338 research outputs found
The Casimir effect: from quantum to critical fluctuations
The Casimir effect in quantum electrodynamics (QED) is perhaps the best-known
example of fluctuation-induced long-ranged force acting on objects (conducting
plates) immersed in a fluctuating medium (quantum electromagnetic field in
vacuum). A similar effect emerges in statistical physics, where the force
acting, e.g., on colloidal particles immersed in a binary liquid mixture is
affected by the classical thermal fluctuations occurring in the surrounding
medium. The resulting Casimir-like force acquires universal features upon
approaching a critical point of the medium and becomes long-ranged at
criticality. In turn, this universality allows one to investigate theoretically
the temperature dependence of the force via representative models and to
stringently test the corresponding predictions in experiments. In contrast to
QED, the Casimir force resulting from critical fluctuations can be easily tuned
with respect to strength and sign by surface treatments and temperature
control. We present some recent advances in the theoretical study of the
universal properties of the critical Casimir force arising in thin films. The
corresponding predictions compare very well with the experimental results
obtained for wetting layers of various fluids. We discuss how the Casimir force
between a colloidal particle and a planar wall immersed in a binary liquid
mixture has been measured with femto-Newton accuracy, comparing these
experimental results with the corresponding theoretical predictions.Comment: Talk delivered at the International Workshop "60 Years of Casimir
Effect", Brasilia, 23-27 June 2008 (17 pages, 7 figures
Normal and lateral critical Casimir forces between colloids and patterned substrates
We study the normal and lateral effective critical Casimir forces acting on a
spherical colloid immersed in a critical binary solvent and close to a
chemically structured substrate with alternating adsorption preference. We
calculate the universal scaling function for the corresponding potential and
compare our results with recent experimental data [Soyka F., Zvyagolskaya O.,
Hertlein C., Helden L., and Bechinger C., Phys. Rev. Lett., 101, 208301
(2008)]. The experimental potentials are properly captured by our predictions
only by accounting for geometrical details of the substrate pattern for which,
according to our theory, critical Casimir forces turn out to be a sensitive
probe.Comment: 6 pages, 3 figure
Effective temperatures in a simple model of non-equilibrium, non-Markovian dynamics
The concept of effective temperatures in nonequilibrium systems is studied
within an exactly solvable model of non-Markovian diffusion. The system is
coupled to two heat baths which are kept at different temperatures: one
('fast') bath associated with an uncorrelated Gaussian noise and a second
('slow') bath with an exponential memory kernel. Various definitions of
effective temperatures proposed in the literature are evaluated and compared.
The range of validity of these definitions is discussed. It is shown in
particular, that the effective temperature defined from the
fluctuation-dissipation relation mirrors the temperature of the slow bath in
parameter regions corresponding to a separation of time scales. On the
contrary, quasi-static and thermodynamic definitions of an effective
temperature are found to display the temperature of the fast bath in most
parameter regions
Hematological Complications in a COVID-19 Patient: A Case Report
Hemophilia A is a hemorrhagic disorder caused by insufficient or inadequate coagulation factor VIII activity. Two different forms are described: congenital, hereditary X-linked, and acquired. Acquired hemophilia A (AHA) is a rare condition and it is defined by the production of autoantibodies neutralizing factor VIII, known as inhibitors. We report the case of a 72-year-old man with a clinical diagnosis of AHA after SARS-CoV-2 infection, which has been described in association with several hematological complications. SARS-CoV-2 infection could represent the immunological trigger for the development of autoantibodies. In our patient, SARS-CoV-2 infection preceded the hemorrhagic complications by 15 days. This lag time is in line with the other cases reported and compatible with the development of an intense immune response with autoantibody production. It is possible that since our patient was affected by type 1 diabetes mellitus, he was more prone to an immune system pathological response against self-antigens. A prompt, appropriate therapeutic intervention with activated recombinant factor VII administration and cyclophosphamide has led to rapid remission of clinical and laboratory findings
Dynamic crossover in the global persistence at criticality
We investigate the global persistence properties of critical systems relaxing
from an initial state with non-vanishing value of the order parameter (e.g.,
the magnetization in the Ising model). The persistence probability of the
global order parameter displays two consecutive regimes in which it decays
algebraically in time with two distinct universal exponents. The associated
crossover is controlled by the initial value m_0 of the order parameter and the
typical time at which it occurs diverges as m_0 vanishes. Monte-Carlo
simulations of the two-dimensional Ising model with Glauber dynamics display
clearly this crossover. The measured exponent of the ultimate algebraic decay
is in rather good agreement with our theoretical predictions for the Ising
universality class.Comment: 5 pages, 2 figure
Critical Casimir Effect in superfluid wetting films
Recent experimental data for the complete wetting behavior of pure 4He and of
3He-4He mixtures exposed to solid substrates show that there is a change of the
corresponding film thicknesses L upon approaching thermodynamically the
lambda-transition and the tricritical end point, respectively, which can be
attributed to critical Casimir forces f_C. We calculate the scaling functions
vartheta of f_C within models representing the corresponding universality
classes. For the mixtures our analysis provides an understanding of the rich
behavior of vartheta deduced from the experimental data and predicts the
crossover behavior between the tricritical point and the lambda-transition of
pure 4He which are connected by a line of critical points. The formation of a
'soft-mode' phase within the wetting films gives rise to a pronounced maximum
of f_C below the tricritical point as observed experimentally. Near the
tricritical point we find logarithmic corrections ~L^(-3)(ln L)^(1/2) for the
leading behavior of vartheta dominating the contributions from the background
dispersion forces.Comment: 32 pages, 12 figure
The bulk correlation length and the range of thermodynamic Casimir forces at Bose-Einstein condensation
The relation between the bulk correlation length and the decay length of
thermodynamic Casimir forces is investigated microscopically in two
three-dimensional systems undergoing Bose-Einstein condensation: the perfect
Bose gas and the imperfect mean-field Bose gas. For each of these systems, both
lengths diverge upon approaching the corresponding condensation point from the
one-phase side, and are proportional to each other. We determine the
proportionality factors and discuss their dependence on the boundary
conditions. The values of the corresponding critical exponents for the decay
length and the correlation length are the same, equal to 1/2 for the perfect
gas, and 1 for the imperfect gas
Critical Langevin dynamics of the O(N)-Ginzburg-Landau model with correlated noise
We use the perturbative renormalization group to study classical stochastic
processes with memory. We focus on the generalized Langevin dynamics of the
\phi^4 Ginzburg-Landau model with additive noise, the correlations of which are
local in space but decay as a power-law with exponent \alpha in time. These
correlations are assumed to be due to the coupling to an equilibrium thermal
bath. We study both the equilibrium dynamics at the critical point and quenches
towards it, deriving the corresponding scaling forms and the associated
equilibrium and non-equilibrium critical exponents \eta, \nu, z and \theta. We
show that, while the first two retain their equilibrium values independently of
\alpha, the non-Markovian character of the dynamics affects the dynamic
exponents (z and \theta) for \alpha < \alpha_c(D, N) where D is the spatial
dimensionality, N the number of components of the order parameter, and
\alpha_c(x,y) a function which we determine at second order in 4-D. We analyze
the dependence of the asymptotic fluctuation-dissipation ratio on various
parameters, including \alpha. We discuss the implications of our results for
several physical situations
Universal scaling functions of critical Casimir forces obtained by Monte Carlo simulations
Effective Casimir forces induced by thermal fluctuations in the vicinity of
bulk critical points are studied by means of Monte Carlo simulations in
three-dimensional systems for film geometries and within the experimentally
relevant Ising and XY universality classes. Several surface universality
classes of the confining surfaces are considered, some of which are relevant
for recent experiments. A novel approach introduced previously EPL 80, 60009
(2007), based inter alia on an integration scheme of free energy differences,
is utilized to compute the universal scaling functions of the critical Casimir
forces in the critical range of temperatures above and below the bulk critical
temperature. The resulting predictions are compared with corresponding
experimental data for wetting films of fluids and with available theoretical
results.Comment: 21 pages, 17 figure
Ageing in the contact process: Scaling behavior and universal features
We investigate some aspects of the ageing behavior observed in the contact
process after a quench from its active phase to the critical point. In
particular we discuss the scaling properties of the two-time response function
and we calculate it and its universal ratio to the two-time correlation
function up to first order in the field-theoretical epsilon-expansion. The
scaling form of the response function does not fit the prediction of the theory
of local scale invariance. Our findings are in good qualitative agreement with
recent numerical results.Comment: 20 pages, 3 figure
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