5,118 research outputs found
Directed polymers on a Cayley tree with spatially correlated disorder
In this paper we consider directed walks on a tree with a fixed branching
ratio K at a finite temperature T. We consider the case where each site (or
link) is assigned a random energy uncorrelated in time, but correlated in the
transverse direction i.e. within the shell. In this paper we take the
transverse distance to be the hierarchical ultrametric distance, but other
possibilities are discussed. We compute the free energy for the case of
quenched disorder and show that there is a fundamental difference between the
case of short range spatial correlations of the disorder which behaves
similarly to the non-correlated case considered previously by Derrida and Spohn
and the case of long range correlations which has a totally different overlap
distribution which approaches a single delta function about q=1 for large L,
where L is the length of the walk. In the latter case the free energy is not
extensive in L for the intermediate and also relevant range of L values,
although in the true thermodynamic limit extensivity is restored. We identify a
crossover temperature which grows with L, and whenever T<T_c(L) the system is
always in the low temperature phase. Thus in the case of long-ranged
correlation as opposed to the short-ranged case a phase transition is absent.Comment: 23 pages, 1 figure, standard latex. J. Phys. A, accepted for
publicatio
Implementation of empirically validated interventions in managed care settings: The premarital relationship enhancement program (PREP)
In an age of managed care, how does the clinician best help couples in marital distress? Do the short-term protocols developed and tested in the laboratory really work in the average clinical setting? This project examined the feasibility and effectiveness of implementing a laboratory-based program designed to prevent the development of relationship distress within a health maintenance organization. Both men and women reported high satisfaction with the program and a subjective sense that it was helpful for their relationships. Specific suggestions are made for assisting therapists in using effective treatments for couples in managed care settings
Implementation of the strongly pronounced non-linear viscoelasticity of an incompressible filled rubber
Filled rubber materials regularly show a pronounced non-linear viscoelasticity with very long relaxation times. In this contribution, a phenomenological description for an incompressible carbon black-filled EPDM (ethylene propylene diene monomer) is given, which also shows the abovementioned characteristic behaviour. In order to represent the non-linear viscoelastic material, the relaxation times of the model are chosen not as constant material parameters but as process-dependent functions.
This contribution presents two different realisations of the model’s implementation. At first, this work provides an implementation of the material model, which is able to describe complex geometries and loading conditions. In this realisation, the three-dimensional model is implemented in the open source finite element library deal.II for finite deformations. Hence, real applications can be represented. In an alternative numerical solution, the model is reduced to the single case of uniaxial tension. The model is simplified to scalar equations, which are quite easy to handle for the implementation. This procedure provides a more simple identification process, but it presents the roblem that the model character is extremely restricted for the individual case of uniaxial tension.
For the numerical realisation, at first, special attention has to be turned on the determination of the inelastic part of the kinematics. A detailed evaluation of the necessary evolution equations is provided in this contribution. Finally, he results of the different implementations are compared with respect to different loading conditions, like relaxation tests or cyclic loading
Large Deviations of the Free-Energy in Diluted Mean-Field Spin-Glass
Sample-to-sample free energy fluctuations in spin-glasses display a markedly
different behaviour in finite-dimensional and fully-connected models, namely
Gaussian vs. non-Gaussian. Spin-glass models defined on various types of random
graphs are in an intermediate situation between these two classes of models and
we investigate whether the nature of their free-energy fluctuations is Gaussian
or not. It has been argued that Gaussian behaviour is present whenever the
interactions are locally non-homogeneous, i.e. in most cases with the notable
exception of models with fixed connectivity and random couplings . We confirm these expectation by means of various analytical
results. In particular we unveil the connection between the spatial
fluctuations of the populations of populations of fields defined at different
sites of the lattice and the Gaussian nature of the free-energy fluctuations.
On the contrary on locally homogeneous lattices the populations do not
fluctuate over the sites and as a consequence the small-deviations of the free
energy are non-Gaussian and scales as in the Sherrington-Kirkpatrick model
Domain wall roughening in dipolar films in the presence of disorder
We derive a low-energy Hamiltonian for the elastic energy of a N\'eel domain
wall in a thin film with in-plane magnetization, where we consider the
contribution of the long-range dipolar interaction beyond the quadratic
approximation. We show that such a Hamiltonian is analogous to the Hamiltonian
of a one-dimensional polaron in an external random potential. We use a replica
variational method to compute the roughening exponent of the domain wall for
the case of two-dimensional dipolar interactions.Comment: REVTEX, 35 pages, 2 figures. The text suffered minor changes and
references 1,2 and 12 were added to conform with the referee's repor
Non-Ergodic Dynamics of the 2D Random-phase Sine-Gordon Model: Applications to Vortex-Glass Arrays and Disordered-Substrate Surfaces
The dynamics of the random-phase sine-Gordon model, which describes 2D
vortex-glass arrays and crystalline surfaces on disordered substrates, is
investigated using the self-consistent Hartree approximation. The
fluctuation-dissipation theorem is violated below the critical temperature T_c
for large time t>t* where t* diverges in the thermodynamic limit. While above
T_c the averaged autocorrelation function diverges as Tln(t), for T<T_c it
approaches a finite value q* proportional to 1/(T_c-T) as q(t) = q* -
c(t/t*)^{-\nu} (for t --> t*) where \nu is a temperature-dependent exponent. On
larger time scales t > t* the dynamics becomes non-ergodic. The static
correlations behave as Tln{x} for T>T_c and for T<T_c when x < \xi* with \xi*
proportional to exp{A/(T_c-T)}. For scales x > \xi*, they behave as (T/m)ln{x}
where m is approximately T/T_c near T_c, in general agreement with the
variational replica-symmetry breaking approach and with recent simulations of
the disordered-substrate surface. For strong- coupling the transition becomes
first-order.Comment: 12 pages in LaTeX, Figures available upon request, NSF-ITP 94-10
Phase Transitions of the Flux Line Lattice in High-Temperature Superconductors with Weak Columnar and Point Disorder
We study the effects of weak columnar and point disorder on the
vortex-lattice phase transitions in high temperature superconductors. The
combined effect of thermal fluctuations and of quenched disorder is
investigated using a simplified cage model. For columnar disorder the problem
maps into a quantum particle in a harmonic + random potential. We use the
variational approximation to show that columnar and point disorder have
opposite effect on the position of the melting line as observed experimentally.
Replica symmetry breaking plays a role at the transition into a vortex glass at
low temperatures.Comment: 4 pages in 2 columns format + 2 eps figs included, uses RevTeX and
multicol.st
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