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 $20\, \rm km$ of single mode telecom fibre. We demonstrate heralded photon amplifier with a gain $>100$ and a heralding probability $>83$, 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 $E_0$ are calculated for static magnetic field $H$ 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 $T>0$, 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 $T>0$, the energy does not go to zero at the reorientation field. Above the Curie temperature $T_C$, the magnon energy gap $E_0(H)$ vanishes for H=0 both in the easy and in the hard case. As $H$ is increased, the gap is found to increase almost linearly with $H$, but with different slopes depending on the field orientation. In particular, the slope is smaller when $H$ is along the hard axis. Such a magnetic anisotropy of the spin-wave energies is shown to persist well above $T_C$ ($T \approx 1.2 T_C$).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 Phänomen, stellt der Befund einer Aplasie von Uterus und Vagina (Mayer-Rokitansky-Küster-Hauser-Syndrom) ein im Allgemeinen wenig bekanntes Thema dar. In der Literatur wird es mit einer Häufigkeit von 1:4000 weiblichen Neugeborenen beschrieben. Die Ursache ist eine Hemmung der Entwicklung, der Ausdifferenzierung und Vereinigung der Müller’schen Gänge, sowie eine fehlende Epithelausknospung aus dem Sinus urogenitalis, wodurch keine Vaginalplatte angelegt und der Anschluss an die Müller’schen Gänge nicht erreicht wird. Die Aplasie von Uterus und Vagina wird in der Regel bei der Abklärung wegen primärer 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 für diese Frauen verschiedene operative Möglichkeiten oder unblutige, bougierende Verfahren zur Verfügung. An der Klinik für Gynäkologie des Departements für Frauenheilkunde des Universitätsspitals Zürich wird zur Neovagina-Bildung die laparoskopisch modifizierte Operationsmethode nach Vecchietti gewählt. Ziel dieser Arbeit war es zu erfahren, ob Frauen nach diesem operativen Eingriff zur Neovagina-Bildung ein für sie normales Sexualleben führen konnten. Dazu wurden von den Patientinnen, die im Zeitraum von 1993 bis 2000 an der Klinik für Gynäkologie 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 durchgeführt worden ist, teilgenommen. Zusammenfassend darf gesagt werden, dass nach der Neovagina-Bildung und nach einer etwas länger dauernden postoperativen Heilungsphase der Geschlechtsverkehr schmerzfrei möglich war und bei ungestörter Libido und ungestörter Orgasmusfähigkeit die Frauen eine insgesamt zufriedenstellende Vita sexualis leben konnten. Das alltägliche Leben hatte sich für die Frauen nicht verändert. Sie betonten aber, dass sie sich durch die Neovagina-Bildung vollkommener fühlten und unbeschwerter eine Beziehung eingehen konnten. Die Operationsmethode war für 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 Überzeugung, dass sie den Eingriff erneut würden machen lassen, da sie das Endresultat als zufriedenstellend empfanden. Ein von allen Patientinnen geäussertes Anliegen war der Wunsch nach einer Selbsthilfegruppe für einen Austausch unter Betroffenen. Für die Gynäkologische Klinik ergab sich als Fazit aus dieser Arbeit die Erkenntnis, dass bei Frauen mit MRKH-Syndrom präoperativ 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 für die Phase der Platzhalteranwendung. Im gesamten betrachtet kann aber der Eingriff bezüglich 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 $\sigma$, 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 $\lambda$-line at which the fluid becomes unstable with respect to periodic density fluctuations. SCOZA fails to yield solutions for state points near such a $\lambda$-line. The propensity to clustering of particles, which is reflected by the presence of a long wavelength $\gg \sigma$, 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 $\lambda$-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 $\lambda$-transition gives rise to pronounced long-wavelength oscillations in the one-body densities at both types of interface.Comment: 14 pages, 11 figure