Abstract:
We introduce an interface growth model exhibiting a nonequilibrium roughening transition (NRT). In the model, particles consist of two species, and deposit or evaporate on one dimensional substrate according to a given dynamic rule. When the dynamics is limited to occur on monolayer, this model has two absorbing states, belonging to the directed Ising (DI) universality class. At criticality, the density of vacant sites at the bottom layer in the growth model decays faster than the DI behavior, however, the dynamic exponent is close to the DI value, suggesting that the dynamics is related to the DI universality class. We also consider an asymmetric version of the growth dynamics, which is according to the directed percolation behavior.

Abstract:
We study an anomalous behavior of the height fluctuation width in the crossover from random to coherent growths of surface for a stochastic model. In the model, random numbers are assigned on perimeter sites of surface, representing pinning strengths of disordered media. At each time, surface is advanced at the site having minimum pinning strength in a random subset of system rather than having global minimum. The subset is composed of a randomly selected site and its $(\ell-1)$ neighbors. The height fluctuation width $W^2(L;\ell)$ exhibits the non-monotonic behavior with $\ell$ and it has a minimum at $\ell^*$. It is found numerically that $\ell^*$ scales as $\ell^*\sim L^{0.59}$, and the height fluctuation width at that minimum, $W^2(L;\ell^*)$, scales as $\sim L^{0.85}$ in 1+1 dimensions. It is found that the subset-size $\ell^*(L)$ is the characteristic size of the crossover from the random surface growth in the KPZ universality, to the coherent surface growth in the directed percolation universality.

Abstract:
We introduce an interface model with q-fold symmetry to study the nonequilibrium phase transition (NPT) from an active to an inactive state at the bottom layer. In the model, q different species of particles are deposited or are evaporated according to a dynamic rule, which includes the interaction between neighboring particles within the same layer. The NPT is classified according to the number of species q. For q=1 and 2, the NPT is characterized by directed percolation, and the directed Ising class, respectively. For $q \ge 3$, the NPT occurs at finite critical probability p_c, and appears to be independent of q; the $q=\infty$ case is related to the Edwards-Wilkinson interface dynamics.

Abstract:
Online auctions have expanded rapidly over the last decade and have become a fascinating new type of business or commercial transaction in this digital era. Here we introduce a master equation for the bidding process that takes place in online auctions. We find that the number of distinct bidders who bid $k$ times, called the $k$-frequent bidder, up to the $t$-th bidding progresses as $n_k(t)\sim tk^{-2.4}$. The successfully transmitted bidding rate by the $k$-frequent bidder is obtained as $q_k(t) \sim k^{-1.4}$, independent of $t$ for large $t$. This theoretical prediction is in agreement with empirical data. These results imply that bidding at the last moment is a rational and effective strategy to win in an eBay auction.

Abstract:
The spectral densities of the weighted Laplacian, random walk and weighted adjacency matrices associated with a random complex network are studied using the replica method. The link weights are parametrized by a weight exponent $\beta$. Explicit results are obtained for scale-free networks in the limit of large mean degree after the thermodynamic limit, for arbitrary degree exponent and $\beta$.

Abstract:
We study discontinuous percolation transitions (PT) in the diffusion-limited cluster aggregation model of the sol-gel transition as an example of real physical systems, in which the number of aggregation events is regarded as the number of bonds occupied in the system. When particles are Brownian, in which cluster velocity depends on cluster size as $v_s \sim s^{\eta}$ with $\eta=-0.5$, a larger cluster has less probability to collide with other clusters because of its smaller mobility. Thus, the cluster is effectively more suppressed in growth of its size. Then the giant cluster size increases drastically by merging those suppressed clusters near the percolation threshold, exhibiting a discontinuous PT. We also study the tricritical behavior by controlling the parameter $\eta$, and the tricritical point is determined by introducing an asymmetric Smoluchowski equation.

Abstract:
Percolation is a paradigmatic model in disordered systems and has been applied to various natural phenomena. The percolation transition is known as one of the most robust continuous transitions. However, recent extensive studies have revealed that a few models exhibit a discontinuous percolation transition (DPT) in cluster merging processes. Unlike the case of continuous transitions, understanding the nature of discontinuous phase transitions requires a detailed study of the system at hand, which has not been undertaken yet for DPTs. Here we examine the cluster size distribution immediately before an abrupt increase in the order parameter of DPT models and find that DPTs induced by cluster merging kinetics can be classified into two types. Moreover, the type of DPT can be determined by the key characteristic of whether the cluster kinetic rule is homogeneous with respect to the cluster sizes. We also establish the necessary conditions for each type of DPT, which can be used effectively when the discontinuity of the order parameter is ambiguous, as in the explosive percolation model.

Abstract:
When a group of people unknown to each other meet and familiarize among themselves, over time they form a community on a macroscopic scale. This phenomenon can be understood in the context of percolation transition (PT) of networks, which takes place continuously in the classical random graph model. Recently, a modified model was introduced in which the formation of the community was suppressed. Then the PT occurs explosively at a delayed transition time. Whether the explosive PT is indeed discontinuous or continuous becomes controversial. Here we show that type of PT depends on a detailed dynamic rule. Thus, when the dynamic rule is designed to suppress the growth of overall clusters, then the explosive PT could be discontinuous.

Abstract:
The evolution of the Erd\H{o}s-R\'enyi (ER) network by adding edges can be viewed as a cluster aggregation process. Such ER processes can be described by a rate equation for the evolution of the cluster-size distribution with the connection kernel $K_{ij}\sim ij$, where $ij$ is the product of the sizes of two merging clusters. Here, we study more general cases in which $K_{ij}$ is sub-linear as $K_{ij}\sim (ij)^{\omega}$ with $0 \le \omega < 1/2$; we find that the percolation transition (PT) is discontinuous. Moreover, PT is also discontinuous when the ER dynamics evolves from proper initial conditions. The rate equation approach for such discontinuous PTs enables us to uncover the mechanism underlying the explosive PT under the Achlioptas process.

Abstract:
Recently increased accessibility of large-scale digital records enables one to monitor human activities such as the interevent time distributions between two consecutive visits to a web portal by a single user, two consecutive emails sent out by a user, two consecutive library loans made by a single individual, etc. Interestingly, those distributions exhibit a universal behavior, $D(\tau)\sim \tau^{-\delta}$, where $\tau$ is the interevent time, and $\delta \simeq 1$ or 3/2. The universal behaviors have been modeled via the waiting-time distribution of a task in the queue operating based on priority; the waiting time follows a power law distribution $P_{\rm w}(\tau)\sim \tau^{-\alpha}$ with either $\alpha=1$ or 3/2 depending on the detail of queuing dynamics. In these models, the number of incoming tasks in a unit time interval has been assumed to follow a Poisson-type distribution. For an email system, however, the number of emails delivered to a mail box in a unit time we measured follows a powerlaw distribution with general exponent $\gamma$. For this case, we obtain analytically the exponent $\alpha$, which is not necessarily 1 or 3/2 and takes nonuniversal values depending on $\gamma$. We develop the generating function formalism to obtain the exponent $\alpha$, which is distinct from the continuous time approximation used in the previous studies.