Bidirectional transport on a dynamic lattice

Bidirectional variants of stochastic many particle models for transport by molecular motors show a strong tendency to form macroscopic clusters on static lattices. Inspired by the fact that the microscopic tracks for molecular motors are dynamical, we study the influence of different types of lattice dynamics on stochastic bidirectional transport. We observe a transition toward efficient transport (corresponding to the dissolution of large clusters) controlled by the lattice dynamics.Comment: 5 pages, 5 figure

Can Path Dependence Explain Institutional Change? Two Approaches Applied to Welfare State Reform

Path dependence as a concept in institutional theories has become increasingly popular in economics and other social sciences. The key idea is that in a sequence of events, the latter events are not (completely) independent from those that occurred in the past. Yet, common usage of the concept often subsumes two markedly different models and approaches to understand historical sequencing. The two main processes of the past shaping the future – diffusion and developmental pathways – must be distinguished analytically. This paper juxtaposes (1) the unplanned "trodden path" that takes shape through the subsequent repeated use by other individuals of that spontaneously chosen path, and (2) the "branching pathways" or juncture at which one of the available alternative pathways must be chosen in order to continue a journey. Furthermore, the typical approaches and their explanatory purchase are discussed in reference to explanations of institutional change. The paper shows that the first path dependence theorem is too deterministic and inflexible, whereas the second approach is sufficiently supple to analyze various forms of institutional change.Pfadabhängigkeit wird in den Sozial- und Wirtschaftswissenschaften zunehmend als Konzept institutioneller Theorien angewendet. Der Grundgedanke ist, dass in einer Sequenz von Ereignissen (oder Entscheidungen) spätere nicht (vollkommen) unabhängig von vorangegangenen sind. Jedoch wird in gängigen Anwendungen des Konzepts oft versäumt, nach zwei äußerst unterschiedlichen Verständnissen historischer Sequenzen zu differenzieren. Die zwei Prozesse der Strukturierung der Zukunft durch die Vergangenheit – Diffusionspfade und Entwicklungspfade – sollen analytisch getrennt werden. Dieser Aufsatz unterscheidet (1) den zufälligen "Trampelpfad", welcher durch die vermehrte Benutzung eines Pfades spontan entsteht, von (2) dem "Scheideweg", an dem einer der möglichen Pfade gewählt werden muss, um den Weg fortzusetzen. Im Weiteren wird der Erklärungsbeitrag der beiden Ansätze in Bezug auf die Erklärung institutionellen Wandels erörtert. Es zeigt sich, dass das erste Pfadabhängigkeitstheorem zu deterministisch und inflexibel ist, der zweite Pfadabhängigkeitsansatz hingegen offen genug, unterschiedliche Formen institutionellen Wandels zu analysieren.1 Introduction 2 Path dependence I: "trodden trail" and the diffusion of an institution 3 Path dependence example I: unintended consequences of early retirement 4 Path dependence II: branching pathways and the structuring of alternatives 5 Path dependence example II: multiple pathways of welfare reform 6 Institutional theories and self-reinforcing mechanisms 7 Conclusion Bibliograph

Particle interactions and lattice dynamics: Scenarios for efficient bidirectional stochastic transport?

Intracellular transport processes driven by molecular motors can be described by stochastic lattice models of self-driven particles. Here we focus on bidirectional transport models excluding the exchange of particles on the same track. We explore the possibility to have efficient transport in these systems. One possibility would be to have appropriate interactions between the various motors' species, so as to form lanes. However, we show that the lane formation mechanism based on modified attachment/detachment rates as it was proposed previously is not necessarily connected to an efficient transport state and is suppressed when the diffusivity of unbound particles is finite. We propose another interaction mechanism based on obstacle avoidance that allows to have lane formation for limited diffusion. Besides, we had shown in a separate paper that the dynamics of the lattice itself could be a key ingredient for the efficiency of bidirectional transport. Here we show that lattice dynamics and interactions can both contribute in a cooperative way to the efficiency of transport. In particular, lattice dynamics can decrease the interaction threshold beyond which lanes form. Lattice dynamics may also enhance the transport capacity of the system even when lane formation is suppressed.Comment: 25 pages, 17 figures, 2 table

Dielectric behavior of Copper Tantalum Oxide

A thorough investigation of the dielectric properties of Cu2Ta4O12, a material crystallizing in a pseudo-cubic, perovskite-derived structure is presented. We measured the dielectric constant and conductivity of single crystals in an exceptionally broad frequency range up to GHz frequencies and at temperatures from 25 - 500 K. The detected dielectric constant is unusually high (reaching values up to 105) and almost constant in a broad frequency and temperature range. Cu2Ta4O12 possesses a crystal structure similar to CaCu3Ti4O12, the compound for which such an unusually high dielectric constant was first observed. An analysis of the results using a simple equivalent circuit and measurements with different types of contact revealed that extrinsic interfacial polarization effects, derived from surface barrier capacitors are the origin of the observed giant dielectric constants. The intrinsic properties of Cu2Ta4O12 are characterized by a (still relatively high) dielectric constant in the order of 100 and by charge transport via hopping conduction of Anderson-localized charge carriers.Comment: 18 pages, 6 figures, submitted to Jouranl of Physical Chemestr

Metal-to-insulator transition and magnetic ordering in CaRu_{1-x}Cu_xO_3

CaRuO_3 is perovskite with an orthorhombic distortion and is believed to be close to magnetic ordering. Magnetic studies of single crystal and polycrystalline CaRu_{1-x}Cu_xO_3 (0\le x \le 15 at.%Cu) reveal that spin-glass-like transition develops for x\le 7 at.%Cu and obtained value for effective magnetic moment p_{eff}=3.55 mu_B for x=5 at.% Cu, single crystal, indicates presence of Ru^{5+}. At higher Cu concentrations more complex magnetic behaviors are observed. Electrical resistivity measured on polycrystalline samples shows metal-to-insulator transition (MIT) at 51 K for only 2 at.% Cu. Charge compensation, which is assumed to be present upon Cu^{2+/3+} substitution, induces appearance of Ru^{5+} and/or creation of oxygen vacancies in crystal structure. Since the observed changes in physical properties are completely attributable to the charge compensation, they cannot be related to behaviors of pure compound where no such mechanism is present. This study provides the criterion for "good" chemical probes for studying Ru-based perovskites.Comment: 12 pages, 7 figure

Wakeful rest alleviates interference-based forgetting

Retroactive interference (RI)—the disruptive influence of events occurring after the formation of a new memory—is one of the primary causes of forgetting. Placing individuals within an environment that postpones interference should, therefore, greatly reduce the likelihood of information being lost from memory. For example, a short period of wakeful rest should diminish interference-based forgetting. To test this hypothesis, participants took part in a foreign language learning activity and were shown English translations of 20 Icelandic words for immediate recall. Half of the participants were then given an 8-min rest before completing a similar or dissimilar interfering distractor task. The other half did not receive a rest until after the distractor task, at which point interference had already taken place. All participants were then asked to translate the Icelandic words for a second time. Results revealed that retention was significantly worse at the second recall test, but being allowed a brief rest before completing the distractor task helped reduce the amount of forgetting. Taking a short, passive break can shield new memories from RI and alleviate forgetting.ERAS Scheme, University of Wolverhampto

A model for bidirectional traffic of cytoskeletal motors

We introduce a stochastic lattice gas model including two particle species and two parallel lanes. One lane with exclusion interaction and directed motion and the other lane without exclusion and unbiased diffusion, mimicking a micotubule filament and the surrounding solution. For a high binding affinity to the filament, jam-like situations dominate the system's behaviour. The fundamental process of position exchange of two particles is approximated. In the case of a many-particle system, we were able to identify a regime in which the system is rather homogenous presenting only small accumulations of particles and a regime in which an important fraction of all particles accumulates in the same cluster. Numerical data proposes that this cluster formation will occur at all densities for large system sizes. Coupling of several filaments leads to an enhanced cluster formation compared to the uncoupled system, suggesting that efficient bidirectional transport on one-dimensional filaments relies on long-ranged interactions and track formation.Comment: 20 pages, 9 figure

Frameworks for logically classifying polynomial-time optimisation problems.

We show that a logical framework, based around a fragment of existential second-order logic formerly proposed by others so as to capture the class of polynomially-bounded P-optimisation problems, cannot hope to do so, under the assumption that P ≠ NP. We do this by exhibiting polynomially-bounded maximisation and minimisation problems that can be expressed in the framework but whose decision versions are NP-complete. We propose an alternative logical framework, based around inflationary fixed-point logic, and show that we can capture the above classes of optimisation problems. We use the inductive depth of an inflationary fixed-point as a means to describe the objective functions of the instances of our optimisation problems