3,246 research outputs found
Fluctuations and Pattern Formation in Fluids with Competing Interactions
One of the most interesting phenomena in the soft-matter realm consists in
the spontaneous formation of super-molecular structures (microphases) in
condition of thermodynamic equilibrium. A simple mechanism responsible for this
self-organization or pattern formation is based on the competition between
attractive and repulsive forces with different length scales in the microscopic
potential, typically, a short-range attraction against a longer-range
repulsion.
We analyse this problem by simulations in 2D fluids. We find that, as the
temperature is lowered, liquid-vapor phase separation is inhibited by the
competition between attraction and repulsion, and replaced by a transition to
non-homogeneous phases. The structure of the fluid shows well defined
signatures of the presence of both intra- and inter-cluster correlations.
Even when the competition between attraction and repulsion is not so strong
as to cause microphase formation, it still induces large density fluctuations
in a wide region of the temperature-density plane. In this large-fluctuation
regime, pattern formation can be triggered by a weak external modulating field.Comment: To appear in the proceedings of the "International workshop on
collective phenomena in macroscopic systems", 2006 Villa Olmo (Como), Ital
Phase diagram of symmetric binary mixtures at equimolar and non-equimolar concentrations: a systematic investigation
We consider symmetric binary mixtures consisting of spherical particles with
equal diameters interacting via a hard-core plus attractive tail potential with
strengths epsilon_{ij}, i,j=1,2, such that epsilon_{11} = epsilon_{22} >
epsilon_{12}. The phase diagram of the system at all densities and
concentrations is investigated as a function of the unlike-to-like interaction
ratio delta = epsilon_{12}/epsilon_{11} by means of the hierarchical reference
theory (HRT). The results are related to those of previous investigations
performed at equimolar concentration, as well as to the topology of the
mean-field critical lines. As delta is increased in the interval 0 < delta < 1,
we find first a regime where the phase diagram at equal species concentration
displays a tricritical point, then one where both a tricritical and a
liquid-vapor critical point are present. We did not find any clear evidence of
the critical endpoint topology predicted by mean-field theory as delta
approaches 1, at least up to delta=0.8, which is the largest value of delta
investigated here. Particular attention was paid to the description of the
critical-plus-tricritical point regime in the whole density-concentration
plane. In this situation, the phase diagram shows, in a certain temperature
interval, a coexistence region that encloses an island of homogeneous,
one-phase fluid.Comment: 27 pages + 20 figure
A complete characterisation of the heralded noiseless amplification of photons
Heralded noiseless amplifcation of photons has recently been shown to provide
a means to overcome losses in complex quantum communication tasks. In
particular, to overcome transmission losses that could allow for the violation
of a Bell inequality free from the detection loophole, for Device Independent
Quantum Key Distribution (DI-QKD). Several implementations of a heralded photon
amplifier have been proposed and the first proof of principle experiments
realised. Here we present the first full characterisation of such a device to
test its functional limits and potential for DI-QKD. This device is tested at
telecom wavelengths and is shown to be capable of overcoming losses
corresponding to a transmission through of single mode telecom
fibre. We demonstrate heralded photon amplifier with a gain and a
heralding probability , required by DI-QKD protocols that use the
Clauser-Horne-Shimony-Holt (CHSH) inequality. The heralded photon amplifier
clearly represents a key technology for the realisation of DI-QKD in the real
world and over typical network distances.Comment: 9 pages, 4 figure
Phase transitions in simple and not so simple binary fluids
Compared to pure fluids, binary mixtures display a very diverse phase
behavior, which depends sensitively on the parameters of the microscopic
potential. Here we investigate the phase diagrams of simple model mixtures by
use of a microscopic implementation of the renormalization group technique.
First, we consider a symmetric mixture with attractive interactions, possibly
relevant for describing fluids of molecules with internal degrees of freedom.
Despite the simplicity of the model, slightly tuning the strength of the
interactions between unlike species drastically changes the topology of the
phase boundary, forcing or inhibiting demixing, and brings about several
interesting features such as double critical points, tricritical points, and
coexistence domains enclosing `islands' of homogeneous, mixed fluid.
Homogeneous phase separation in mixtures can be driven also by purely repulsive
interactions. As an example, we consider a model of soft particles which has
been adopted to describe binary polymer solutions. This is shown to display
demixing (fluid-fluid) transition at sufficiently high density. The nature and
the physical properties of the corresponding phase transition are investigated.Comment: 6 pages + 3 figures, presented at the 5th EPS Liquid Matter
Conference, Konstanz, 14-18 September 200
Anisotropy effects on the magnetic excitations of a ferromagnetic monolayer below and above the Curie temperature
The field-driven reorientation transition of an anisotropic ferromagnetic
monolayer is studied within the context of a finite-temperature Green's
function theory. The equilibrium state and the field dependence of the magnon
energy gap are calculated for static magnetic field applied in plane
along an easy or a hard axis. In the latter case, the in-plane reorientation of
the magnetization is shown to be continuous at T=0, in agreement with free spin
wave theory, and discontinuous at finite temperature , in contrast with
the prediction of mean field theory. The discontinuity in the orientation angle
creates a jump in the magnon energy gap, and it is the reason why, for ,
the energy does not go to zero at the reorientation field. Above the Curie
temperature , the magnon energy gap vanishes for H=0 both in the
easy and in the hard case. As is increased, the gap is found to increase
almost linearly with , but with different slopes depending on the field
orientation. In particular, the slope is smaller when is along the hard
axis. Such a magnetic anisotropy of the spin-wave energies is shown to persist
well above ().Comment: Final version accepted for publication in Physical Review B (with
three figures
Sexuelle Zufriedenheit von Frauen mit vaginaler Aplasie nach operativer Neovagina-Bildung
Obwohl kein sehr seltenes PhĂ€nomen, stellt der Befund einer Aplasie von Uterus und Vagina (Mayer-Rokitansky-KĂŒster-Hauser-Syndrom) ein im Allgemeinen wenig bekanntes Thema dar.
In der Literatur wird es mit einer HĂ€ufigkeit von 1:4000 weiblichen Neugeborenen beschrieben.
Die Ursache ist eine Hemmung der Entwicklung, der Ausdifferenzierung und Vereinigung der MĂŒllerâschen GĂ€nge, sowie eine fehlende Epithelausknospung aus dem Sinus urogenitalis, wodurch keine Vaginalplatte angelegt und der Anschluss an die MĂŒllerâschen GĂ€nge nicht erreicht wird.
Die Aplasie von Uterus und Vagina wird in der Regel bei der AbklÀrung wegen primÀrer Amenorrhoe diagnostiziert. Frauen mit dieser Fehlbildung sind in zweifacher Weise betroffen: Einerseits beim Fehlen der Vagina durch die Unmöglichkeit, vaginalen Geschlechtsverkehr haben zu können und andererseits beim Fehlen des Uterus durch die Unmöglichkeit, eine Schwangerschaft erleben zu können.
Zur Bildung einer Neovagina stehen fĂŒr diese Frauen verschiedene operative Möglichkeiten oder unblutige, bougierende Verfahren zur VerfĂŒgung.
An der Klinik fĂŒr GynĂ€kologie des Departements fĂŒr Frauenheilkunde des UniversitĂ€tsspitals ZĂŒrich wird zur Neovagina-Bildung die laparoskopisch modifizierte Operationsmethode nach Vecchietti gewĂ€hlt.
Ziel dieser Arbeit war es zu erfahren, ob Frauen nach diesem operativen Eingriff zur Neovagina-Bildung ein fĂŒr sie normales Sexualleben fĂŒhren konnten.
Dazu wurden von den Patientinnen, die im Zeitraum von 1993 bis 2000 an der Klinik fĂŒr GynĂ€kologie im USZ im Sinne einer Neovagina-Bildung nach Vecchietti operiert worden waren, die Krankengeschichten gesichtet und die Frauen zwecks Befragung kontaktiert.
Von den 13 Patientinnen haben 10 an der Befragung, die in Interviewform unter Verwendung eines Fragebogens durchgefĂŒhrt worden ist, teilgenommen. Zusammenfassend darf gesagt werden, dass nach der Neovagina-Bildung und nach einer etwas lĂ€nger dauernden postoperativen Heilungsphase der Geschlechtsverkehr schmerzfrei möglich war und bei ungestörter Libido und ungestörter OrgasmusfĂ€higkeit die Frauen eine insgesamt zufriedenstellende Vita sexualis leben konnten. Das alltĂ€gliche Leben hatte sich fĂŒr die Frauen nicht verĂ€ndert. Sie betonten aber, dass sie sich durch die Neovagina-Bildung vollkommener fĂŒhlten und unbeschwerter eine Beziehung eingehen konnten.
Die Operationsmethode war fĂŒr die befragten Frauen gut akzeptabel, wurde aber durchwegs als schmerzhaft erlebt, vor allem die Phase der langsamen postoperativen Scheidendehnung.
8 der befragten 10 Frauen waren der Ăberzeugung, dass sie den Eingriff erneut wĂŒrden machen lassen, da sie das Endresultat als zufriedenstellend empfanden. Ein von allen Patientinnen geĂ€ussertes Anliegen war der Wunsch nach einer Selbsthilfegruppe fĂŒr einen Austausch unter Betroffenen.
FĂŒr die GynĂ€kologische Klinik ergab sich als Fazit aus dieser Arbeit die Erkenntnis, dass bei Frauen mit MRKH-Syndrom prĂ€operativ noch mehr zu betonen ist, wie sehr die postoperative Phase von Schmerzen begleitet sein wird und dass insgesamt eine gute ambulante Begleitung der Frauen auch nach der Entlassung aus dem Spital angeboten werden muss, speziell fĂŒr die Phase der Platzhalteranwendung. Im gesamten betrachtet kann aber der Eingriff bezĂŒglich sexueller Zufriedenheit postoperativ empfohlen werden
Recent developments of the Hierarchical Reference Theory of Fluids and its relation to the Renormalization Group
The Hierarchical Reference Theory (HRT) of fluids is a general framework for
the description of phase transitions in microscopic models of classical and
quantum statistical physics. The foundations of HRT are briefly reviewed in a
self-consistent formulation which includes both the original sharp cut-off
procedure and the smooth cut-off implementation, which has been recently
investigated. The critical properties of HRT are summarized, together with the
behavior of the theory at first order phase transitions. However, the emphasis
of this presentation is on the close relationship between HRT and non
perturbative renormalization group methods, as well as on recent
generalizations of HRT to microscopic models of interest in soft matter and
quantum many body physics.Comment: 17 pages, 5 figures. Review paper to appear in Molecular Physic
A model colloidal fluid with competing interactions: bulk and interfacial properties
Using a simple mean-field density functional theory theory (DFT), we
investigate the structure and phase behaviour of a model colloidal fluid
composed of particles interacting via a pair potential which has a hard core of
diameter , is attractive Yukawa at intermediate separations and
repulsive Yukawa at large separations. We analyse the form of the asymptotic
decay of the bulk fluid correlation functions, comparing results from our DFT
with those from the self consistent Ornstein-Zernike approximation (SCOZA). In
both theories we find rich crossover behaviour, whereby the ultimate decay of
correlation functions changes from monotonic to long-wavelength damped
oscillatory decay on crossing certain lines in the phase diagram, or sometimes
from oscillatory to oscillatory with a longer wavelength. For some choices of
potential parameters we find, within the DFT, a -line at which the
fluid becomes unstable with respect to periodic density fluctuations. SCOZA
fails to yield solutions for state points near such a -line. The
propensity to clustering of particles, which is reflected by the presence of a
long wavelength , slowly decaying oscillatory pair correlation
function, and a structure factor that exhibits a very sharp maximum at small
but non zero wavenumbers, is enhanced in states near the -line. We
present density profiles for the planar liquid-gas interface and for fluids
adsorbed at a planar hard wall. The presence of a nearby -transition
gives rise to pronounced long-wavelength oscillations in the one-body densities
at both types of interface.Comment: 14 pages, 11 figure
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