18,606 research outputs found
On the Melting of Bosonic Stripes
We use quantum Monte Carlo simulations to determine the finite temperature
phase diagram and to investigate the thermal and quantum melting of stripe
phases in a two-dimensional hard-core boson model. At half filling and low
temperatures the stripes melt at a first order transition. In the doped system,
the melting transitions of the smectic phase at high temperatures and the
superfluid smectic (supersolid) phase at low temperatures are either very
weakly first order, or of second order with no clear indications for an
intermediate nematic phase.Comment: 4 pages, 5 figure
Double Entropic Stochastic Resonance
We demonstrate the appearance of a purely entropic stochastic resonance (ESR)
occurring in a geometrically confined system, where the irregular boundaries
cause entropic barriers. The interplay between a periodic input signal, a
constant bias and intrinsic thermal noise leads to a resonant ESR-phenomenon in
which feeble signals become amplified. This new phenomenon is characterized by
the presence of two peaks in the spectral amplification at corresponding
optimal values of the noise strength. The main peak is associated with the
manifest stochastic resonance synchronization mechanism involving the
inter-well noise-activated dynamics while a second peak relates to a regime of
optimal sensitivity for intra-well dynamics. The nature of ESR, occurring when
the origin of the barrier is entropic rather than energetic, offers new
perspectives for novel investigations and potential applications. ESR by itself
presents yet another case where one constructively can harvest noise in driven
nonequilibrium systems.Comment: 6 pages, 7 figures ; Europhys. Lett., in press (2009
Searching atomic spin contrast on nickel oxide (001) by force microscopy
The (001) surface of NiO, an antiferromagnet at room temperature, was
investigated under ultra-high vacuum conditions with frequency modulation
atomic force microscopy (FM-AFM). The antiferromagnetic coupling between ions
leads to a spin superstructure on (001) surfaces. Exchange interaction between
the probe of a force microscope and the NiO (001) surface should allow to image
spin superstructures in real space. The surface was imaged with three different
probing tips: nonmagnetic W tips, ferromagnetic Co tips and antiferromagnetic
NiO tips - and atomic resolution was achieved with all three of them in various
distance regimes and in several channels. Evidence for spin contrast was
obtained in experiments that utilize NiO tips and oscillation amplitudes in the
\AA-regime, where optimal signal-to-noise ratio is expected. The spin contrast
is weaker than expected and only visible in Fourier space images.Comment: 7 pages, 6 figures, submitted to Physical Review
Entropic stochastic resonance: the constructive role of the unevenness
We demonstrate the existence of stochastic resonance (SR) in confined systems
arising from entropy variations associated to the presence of irregular
boundaries. When the motion of a Brownian particle is constrained to a region
with uneven boundaries, the presence of a periodic input may give rise to a
peak in the spectral amplification factor and therefore to the appearance of
the SR phenomenon. We have proved that the amplification factor depends on the
shape of the region through which the particle moves and that by adjusting its
characteristic geometric parameters one may optimize the response of the
system. The situation in which the appearance of such entropic stochastic
resonance (ESR) occurs is common for small-scale systems in which confinement
and noise play an prominent role. The novel mechanism found could thus
constitute an important tool for the characterization of these systems and can
put to use for controlling their basic properties.Comment: 8 pages, 8 figure
Biased diffusion in confined media: Test of the Fick-Jacobs approximation and validity criteria
We study biased, diffusive transport of Brownian particles through narrow,
spatially periodic structures in which the motion is constrained in lateral
directions. The problem is analyzed under the perspective of the Fick-Jacobs
equation which accounts for the effect of the lateral confinement by
introducing an entropic barrier in a one dimensional diffusion. The validity of
this approximation, being based on the assumption of an instantaneous
equilibration of the particle distribution in the cross-section of the
structure, is analyzed by comparing the different time scales that characterize
the problem. A validity criterion is established in terms of the shape of the
structure and of the applied force. It is analytically corroborated and
verified by numerical simulations that the critical value of the force up to
which this description holds true scales as the square of the periodicity of
the structure. The criterion can be visualized by means of a diagram
representing the regions where the Fick-Jacobs description becomes inaccurate
in terms of the scaled force versus the periodicity of the structure.Comment: 20 pages, 7 figure
Understanding the dynamical structure of pulsating stars. HARPS spectroscopy of the delta Scuti stars rho Pup and DX Cet
High-resolution spectroscopy is a powerful tool to study the dynamical
structure of pulsating stars atmosphere. We aim at comparing the line asymmetry
and velocity of the two delta Sct stars rho Pup and DX Cet with previous
spectroscopic data obtained on classical Cepheids and beta Cep stars. We
obtained, analysed and discuss HARPS high-resolution spectra of rho Pup and DX
Cet. We derived the same physical quantities as used in previous studies, which
are the first-moment radial velocities and the bi-Gaussian spectral line
asymmetries. The identification of f=7.098 (1/d) as a fundamental radial mode
and the very accurate Hipparcos parallax promote rho Pup as the best standard
candle to test the period-luminosity relations of delta Sct stars. The action
of small-amplitude nonradial modes can be seen as well-defined cycle-to-cycle
variations in the radial velocity measurements of rho Pup. Using the
spectral-line asymmetry method, we also found the centre-of-mass velocities of
rho Pup and DX Cet, V_gamma = 47.49 +/- 0.07 km/s and V_gamma = 25.75 +/- 0.06
km/s, respectively. By comparing our results with previous HARPS observations
of classical Cepheids and beta Cep stars, we confirm the linear relation
between the atmospheric velocity gradient and the amplitude of the radial
velocity curve, but only for amplitudes larger than 22.5 km/s. For lower values
of the velocity amplitude (i.e., < 22.5 km/s), our data on rho Pup seem to
indicate that the velocity gradient is null, but this result needs to be
confirmed with additional data. We derived the Baade-Wesselink projection
factor p = 1.36 +/- 0.02 for rho Pup and p = 1.39 +/- 0.02 for DX Cet. We
successfully extended the period-projection factor relation from classical
Cepheids to delta Scuti stars.Comment: Accepted for publication in A&A (in press
Spin nematics in the bilinear-biquadratic S=1 spin chain
We report the existence of an extended critical, nondimerized region in the
phase diagram of the bilinear-biquadratic spin-one chain. The dominant power
law correlations are ferroquadrupolar, i.e. spin nematic in character. Another
known critical region is also characterized by dominant quadrupolar
correlations, although with a different wave vector. Our results show that spin
nematic correlations play an important role in quantum magnets with spin S >= 1
in regions between antiferromagnetic and ferromagnetic phases.Comment: 4 pages, 7 figure
Nonverbal communication in health settings
The goal of the present chapter is to provide an overview of the existing literature on nonverbal communication in the medical encounter. After introducing the different functions of nonverbal behavior and its importance in the medical context, we present empirical evidence showing how physician nonverbal behavior relates to different patient outcomes such as satisfaction, trust, or adherence. We then present different models and assessment tools with which nonverbal behavior in the medical encounter can be studied. The physician's ability to pick up and correctly interpret the patient's nonverbal cues (interpersonal sensitivity) and its impact on patient outcomes will be reviewed. We close the chapter by providing a summary of the main results and an outlook on open questions in the field
A variational framework for flow optimization using semi-norm constraints
When considering a general system of equations describing the space-time
evolution (flow) of one or several variables, the problem of the optimization
over a finite period of time of a measure of the state variable at the final
time is a problem of great interest in many fields. Methods already exist in
order to solve this kind of optimization problem, but sometimes fail when the
constraint bounding the state vector at the initial time is not a norm, meaning
that some part of the state vector remains unbounded and might cause the
optimization procedure to diverge. In order to regularize this problem, we
propose a general method which extends the existing optimization framework in a
self-consistent manner. We first derive this framework extension, and then
apply it to a problem of interest. Our demonstration problem considers the
transient stability properties of a one-dimensional (in space) averaged
turbulent model with a space- and time-dependent model "turbulent viscosity".
We believe this work has a lot of potential applications in the fluid
dynamics domain for problems in which we want to control the influence of
separate components of the state vector in the optimization process.Comment: 30 page
Observation of asymmetric spectrum broadening induced by silver nanoparticles in a heavy-metal oxide glass
We demonstrate experimentally and support by a theoretical analysis an effect
of asymmetric spectrum broadening, which results from doping of silver
nanoparticles into a heavy-glass matrix, 90(0.5WO3-0.3SbPO4-0.2PbO)-10AgCl. The
strong dispersion of the effective nonlinear coefficient of the composite
significantly influences the spectral broadening via the self-phase modulation,
and leads to a blue upshift of the spectrum. Further extension of the spectrum
towards shorter wavelengths is suppressed by a growing loss caused by the
plasmon resonance in the silver particles. The red-edge spectral broadening is
dominated by the stimulated Raman Scattering.Comment: Accepted for publishing epl13477; EPL Journal 201
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