1,675 research outputs found
Dynamics of individual Brownian rods in a microchannel flow
We study the orientational dynamics of heavy silica microrods flowing through
a microfluidic channel. Comparing experiments and Brownian dynamics simulations
we identify different particle orbits, in particular in-plane tumbling
behavior, which cannot be explained by classical Jeffery theory, and we relate
this behavior to the rotational diffusion of the rods. By constructing the
full, three-dimensional, orientation distribution, we describe the rod
trajectories and quantify the persistence of Jeffery orbits using temporal
correlation functions of the Jeffery constant. We find that our colloidal rods
lose memory of their initial configuration in about a second, corresponding to
half a Jeffery period.Comment: 5 pages, 4 figure
Vacuum Polarization and Dynamical Chiral Symmetry Breaking: Phase Diagram of QED with Four-Fermion Contact Interaction
We study chiral symmetry breaking for fundamental charged fermions coupled
electromagnetically to photons with the inclusion of four-fermion contact
self-interaction term. We employ multiplicatively renormalizable models for the
photon dressing function and the electron-photon vertex which minimally ensures
mass anomalous dimension = 1. Vacuum polarization screens the interaction
strength. Consequently, the pattern of dynamical mass generation for fermions
is characterized by a critical number of massless fermion flavors above which
chiral symmetry is restored. This effect is in diametrical opposition to the
existence of criticality for the minimum interaction strength necessary to
break chiral symmetry dynamically. The presence of virtual fermions dictates
the nature of phase transition. Miransky scaling laws for the electromagnetic
interaction strength and the four-fermion coupling, observed for quenched QED,
are replaced by a mean-field power law behavior corresponding to a second order
phase transition. These results are derived analytically by employing the
bifurcation analysis, and are later confirmed numerically by solving the
original non-linearized gap equation. A three dimensional critical surface is
drawn to clearly depict the interplay of the relative strengths of interactions
and number of flavors to separate the two phases. We also compute the
beta-function and observe that it has ultraviolet fixed point. The power law
part of the momentum dependence, describing the mass function, reproduces the
quenched limit trivially. We also comment on the continuum limit and the
triviality of QED.Comment: 9 pages, 10 figure
Magnetic properties and critical behavior of disordered Fe_{1-x}Ru_x alloys: a Monte Carlo approach
We study the critical behavior of a quenched random-exchange Ising model with
competing interactions on a bcc lattice. This model was introduced in the study
of the magnetic behavior of Fe_{1-x}Ru_x alloys for ruthenium concentrations
x=0%, x=4%, x=6%, and x=8%. Our study is carried out within a Monte Carlo
approach, with the aid of a re-weighting multiple histogram technique. By means
of a finite-size scaling analysis of several thermodynamic quantities, taking
into account up to the leading irrelevant scaling field term, we find estimates
of the critical exponents \alpha, \beta, \gamma, and \nu, and of the critical
temperatures of the model. Our results for x=0% are in excellent agreement with
those for the three-dimensional pure Ising model in the literature. We also
show that our critical exponent estimates for the disordered cases are
consistent with those reported for the transition line between paramagnetic and
ferromagnetic phases of both randomly dilute and Ising models. We
compare the behavior of the magnetization as a function of temperature with
that obtained by Paduani and Branco (2008), qualitatively confirming the
mean-field result. However, the comparison of the critical temperatures
obtained in this work with experimental measurements suggest that the model
(initially obtained in a mean-field approach) needs to be modified
An Upsilon Point in a Spin Model
We present analytic evidence for the occurrence of an upsilon point, an
infinite checkerboard structure of modulated phases, in the ground state of a
spin model. The structure of the upsilon point is studied by calculating
interface--interface interactions using an expansion in inverse spin
anisotropy.Comment: 18 pages ReVTeX file, including 6 figures encoded with uufile
Critical behavior of repulsive linear -mers on triangular lattices
Monte Carlo (MC) simulations and finite-size scaling analysis have been
carried out to study the critical behavior in a submonolayer two-dimensional
gas of repulsive linear -mers on a triangular lattice at coverage
. A low-temperature ordered phase, characterized by a repetition of
alternating files of adsorbed -mers separated by adjacent empty sites,
is separated from the disordered state by a order-disorder phase transition
occurring at a finite critical temperature, . The MC technique was
combined with the recently reported Free Energy Minimization Criterion Approach
(FEMCA), [F. Rom\'a et al., Phys. Rev. B, 68, 205407, (2003)], to predict the
dependence of the critical temperature of the order-disorder transformation.
The dependence on of the transition temperature, , observed in MC
is in qualitative agreement with FEMCA. In addition, an accurate determination
of the critical exponents has been obtained for adsorbate sizes ranging between
and . For , the results reveal that the system does not belong
to the universality class of the two-dimensional Potts model with (,
monomers). Based on symmetry concepts, we suggested that the behavior observed
for and 3 could be generalized to include larger particle sizes ().Comment: 17 pages, 13 figure
Destruction of first-order phase transition in a random-field Ising model
The phase transitions that occur in an infinite-range-interaction Ising
ferromagnet in the presence of a double-Gaussian random magnetic field are
analyzed. Such random fields are defined as a superposition of two Gaussian
distributions, presenting the same width . Is is argued that this
distribution is more appropriate for a theoretical description of real systems
than its simpler particular cases, i.e., the bimodal () and the
single Gaussian distributions. It is shown that a low-temperature first-order
phase transition may be destructed for increasing values of , similarly
to what happens in the compound , whose
finite-temperature first-order phase transition is presumably destructed by an
increase in the field randomness.Comment: 13 pages, 3 figure
Glassy states in lattice models with many coexisting crystalline phases
We study the emergence of glassy states after a sudden cooling in lattice
models with short range interactions and without any a priori quenched
disorder. The glassy state emerges whenever the equilibrium model possesses a
sufficient number of coexisting crystalline phases at low temperatures,
provided the thermodynamic limit be taken before the infinite time limit. This
result is obtained through simulations of the time relaxation of the standard
Potts model and some exclusion models equipped with a local stochastic dynamics
on a square lattice.Comment: 12 pages, 4 figure
Sceptical Employees as CSR Ambassadors in Times of Financial Uncertainty
This chapter offers new insights into the understanding of internal (employee) perceptions of organizational corporate social responsibility (CSR) policies and strategies. This study explores the significance of employeesâ involvement and scepticism upon CSR initiatives and focuses on the effects it may have upon word of mouth (WOM) and the development of employeeâorganisation relationships. Desk research introduces the research questions. Data for the research questions were gathered through a self-completion questionnaire distributed in a hardcopy form to the sample. An individualâs level of scepticism and involvement appears to affect the development of a positive effect on employeesâ WOM. Involvement with the domain of the investment may be a central factor affecting relationship building within the organization, and upon generation of positive WOM. The chapter offers a conceptual framework to public relations (PR) and corporate communications practitioners, which may enrich their views and understanding of the use and value of CSR for communication strategies and practices. For-profit organisations are major institutions in todayâs society. CSR is proffered as presenting advantages for (at macro level) society and (micro level) the organization and its employees. Concepts, such as involvement and scepticism, which have not been rigorously examined in PR and corporate communication literature, are addressed. By examining employee perceptions, managers and academic researchers gain insights into the acceptance, appreciation and effectiveness of CSR policies and activities upon the employee stakeholder group. This will affect current and future CSR communication strategies. The knowledge acquired from this chapter may be transferable outside the for-profit sector
Distribution of shortest cycle lengths in random networks
We present analytical results for the distribution of shortest cycle lengths
(DSCL) in random networks. The approach is based on the relation between the
DSCL and the distribution of shortest path lengths (DSPL). We apply this
approach to configuration model networks, for which analytical results for the
DSPL were obtained before. We first calculate the fraction of nodes in the
network which reside on at least one cycle. Conditioning on being on a cycle,
we provide the DSCL over ensembles of configuration model networks with degree
distributions which follow a Poisson distribution (Erdos-R\'enyi network),
degenerate distribution (random regular graph) and a power-law distribution
(scale-free network). The mean and variance of the DSCL are calculated. The
analytical results are found to be in very good agreement with the results of
computer simulations.Comment: 44 pages, 11 figure
Complete wetting in the three-dimensional transverse Ising model
We consider a three-dimensional Ising model in a transverse magnetic field,
and a bulk field . An interface is introduced by an appropriate choice
of boundary conditions. At the point spin configurations
corresponding to different positions of the interface are degenerate. By
studying the phase diagram near this multiphase point using quantum-mechanical
perturbation theory we show that that quantum fluctuations, controlled by ,
split the multiphase degeneracy giving rise to an infinite sequence of layering
transitions.Comment: 16 pages (revtex) including 8 figs; to appear in J. Stat. Phy
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