T< 1.1T_g, while smaller values E_a=2.5eV are observed for moderate salt contents. The correlation functions are most nonexponential for 15:1 PPO-LiClO4, whereas the stretching is reduced for higher and lower salt concentrations. A similar dependence of the correlation functions on the evolution time in the presence and in the absence of ions indicates that addition of salt hardly affects the reorientational mechanism. For all compositions, mean jump angles of about 15 degree characterize the segmental reorientation. In addition, comparison of results from 2H and 7Li NMR stimulated-echo experiments suggests a coupling of ion and polymer dynamics in 15:1 PPO-LiClO4.

cond-mat.mtrl-sci↗

arXiv2004

A complete identification of lithium sites in a model of LiPO$_3$ glass: effects of the local structure and energy landscape on ionic jump dynamics

We perform molecular dynamics simulations to study lithium dynamics in a model of LiPO$_3$ glass at temperatures below the glass transition. A straightforward analysis of the ionic trajectories shows that lithium diffusion results from jumps between sites that are basically unmodified on the time scale of the lithium ionic relaxation. This allows us a detailed identification and characterization of the sites. The results indicate that the number of lithium sites is only slightly bigger than the number of lithium ions so that the fraction of vacant sites is very limited at every instant. Mapping the ionic trajectories onto series of jumps between the sites provides direct access to lithium jump dynamics. For each site, we determine the mean residence time $τ_s$ and the probability $p_s^b$ that a jump from this site to another site is followed by a direct backjump. While a broad distribution $G(\lg τ_s)$ shows that different sites feature diverse lithium dynamics, high values of $p_s^b$ give direct evidence for back-and-forth jumps. We further study how the local glass structure and the local energy landscape affect lithium jump dynamics. We observe substantial effects due to the energy landscape, which are difficult to capture within single-particle approaches.

cond-mat.soft↗

arXiv2004

Spatially heterogeneous dynamics and dynamic facilitation in a model of viscous silica

Performing molecular dynamics simulations, we find that the structural relaxation dynamics of viscous silica, the prototype of a strong glass former, are spatially heterogeneous and cannot be understood as a statistical bond breaking process. Further, we show that high particle mobility predominantly propagates continuously through the melt, supporting the concept of dynamic facilitation emphasized in recent theoretical work.

cond-mat.dis-nn↗

arXiv2003

Particle rearrangements during transitions between local minima of the potential energy landscape of a supercooled Lennard-Jones liquid

The potential energy landscape (PEL) of supercooled binary Lennard-Jones (BLJ) mixtures exhibits local minima, or inherent structures (IS), which are organized into meta-basins (MB). We study the particle rearrangements related to transitions between both successive IS and successive MB for a small 80:20 BLJ system near the mode-coupling temperature T_MCT. The analysis includes the displacements of individual particles, the localization of the rearrangements and the relevance of string-like motion. We find that the particle rearrangements during IS and MB transitions do not change significantly at T_MCT. Further, it is demonstrated that IS and MB dynamics are spatially heterogeneous and facilitated by string-like motion. To investigate the mechanism of string-like motion, we follow the particle rearrangements during suitable sequences of IS transitions. We find that most strings observed after a series of transitions do not move coherently during a single transition, but subunits of different sizes are active at different times. Several findings suggest that the occurrence of a successful string enables the system to exit a MB. Moreover, we show that the particle rearrangements during two consecutive MB transitions are basically uncorrelated. Specifically, different groups of particles are highly mobile during subsequent MB transitions. Finally, the relation between the features of the PEL and the relaxation processes in supercooled liquids is discussed.

cond-mat↗

arXiv2003

Complex lithium ion dynamics in simulated LiPO3 glass studied by means of multi-time correlation functions

Molecular dynamics simulations are performed to study the lithium jumps in LiPO3 glass. In particular, we calculate higher-order correlation functions that probe the positions of single lithium ions at several times. Three-time correlation functions show that the non-exponential relaxation of the lithium ions results from both correlated back-and-forth jumps and the existence of dynamical heterogeneities, i.e., the presence of a broad distribution of jump rates. A quantitative analysis yields that the contribution of the dynamical heterogeneities to the non-exponential depopulation of the lithium sites increases upon cooling. Further, correlated back-and-forth jumps between neighboring sites are observed for the fast ions of the distribution, but not for the slow ions and, hence, the back-jump probability depends on the dynamical state. Four-time correlation functions indicate that an exchange between fast and slow ions takes place on the timescale of the jumps themselves, i.e., the dynamical heterogeneities are short-lived. Hence, sites featuring fast and slow lithium dynamics, respectively, are intimately mixed. In addition, a backward correlation beyond the first neighbor shell for highly mobile ions and the presence of long-range dynamical heterogeneities suggest that fast ion migration occurs along preferential pathways in the glassy matrix. In the melt, we find no evidence for correlated back-and-forth motions and dynamical heterogeneities on the length scale of the next-neighbor distance.

cond-mat↗

arXiv2002

Backward correlations and dynamic heterogeneities: a computer study of ion dynamics

We analyse the correlated back and forth dynamics and dynamic heterogeneities, i.e. the presence of fast and slow ions, for a lithium metasilicate system via computer simulations. For this purpose we define, in analogy to previous work in the field of glass transition, appropriate three-time correlation functions. They contain information about the dynamics during two successive time intervals. First we apply them to simple model systems in order to clarify their information content. Afterwards we use this formalism to analyse the lithium trajectories. A strong back-dragging effect is observed, which also fulfills the time-temperature superposition principle. Furthermore, it turns out that the back-dragging effect is long-ranged and exceeds the nearest neighbor position. In contrast, the strength of the dynamic heterogeneities does not fulfill the time-temperature superposition principle. The lower the temperature, the stronger the mobility difference between fast and slow ions. The results are then compared with the simple model systems considered here as well as with some lattice models of ion dynamics.

cond-mat.dis-nn↗

arXiv2002

Characterization of the complex ion dynamics in lithium silicate glasses via computer simulations

We present results of molecular dynamics simulations on lithium metasilicate over a broad range of temperatures for which the silicate network is frozen in but the lithium ions can still be equilibrated. The lithium dynamics is studied via the analysis of different correlation functions. The activation energy for the lithium mobility agrees very well with experimental data. The correlation of the dynamics of adjacent ions is weak. At low temperatures the dynamics can be separated into local vibrational dynamics and hopping events between adjacent lithium sites. The derivative of the mean square displacement displays several characteristic time regimes. They can be directly mapped onto respective frequency regimes for the conductivity. In particular it is possible to identify time regimes dominated by localized dynamics and long-range dynamics, respectively. The question of time-temperature superposition is discussed for the mean square displacement and the incoherent scattering function.

cond-mat.dis-nn↗

arXiv1999

Impurity spin relaxation in S=1/2 XX chains

Dynamic autocorrelations $ $ (α=x,z) of an isolated impurity spin in a S=1/2 XX chain are calculated. The impurity spin, defined by a local change in the nearest-neighbor coupling, is either in the bulk or at the boundary of the open-ended chain. The exact numerical calculation of the correlations employs the Jordan-Wigner mapping from spin operators to Fermi operators; effects of finite system size can be eliminated. Two distinct temperature regimes are observed in the long-time asymptotic behavior. At T=0 only power laws are present. At high T the x correlation decays exponentially (except at short times) while the z correlation still shows an asymptotic power law (different from the one at T=0) after an intermediate exponential phase. The boundary impurity correlations follow power laws at all T. The power laws for the z correlation and the boundary correlations can be deduced from the impurity-induced changes in the properties of the Jordan-Wigner fermion states.

cond-mat.stat-mech↗