48 research outputs found
The role of three-body interactions in two-dimensional polymer collapse
Various interacting lattice path models of polymer collapse in two dimensions
demonstrate different critical behaviours. This difference has been without a
clear explanation. The collapse transition has been variously seen to be in the
Duplantier-Saleur -point university class (specific heat cusp), the
interacting trail class (specific heat divergence) or even first-order. Here we
study via Monte Carlo simulation a generalisation of the Duplantier-Saleur
model on the honeycomb lattice and also a generalisation of the so-called
vertex-interacting self-avoiding walk model (configurations are actually
restricted trails known as grooves) on the triangular lattice. Crucially for
both models we have three and two body interactions explicitly and
differentially weighted. We show that both models have similar phase diagrams
when considered in these larger two-parameter spaces. They demonstrate regions
for which the collapse transition is first-order for high three body
interactions and regions where the collapse is in the Duplantier-Saleur
-point university class. We conjecture a higher order multiple critical
point separating these two types of collapse.Comment: 17 pages, 20 figure
Exact Solution of the Discrete (1+1)-dimensional RSOS Model in a Slit with Field and Wall Interactions
We present the solution of a linear Restricted Solid--on--Solid (RSOS) model
confined to a slit. We include a field-like energy, which equivalently weights
the area under the interface, and also include independent interaction terms
with both walls. This model can also be mapped to a lattice polymer model of
Motzkin paths in a slit interacting with both walls and including an osmotic
pressure. This work generalises previous work on the RSOS model in the
half-plane which has a solution that was shown recently to exhibit a novel
mathematical structure involving basic hypergeometric functions .
Because of the mathematical relationship between half-plane and slit this work
hence effectively explores the underlying -orthogonal polynomial structure
to that solution. It also generalises two other recent works: one on Dyck paths
weighted with an osmotic pressure in a slit and another concerning Motzkin
paths without an osmotic pressure term in a slit
Monte Carlo Investigation of Lattice Models of Polymer Collapse in Five Dimensions
Monte Carlo simulations, using the PERM algorithm, of interacting
self-avoiding walks (ISAW) and interacting self-avoiding trails (ISAT) in five
dimensions are presented which locate the collapse phase transition in those
models. It is argued that the appearance of a transition (at least) as strong
as a pseudo-first-order transition occurs in both models. The values of various
theoretically conjectured dimension-dependent exponents are shown to be
consistent with the data obtained. Indeed the first-order nature of the
transition is even stronger in five dimensions than four. The agreement with
the theory is better for ISAW than ISAT and it cannot be ruled out that ISAT
have a true first-order transition in dimension five. This latter difference
would be intriguing if true. On the other hand, since simulations are more
difficult for ISAT than ISAW at this transition in high dimensions, any
discrepancy may well be due to the inability of the simulations to reach the
true asymptotic regime.Comment: LaTeX file, 16 pages incl. 7 figure
Stretching of a chain polymer adsorbed at a surface
In this paper we present simulations of a surface-adsorbed polymer subject to
an elongation force. The polymer is modelled by a self-avoiding walk on a
regular lattice. It is confined to a half-space by an adsorbing surface with
attractions for every vertex of the walk visiting the surface, and the last
vertex is pulled perpendicular to the surface by a force. Using the recently
proposed flatPERM algorithm, we calculate the phase diagram for a vast range of
temperatures and forces. The strength of this algorithm is that it computes the
complete density of states from one single simulation. We simulate systems of
sizes up to 256 steps.Comment: 13 pages, 7 figure
Exact Solution of Semi-Flexible and Super-Flexible Interacting Partially Directed Walks
We provide the exact generating function for semi-flexible and super-flexible
interacting partially directed walks and also analyse the solution in detail.
We demonstrate that while fully flexible walks have a collapse transition that
is second order and obeys tricritical scaling, once positive stiffness is
introduced the collapse transition becomes first order. This confirms a recent
conjecture based on numerical results. We note that the addition of an
horizontal force in either case does not affect the order of the transition. In
the opposite case where stiffness is discouraged by the energy potential
introduced, which we denote the super-flexible case, the transition also
changes, though more subtly, with the crossover exponent remaining unmoved from
the neutral case but the entropic exponents changing