25,026 research outputs found
Potts Models on Feynman Diagrams
We investigate numerically and analytically Potts models on ``thin'' random
graphs -- generic Feynman diagrams, using the idea that such models may be
expressed as the N --> 1 limit of a matrix model. The thin random graphs in
this limit are locally tree-like, in distinction to the ``fat'' random graphs
that appear in the planar Feynman diagram limit, more familiar from discretized
models of two dimensional gravity.
The interest of the thin graphs is that they give mean field theory behaviour
for spin models living on them without infinite range interactions or the
boundary problems of genuine tree-like structures such as the Bethe lattice.
q-state Potts models display a first order transition in the mean field for
q>2, so the thin graph Potts models provide a useful test case for exploring
discontinuous transitions in mean field theories in which many quantities can
be calculated explicitly in the saddle point approximation.Comment: 10 pages, latex, + 6 postscript figure
Phase diagram of the mean field model of simplicial gravity
We discuss the phase diagram of the balls in boxes model, with a varying
number of boxes. The model can be regarded as a mean-field model of simplicial
gravity. We analyse in detail the case of weights of the form , which correspond to the measure term introduced in the simplicial
quantum gravity simulations. The system has two phases~: {\em elongated} ({\em
fluid}) and {\em crumpled}. For the transition between
these two phases is first order, while for it is continuous.
The transition becomes softer when approaches unity and eventually
disappears at . We then generalise the discussion to an arbitrary set
of weights. Finally, we show that if one introduces an additional kinematic
bound on the average density of balls per box then a new {\em condensed} phase
appears in the phase diagram. It bears some similarity to the {\em crinkled}
phase of simplicial gravity discussed recently in models of gravity interacting
with matter fields.Comment: 15 pages, 5 figure
Decorating Random Quadrangulations
On various regular lattices (simple cubic, body centred cubic..) decorating
an edge with an Ising spin coupled by bonds of strength L to the original
vertex spins and competing with a direct anti-ferromagnetic bond of strength
alpha L can give rise to three transition temperatures for suitable alpha. The
system passes through ferromagnetic, paramagnetic, anti-ferromagnetic and
paramagnetic phases respectively as the temperature is increased.
For the square lattice on the other hand multiple decoration is required to
see this effect. We note here that a single decoration suffices for the Ising
model on planar random quadrangulations (coupled to 2D quantum gravity). Other
random bipartite lattices such as the Penrose tiling are more akin to the
regular square lattice and require multiple decoration to have any affect.Comment: 6 pages + 5 figure
The effects of arbuscular mycorrhizal fungi (AMF) and Rhizophagus irregularis on soil microorganisms assessed by metatranscriptomics and metaproteomics
Arbuscular mycorrhizal fungi (AMF) form symbioses with approximately 80% of plant species and potentially benefit their hosts (e.g. nutrient acquisition) and the soil environment (e.g. soil aggregation). AMF also affect soil microbiota and soil multifunctionality. We manipulated AMF presence (via inoculation of non-sterile soil with Rhizophagus irregularis and using a hyphal compartment design) and used RNA-seq and metaproteomics to assess AMF roles in soil. The results indicated that AMF drove an active soil microbial community expressing transcripts and proteins related to nine metabolic functions, including the metabolism of C and N. We suggest two possible mechanisms: 1) the AMF hyphae produce exudates that select a beneficial community, or, 2) the hyphae compete with other soil microbes for available nutrients and consequently induce the community to mineralize nutrients from soil organic matter. We also identified candidate proteins that are potentially related to soil aggregation, such as Lpt and HSP60. Our results bridge microbial ecology and ecosystem functioning. We show that the AMF hyphosphere contains an active community related to soil respiration and nutrient cycling, thus potentially improving nutrient mineralization from soil organic matter and nutrient supply to the plants
Potts Models with (17) Invisible States on Thin Graphs
The order of a phase transition is usually determined by the nature of the
symmetry breaking at the phase transition point and the dimension of the model
under consideration. For instance, q-state Potts models in two dimensions
display a second order, continuous transition for q = 2,3,4 and first order for
higher q.
Tamura et al recently introduced Potts models with "invisible" states which
contribute to the entropy but not the internal energy and noted that adding
such invisible states could transmute continuous transitions into first order
transitions. This was observed both in a Bragg-Williams type mean-field
calculation and 2D Monte-Carlo simulations. It was suggested that the invisible
state mechanism for transmuting the order of a transition might play a role
where transition orders inconsistent with the usual scheme had been observed.
In this paper we note that an alternative mean-field approach employing
3-regular random ("thin") graphs also displays this change in the order of the
transition as the number of invisible states is varied, although the number of
states required to effect the transmutation, 17 invisible states when there are
2 visible states, is much higher than in the Bragg-Williams case. The
calculation proceeds by using the equivalence of the Potts model with 2 visible
and r invisible states to the Blume-Emery-Griffiths (BEG) model, so a
by-product is the solution of the BEG model on thin random graphs.Comment: (2) Minor typos corrected, references update
The Gonihedric Ising Model and Glassiness
The Gonihedric 3D Ising model is a lattice spin model in which planar Peierls
boundaries between + and - spins can be created at zero energy cost. Instead of
weighting the area of Peierls boundaries as the case for the usual 3D Ising
model with nearest neighbour interactions, the edges, or "bends" in an
interface are weighted, a concept which is related to the intrinsic curvature
of the boundaries in the continuum.
In these notes we follow a roughly chronological order by first reviewing the
background to the formulation of the model, before moving on to the elucidation
of the equilibrium phase diagram by various means and then to investigation of
the non-equilibrium, glassy behaviour of the model.Comment: To appear as Chapter 7 in Rugged Free-Energy Landscapes - An
Introduction, Springer Lecture Notes in Physics, 736, ed. W. Janke, (2008
Magnetic properties of strongly disordered electronic systems
We present a unified, global perspective on the magnetic properties of
strongly disordered electronic systems, with special emphasis on the case where
the ground state is metallic. We review the arguments for the instability of
the disordered Fermi liquid state towards the formation of local magnetic
moments, and argue that their singular low temperature thermodynamics are the
``quantum Griffiths'' precursors of the quantum phase transition to a metallic
spin glass; the local moment formation is therefore not directly related to the
metal-insulator transition. We also review the the mean-field theory of the
disordered Fermi liquid to metallic spin glass transition and describe the
separate regime of ``non-Fermi liquid'' behavior at higher temperatures near
the quantum critical point. The relationship to experimental results on doped
semiconductors and heavy-fermion compounds is noted.Comment: 25 pages; Contribution to the Royal Society Discussion Meeting on
"The Metal-Non Metal Transition in Macroscopic and Microscopic Systems",
March 5-6, 199
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