665 research outputs found
Contact forces in regular 3D granular pile
We present exact results for the contact forces in a three dimensional static
piling of identical, stiff and frictionless spheres. The pile studied is a
pyramid of equilateral triangular base (``stack of cannonballs'') with a FCC
(face centered cubic) structure. We show in particular that, as for the two
dimensional case, the pressure on the base of such a pile is uniform.Comment: 10 pages, 5 figure
Incipient order in the t-J model at high temperatures
We analyze the high-temperature behavior of the susceptibilities towards a
number of possible ordered states in the t-J-V model using the high-temperature
series expansion. From all diagrams with up to ten edges, reliable results are
obtained down to temperatures of order J, or (with some optimism) to J/2. In
the unphysical regime, t<J, large superconducting susceptibilities are found,
which moreover increase with decreasing temperatures, but for t>J, these
susceptibilities are small and decreasing with decreasing temperature; this
suggests that the t-J model does not support high-temperature
superconductivity. We also find modest evidence of a tendency toward nematic
and d-density wave orders.
ERRATUM: Due to an error in the calculation, the series for d-wave
supeconducting and extended s-wave superconducting orders were incorrect. We
recalculate the series and give the replacement figures. In agreement with our
earlier findings, we still find no evidence of any strong enhancement of the
superconducting susceptibility with decreasing temperature. However, because
different Pade approximants diverge from each other at somewhat higher
temperatures than we originally found, it is less clear what this implies
concerning the presence or absence of high-temperature superconductivity in the
t-J model.Comment: 4 pages, 5 eps figures included; ERRATUM 2 pages, 3 eps figures
correcting the error in the series for superconducting susceptibilitie
Stripes Disorder and Correlation lengths in doped antiferromagnets
For stripes in doped antiferromagnets, we find that the ratio of spin and
charge correlation lenghts, , provide a sharp criterion for
determining the dominant form of disorder in the system. If stripes disorder is
controlled by topological defects then . In contast,
if stripes correlations are disordered primarily by non-topological elastic
deformations (i.e., a Bragg-Glass type of disorder) then is expected. Therefore, the observation of in and in invariably implies that the stripes
are in a Bragg glass type state, and topological defects are much less relevant
than commonly assumed. Expected spectral properties are discussed. Thus, we
establish the basis for any theoretical analysis of the experimentally
obsereved glassy state in these material.Comment: 4 pages, 2 figure
Quasi-rigidity: some uniqueness issues
Quasi-rigidity means that one builds a theory for assemblies of grains under
a slowly changing external load by using the deformation of those grains as a
small parameter. Is quasi-rigidity a complete theory for these granular
assemblies? Does it provide unique predictions of the assembly's behavior, or
must some other process be invoked to decide between several possibilities? We
provide evidence that quasi-rigidity is a complete theory by showing that two
possible sources of indeterminacy do not exist for the case of disk shaped
grains. One possible source of indeterminacy arises from zero-frequency modes
present in the packing. This problem can be solved by considering the
conditions required to obtain force equilibrium. A second possible source of
indeterminacy is the necessity to choose the status (sliding or non-sliding) at
each contact. We show that only one choice is permitted, if contacts slide only
when required by Coulomb friction.Comment: 14 pages, 3 figures, submitted to Phys Rev E (introduction and
conclusion revised
On the principal bifurcation branch of a third order nonlinear long-wave equation
We study the principal bifurcation curve of a third order equation which
describes the nonlinear evolution of several systems with a long--wavelength
instability. We show that the main bifurcation branch can be derived from a
variational principle. This allows to obtain a close estimate of the complete
branch. In particular, when the bifurcation is subcritical, the large amplitude
stable branch can be found in a simple manner.Comment: 11 pages, 3 figure
Dewetting of thin polymer films near the glass transition
Dewetting of ultra-thin polymer films near the glass transition exhibits
unexpected front morphologies [G. Reiter, Phys. Rev. Lett., 87, 186101 (2001)].
We present here the first theoretical attempt to understand these features,
focusing on the shear-thinning behaviour of these films. We analyse the profile
of the dewetting film, and characterize the time evolution of the dry region
radius, , and of the rim height, . After a transient time
depending on the initial thickness, grows like while
increases like . Different regimes of growth are
expected, depending on the initial film thickness and experimental time range.Comment: 4 pages, 5 figures Revised version, published in Physical Review
Letters: F. Saulnier, E. Raphael and P.-G. de Gennes, Phys. Rev. Lett. 88,
196101 (2002
Noise auto-correlation spectroscopy with coherent Raman scattering
Ultrafast lasers have become one of the most powerful tools in coherent
nonlinear optical spectroscopy. Short pulses enable direct observation of fast
molecular dynamics, whereas broad spectral bandwidth offers ways of controlling
nonlinear optical processes by means of quantum interferences. Special care is
usually taken to preserve the coherence of laser pulses as it determines the
accuracy of a spectroscopic measurement. Here we present a new approach to
coherent Raman spectroscopy based on deliberately introduced noise, which
increases the spectral resolution, robustness and efficiency. We probe laser
induced molecular vibrations using a broadband laser pulse with intentionally
randomized amplitude and phase. The vibrational resonances result in and are
identified through the appearance of intensity correlations in the noisy
spectrum of coherently scattered photons. Spectral resolution is neither
limited by the pulse bandwidth, nor sensitive to the quality of the temporal
and spectral profile of the pulses. This is particularly attractive for the
applications in microscopy, biological imaging and remote sensing, where
dispersion and scattering properties of the medium often undermine the
applicability of ultrafast lasers. The proposed method combines the efficiency
and resolution of a coherent process with the robustness of incoherent light.
As we demonstrate here, it can be implemented by simply destroying the
coherence of a laser pulse, and without any elaborate temporal scanning or
spectral shaping commonly required by the frequency-resolved spectroscopic
methods with ultrashort pulses.Comment: To appear in Nature Physic
Dewetting of Glassy Polymer Films
Dynamics and morphology of hole growth in a film of power hardening
viscoplastic solid (yield stress ~ [strain-rate]^n) is investigated. At
short-times the growth is exponential and depends on the initial hole size. At
long-times, for n > 1/3, the growth is exponential with a different exponent.
However, for n < 1/3, the hole growth slows; the hole radius approaches an
asymptotic value as time tends to infinity. The rim shape is highly asymmetric,
the height of which has a power law dependence on the hole radius (exponent
close to unity for 0.25 < n < 0.4). The above results explain recent intriguing
experiments of Reiter, Phys. Rev. Lett, 87, 186101 (2001).Comment: 4 pages, 5 figures, RevTe
The reversible polydisperse Parking Lot Model
We use a new version of the reversible Parking Lot Model to study the
compaction of vibrated polydisperse media. The particle sizes are distributed
according to a truncated power law. We introduce a self-consistent desorption
mechanism with a hierarchical initialization of the system. In this way, we
approach densities close to unity. The final density depends on the
polydispersity of the system as well as on the initialization and will reach a
maximum value for a certain exponent in the power law.Comment: 7 pages, Latex, 12 figure
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