501 research outputs found
Phase space gaps and ergodicity breaking in systems with long range interactions
We study a generalized isotropic XY-model which includes both two-spin and
four-spin mean-field interactions. This model can be solved in the
microcanonical ensemble. It is shown that in certain parameter regions the
model exhibits gaps in the magnetization at fixed energy, resulting in
ergodicity breaking. This phenomenon has previously been reported in
anisotropic and discrete spin models. The entropy of the model is calculated
and the microcanonical phase diagram is derived, showing the existence of first
order phase transitions from the ferromagnetic to a paramagnetic disordered
phase. It is found that ergodicity breaking takes place both in the
ferromagnetic and the paramagnetic phases. As a consequence, the system can
exhibit a stable ferromagnetic phase within the paramagnetic region, and
conversely a disordered phase within the magnetically ordered region
Finite-size effects on the Hamiltonian dynamics of the XY-model
The dynamical properties of the finite-size magnetization M in the critical
region T<T_{KTB} of the planar rotor model on a L x L square lattice are
analyzed by means of microcanonical simulations . The behavior of the q=0
structure factor at high frequencies is consistent with field-theoretical
results, but new additional features occur at lower frequencies. The motion of
M determines a region of spectral lines and the presence of a central peak,
which we attribute to phase diffusion. Near T_{KTB} the diffusion constant
scales with system size as D ~ L^{-1.6(3)}.Comment: To be published in Europhysics Letter
Long-Range Effects in Layered Spin Structures
We study theoretically layered spin systems where long-range dipolar
interactions play a relevant role. By choosing a specific sample shape, we are
able to reduce the complex Hamiltonian of the system to that of a much simpler
coupled rotator model with short-range and mean-field interactions. This latter
model has been studied in the past because of its interesting dynamical and
statistical properties related to exotic features of long-range interactions.
It is suggested that experiments could be conducted such that within a specific
temperature range the presence of long-range interactions crucially affect the
behavior of the system
Ensemble inequivalence in systems with long-range interactions
Ensemble inequivalence has been observed in several systems. In particular it
has been recently shown that negative specific heat can arise in the
microcanonical ensemble in the thermodynamic limit for systems with long-range
interactions. We display a connection between such behaviour and a mean-field
like structure of the partition function. Since short-range models cannot
display this kind of behaviour, this strongly suggests that such systems are
necessarily non-mean field in the sense indicated here. We illustrate our
results showing an application to the Blume-Emery-Griffiths model. We further
show that a broad class of systems with non-integrable interactions are indeed
of mean-field type in the sense specified, so that they are expected to display
ensemble inequivalence as well as the peculiar behaviour described above in the
microcanonical ensemble.Comment: 12 pages, no figure
Obese patients with a binge eating disorder have an unfavorable metabolic and inflammatory profile
To evaluate whether obese patients with a binge eating disorder (BED) have an altered metabolic and inflammatory profile related to their eating behaviors compared with non-BED obese.A total of 115 White obese patients consecutively recruited underwent biochemical, anthropometrical evaluation, and a 75-g oral glucose tolerance test. Patients answered the Binge Eating Scale and were interviewed by a psychiatrist. The patients were subsequently divided into 2 groups according to diagnosis: non-BED obese (n = 85) and BED obese (n = 30). Structural equation modeling analysis was performed to elucidate the relation between eating behaviors and metabolic and inflammatory profile.BED obese exhibited significantly higher percentages of altered eating behaviors, body mass index (P < 0.001), waist circumference (P < 0.01), fat mass (P < 0.001), and a lower lean mass (P < 0.001) when compared with non-BED obese. Binge eating disorder obese also had a worse metabolic and inflammatory profile, exhibiting significantly lower high-density lipoprotein cholesterol levels (P < 0.05), and higher levels of glycated hemoglobin (P < 0.01), uric acid (P < 0.05), erythrocyte sedimentation rate (P < 0.001), high-sensitive C-reactive protein (P < 0.01), and white blood cell counts (P < 0.01). Higher fasting insulin (P < 0.01) and higher insulin resistance (P < 0.01), assessed by homeostasis model assessment index and visceral adiposity index (P < 0.001), were observed among BED obese. All differences remained significant after adjusting for body mass index. No significant differences in fasting plasma glucose or 2-hour postchallenge plasma glucose were found. Structural equation modeling analysis confirmed the relation between the altered eating behaviors of BED and the metabolic and inflammatory profile.Binge eating disorder obese exhibited an unfavorable metabolic and inflammatory profile, which is related to their characteristic eating habits
Driven Macroscopic Quantum Tunneling of Ultracold Atoms in Engineered Optical Lattices
Coherent macroscopic tunneling of a Bose-Einstein condensate between two
parts of an optical lattice separated by an energy barrier is theoretically
investigated. We show that by a pulsewise change of the barrier height, it is
possible to switch between tunneling regime and a self-trapped state of the
condensate. This property of the system is explained by effectively reducing
the dynamics to the nonlinear problem of a particle moving in a double square
well potential. The analysis is made for both attractive and repulsive
interatomic forces, and it highlights the experimental relevance of our
findings
Exploring the thermodynamic limit of Hamiltonian models: convergence to the Vlasov equation
We here discuss the emergence of Quasi Stationary States (QSS), a universal
feature of systems with long-range interactions. With reference to the
Hamiltonian Mean Field (HMF) model, numerical simulations are performed based
on both the original -body setting and the continuum Vlasov model which is
supposed to hold in the thermodynamic limit. A detailed comparison
unambiguously demonstrates that the Vlasov-wave system provides the correct
framework to address the study of QSS. Further, analytical calculations based
on Lynden-Bell's theory of violent relaxation are shown to result in accurate
predictions. Finally, in specific regions of parameters space, Vlasov numerical
solutions are shown to be affected by small scale fluctuations, a finding that
points to the need for novel schemes able to account for particles
correlations.Comment: 5 pages, 3 figure
Kinetic theory for non-equilibrium stationary states in long-range interacting systems
We study long-range interacting systems perturbed by external stochastic
forces. Unlike the case of short-range systems, where stochastic forces usually
act locally on each particle, here we consider perturbations by external
stochastic fields. The system reaches stationary states where external forces
balance dissipation on average. These states do not respect detailed balance
and support non-vanishing fluxes of conserved quantities. We generalize the
kinetic theory of isolated long-range systems to describe the dynamics of this
non-equilibrium problem. The kinetic equation that we obtain applies to
plasmas, self-gravitating systems, and to a broad class of other systems. Our
theoretical results hold for homogeneous states, but may also be generalized to
apply to inhomogeneous states. We obtain an excellent agreement between our
theoretical predictions and numerical simulations. We discuss possible
applications to describe non-equilibrium phase transitions.Comment: 11 pages, 2 figures; v2: small changes, close to the published
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