Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S15: Spins and Complex Systems |
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Sponsoring Units: GSNP GMAG Chair: Katherine Klymko, University of California, Berkeley Room: 274 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S15.00001: Entangled fractal clusters forming the lattice animals in irreversible DLCA binary systems Zakiya Shireen, Sujin Babu Irreversible DLCA of binary spheres was simulated by modifying the Brownian Cluster Dynamics. Volume fraction of randomly distributed $N$ spheres in a box of size $L$ is given by ${\Phi_{tot}=\frac{\pi}{6} N_{tot}/L^3}$. ${N_{A}}$ and ${N_{B}}$ number of spheres of same size are defined as species $A$ and $B$. Intra-species form irreversible bonds, while inter-species interaction is through hard core repulsions. We kept ${N_{B}}$ $\geq$ ${N_{A}}$, and observed bigel for certain fraction of $A$ species. By tuning the $\Phi_{tot}$ and fraction of $A$ and $B$ species we were able to regulate the size of the cage and designed clusters of a specific size. We found that the accessible volume of the system increases when compared to the monomeric case, which means that species $A$ is aggregating inside the cage created by $B$. Unlike monomeric DLCA we observe that for moderate ${\Phi_{tot}}$ both the species undergo a transition from lattice animal(fractal dimension,$d_f=2.0$) to the percolation region($d_f=2.5$). We found that $A$ clusters are stuck inside the $B$ percolating cluster and always have a fractal dimension of $2$, thus having $2$ characteristic length scale for binary system. Also, diffusion of one species are hindered by the presence of the other species forming cages. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S15.00002: Crackling to periodic transition in a granular stick-slip experiment Aghil Abed Zadeh, Jonathan Barés, Robert Behringer We perform a stick-slip experiment to characterize avalanches in time and space for granular materials. In our experiment, a constant speed stage pulls a slider which rests on a vertical bed of circular photo-elastic particles in a 2D system. The stage is connected to the slider by a spring. We measure the force on the spring by a force sensor attached to the spring. We study the avalanche size statistics, and other seismicity laws of slip avalanches. Using the power spectrum of the force signal and avalanche statistics, we analyze the effect of the loading speed and of the spring stiffness and we capture a transition from crackling to periodic regime by changing these parameters. From a more local point of view and by using a high speed camera and the photo-elastic properties of our particles, we characterize the local stress change and flow of particles during slip avalanches. By image processing, we detect the local avalanches as connected components in space and time, and we study the avalanche size probability density functions (PDF). The PDF of avalanches obey power laws both at global and local scales, but with different exponents. We try to understand the correlation of local avalanches in space and the way they coarse grain to the global avalanches. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S15.00003: Gap Statistics of Avalanches in Disordered Spin Models Jishnu Nampoothiri, Kabir Ramola, Bulbul Chakraborty, Sanjib Sabhapandit Recent studies of the statistics of avalanches in amorphous materials subjected to shear have revealed that a characteristic difference in the yielding of amorphous solids and depinning is in the statistics of `gaps' in strain between successive avalanche events. The difference is characterized by an exponent theta which is always zero in the depinning model but is nonzero in some regions of the driving field in the yielding process. Disordered spin models have been model systems for studying avalanche responses and motivated by the yielding process, we have investigated the gap statistics in two disordered spin models in one dimension namely the standard random field Ising model with nearest neighbor ferromagnetic coupling and a long range antiferromagnetic random field Ising model with power law antiferromagnetic couplings. Our studies show that the theta exponent is always zero in the first model and the we obtain a non-zero theta in the second model. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S15.00004: Dynamics Of Human Motion The Case Study of an Examination Hall Samuel Ogunjo, Oluwaseyi Ajayi, Ibiyinka Fuwape, Emmanuel Dansu Human behaviour is difficult to characterize and generalize due to ITS complex nature. Advances in mathematical models have enabled human systems such as love interaction, alcohol abuse, admission problem to be described using models. This study investigates one of such problems, the dynamics of human motion in an examination hall with limited computer systems such that students write their examination in batches. The examination is characterized by time (t) allocated to each students and difficulty level (dl) associated with the examination. A stochastic model based on the difficulty level of the examination was developed for the prediction of student's motion around the examination hall. A good agreement was obtained between theoretical predictions and numerical simulation. The result obtained will help in better planning of examination session to maximize available resources. Furthermore, results obtained in the research can be extended to other areas such as banking hall, customer service points where available resources will be shared amongst many users. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S15.00005: Chiral Potts Spin Glass in $d=2$ and $3$ Tolga Caglar, A. Nihat Berker The chiral spin-glass Potts system with $q=3$ states is studied in $d=2$ and $3$ by renormalization-group theory.[1] Global phase diagrams are calculated in temperature, chirality concentration $p$, and chirality-breaking concentration $c$, with determination of phase chaos and phase-boundary chaos. In $d=3$, the system has ferromagnetic, left-chiral, right-chiral, chiral spin-glass, and disordered phases. The boundaries to ferromagnetic, left- and right-chiral phases show, differently, and unusual, fibrous patchwork (microreentrances) of all four (ferromagnetic, left- and right-chiral, chiral spin-glass) ordered phases, especially in the multicritical region. The chaotic behavior of the interactions under scale change are determined in the chiral spin-glass phase and on the boundary between the chiral spin-glass and disordered phases, showing Lyapunov exponents in magnitudes reversed from usual ferromagnetic-antiferromagnetic spin-glass systems. In $d=2$, the chiral spin-glass Potts system does not have a spin-glass phase. \\[4pt] [1] T. \c{C}a\u{g}lar and A.N. Berker, Phys. Rev. E \underline{94}, 032121 (2016) [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S15.00006: Phase transitions in glassy systems via convolutional neural networks Chao Fang Machine learning is a powerful approach commonplace in industry to tackle large data sets. Most recently, it has found its way into condensed matter physics, allowing for the first time the study of, e.g., topological phase transitions and strongly-correlated electron systems. The study of spin glasses is plagued by finite-size effects due to the long thermalization times needed. Here we use convolutional neural networks in an attempt to detect a phase transition in three-dimensional Ising spin glasses. Our results are compared to traditional approaches. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S15.00007: Dual time scales in simulated annealing of a two-dimensional Ising Spin Glass Na Xu, Shanon Rubin, Anders Sandvik We apply simulated annealing to a 2D Ising spin glass (2DISG) with bimodal random couplings and analyze results for different system sizes and velocities based on a generalized Kibble-Zurek (KZ) scaling ansatz [1]. As the system approaches T$=$0, the critical temperature, we find two different time scales (dynamic exponents) governing the relaxation of the spin-glass order parameter and the excess energy. The energy relaxes slower than the order parameter. We argue that this unusual behavior is in accord with the entropy-enhanced ordering mechanism based on the droplet theory in 2DISG [2]. We discuss the relevance of our findings to optimization theory. [1] Shanon J. Rubin, Na Xu, Anders W. Sandvik, arXiv:1609. 09024. [2] C. K. Thomas, D. A. Huse, and A. A. Middleton, Phys.Rev. Lett. 107, 047203 (2011). [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S15.00008: Environment overwhelms both nature and nurture in a model spin glass A. Alan Middleton, Jie Yang We are interested in exploring what information determines the particular history of the glassy long term dynamics in a disordered material. We study the effect of initial configurations and the realization of stochastic dynamics on the long time evolution of configurations in a two-dimensional Ising spin glass model. The evolution of nearest neighbor correlations is computed using patchwork dynamics, a coarse-grained numerical heuristic for temporal evolution. The dependence of the nearest neighbor spin correlations at long time on both initial spin configurations and noise histories are studied through cross-correlations of long-time configurations and the spin correlations are found to be independent of both. We investigate how effectively rigid bond clusters coarsen. Scaling laws are used to study the convergence of configurations and the distribution of sizes of nearly rigid clusters. The implications of the computational results on simulations and phenomenological models of spin glasses are discussed. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S15.00009: Configuration Memory in Patchwork Dynamics for Low-dimensional Spin Glasses Jie Yang, A. Alan Middleton A patchwork dynamics method is used to study the loss and recovery of an initial configuration in spin glass models in dimensions $d=1$ and $d=2$. This method is used as a heuristic to accelerate the dynamics and to investigate how these models might reproduce the remarkable memory effects seen in experiment. Starting from a ground state configuration at one choice of couplings, a sample is aged up to a given scale under an independent choice of couplings, leading to the partial erasure of the original state. The memory of the original ground state is then computed when the couplings are reset to the original choice and patchwork coarsening is again applied. Recovery of the original ground state with coarsening is found for two-dimensional Ising spin glasses and one-dimensional Potts models, while one-dimensional Ising glasses neither lose nor gain overlap with coarsening. The recovery curves for the two-dimensional Ising spin glasses are consistent with scaling relations that define a recovery length scale that grows as a power of the aging length scale. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S15.00010: The role of fluctuations and interactions in pedestrian dynamics Alessandro Corbetta, Jasper Meeusen, Roberto Benzi, Chung-min Lee, Federico Toschi Understanding quantitatively the statistical behaviour of pedestrians walking in crowds is a major scientific challenge of paramount societal relevance. Walking humans exhibit a rich (stochastic) dynamics whose small and large deviations are driven, among others, by own will as well as by environmental conditions. Via 24/7 automatic pedestrian tracking from multiple overhead Microsoft Kinect depth sensors, we collected large ensembles of pedestrian trajectories (in the order of tens of millions) in different real-life scenarios. These scenarios include both narrow corridors and large urban hallways, enabling us to cover and compare a wide spectrum of typical pedestrian dynamics. We investigate the pedestrian motion measuring the PDFs, e.g. those of position, velocity and acceleration, and at unprecedentedly high statistical resolution. We consider the dependence of PDFs on flow conditions, focusing on diluted dynamics and pair-wise interactions ("collisions") for mutual avoidance. By means of Langevin-like models we provide models for the measured data, inclusive typical fluctuations and rare events. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S15.00011: Pathways towards instability in financial networks Guido Caldarelli, Marco Bardoscia, Fabio Caccioli, Stefano Battiston There is growing consensus that processes of market integration and risk diversification may come at the price of more systemic risk. Indeed, financial institutions are interconnected in a network of contracts where distress can either be amplified or dampened. However, a mathematical understanding of instability in relation to the network topology is still lacking. In a model financial network, we show that the origin of instability resides in the presence of specific types of cyclical structures, regardless of many of the details of the distress propagation mechanism. In particular, we show the existence of trajectories in the space of graphs along which a complex network turns from stable to unstable, although at each point along the trajectory its nodes satisfy constraints that would apparently make them individually stable. In the financial context, our findings have important implications for policies aimed at increasing financial stability. We illustrate the propositions on a sample dataset for the top 50 EU listed banks between 2008 and 2013. More in general, our results shed light on previous findings on the instability of model ecosystems and are relevant for a broad class of dynamical processes on complex networks. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S15.00012: Generic spin model on a pyrochlore lattice GiBaik Sim, SungBin Lee Motivated by several pyrochlores, we discuss generic spin model considering nearest and next nearest neighbors. Both Luttinger-Tisza analysis and simulated annealing, we analyze the phase diagram of classical spin model and discuss new types of non-coplanar order induced by anisotropic interactions. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S15.00013: Magnetic Polarons in EuB6 Gabrielle Beaudin, Andrea Bianchi, Alexandre Désilets-Benoit, Mark Laver, Robert Arnold, Steve Samothrakitis, Michel Kenzelmann, Jorge L. Gavilano, Simon Gerber, Robert Cubitt, Charles Dewhurst We present results of small-angle neutron scattering (SANS) experiments on the rare-earth, magnetic semiconductor $\mathrm{EuB_6}$. This compound exhibits two phase transitions: Upon cooling from an insulating state at high temperature, it first becomes metallic with a drop at a $T_M$ of 14.5 K, after which it orders ferromagnetically with a $T_C$ of 11.8 K. We carried out SANS experiments over a large range of scattering wave vectors $q$ from 0.006 to 0.140 \AA$^{-1}$ and temperatures from 2 to 60 K. The experiments show a Lorentzian dependence on the wave vector in the magnetic scattering intensity for temperatures just above $T_C$, which demonstrates the presence of magnetic polarons. Below $T_C$, the polarons merge together, and most of the observed intensity is, as indicated by a Lorentzian square dependence, from scattering of wall domains. We calculated a correlation length from this Lorentzian fit and obtained a range of $10^2$ to $10^3$ \AA{} for the size of the magnetic polarons. [Preview Abstract] |
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