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Journal articleRochester CC, Pruessner G, Kornyshev AA, 2015,
Statistical mechanics of 'Unwanted Electroactuation' in nanoporous supercapacitors
, Electrochimica Acta, Vol: 174, Pages: 978-984, ISSN: 0013-4686Nanoporous electrodes have the potential to increase the surface electrode interfacial area and the stored energy density of a supercapacitor. However, structural deformation of the electrode can become apparent when the size of the pore is comparable to the size of a charging ion. After many cycles this could cause wear and degradation. We present a theoretical study of this ‘Unwanted Electroactuation’ in a carbon electrode wetted with an ionic liquid. We incorporate changes of the carbon-carbon bond length due to electrochemical doping of the pore walls and steric effects related to counterion insertion into the pore via a modified Ising model of charge storage. When considering the total electrode deformation these effects either complement or compete with each other, depending on the polarisation of the electrode. Our model shows qualitative agreement with the features of the experimentally observed expansion caused by variation of electrode potential.
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Journal articleWillis G, Pruessner G, Keelan J, 2015,
Minimalistic real-space Renormalization of 4x4 Ising and Potts Models in two dimensions
, Frontiers in Physics, Vol: 3, ISSN: 2296-424XWe introduce and discuss a real-space renormalization group (RSRG) procedure on verysmall lattices, which in principle does not require any of the usual approximations, e.g., acut-off in the expansion of the Hamiltonian in powers of the field. The procedure is carriedout numerically on very small lattices (4×4 to 2×2) and implemented for the Ising Modeland the q = 3, 4, 5-state Potts Models. Nevertheless, the resulting estimates of thecorrelation length exponent and the magnetization exponent are typically within 3–7% ofthe exact values. The 4-state Potts Model generates a third magnetic exponent, whichseems to be unknown in the literature. A number of questions about the meaning ofcertain exponents and the procedure itself arise from its use of symmetry principles andits application to the q = 5 Potts Model.
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Journal articleClough JR, Gollings J, Loach TV, et al., 2015,
Transitive reduction of citation networks
, Journal of Complex Networks, Vol: 3, Pages: 189-203, ISSN: 2051-1310In many complex networks, the vertices are ordered in time, and edges represent causal connections. We propose methods of analysing such directed acyclic graphs taking into account the constraints of causality and highlighting the causal structure. We illustrate our approach using citation networks formed from academic papers, patents and US Supreme Court verdicts. We show how transitive reduction (TR) reveals fundamental differences in the citation practices of different areas, how it highlights particularly interesting work, and how it can correct for the effect that the age of a document has on its citation count. Finally, we transitively reduce null models of citation networks with similar degree distributions and show the difference in degree distributions after TR to illustrate the lack of causal structure in such models.
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Journal articleMcAteer RTJ, Aschwanden MJ, Dimitropoulou M, et al., 2015,
25 Years of self-organized criticality: numerical detection methods
, Space Science Reviews, Vol: 198, Pages: 217-266, ISSN: 1572-9672The detection and characterization of self-organized criticality (SOC), in bothreal and simulated data, has undergone many significant revisions over the past 25 years. The explosive advances in the many numerical methods available for detecting, discriminating, and ultimately testing, SOC have played a critical role in developing our understanding of how systems experience and exhibit SOC. In this article, methods of detecting SOC are reviewed; from correlations to complexity to critical quantities. A description of the basicautocorrelation method leads into a detailed analysis of application-oriented methods developed in the last 25 years. In the second half of this manuscript space-based, time-based and spatial-temporal methods are reviewed and the prevalence of power laws in nature is described, with an emphasis on event detection and characterization. The search for numericalmethods to clearly and unambiguously detect SOC in data often leads us outside the comfort zone of our own disciplines—the answers to these questions are often obtained by studying the advances made in other fields of study. In addition, numerical detection methods often provide the optimum link between simulations and experiments in scientific research. We seek to explore this boundary where the rubber meets the road, to review this expandingfield of research of numerical detection of SOC systems over the past 25 years, and to iterate forwards so as to provide some foresight and guidance into developing breakthroughs in this subject over the next quarter of a century.
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Journal articleFont-Clos F, Pruessner G, Moloney NR, et al., 2015,
The perils of thresholding
, New Journal of Physics, Vol: 17, ISSN: 1367-2630The thresholding of time series of activity or intensity is frequently used to define and differentiateevents. This is either implicit, for example due to resolution limits, or explicit, in order to filter certainsmall scale physics from the supposed true asymptotic events. Thresholding the birth–death process,however, introduces a scaling region into the event size distribution, which is characterized by anexponent that is unrelated to the actual asymptote and is rather an artefact of thresholding. As a result,numerical fits of simulation data produce a range of exponents, with the true asymptote visible only inthe tail of the distribution. This tail is increasingly difficult to sample as the threshold is increased. Inthe present case, the exponents and the spurious nature of the scaling region can be determinedanalytically, thus demonstrating the way in which thresholding conceals the true asymptote. Theanalysis also suggests a procedure for detecting the influence of the threshold by means of a datacollapse involving the threshold-imposed scale.
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Conference paperGoldberg SR, Anthony H, Evans TS, 2015,
Do We Need Global and Local Knowledge of the Citation Network?
, 15th International Conference of the International-Society-for-Scientometrics-and-Informetrics (ISSI) on Scientometrics and Informetrics, Publisher: INT SOC SCIENTOMETRICS & INFORMETRICS-ISSI, Pages: 282-283, ISSN: 2175-1935 -
Conference paperClough JR, Evans TS, 2015,
Time & Citation Networks
, 15th International Conference of the International-Society-for-Scientometrics-and-Informetrics (ISSI) on Scientometrics and Informetrics, Publisher: INT SOC SCIENTOMETRICS & INFORMETRICS-ISSI, Pages: 1073-1078, ISSN: 2175-1935 -
Conference paperLoach TV, Evans TS, 2015,
Ranking Journals Using Altmetrics
, 15th International Conference of the International-Society-for-Scientometrics-and-Informetrics (ISSI) on Scientometrics and Informetrics, Publisher: INT SOC SCIENTOMETRICS & INFORMETRICS-ISSI, Pages: 89-94, ISSN: 2175-1935- Author Web Link
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Journal articleGastner MT, Markou N, Pruessner G, et al., 2014,
Opinion Formation Models on a Gradient
, PLOS One, Vol: 9, ISSN: 1932-6203Statistical physicists have become interested in models of collective social behaviorsuch as opinion formation, where individuals change their inherently preferredopinion if their friends disagree. Real preferences often depend on regional culturaldifferences, which we model here as a spatial gradient g in the initial opinion. Thegradient does not only add reality to the model. It can also reveal that opinionclusters in two dimensions are typically in the standard (i.e., independent)percolation universality class, thus settling a recent controversy about a nonconsensusmodel. However, using analytical and numerical tools, we also present amodel where the width of the transition between opinions scales !g{1=4, not!g{4=7 as in independent percolation, and the cluster size distribution isconsistent with first-order percolation.
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Journal articlePruessner G, Cheang S, Jensen HJ, 2014,
Synchronization by small time delays
, Physica A: Statistical Mechanics and its Applications, Vol: 420, Pages: 8-13, ISSN: 1873-2119Synchronization is a phenomenon observed in all of the living and in much of the nonliving world, for example in the heart beat, Huygens’ clocks, the flashing of fireflies and the clapping of audiences. Depending on the number of degrees of freedom involved, different mathematical approaches have been used to describe it, most prominently integrateand-fire oscillators and the Kuramoto model of coupled oscillators. In the present work, we study a very simple and general system of smoothly evolving oscillators, which continue to interact even in the synchronized state. We find that under very general circumstances, synchronization generically occurs in the presence of a (small) time delay. Strikingly, the synchronization time is inversely proportional to the time delay.
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