Universality from disorder in the random-bond Blume-Capel model.

Using high-precision Monte Carlo simulations and finite-size scaling we study the effect of quenched disorder in the exchange couplings on the Blume-Capel model on the square lattice. The first-order transition for large crystal-field coupling is softened to become continuous, with a divergent correlation length. An analysis of the scaling of the correlation length as well as the susceptibility and specific heat reveals that it belongs to the universality class of the Ising model with additional logarithmic corrections which is also observed for the Ising model itself if coupled to weak disorder. While the leading scaling behavior of the disordered system is therefore identical between the second-order and first-order segments of the phase diagram of the pure model, the finite-size scaling in the ex-first-order regime is affected by strong transient effects with a crossover length scale L^{*}≈32 for the chosen parameters.

Persistence-length renormalization of polymers in a crowded environment of hard disks.

The most conspicuous property of a semiflexible polymer is its persistence length, defined as the decay length of tangent correlations along its contour. Using an efficient stochastic growth algorithm to sample polymers embedded in a quenched hard-disk fluid, we find apparent wormlike chain statistics with a renormalized persistence length. We identify a universal form of the disorder renormalization that suggests itself as a quantitative measure of molecular crowding.

Polymer adsorption on reconstructed Au(001): a statistical description of P3HT by scanning tunneling microscopy and coarse-grained Monte Carlo simulations.

We report on a combined theoretical and experimental characterization of isolated Poly(3-hexylthiophene) (P3HT) chains weakly adsorbed on a reconstructed Au(001) surface. The local chain conformations of in situ deposited P3HT molecules were investigated by means of scanning tunneling microscopy. For comparison, Monte Carlo simulations of the system were performed up to a maximum chain length of 60 monomer units. The dependence of the end-to-end distance and the radius of gyration on the polymer chain length shows a good agreement between experiment and Monte Carlo simulations using simple updates for short chains.

Tuning the shape of the condensate in spontaneous symmetry breaking.

We investigate the conditions which determine the shape of a particle condensate in situations when it emerges as a result of spontaneous breaking of translational symmetry. We consider a model with particles hopping between sites of a one-dimensional grid and interacting if they are at the same site or at neighboring sites. We predict the envelope of the condensate and the scaling of its width with the system size for various interaction potentials and show how to tune the shape from a delta peak to a rectangular or paraboliclike form.

Ground-state properties of tubelike flexible polymers.

In this work we investigate the structural properties of native states of a simple model for short flexible homopolymers, where the steric influence of monomeric side chains is effectively introduced by a thickness constraint. This geometric constraint is implemented through the concept of the global radius of curvature and affects the conformational topology of ground-state structures. A systematic analysis allows for a thickness-dependent classification of the dominant ground-state topologies. It turns out that helical structures, strands, rings, and coils are natural, intrinsic geometries of such tubelike objects.

Monte Carlo study of the droplet formation-dissolution transition on different two-dimensional lattices.

In 2003 Biskup [Commun. Math. Phys. 242, 137 (2003)] gave a rigorous proof for the behavior of equilibrium droplets in the two-dimensional (2D) spin-1/2 Ising model (or, equivalently, a lattice gas of particles) on a finite square lattice of volume V with a given excess delta M identical with M-M 0 of magnetization compared to the spontaneous magnetization M 0=m0V . By identifying a dimensionless parameter Delta(delta M) and a universal constant Delta c , they showed in the limit of large system sizes that for DeltaDelta c a droplet of the minority phase occurs ("condensed" system). By minimizing the free energy of the system, they derived an explicit formula for the fraction lambda(Delta) of excess magnetization forming the droplet. To check the applicability of the asymptotic analytical results to much smaller, practically accessible, system sizes, we performed several Monte Carlo simulations of the 2D Ising model with nearest-neighbor couplings on a square lattice at fixed magnetization M . Thereby, we measured the largest minority droplet, corresponding to the condensed phase in the lattice-gas interpretation, at various system sizes (L=40,80,...,640) . With analytical values for the spontaneous magnetization density m 0 , the susceptibility chi , and the Wulff interfacial free energy density tau W for the infinite system, we were able to determine lambda numerically in very good agreement with the theoretical prediction. Furthermore, we did simulations for the spin-1/2 Ising model on a triangular lattice and with next-nearest-neighbor couplings on a square lattice. Again finding a very good agreement with the analytic formula, we demonstrate the universal aspects of the theory with respect to the underlying lattice and type of interaction. For the case of the next-nearest-neighbor model, where tau W is unknown analytically, we present different methods to obtain it numerically by fitting to the distribution of the magnetization density P(m) .

Approaching the thermodynamic limit in equilibrated scale-free networks.

We discuss how various models of scale-free complex networks approach their limiting properties when the size N of the network grows. We focus mainly on equilibrated networks and their finite-size degree distributions. Our results show that the position of the cutoff in the degree distribution, k_{cutoff} , scales with N in a different way than predicted for N-->infinity ; that is, subleading corrections to the scaling k_{cutoff} approximately N;{alpha} are strong even for networks of order N approximately 10;{9} nodes. We observe also a logarithmic correction to the scaling for degenerated graphs with the degree distribution pi(k) approximately k;{-3} . On the other hand, the distribution of the maximal degree k_{max} may have a different scaling than the cutoff and, moreover, it approaches the thermodynamic limit much faster. We argue that k_{max} approximately N;{alpha;{'}} with an exponent alpha;{'}=min[alpha,1(gamma-1)] , where gamma is the exponent in the power law pi(k) approximately k;{-gamma} . We also present some results on the cutoff function and the distribution of the maximal degree in equilibrated networks.

Condensation in zero-range processes on inhomogeneous networks.

We investigate the role of inhomogeneities in zero-range processes in condensation dynamics. We consider the dynamics of balls hopping between nodes of a network with one node of degree k_{1} much higher than a typical degree k , and find that the condensation is triggered by the inhomogeneity and that it depends on the ratio k_{1}k . Although, on the average, the condensate takes an extensive number of balls, its occupation can oscillate in a wide range. We show that in systems with strong inhomogeneity, the typical melting time of the condensate grows exponentially with the number of balls.

Balls-in-boxes condensation on networks.

We discuss two different regimes of condensate formation in zero-range processes on networks: on a q-regular network, where the condensate is formed as a result of a spontaneous symmetry breaking, and on an irregular network, where the symmetry of the partition function is explicitly broken. In the latter case we consider a minimal irregularity of the q-regular network introduced by a single Q node with degree Q>q. The statics and dynamics of the condensation depend on the parameter alpha=ln Q/q, which controls the exponential falloff of the distribution of particles on regular nodes and the typical time scale for melting of the condensate on the Q node, which increases exponentially with the system size N. This behavior is different than that on a q-regular network, where alpha=0 and where the condensation results from the spontaneous symmetry breaking of the partition function, which is invariant under a permutation of particle occupation numbers on the q nodes of the network. In this case the typical time scale for condensate melting is known to increase typically as a power of the system size.

Self-consistent scaling theory for logarithmic-correction exponents.

Multiplicative logarithmic corrections frequently characterize critical behavior in statistical physics. Here, a recently proposed theory relating the exponents of such terms is extended to account for circumstances which often occur when the leading specific-heat critical exponent vanishes. Also, the theory is widened to encompass the correlation function. The new relations are then confronted with results from the literature, and some new predictions for logarithmic corrections in certain models are made.

Scaling relations for logarithmic corrections.

Multiplicative logarithmic corrections to scaling are frequently encountered in the critical behavior of certain statistical-mechanical systems. Here, a Lee-Yang zero approach is used to systematically analyze the exponents of such logarithms and to propose scaling relations between them. These proposed relations are then confronted with a variety of results from the literature.

Multicanonical Monte Carlo study and analysis of tails for the order-parameter distribution of the two-dimensional Ising model.

The tails of the critical order-parameter distribution of the two-dimensional Ising model are investigated through extensive multicanonical Monte Carlo simulations. Results for fixed boundary conditions are reported here, and compared with known results for periodic boundary conditions. Clear numerical evidence for "fat" stretched exponential tails exists below the critical temperature, indicating the possible presence of fat tails at the critical temperature. Our work suggests that the true order-parameter distribution at the critical temperature must be considered to be unknown at present.

Information geometry of the spherical model.

Motivated by the observation that geometrizing statistical mechanics offers an interesting alternative to more standard approaches, we calculate the scaling behavior of the curvature R of the information geometry metric for the spherical model. We find that R approximately epsilon(-2), where epsilon=beta(c)-beta is the distance from criticality. The discrepancy from the naively expected scaling R approximately epsilon(-3) is explained and compared with that for the Ising model on planar random graphs, which shares the same critical exponents.

Information geometry of the ising model on planar random graphs.

It has been suggested that an information geometric view of statistical mechanics in which a metric is introduced onto the space of parameters provides an interesting alternative characterization of the phase structure, particularly in the case where there are two such parameters, such as the Ising model with inverse temperature beta and external field h. In various two-parameter calculable models, the scalar curvature R of the information metric has been found to diverge at the phase transition point beta(c) and a plausible scaling relation postulated: R approximately |beta-beta(c)|(alpha-2). For spin models the necessity of calculating in nonzero field has limited analytic consideration to one-dimensional, mean-field and Bethe lattice Ising models. In this paper we use the solution in field of the Ising model on an ensemble of planar random graphs (where alpha=-1, beta=1/2, gamma=2) to evaluate the scaling behavior of the scalar curvature, and find R approximately |beta-beta(c)|(-2). The apparent discrepancy is traced back to the effect of a negative alpha.

Softening of first-order transition in three-dimensions by quenched disorder.

We study by extensive Monte Carlo simulations the effect of random bond dilution on the phase transition of the three-dimensional four-state Potts model that is known to exhibit a strong first-order transition in the pure case. The phase diagram in the dilution-temperature plane is determined from the peaks of the susceptibility for sufficiently large system sizes. In the strongly disordered regime, numerical evidence for softening to a second-order transition induced by randomness is given. Here a large-scale finite-size scaling analysis, made difficult due to strong crossover effects presumably caused by the percolation fixed point, is performed.

[Experimental study of the effect of acetylsalicylic acid combined with caffeine regarding possible abuse potential]. / Experimentelle Untersuchung zur Wirkung von Acetylsalicylsäure kombiniert mit Coffein unter dem Aspekt eines Missbrauchspotentials.

Caffeine is suspected to initiate the abuse of caffeine-containing analgesics. To clarify this matter the study investigated the effects of aspirin alone and in combination with caffeine on mood and cardiovascular parameters before, during, and after pain induction through mechanical stimulation. 96 healthy middle-aged women--all habitual coffee drinkers--were randomly designed to receive 0 or 500 mg aspirin together with 0, 50, or 100 mg caffeine administered in a double-blind design. Aspirin did not change ratings of mood scales. Caffeine decreased scores on negative mood scales, especially on the anxiety/sadness scale. The combination of aspirin and caffeine failed to increase any score on scales measuring positive moods. Only a few interactions are noticed. The results do not support the suggestion that caffeine increases the potential for abuse of aspirin.

Scanning behavior of rats during eating under stressful noise.

Behavioral ecological theories postulate that threatening environments should increase eating speed and vigilance during feeding. In the present experiment, eating speed and scanning behavior during eating were measured in 36 rats in 5 consecutive test sessions under stressful noise (95 dB white noise, n = 18) and control conditions (60 dB, n = 18) after the animals had been habituated to the test environment. Intense noise induced an increase of scanning rate and eating speed. These effects are similar to those reported for novel and light environments.

Effects of stressful noise on eating and non-eating behavior in rats.

Eating and other behaviors were measured in 36 food-deprived rats on 15 consecutive days during 20 min test sessions. During training sessions 1 to 5 all animals were habituated to the test boxes with white noise of 55 dB intensity. For sessions 6 to 10 noise intensity was increased to 95 dB for the experimental rats and to 60 dB for the control rats. The food intake of experimental rats was lower for stress session 1. The duration of eating behavior was lower, and durations of exploring, grooming and resting behaviors were higher for all stress sessions for rats exposed to 95 dB white noise. Speed of eating behavior was higher for all stress sessions in the experimental group. Defecation rate of the experimental rats was higher for all stress sessions. On post-stress sessions 11 to 15 animals were again tested under the stimulus conditions of the training period (55 dB). No significant effects were observed for this period. The results are discussed with respect to models of "stress-induced" eating and behavioral ecology.

Experimental models for aggression and inventories for the assessment of aggressive and autoaggressive behavior.

This paper reviews principles realized in questionnaires for the assessment of aggression as well as in experimental models suitable for inducing aggression for the validation of questionnaire scales and for providing experimental models for testing aggression-reducing drug effects. Existing self-rating scales based on factor analysis were shown to measure certain parameters of reactions concerning modes of expression of aggression and its objects. In observer rating scales situations are usually also specified. A final scale containing 9 situations and 17 reactions grouped into seven factors is presented. It could be shown to differentiate between certain types of aggression provoking situations. Furthermore, models suitable for eliciting aggression were developed in three different departments of psychology. They are based on frustration by blockade of goals and critique or subtraction of positive reinforcers ("Tower of Hanoi" and "Superball Game" in Würzburg, "Unsolvable Maze Computer Task" in Berlin) and by a competitive condition combined with application of aversive stimuli by a coplayer ("Modified Buss Machine" in Giessen). All experimental conditions were suitable for inducing anger and emotional arousal, negative ratings of confederates or experimenters, and partly also physiological changes. The results seem promising enough to test the relationship between artificially induced aggression and pathologiocal aggression in further research.