14,027 research outputs found
Dissipative dynamics of a two - level system resonantly coupled to a harmonic mode
We propose an approximation scheme to describe the dynamics of the spin-boson
model when the spectral density of the environment shows a peak at a
characteristic frequency which can be very close (or even equal) to
the spin Zeeman frequency . Mapping the problem onto a two-state system
(TSS) coupled to a harmonic oscillator (HO) with frequency we show
that the representation of displaced HO states provides an appropriate basis to
truncate the Hilbert space of the TSS-HO system and therefore a better picture
of the system dynamics. We derive an effective Hamiltonian for the TSS-HO
system, and show it furnishes a very good approximation for the system dynamics
even when its two subsystems are moderately coupled. Finally, assuming the
regime of weak HO-bath coupling and low temperatures, we are able to
analytically evaluate the dissipative TSS dynamics.Comment: 12 pages, 2 figures; V2: Published versio
On the dependence of the avalanche angle on the granular layer thickness
A layer of sand of thickness h flows down a rough surface if the inclination
is larger than some threshold value theta which decreases with h. A tentative
microscopic model for the dependence of theta with h is proposed for rigid
frictional grains, based on the following hypothesis: (i) a horizontal layer of
sand has some coordination z larger than a critical value z_c where mechanical
stability is lost (ii) as the tilt angle is increased, the configurations
visited present a growing proportion $_s of sliding contacts. Instability with
respect to flow occurs when z-z_s=z_c. This criterion leads to a prediction for
theta(h) in good agreement with empirical observations.Comment: 6 pages, 2 figure
The distribution of forces affects vibrational properties in hard sphere glasses
We study theoretically and numerically the elastic properties of hard sphere
glasses, and provide a real-space description of their mechanical stability. In
contrast to repulsive particles at zero-temperature, we argue that the presence
of certain pairs of particles interacting with a small force soften elastic
properties. This softening affects the exponents characterizing elasticity at
high pressure, leading to experimentally testable predictions. Denoting
the force distribution of such pairs and the
packing fraction at which pressure diverges, we predict that (i) the density of
states has a low-frequency peak at a scale , rising up to it as
, and decaying above as where and is the frequency,
(ii) shear modulus and mean-squared displacement are inversely proportional
with where
, and (iii) continuum elasticity breaks down on a
scale where
and , where is the
coordination and the spatial dimension. We numerically test (i) and provide
data supporting that in our bi-disperse system,
independently of system preparation in two and three dimensions, leading to
, , and . Our results for the
mean-square displacement are consistent with a recent exact replica computation
for , whereas some observations differ, as rationalized by the
present approach.Comment: 5 pages + 4 pages supplementary informatio
On the rigidity of a hard sphere glass near random close packing
We study theoretically and numerically the microscopic cause of the
mechanical stability of hard sphere glasses near their maximum packing. We show
that, after coarse-graining over time, the hard sphere interaction can be
described by an effective potential which is exactly logarithmic at the random
close packing . This allows to define normal modes, and to apply recent
results valid for elastic networks: mechanical stability is a non-local
property of the packing geometry, and is characterized by some length scale
which diverges at [1, 2]. We compute the scaling of the bulk and
shear moduli near , and speculate on the possible implications of these
results for the glass transition.Comment: 7 pages, 4 figures. Figure 4 had a wrong unit in abscissa, which was
correcte
Four-dimensional gravity on supersymmetric dilatonic domain walls
We investigate the localization of four-dimensional metastable gravity in
supersymmetric dilatonic domain walls through massive modes by considering
several scenarios in the model. We compute corrections to the Newtonian
potential for small and long distances compared with a crossover scale given in
terms of the dilatonic coupling. 4D gravity behavior is developed on the brane
for distance very much below the crossover scale, while for distance much
larger, the 5D gravity is recovered. Whereas in the former regime gravity is
always attractive, in the latter regime due to non-normalizable unstable
massive graviton modes present on the spectrum, in some special cases, gravity
appears to be repulsive and signalizes a gravitational confining phase which is
able to produce an inflationary phase of the Universe.Comment: 11 pages, 4 figures, Latex. Version to appear in PL
On the Modeling of Droplet Evaporation on Superhydrophobic Surfaces
When a drop of water is placed on a rough surface, there are two possible
extreme regimes of wetting: the one called Cassie-Baxter (CB) with air pockets
trapped underneath the droplet and the one characterized by the homogeneous
wetting of the surface, called the Wenzel (W) state. A way to investigate the
transition between these two states is by means of evaporation experiments, in
which the droplet starts in a CB state and, as its volume decreases, penetrates
the surface's grooves, reaching a W state. Here we present a theoretical model
based on the global interfacial energies for CB and W states that allows us to
predict the thermodynamic wetting state of the droplet for a given volume and
surface texture. We first analyze the influence of the surface geometric
parameters on the droplet's final wetting state with constant volume, and show
that it depends strongly on the surface texture. We then vary the volume of the
droplet keeping fixed the geometric surface parameters to mimic evaporation and
show that the drop experiences a transition from the CB to the W state when its
volume reduces, as observed in experiments. To investigate the dependency of
the wetting state on the initial state of the droplet, we implement a cellular
Potts model in three dimensions. Simulations show a very good agreement with
theory when the initial state is W, but it disagrees when the droplet is
initialized in a CB state, in accordance with previous observations which show
that the CB state is metastable in many cases. Both simulations and theoretical
model can be modified to study other types of surface.Comment: 23 pages, 7 figure
Structure and dynamics in glass-formers: predictability at large length scales
Dynamic heterogeneity in glass-formers has been related to their static
structure using the concept of dynamic propensity. We re-examine this
relationship by analyzing dynamical fluctuations in two atomistic glass-formers
and two theoretical models. We introduce quantitative statistical indicators
which show that the dynamics of individual particles cannot be predicted on the
basis of the propensity, nor by any structural indicator. However, the spatial
structure of the propensity field does have predictive power for the spatial
correlations associated with dynamic heterogeneity. Our results suggest that
the quest for a connection between static and dynamic properties of
glass-formers at the particle level is vain, but they demonstrate that such
connection does exist on larger length scales.Comment: 7 pages; 4 figs - Extended, clarified versio
Innovation-based Nets as Collective Actors: A Heterarchization Case Study from the Automotive Industry
Cooperation and collaboration between companies represents a key issue within the conceptual framework developed by the IMP Group. However, little attention has been paid to a phenomenon which can result from such collaboration, i.e. collective action. This involves cooperative activities undertaken by a significant number of actors sharing a common aim. This research uses the concept of issue-based net to open new avenues to understand collective action in the context of innovation activities, specifically by analyzing a case study of an innovation-based net in the automotive industry. Two main objectives are addressed in this study: Related to this discussion of different development paths of collective actors, the case study analysis focuses on how issue-based nets emerge and evolve in situations of innovation, specifically, what kind of structure and process issues characterize a heterarchization development path. Furthermore, the analysis addressed how issue-based nets change the positioning of individual member firms, a well as that of the collective actor within the overall network.Innovation, collective actor, issue-based nets, heterarchization, case study, automotive industry
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