Search

Driven lattice gases serve as canonical models for investigating collective transport phenomena and properties of non-equilibrium steady states (NESS). Here we study one-dimensional transport with nearest-neighbor interactions both in closed bulk systems and in open channels coupled to two particle reservoirs at the ends of the channel. For the widely employed Glauber rates we derive an exact current-density relation in the bulk for unidirectional hopping. An approach based on time-dependent density functional theory provides a good description of the kinetics. For open systems, the system-reservoir couplings are shown to have a striking influence on boundary-induced phase diagrams. The role of particle-hole symmetry is discussed and its consequence on the topology of the phase diagrams. It is furthermore demonstrated that systems with weak bias can be mapped onto systems with unidirectional hopping., Comment: 12 pages, 7 figures

Spatiotemporal excitation patterns in the FitzHugh-Nagumo model are studied, which result from the disturbance of a primary pacemaker by a secondary pacemaker. The primary and secondary pacemakers generate regular waves with frequencies f(pace) and f(pert), respectively. The pacemakers are spatially separated, but waves emanating from them encounter each other via a small bridge. This leads to three different types I-III of irregular excitation patterns in disjunct domains of the f(pace)-f(pert) plane. Types I and II are caused by detachments of waves coming from the two pacemakers at corners of the bridge. Type III irregularities are confined to a boundary region of the system and originate from a partial penetration of the primary waves into a space, where circular wave fronts from the secondary pacemaker prevail. For this type, local frequencies can significantly exceed f(pace) and f(pert). The degree of irregularity found for the three different types is quantified by the entropy of the local frequency distribution and an order parameter for phase coherence.

Molecular self-assembly constitutes a versatile strategy for creating functional structures on surfaces. Tuning the subtle balance between intermolecular and molecule-surface interactions allows structure formation to be tailored at the single-molecule level. While metal surfaces usually exhibit interaction strengths in an energy range that favors molecular self-assembly, dielectric surfaces having low surface energies often lack sufficient interactions with adsorbed molecules. As a consequence, application-relevant, bulk insulating materials pose significant challenges when considering them as supporting substrates for molecular self-assembly. Here, the current status of molecular self-assembly on surfaces of wide-bandgap dielectric crystals, investigated under ultrahigh vacuum conditions at room temperature, is reviewed. To address the major issues currently limiting the applicability of molecular self-assembly principles in the case of dielectric surfaces, a systematic discussion of general strategies is provided for anchoring organic molecules to bulk insulating materials.

We investigate the distribution of work performed on a Brownian particle in a time-dependent asymmetric potential well. The potential has a harmonic component with a time-dependent force constant and a time-independent logarithmic barrier at the origin. For an arbitrary driving protocol, the problem of solving the Fokker–Planck equation for the joint probability density of work and particle position is reduced to the solution of the Riccati differential equation. For a particular choice of the driving protocol, an exact solution of the Riccati equation is presented. An asymptotic analysis of the resulting expression yields the tail behavior of the work distribution for small and large work values. In the limit of a vanishing logarithmic barrier, the work distribution for the breathing parabola model is obtained.

Analysis of signal fluctuations of a locally fixed probe, caused by molecules diffusing under the probe, can be used to determine diffusion coefficients. Theoretical treatments so far have been limited to point-like particles or to molecules with circle-like shapes. Here we extend these treatments to molecules with rectangle-like shapes, for which also rotational diffusion needs to be taken into account. Focusing on the distribution of peak widths in the signal, we show how translational as well as rotational diffusion coefficients can be determined. We address also the question, how the distribution of interpeak time intervals and autocorrelation function can be employed for determining diffusion coefficients. Our approach is validated against kinetic Monte Carlo simulations., Comment: 7 pages, 7 figures

DNA hairpin molecules with periodic base sequences can be expected to exhibit a regular coarse-grained free energy landscape (FEL) as function of the number of open base pairs and applied mechanical force. Using a commonly employed model, we first analyse for which types of sequences a particularly simple landscape structure is predicted, where forward and backward energy barriers between partly unfolded states are decreasing linearly with force. Stochastic unfolding trajectories for such molecules with simple FEL are subsequently generated by kinetic Monte Carlo simulations. Introducing probabilities that can be sampled from these trajectories, it is shown how the parameters characterising the FEL can be estimated. Already 300 trajectories, as typically generated in experiments, provide faithful results for the FEL parameters., Comment: 16 pages, 9 figures

We illustrate the general principle that in biophysics, econophysics and possibly even city growth, the conceptual framework provided by scaling and universality may be of use in making sense of complex statistical data. Specifically, we discuss recent work on DNA sequences, heartbeat intervals, avalanche-like lung inflation, urban growth, and company growth. Although our main focus is on data, we also discuss statistical mechanical models.

To explore the origin of the recently discovered non-Arrhenius behavior of the dc conductivity in glassy fast ionic conductors at high temperatures, we investigate by Monte Carlo simulations the transport of charged particles in an energetically disordered structure. We show that the combined effect of Coulomb interaction and disorder can account for the experimental findings. Our results suggest that glassy superionic conductors can be optimized by lowering the strength of the energetic disorder but that the ionic interaction effects set an upper bound for the conductivity at high temperatures.

We develop a microscopic model for the mixed alkali effect in crystals of the $\ensuremath{\beta}$- and ${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-alumina type, which is based on the formation of certain mobile defects and the fact that different types of mobile ions have a different preference to become part of these defects. Our computer simulations for the ionic conductivity are in excellent agreement with experimental data. Our model predicts a common shape of the rescaled conductivity isotherms independent of temperature and the specific type of probe, which is also in agreement with the available experimental results.

We discuss examples of complex systems composed of many interacting subsystems. We focus on those systems displaying nontrivial long-range correlations. These include the one-dimensional sequence of base pairs in DNA, the sequence of flight times of the large seabird Wandering Albatross, and the annual fluctuations in the growth rate of business firms. We review formal analogies in the models that describe the observed long-range correlations, and conclude by discussing the possibility that behavior of large numbers of humans (as measured, e.g., by economic indices) might conform to analogs of the scaling laws that have proved useful in describing systems composed of large numbers of inanimate objects.

We investigate by Monte Carlo simulations the frequency dependent conductivity σ(ω,T) and the spin lattice relaxation rate 1/T1(ω,T) due to the hopping motion of noninteracting particles in an uncorrelated and correlated disordered energy landscape. The conductivity shows a pronounced dispersion at low temperatures in both the correlated and the uncorrelated case, reminiscent of the non‐Debye behavior observed in many disordered materials. In contrast, 1/T1(ω,T) exhibits the typical non‐BPP form for uncorrelated energy disorder, but obeys an ordinary BPP‐type behavior in the presence of finite correlations. Our results show that in general a simple relation between σ(ω,T) and 1/T1(ω,T) does not exist and that it might be difficult to account for the experimental findings, if interactions between the mobile particles are neglected.

We investigate by Monte Carlo simulations the diffusion of ions in ordered and structurally disordered lattices, where the Coulomb interaction between the mobile ions is explicitly taken into account. Results for tracer diffusion, conductivity, the diffusion-induced spin-lattice relaxation rate, and the incoherent intermediate scattering function are discussed in detail. It is shown that the presence of both disorder and Coulomb interactions leads to the typical deviations from the standard behavior of the relevant transport quantities (non-BPP, where BPP indicates Bloembergen, Purcell, and Pound behavior of the spin-lattice relaxation rate, strong dispersion of the conductivity, nonexponential decay of relaxation functions, etc.) that are experimentally observed in a wide class of ion-conducting materials.

The capture numbers entering the rate equations (RE) for submonolayer film growth are determined from extensive kinetic Monte Carlo (KMC) simulations for simple representative growth models yielding point, compact, and fractal island morphologies. The full dependence of the capture numbers on island size, and on both the coverage and the D/F ratio between the adatom diffusion coefficient D and deposition rate F is determined. Based on this information, the RE are solved to give the RE island size distribution (RE-ISD). The RE-ISDs are shown to agree well with the corresponding KMC-ISDs for all island morphologies. For compact morphologies, however, this agreement is only present for coverages smaller than about 5% due to a significantly increased coalescence rate compared to fractal morphologies. As found earlier, the scaled KMC-ISDs as a function of scaled island size approach, for fixed coverage, a limiting curve for D/F going to infinity. Our findings provide evidence that the limiting curve is independent of the coverage for point islands, while the results for compact and fractal island morphologies indicate a dependence on the coverage., Comment: 13 pages, 12 figures

We report the complete assignment of the vibrational spectrum of dipalmitoylphosphatidylcholine (DPPC), which belongs to the most ubiquitous membrane phospholipid family, phosphatidylcholine. We find that water hydrating the lipid headgroups enables efficient energy transfer across membrane leaflets on sub-picosecond time scales. The emergence of spatially extended vibrational modes upon hydration, underlies this phenomenon. Our findings illustrate the importance of collective molecular behavior of biomembranes and reveal that hydrated lipid membranes can act as efficient media for the transfer of vibrational energy.

Using a Markov chain approach we rederive the exact density functional for hard rod mixtures on a one-dimensional lattice, which forms the basis of the lattice fundamental measure theory. The transition probability in the Markov chain depends on a set of occupation numbers, which reflects the property of a zero-dimensional cavity to hold at most one particle. For given mean occupation numbers (density profile), an exact expression for the equilibrium distribution of microstates is obtained, that means an expression for the unique external potential that generates the density profile in equilibrium. By considering the rod ends to fall onto lattice sites, the mixture is always additive., 4 pages, 1 figure

Methods of determining surface diffusion coefficients of molecules from signal fluctuations of a locally fixed probe are revisited and refined. Particular emphasis is put on the influence of the molecule's extent. In addition to the formerly introduced autocorrelation method and residence time method, we develop a further method based on the distribution of intervals between successive peaks in the signal. The theoretical findings are applied to STM measurements of copper phthalocyanine (CuPc) molecules on the Ag(100) surface. We discuss advantages and disadvantages of each method and suggest a combination to obtain accurate results for diffusion coefficients., Comment: 10 pages, 8 figures

The diffusion of ions in a structurally disordered lattice (percolation model), where the Coulom interaction between the mobile ions is explicitely taken into account, is investigated by Monte Carlo simulations. It is shown that both the dispersion of the conductivity and the deviations of the spin-lattice relaxation rate from the Bloembergen-Purcell-Pound behaviour are caused by the interplay between structural disorder and long range Coulomb interaction. Specifically, it is shown that the characteristic quantities describing σ(ω, T) and 1/T1(ω, T) (exponents nσ, nSLR, and relaxation times τσ, τSLR) show a different temperature dependence. When enhancing the disorder and approaching pc, a characteristic slowing of the spin-lattice relaxation time, τSLR, is observed, while the conductivity relaxation time, τσ, remains practically unchanged.

A model is developed for ion transport in glass which involves the creation of fluctuating pathways within a dynamically determined structure. Key features include a site memory effect which introduces vacancies appropriate to each kind of mobile ion, and a mismatch energy which emerges whenever an ion attempts to enter a different kind of site. The exploration of this model by numerical methods leads (i) to a power law relationship between ionic conductivity and cation content (now confirmed in the literature) and (ii) to the elucidation of many facets of the mixed alkali effect. It is suggested that this ‘dynamic structure’ model could form the basis for a comprehensive theory of vitreous electrolytes.

Deposition of fullerenes on the CaF(2)(111) surface yields peculiar island morphologies with close similarities to previous findings for (100) surfaces of other ionic crystals. By means of noncontact atomic force microscopy we find a smooth transition from compact, triangular islands to branched hexagonal islands upon lowering the temperature. While triangular islands are two monolayers high, hexagonal islands have a base of one monolayer and exhibit a complicated structure with a second-layer outer rim and trenches oriented towards the interior. By developing a kinetic growth model we unravel the microscopic mechanisms of the structure formation.

We study nonequilibrium steady states of lattice gases with nearest-neighbor interactions that are driven between two reservoirs. Density profiles in these systems exhibit oscillations close to the reservoirs. We demonstrate that an approach based on time-dependent density functional theory copes with these oscillations and predicts phase diagrams of bulk densities to a good approximation under arbitrary boundary-reservoir couplings. The minimum or maximum current principles can be applied only for specific bulk-adapted couplings. We show that they generally fail to give the correct topology of phase diagrams but can still be useful for getting insight into the mutual arrangement of different phases., Comment: 4 pages, 3 figures

We discuss a recently developed random walk model for ionic transport in glass, which offers a simple explanation to the so far unexplained transport anomalies in glassy ionic conductors: The set of anomalies combined under the name mixed alkali effect and the drastic increase of the ionic conductivity with the concentration of mobile ions. The model is based on the experimental evidence that ions maintain their distinct local environments and leads to the formation of fluctuating structural pathways. The connectivity of these pathways determines the ion mobility in the glassy network.

We present a new method to describe the kinetics of driven lattice gases with particle-particle interactions beyond hard-core exclusions. The method is based on the time-dependent density functional theory for lattice systems and allows one to set up closed evolution equations for mean site occupation numbers in a systematic manner. Application of the method to a totally asymmetric site exclusion process with nearest-neighbor interactions yields predictions for the current-density relation in the bulk, the phase diagram of non-equilibrium steady states and the time evolution of density profiles that are in good agreement with results from kinetic Monte Carlo simulations., 11 pages, 3 figures

We show that mean-field rate equations for submonolayer growth can successfully predict island size distributions in the pre-coalescence regime if the full dependence of capture numbers on both the island size and the coverage is taken into account. This is demonstrated by extensive Kinetic Monte Carlo simulations for a growth kinetics with hit and stick aggregation. A detailed analysis of the capture numbers reveals a nonlinear dependence on the island size for small islands. This nonlinearity turns out to be crucial for the successful prediction of the island size distribution and renders an analytical treatment based on a continuum limit of the mean-field rate equations difficult., Comment: 4 pages, 4 figues

We investigate for the first time spin-lattice relaxation caused by particles diffusing in disordered environments. For noninteracting particles on a fractal substrate with fractal dimension d f and diffusion exponent d w , we find that the NMR correlation functions G ( q ) ( t ) are asymptotically described by G ( q ) ( t )∼ t − y , with γ= 2 d w in d =2 and γ= (6−d f ) d w in d = 3. This behavior leads to an anomalous frequency dependence of 1/ T 1 at high temperatures, 1 T 1 ∼ω −(1− Gg ) . For charged particles, we have investigated G ( q ) ( t ) by Monte Carlo simulations. We find that due to the interplay between structural disorder and Coulomb interaction the decay of G ( q ) ( t ) is dramatically slowed down when p c is approached.

We discuss a recently developed theoretical model for ionic transport in glass, which offers a simple explanation to two long lasting problems in the physics of glassy ionic conductors: the anomalous dependence of conductivity on the modifier content and the mixed alkali effect. The model is based on the experimental evidence that ions maintain their distinct local environments and leads to the formation of fluctuating structural pathways. The connectivity of these pathways determines the ion mobility in the glassy network.

Master equations can be conveniently used to investigate many particle systems driven out of equilibrium by time-dependent external fields. This topic is of vital interest in connection with fluctuation theorems and the associated microscopic work and heat distributions. We present an exact Monte Carlo simulation algorithm, which allows us to study interaction effects on these distributions. The method is applied to an Ising chain with Glauber dynamics. We find that the distributions are characterized by $\ensuremath{\delta}$ and step functions with a smooth part in between. With decreasing sweeping rate of the external field or decreasing interaction strength, the singular part becomes less dominant and the Gaussian fluctuation regime is approached.

Mixing two types of glass formers in ion conducting glasses can be exploited to lower conductivity activation energy and thereby increasing the ionic conductivity, a phenomenon known as the mixed glass former effect (MGFE). We develop a model for this MGFE, where activation barriers for individual ion jumps get lowered in inhomogeneous environments containing both types of network forming units. Fits of the model to experimental data allow one to estimate the strength of the barrier reduction, and they indicate a spatial clustering of the two types of network formers. The model predicts a time-temperature superposition of conductivity spectra onto a common master curve independent of the mixing ratio.

We present a novel model for ionic transport in glasses which we call diffusion limited percolation. The model focuses on the fact that each mobile ion prefers and maintains its own distinct characteristic environment in the glassy network. This leads to mismatch effects and to the existence of a memory effect which strongly influences the process of ion migration. We show that by our simple model a variety of so far imperfectly understood phenomena can be explained, among them the drastic increase of the dc conductivity of oxide glasses with the modifier content and the mixed alkali effect.

Ionic conductivities in glass depend very strongly on temperature, frequency and glass composition. In the absence of well-defined structure, the behaviour of vitreous electrolytes may be strongly influenced by various “memory effects”. These include the classical ion-atmosphere effect which tends to return ions to their starting positions – and hence causes a reduction in ion mobilities – and a newly formulated site memory effect, which effectively creates “cation vacancies”, and leads to conductivity enhancement. The site memory effect gives a straightforward explanation for compositional variations in conductivity (including the mixed alkali effect) and points up a connection with “strength” and “fragility” concepts of glass/melt structure.

The authors present a novel theory of the mixed alkali effect which focuses on the fact that each alkali prefers and maintains its own distinct characteristic environment in the mixed glass. This leads to mismatch between cations and sites recently occupied by unlike cations, and the existence of a memory effect which strongly influences the process of ion migration. This view of glass is consistent with both EXAFS and infrared measurements, and computer simulations reproduce the essential features of the mixed alkali effect including the mobility crossover and the conductivity minimum.

A new model theory of the mixed alkali effect that is consistent with the most recent EXAFS data of cation environments in mixed alkali silicate glass is presented. The dynamics of ion transport and the memory effects of site occupancy are brought out by computer simulations based on the model in an infinite percolation cluster. The salient features of the mixed alkali effect in the diffusion coefficients of the two cations are reproduced in the results of computer simulations.

Transport in disordered systems, where the mobile particles are coupled by nearest-neighbor interactions, is discussed. It is found that due to the mutual interaction, novel crossover phenomena occur in the density distribution and the mean square displacement R2(t) of tagged particles. At intermediate time-scales t1 ⪡ t ⪡ t2, R2(t) is described by a new exponent k′, enhanced. The frequency dependent conductivity scales as σ(ω) ∼ ωn′, with n′ = 1 − k′, at intermediate frequencies. In agreement with the experimental situation, n′ can be considerably larger than the asymptotic exponent n = 1−k which is the same as for non-interacting particles.

We perform Monte Carlo simulations for diffusion of charged particles in structurally disorderd lattices. As a consequence of the Coulomb interaction, pronounced backward correlations occur which lead to power-law behavior of the relevant transport quantities at intermediate time and frequency scales. Both the exponents and the size of the dispersive regime depend on temperature. Our simulations suggest that the combined effect of disorder and Coulomb interaction provides a new mechanism for understanding non-Debye relaxation, which is experimentally known for a wide class of materials. © 1991 The American Physical Society.

Transforming growth factor-beta (TGF-beta) is abundantly expressed in malignant gliomas and is crucial for the tumor micromilieu. TGF-beta not only enhances migration and invasion of glioma cells but also inhibits an effective anti-glioma immune response. TGF-beta mediates its biologic effects through interactions with TGF-beta receptors (TbetaR)-I to -III. Binding of TGF-beta leads to the activation of an intracellular signaling cascade and subsequent phosphorylation of Sma and MAD-related proteins (SMAD). Soluble TGF-beta receptors (TbetaRs) abrogate the TGF-beta effect by competing for the binding of the ligand to its receptor. Here we used adenoviral gene transfer to express TbetaR-IIs and -IIIs in human glioma cell lines. TbetaR-IIs reduced SMAD2 phosphorylation and TGF-beta-dependent reporter activity. Furthermore, it enhanced glioma cell lysis by natural killer cells. TbetaR-IIIs alone were inactive in these assays, but enhanced the effects of TbetaR-IIs. Transduction of LN-308 cells with TbetaRs markedly delayed growth of intracerebral xenografts in nude mice in vivo. These data commend TbetaRs for possible experimental therapy of gliomas.

Aging phenomena in the plastic response of amorphous solids are studied within the theory of shear transformation zones (STZs), which describes the kinetic rearrangement of localized defects in response to external stress. To account for the slow, nonequilibrium dynamics after a quench below the glass transition temperature, two possible models are considered. In the first model, transition rates between the internal states of STZs decrease with time, while in the second model aging occurs due to the relaxation of an effective temperature that determines the number density of STZs and other out-of-equilibium degrees of freedom. It is shown that for reasonable choices of parameters, both models capture qualitatively typical aging features seen in computer simulations and experiments: (i) compliance curves measured for different waiting times t_{w} after the quench can be superimposed, when the observation times are rescaled with relaxation times proportional, variantt_{w};{mu} , 0

The slow non‐equilibrium dynamics in various systems show common physical aging features and sometimes surprising rejuvenation and memory effects. We present two theoretical approaches to understand these phenomena on the basis of coarse grained models. In the first approach a system in configuration space is considered that changes its state by thermally activated jumps in a random energy landscape. In the second approach an effective dynamics for the Edwards‐Anderson spin‐glass model on a hierarchical lattice is studied by means of real‐space renormalization concepts.

We study the random hopping motion of a particle, for which the jump directions vary randomly in time but the jump lengths l (“noise amplitudes”) are fixed in space (quenched disorder). Two cases are considered: I the distribution p(l) of jump lengths has a finite second moment, and II p(l) decays slowly according to a Levy distribution, p(l)∼l −1−f with 0

We study the nucleation kinetics of binary alloys formed by codeposition of A and B atoms onto a planar substrate with fluxes F(alpha) (alpha=A, B). Based on a generalization of mean-field rate equations, we derive scaling relations for the dependence of the number density of stable islands on the ratios D(alpha)/F(alpha), where D(alpha) are the diffusion coefficients of two types of adatoms. Novel scaling laws are predicted for different situations of cluster stabilities with respect to their size and composition. The predictions are validated by kinetic Monte Carlo simulations and can be tested in experiments.

Stochastic growth of binary alloys on a weakly interacting substrate is studied by kinetic Monte Carlo simulation. The underlying lattice model relates to fcc alloys, and the kinetics are based on deposition, atomic migration with bond-breaking processes, and exchange processes mediated by nearest neighbor hopping steps. We investigate the interrelation between surface processes and the emerging nonequilibrium structure at and below the growing surface under conditions where atoms in the bulk can be regarded as immobile. The parameters of the model are adapted to ${\mathrm{CoPt}}_{3}$ alloys. Growing nanoclusters exhibit an anisotropic short range order, primarily caused by Pt segregation at the surface. The overall structural anisotropy depends on both Pt surface segregation and cluster shape, and can explain the perpendicular magnetic anisotropy (PMA) recently measured in ${\mathrm{CoPt}}_{3}$ nanoclusters on a van der Waals substrate. The onset of $L{1}_{2}$ ordering in the cluster is induced by surface processes. The same kinetic model is applied also to continuous thin films, which in addition can exhibit a small bulk contribution to PMA.

Raman spectra of thin solid electrolyte films obtained by sputtering a lithium sulfate target in nitrogen plasma are measured and compared to ab initio electronic structure calculations for clusters composed of 28 atoms. Agreement between measured and calculated spectra is obtained when oxygen atoms are replaced by nitrogen atoms and when the nitrogen atoms form bonds with each other. This suggests that the incorporation of nitrogen during the sputtering process leads to structures in the film, which prevent crystallization of these thin film salt glasses., Comment: 5 pages, 4 figures

A soft particle model for diblock (AB) copolymer melts is proposed. Each molecule is mapped onto two soft spheres built by Gaussian A- and B-monomer distributions. An approximate analytical expression for the joint distribution function for the distance between both spheres and their radii of gyration is derived, which determines the entropic contribution to the intramolecular free energy. Adding a mean-field expression for the intermolecular interactions, we obtain the total free energy of the system. Based on this free energy, Monte Carlo simulations are carried out to study the kinetics of microphase ordering in the bulk and its effect on molecular diffusion. This is followed by an analysis of thin films with emphasis on pattern transfer from walls with a periodic structure. It is shown that the level of coarse graining in the soft particle model is suitable to describe structural and kinetic properties of copolymers on mesoscopic scales., Comment: 20 pages, 13 figures

The work distribution of a driven Brownian particle in an anharmonic potential is studied. The potential consists of two components: a harmonic part with a time-dependent stiffness and a time-independent logarithmic part. For arbitrary driving of the stiffness, the problem of solving the evolution equation for the joint probability density of work and particle position reduces to the solution of a Riccati differential equation. For a particular driving protocol, the Riccati equation can be solved and the exact large-work representation of the work distribution can be calculated. We propose a general form of the tail behavior. The asymptotic analysis of the work distribution is of vital importance for obtaining equilibrium free energy differences in experiments based on the Jarzynski identity. In the absence of the logarithmic component, our results agree with the work distribution for driven Brownian motion in a harmonic potential.

We develop two methods for determining anisotropic diffusion properties of molecules on surfaces. These methods are based on the analysis of signal fluctuations recorded with moving probe tips, of, for example, a scanning tunneling microscope. In the first method, the probe tip oscillates along a straight line, which allows to quantify the effective diffusion perpendicular to the line. By varying the line orientation, the diffusion tensor is obtained. In the second method, the probe tip rotates along a circle and the diffusion tensor is determined by analysis of cross-correlations between antipodal points. Both methods are successfully validated against surrogate data from kinetic Monte Carlo simulations.

The statistical properties of heart beat intervals of 130 long-term surface electrocardiogram recordings during atrial fibrillation (AF) are investigated. We find that the distribution of interbeat intervals exhibits a characteristic exponential tail, which is absent during sinus rhythm, as tested in a corresponding control study with 72 healthy persons. The rate of the exponential decay lies in the range 3-12 Hz and shows diurnal variations. It equals, up to statistical uncertainties, the level of the previously uncovered white noise part in the power spectrum, which is also characteristic for AF. The overall statistical features can be described by decomposing the intervals into two statistically independent times, where the first one is associated with a correlated process with 1/f noise characteristics, while the second one belongs to an uncorrelated process and is responsible for the exponential tail. It is suggested to use the rate of the exponential decay as a further parameter for a better classification of AF and for the medical diagnosis. The relevance of the findings with respect to a general understanding of AF is pointed out.

Based on molecular dynamics simulations of a lithium metasilicate glass we study the potential of bond valence sum calculations to identify sites and diffusion pathways of mobile Li ions in a glassy silicate network. We find that the bond valence method is not well suitable to locate the sites, but allows one to estimate the number of sites. Spatial regions of the glass determined as accessible for the Li ions by the bond valence method can capture up to 90% of the diffusion path. These regions however entail a significant fraction that does not belong to the diffusion path. Because of this low specificity, care must be taken to determine the diffusive motion of particles in amorphous systems based on the bond valence method. The best identification of the diffusion path is achieved by using a modified valence mismatch in the BV analysis that takes into account that a Li ion favors equal partial valences to the neighboring oxygen ions. Using this modified valence mismatch it is possible to replace hard geometric constraints formerly applied in the BV method. Further investigations are necessary to better understand the relation between the complex structure of the host network and the ionic diffusion paths., Comment: 16 pages, 10 figures

The relaxation properties of dynamical probes in random dipolar systems exhibit pronounced non‐Debye features even at moderate temperatures. These non‐Debye features have their origin in broad Levy type local field distributions and strong correlations in the collective dynamics of nearby dipoles. We discuss two examples of experimental relevance: The dephasing and decay of polarization of an ensemble of spin‐polarized particles brought into a random assembly of superparamagnetic clusters, and the nearly constant loss behavior widely observed in the dielectric response of disordered materials.

Based on a hopping model we show how the mixed alkali effect in glasses can be understood if only a small fraction c_V ofthe available sites for the mobile ions is vacant. In particular, we reproduce the peculiar behavior of the internal friction and the steep fall (''vulnerability'') of the mobility of the majority ion upon small replacements by the minority ion. The single and mixed alkali internal friction peaks are caused by ion-vacancy and ion-ion exchange processes. If c_V is small, they can become comparable in height even at small mixing ratios. The large vulnerability is explained by a trapping of vacancies induced by the minority ions. Reasonable choices of model parameters yield typical behaviors found in experiments., Comment: 4 pages, 4 figures

The low-frequency dynamic response of many non-metallic materials is governed by the transport of mobile ions or other charged mobile defects. The classes of such materials include traditional ionic glasses, polymeric and glassy superionic conductors, highly defective crystals or even highly viscous liquids such as glassforming melts. To get an understanding of the microscopic transport mechanism in these materials, a large number of experimental techniques has been applied, among them are tracer experiments (Chap. 1 and [1]), conductivity measurements including impedance spectroscopy (Chap. 21, [2–5]), nuclear magnetic resonance (NMR) relaxation (Chap. 9, [6–10]), quasielastic neutron scattering (Chaps. 2, 3 and 13, [11, 12]), internal friction and ultrasonic absorption measurements (Brillouin scattering) (Chap. 11, [13,14]). In all these experiments the measured quantities show characteristic deviations from the standard behavior that one would expect for a purely random motion of the mobile ions. For example, the dynamic conductivity σ(ω) in disordered crystalline and glassy ionic conductors exhibits, for fixed temperature T , a dc-plateau at low frequencies (below some crossover frequency 1/τσ), and follows an approximate power law behavior at larger frequencies [15]

A SUCCESSFUL theory of corporate growth should include both the external and internal factors that affect the growth of a company1–18. Whereas traditional models emphasize production-related influences such as investment in physical capital and in research and development18, recent models10–20 recognize the equal importance of organizational infrastructure. Unfortunately, no exhaustive empirical account of the growth of companies exists by which these models can be tested. Here we present a broad, phenomenological picture of the dependence of growth on company size, derived from data for all publicly traded US manufacturing companies between 1975 and 1991. We find that, for firms with similar sales, the distribution of annual (logarithmic) growth rates has an exponential form; the spread in the distribution of rates decreases with increasing sales as a power law over seven orders of magnitude. A model wherein the probability of a company's growth depends on its past as well as present sales accounts for the former observation. As the latter observation applies to companies that manufacture products of all kinds, organizational structures common to all firms might well be stronger determinants of growth than production-related factors, which differ for companies producing different goods.