3,307 research outputs found
Metastability in the two-dimensional Ising model with free boundary conditions
We investigate metastability in the two dimensional Ising model in a square
with free boundary conditions at low temperatures. Starting with all spins down
in a small positive magnetic field, we show that the exit from this metastable
phase occurs via the nucleation of a critical droplet in one of the four
corners of the system. We compute the lifetime of the metastable phase
analytically in the limit , and via Monte Carlo simulations at
fixed values of and and find good agreement. This system models the
effects of boundary domains in magnetic storage systems exiting from a
metastable phase when a small external field is applied.Comment: 24 pages, TeX fil
A combinatorial proof of tree decay of semi-invariants
We consider finite range Gibbs fields and provide a purely combinatorial
proof of the exponential tree decay of semi--invariants, supposing that the
logarithm of the partition function can be expressed as a sum of suitable local
functions of the boundary conditions. This hypothesis holds for completely
analytical Gibbs fields; in this context the tree decay of semi--invariants has
been proven via analyticity arguments. However the combinatorial proof given
here can be applied also to the more complicated case of disordered systems in
the so called Griffiths' phase when analyticity arguments fail
Renormalization Group in the uniqueness region: weak Gibbsianity and convergence
We analyze the block averaging transformation applied to lattice gas models
with short range interaction in the uniqueness region below the critical
temperature. We prove weak Gibbsianity of the renormalized measure and
convergence of the renormalized potential in a weak sense. Since we are
arbitrarily close to the coexistence region we have a diverging characteristic
length of the system: the correlation length or the critical length for
metastability, or both. Thus, to perturbatively treat the problem we have to
use a scale-adapted expansion. Moreover, such a model below the critical
temperature resembles a disordered system in presence of Griffiths'
singularity. Then the cluster expansion that we use must be graded with its
minimal scale length diverging when the coexistence line is approached
Kink Localization under Asymmetric Double-Well Potential
We study diffuse phase interfaces under asymmetric double-well potential
energies with degenerate minima and demonstrate that the limiting sharp
profile, for small interface energy cost, on a finite space interval is in
general not symmetric and its position depends exclusively on the second
derivatives of the potential energy at the two minima (phases). We discuss an
application of the general result to porous media in the regime of solid-fluid
segregation under an applied pressure and describe the interface between a
fluid-rich and a fluid-poor phase. Asymmetric double-well potential energies
are also relevant in a very different field of physics as that of Brownian
motors. An intriguing analogy between our result and the direction of the dc
soliton current in asymmetric substrate driven Brownian motors is pointed out
Relaxation Height in Energy Landscapes: an Application to Multiple Metastable States
The study of systems with multiple (not necessarily degenerate) metastable
states presents subtle difficulties from the mathematical point of view related
to the variational problem that has to be solved in these cases. We introduce
the notion of relaxation height in a general energy landscape and we prove
sufficient conditions which are valid even in presence of multiple metastable
states. We show how these results can be used to approach the problem of
multiple metastable states via the use of the modern theories of metastability.
We finally apply these general results to the Blume--Capel model for a
particular choice of the parameters ensuring the existence of two multiple, and
not degenerate in energy, metastable states
Superconducting nanowire quantum interference device based on Nb ultrathin films deposited on self-assembled porous Si templates
Magnetoresistance oscillations were observed on networks of superconducting
ultrathin Nb nanowires presenting evidences of either thermal or quantum
activated phase slips. The magnetic transport data, discussed in the framework
of different scenarios, reveal that the system behaves coherently in the
temperature range where the contribution of the fluctuations is important.Comment: accepted for publication on Nanotechnolog
Interface Transparency of Nb/Pd Layered Systems
We have investigated, in the framework of proximity effect theory, the
interface transparency T of superconducting/normal metal layered systems which
consist of Nb and high paramagnetic Pd deposited by dc magnetron sputtering.
The obtained T value is relatively high, as expected by theoretical arguments.
This leads to a large value of the ratio although Pd does
not exhibit any magnetic ordering.Comment: To be published on Eur. Phys. J.
Effect of metal clusters on the swelling of gold-fluorocarbon-polymer composite films
We have investigated the phenomenon of swelling due to acetone diffusion in
fluorocarbon polymer films doped with different gold concentrations below the
percolation threshold. The presence of the gold clusters in the polymer is
shown to improve the mixing between the fluorocarbon polymer and the acetone,
which is not a good solvent for this kind of polymers. In order to explain the
experimental results the stoichiometry and the morphology of the polymer--metal
system have been studied and a modified version of the Flory--Huggins model has
been developed
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