1,933 research outputs found
Preroughening transitions in a model for Si and Ge (001) type crystal surfaces
The uniaxial structure of Si and Ge (001) facets leads to nontrivial
topological properties of steps and hence to interesting equilibrium phase
transitions. The disordered flat phase and the preroughening transition can be
stabilized without the need for step-step interactions. A model describing this
is studied numerically by transfer matrix type finite-size-scaling of interface
free energies. Its phase diagram contains a flat, rough, and disordered flat
phase, separated by roughening and preroughening transition lines. Our estimate
for the location of the multicritical point where the preroughening line merges
with the roughening line, predicts that Si and Ge (001) undergo preroughening
induced simultaneous deconstruction transitions.Comment: 13 pages, RevTex, 7 Postscript Figures, submitted to J. Phys.
Reconstructed Rough Growing Interfaces; Ridgeline Trapping of Domain Walls
We investigate whether surface reconstruction order exists in stationary
growing states, at all length scales or only below a crossover length, . The later would be similar to surface roughness in growing crystal
surfaces; below the equilibrium roughening temperature they evolve in a
layer-by-layer mode within a crossover length scale , but are always
rough at large length scales. We investigate this issue in the context of KPZ
type dynamics and a checker board type reconstruction, using the restricted
solid-on-solid model with negative mono-atomic step energies. This is a
topology where surface reconstruction order is compatible with surface
roughness and where a so-called reconstructed rough phase exists in
equilibrium. We find that during growth, reconstruction order is absent in the
thermodynamic limit, but exists below a crossover length , and that this local order fluctuates critically. Domain walls become
trapped at the ridge lines of the rough surface, and thus the reconstruction
order fluctuations are slaved to the KPZ dynamics
Can exercise limits prevent post-exertional malaise in chronic fatigue syndrome? An uncontrolled clinical trial.
<b>Objective</b>: It was hypothesized that the use of exercise limits prevents symptom increases and worsening of their health status following a walking exercise in people with Chronic Fatigue Syndrome (CFS).
<b>Design</b>: An uncontrolled clinical trial (semi-experimental design).
<b>Setting</b>: Outpatient clinic of a university department.
<b>Subjects</b>: 24 patients with CFS.
<b>Interventions</b>: Subjects undertook a walking test with the two concurrent exercise limits. Each subject walked at an <i>intensity</i> where the maximum heart rate was determined by heart rate corresponding to the respiratory exchange ratio =1.0 derived from a previous sub-maximal exercise test and for a duration calculated from how long each patient felt they were able to walk.
<b>Main outcome measures</b>: The Short Form 36 Health Survey or SF-36, the CFS Symptom List, and the CFS-Activities and Participation Questionnaire were filled in prior to, immediately and 24 hours post-exercise.
<b>Results</b>: The fatigue increase observed immediately post-exercise (p=0.006) returned to pre-exercise levels 24 hours post-exercise. The increase in pain observed immediately post-exercise was retained at 24 hours post-exercise (p=0.03). Fourteen of 24 subjects experienced a clinically meaningful change in bodily pain (change of SF-36 bodily pain score ³10). Six of 24 participants indicated that the exercise bout had slightly worsened their health status, and 2 of 24 had a clinically meaningful decrease in vitality (change of SF-36 vitality score ³20). There was no change in activity limitations/participation restrictions.
<b>Conclusion</b>: It was shown that the use of exercise limits (limiting both the intensity and duration of exercise) prevents important health status changes following a walking exercise in people with CFS, but was unable to prevent short-term symptom increases
Dynamic instability transitions in 1D driven diffusive flow with nonlocal hopping
One-dimensional directed driven stochastic flow with competing nonlocal and
local hopping events has an instability threshold from a populated phase into
an empty-road (ER) phase. We implement this in the context of the asymmetric
exclusion process. The nonlocal skids promote strong clustering in the
stationary populated phase. Such clusters drive the dynamic phase transition
and determine its scaling properties. We numerically establish that the
instability transition into the ER phase is second order in the regime where
the entry point reservoir controls the current and first order in the regime
where the bulk is in control. The first order transition originates from a
turn-about of the cluster drift velocity. At the critical line, the current
remains analytic, the road density vanishes linearly, and fluctuations scale as
uncorrelated noise. A self-consistent cluster dynamics analysis explains why
these scaling properties remain that simple.Comment: 11 pages, 14 figures (25 eps files); revised as the publised versio
Preroughening, Diffusion, and Growth of An FCC(111) Surface
Preroughening of close-packed fcc(111) surfaces, found in rare gas solids, is
an interesting, but poorly characterized phase transition. We introduce a
restricted solid-on-solid model, named FCSOS, which describes it. Using mostly
Monte Carlo, we study both statics, including critical behavior and scattering
properties, and dynamics, including surface diffusion and growth. In antiphase
scattering, it is shown that preroughening will generally show up at most as a
dip. Surface growth is predicted to be continuous at preroughening, where
surface self-diffusion should also drop. The physical mechanism leading to
preroughening on rare gas surfaces is analysed, and identified in the step-step
elastic repulsion.Comment: Revtex + uuencoded figures, to appear in Physical Review Letter
Roughening Induced Deconstruction in (100) Facets of CsCl Type Crystals
The staggered 6-vertex model describes the competition between surface
roughening and reconstruction in (100) facets of CsCl type crystals. Its phase
diagram does not have the expected generic structure, due to the presence of a
fully-packed loop-gas line. We prove that the reconstruction and roughening
transitions cannot cross nor merge with this loop-gas line if these degrees of
freedom interact weakly. However, our numerical finite size scaling analysis
shows that the two critical lines merge along the loop-gas line, with strong
coupling scaling properties. The central charge is much larger than 1.5 and
roughening takes place at a surface roughness much larger than the conventional
universal value. It seems that additional fluctuations become critical
simultaneously.Comment: 31 pages, 9 figure
An exact universal amplitude ratio for percolation
The universal amplitude ratio for percolation in two
dimensions is determined exactly using results for the dilute A model in regime
1, by way of a relationship with the q-state Potts model for q<4.Comment: 5 pages, LaTeX, submitted to J. Phys. A. One paragraph rewritten to
correct error
Quantum Hall Transition in the Classical Limit
We study the quantum Hall transition using the density-density correlation
function. We show that in the limit h->0 the electron density moves along the
percolating trajectories, undergoing normal diffusion. The localization
exponent coincides with its percolation value \nu=4/3. The framework provides a
natural way to study the renormalization group flow from percolation to quantum
Hall transition. We also confirm numerically that the critical conductivity of
a classical limit of quantum Hall transition is \sigma_{xx} = \sqrt{3}/4.Comment: 8 pages, 4 figures; substantial changes include the critical
conductivity calculatio
Crossover Scaling Functions in One Dimensional Dynamic Growth Models
The crossover from Edwards-Wilkinson () to KPZ () type growth is
studied for the BCSOS model. We calculate the exact numerical values for the
and massgap for using the master equation. We predict
the structure of the crossover scaling function and confirm numerically that
and , with . KPZ type growth is
equivalent to a phase transition in meso-scopic metallic rings where attractive
interactions destroy the persistent current; and to endpoints of facet-ridges
in equilibrium crystal shapes.Comment: 11 pages, TeX, figures upon reques
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