409 research outputs found
Phase transitions in generalized chiral or Stiefel's models
We study the phase transition in generalized chiral or Stiefel's models using
Monte Carlo simulations. These models are characterized by a breakdown of
symmetry O(N)/O(N-P). We show that the phase transition is clearly first order
for N >= 3 when P=N and P=N-1, contrary to predictions based on the
Renormalization Group in 4-\epsilon expansion but in agreement with a recent
non perturbative Renormalization Group approach.Comment: 9 pages, 8 figure
First and second order transition of frustrated Heisenberg spin systems
Starting from the hypothesis of a second order transition we have studied
modifications of the original Heisenberg antiferromagnet on a stacked
triangular lattice (STA-model) by the Monte Carlo technique. The change is a
local constraint restricting the spins at the corners of selected triangles to
add up to zero without stopping them from moving freely (STAR-model). We have
studied also the closely related dihedral and trihedral models which can be
classified as Stiefel models. We have found indications of a first order
transition for all three modified models instead of a universal critical
behavior. This is in accordance with the renormalization group investigations
but disagrees with the Monte Carlo simulations of the original STA-model
favoring a new universality class. For the corresponding x-y antiferromagnet
studied before, the second order nature of the transition could also not be
confirmed.Comment: 31 pages, 13 figures, to be published in Euro. J. Phys.
Frustrated magnets in three dimensions: a nonperturbative approach
Frustrated magnets exhibit unusual critical behaviors: they display scaling
laws accompanied by nonuniversal critical exponents. This suggests that these
systems generically undergo very weak first order phase transitions. Moreover,
the different perturbative approaches used to investigate them are in conflict
and fail to correctly reproduce their behavior. Using a nonperturbative
approach we explain the mismatch between the different perturbative approaches
and account for the nonuniversal scaling observed.Comment: 7 pages, 1 figure. IOP style files included. To appear in Journal of
Physics: Condensed Matter. Proceedings of the conference HFM 2003, Grenoble,
Franc
Critical behavior of frustrated systems: Monte Carlo simulations versus Renormalization Group
We study the critical behavior of frustrated systems by means of Pade-Borel
resummed three-loop renormalization-group expansions and numerical Monte Carlo
simulations. Amazingly, for six-component spins where the transition is second
order, both approaches disagree. This unusual situation is analyzed both from
the point of view of the convergence of the resummed series and from the
possible relevance of non perturbative effects.Comment: RevTex, 10 pages, 3 Postscript figure
Pores in Bilayer Membranes of Amphiphilic Molecules: Coarse-Grained Molecular Dynamics Simulations Compared with Simple Mesoscopic Models
We investigate pores in fluid membranes by molecular dynamics simulations of
an amphiphile-solvent mixture, using a molecular coarse-grained model. The
amphiphilic membranes self-assemble into a lamellar stack of amphiphilic
bilayers separated by solvent layers. We focus on the particular case of
tension less membranes, in which pores spontaneously appear because of thermal
fluctuations. Their spatial distribution is similar to that of a random set of
repulsive hard discs. The size and shape distribution of individual pores can
be described satisfactorily by a simple mesoscopic model, which accounts only
for a pore independent core energy and a line tension penalty at the pore
edges. In particular, the pores are not circular: their shapes are fractal and
have the same characteristics as those of two dimensional ring polymers.
Finally, we study the size-fluctuation dynamics of the pores, and compare the
time evolution of their contour length to a random walk in a linear potential
The critical behavior of frustrated spin models with noncollinear order
We study the critical behavior of frustrated spin models with noncollinear
order, including stacked triangular antiferromagnets and helimagnets. For this
purpose we compute the field-theoretic expansions at fixed dimension to six
loops and determine their large-order behavior. For the physically relevant
cases of two and three components, we show the existence of a new stable fixed
point that corresponds to the conjectured chiral universality class. This
contradicts previous three-loop field-theoretical results but is in agreement
with experiments.Comment: 4 pages, RevTe
- …