Comment on: "Roughness of Interfacial Crack Fronts: Stress-Weighted Percolation in the Damage Zone"

This is a comment on J. Schmittbuhl, A. Hansen, and G. G. Batrouni, Phys. Rev. Lett. 90, 045505 (2003). They offer a reply, in turn.Comment: 1 page, 1 figur

Optimization in random field Ising models by quantum annealing

We investigate the properties of quantum annealing applied to the random field Ising model in one, two and three dimensions. The decay rate of the residual energy, defined as the energy excess from the ground state, is find to be $e_{res}\sim \log(N_{MC})^{-\zeta}$ with $\zeta$ in the range $2...6$, depending on the strength of the random field. Systems with ``large clusters'' are harder to optimize as measured by $\zeta$. Our numerical results suggest that in the ordered phase $\zeta=2$ whereas in the paramagnetic phase the annealing procedure can be tuned so that $\zeta\to6$.Comment: 7 pages (2 columns), 9 figures, published with minor changes, one reference updated after the publicatio

Creep of a fracture line in paper peeling

The slow motion of a crack line is studied via an experiment in which sheets of paper are split into two halves in a ``peel-in-nip'' (PIN) geometry under a constant load, in creep. The velocity-force relation is exponential. The dynamics of the fracture line exhibits intermittency, or avalanches, which are studied using acoustic emission. The energy statistics is a power-law, with the exponent $\beta \sim 1.8 \pm 0.1$. Both the waiting times between subsequent events and the displacement of the fracture line imply complicated stick-slip dynamics. We discuss the correspondence to tensile PIN tests and other similar experiments on in-plane fracture and the theory of creep for elastic manifolds

Quenched noise and over-active sites in sandpile dynamics

The dynamics of sandpile models are mapped to discrete interface equations. We study in detail the Bak-Tang-Wiesenfeld model, a stochastic model with random thresholds, and the Manna model. These are, respectively, discretizations of the quenched Edwards-Wilkinson equation with columnar, point-like and correlated noise, with the constraint that the interface velocity is either zero or exactly one. The constraint, embedded in the sandpile rules, gives rise to another noise component. This term has for the Bak-Tang-Wiesenfeld model long-range on-site correlations and reveals that with open boundary conditions there is no spatial translational invariance.Comment: 4 pages, 3 figure

Phase transitions in a disordered system in and out of equilibrium

The equilibrium and non--equilibrium disorder induced phase transitions are compared in the random-field Ising model (RFIM). We identify in the demagnetized state (DS) the correct non-equilibrium hysteretic counterpart of the T=0 ground state (GS), and present evidence of universality. Numerical simulations in d=3 indicate that exponents and scaling functions coincide, while the location of the critical point differs, as corroborated by exact results for the Bethe lattice. These results are of relevance for optimization, and for the generic question of universality in the presence of disorder.Comment: Accepted for publication in Phys. Rev. Let

Scaling in self-organized criticality from interface depinning?

The avalanche properties of models that exhibit 'self-organized criticality' (SOC) are still mostly awaiting theoretical explanations. A recent mapping (Europhys. Lett.~53, 569) of many sandpile models to interface depinning is presented first, to understand how to reach the SOC ensemble and the differences of this ensemble with the usual depinning scenario. In order to derive the SOC avalanche exponents from those of the depinning critical point, a geometric description is discussed, of the quenched landscape in which the 'interface' measuring the integrated activity moves. It turns out that there are two main alternatives concerning the scaling properties of the SOC ensemble. These are outlined in one dimension in the light of scaling arguments and numerical simulations of a sandpile model which is in the quenched Edwards-Wilkinson universality class.Comment: 7 pages, 3 figures, Statphys satellite meeting in Merida, July 200

Power spectra of self-organized critical sandpiles

We analyze the power spectra of avalanches in two classes of self-organized critical sandpile models, the Bak-Tang-Wiesenfeld model and the Manna model. We show that these decay with a $1/f^\alpha$ power law, where the exponent value $\alpha$ is significantly smaller than 2 and equals the scaling exponent relating the avalanche size to its duration. We discuss the basic ingredients behind this result, such as the scaling of the average avalanche shape.Comment: 7 pages, 3 figures, submitted to JSTA

The effect of thresholding on temporal avalanche statistics

We discuss intermittent time series consisting of discrete bursts or avalanches separated by waiting or silent times. The short time correlations can be understood to follow from the properties of individual avalanches, while longer time correlations often present in such signals reflect correlations between triggerings of different avalanches. As one possible source of the latter kind of correlations in experimental time series, we consider the effect of a finite detection threshold, due to e.g. experimental noise that needs to be removed. To this end, we study a simple toy model of an avalanche, a random walk returning to the origin or a Brownian bridge, in the presence and absence of superimposed delta-correlated noise. We discuss the properties after thresholding of artificial timeseries obtained by mixing toy avalanches and waiting times from a Poisson process. Most of the resulting scalings for individual avalanches and the composite timeseries can be understood via random walk theory, except for the waiting time distributions when strong additional noise is added. Then, to compare with a more complicated case we study the Manna sandpile model of self-organized criticality, where some further complications appear.Comment: 15 pages, 12 figures, submitted to J. Stat. Mech., special issue of the UPoN2008 conferenc

Comment on: `Pipe Network Model for Scaling of Dynamic Interfaces in Porous Media'

We argue that a proposed exponent identity [Phys. Rev. Lett 85, 1238 (2000)] for interface roughening in spontaneous imbibition is wrong. It rests on the assumption that the fluctuations are controlled by a single time scale, but liquid conservation imposes two distinct time scales.Comment: 1 page, to appear in Phys. Rev. Let