34 research outputs found
The Extent of Sectarianism Online : Project Report produced for Nil by Mouth
Study focusing on the extent and nature of online sectarianism often connected with football in Scotland
The Extent of Sectarianism Online
Study focusing on the extent and nature of online sectarianism often connected with football in Scotland
Spontaneous Jamming in One-Dimensional Systems
We study the phenomenon of jamming in driven diffusive systems. We introduce
a simple microscopic model in which jamming of a conserved driven species is
mediated by the presence of a non-conserved quantity, causing an effective long
range interaction of the driven species. We study the model analytically and
numerically, providing strong evidence that jamming occurs; however, this
proceeds via a strict phase transition (with spontaneous symmetry breaking)
only in a prescribed limit. Outside this limit, the nearby transition
(characterised by an essential singularity) induces sharp crossovers and
transient coarsening phenomena. We discuss the relevance of the model to two
physical situations: the clustering of buses, and the clogging of a suspension
forced along a pipe.Comment: 8 pages, 4 figures, uses epsfig. Submitted to Europhysics Letter
Analysis of a convenient information bound for general quantum channels
Open questions from Sarovar and Milburn (2006 J.Phys. A: Math. Gen. 39 8487)
are answered. Sarovar and Milburn derived a convenient upper bound for the
Fisher information of a one-parameter quantum channel. They showed that for
quasi-classical models their bound is achievable and they gave a necessary and
sufficient condition for positive operator-valued measures (POVMs) attaining
this bound. They asked (i) whether their bound is attainable more generally,
(ii) whether explicit expressions for optimal POVMs can be derived from the
attainability condition. We show that the symmetric logarithmic derivative
(SLD) quantum information is less than or equal to the SM bound, i.e.\
and we find conditions for equality. As
the Fisher information is less than or equal to the SLD quantum information,
i.e. , we can deduce when equality holds in
. Equality does not hold for all
channels. As a consequence, the attainability condition cannot be used to test
for optimal POVMs for all channels. These results are extended to
multi-parameter channels.Comment: 16 pages. Published version. Some of the lemmas have been corrected.
New resuts have been added. Proofs are more rigorou
Phase Transition in Two Species Zero-Range Process
We study a zero-range process with two species of interacting particles. We
show that the steady state assumes a simple factorised form, provided the
dynamics satisfy certain conditions, which we derive. The steady state exhibits
a new mechanism of condensation transition wherein one species induces the
condensation of the other. We study this mechanism for a specific choice of
dynamics.Comment: 8 pages, 3 figure
Anomalous aging phenomena caused by drift velocities
We demonstrate via several examples that a uniform drift velocity gives rise
to anomalous aging, characterized by a specific form for the two-time
correlation functions, in a variety of statistical-mechanical systems far from
equilibrium. Our first example concerns the oscillatory phase observed recently
in a model of competitive learning. Further examples, where the proposed theory
is exact, include the voter model and the Ohta-Jasnow-Kawasaki theory for
domain growth in any dimension, and a theory for the smoothing of sandpile
surfaces.Comment: 7 pages, 3 figures. To appear in Europhysics Letter
Factorised Steady States in Mass Transport Models
We study a class of mass transport models where mass is transported in a
preferred direction around a one-dimensional periodic lattice and is globally
conserved. The model encompasses both discrete and continuous masses and
parallel and random sequential dynamics and includes models such as the
Zero-range process and Asymmetric random average process as special cases. We
derive a necessary and sufficient condition for the steady state to factorise,
which takes a rather simple form.Comment: 6 page
Collective traffic-like movement of ants on a trail: dynamical phases and phase transitions
The traffic-like collective movement of ants on a trail can be described by a
stochastic cellular automaton model. We have earlier investigated its unusual
flow-density relation by using various mean field approximations and computer
simulations. In this paper, we study the model following an alternative
approach based on the analogy with the zero range process, which is one of the
few known exactly solvable stochastic dynamical models. We show that our theory
can quantitatively account for the unusual non-monotonic dependence of the
average speed of the ants on their density for finite lattices with periodic
boundary conditions. Moreover, we argue that the model exhibits a continuous
phase transition at the critial density only in a limiting case. Furthermore,
we investigate the phase diagram of the model by replacing the periodic
boundary conditions by open boundary conditions.Comment: 8 pages, 6 figure
From urn models to zero-range processes: statics and dynamics
The aim of these lecture notes is a description of the statics and dynamics
of zero-range processes and related models. After revisiting some conceptual
aspects of the subject, emphasis is then put on the study of the class of
zero-range processes for which a condensation transition arises.Comment: Lecture notes for the Luxembourg Summer School 200
Hydrodynamics of the zero-range process in the condensation regime
We argue that the coarse-grained dynamics of the zero-range process in the
condensation regime can be described by an extension of the standard
hydrodynamic equation obtained from Eulerian scaling even though the system is
not locally stationary. Our result is supported by Monte Carlo simulations.Comment: 14 pages, 3 figures. v2: Minor alteration