On the surface tension of fluctuating quasi-spherical vesicles

We calculate the stress tensor for a quasi-spherical vesicle and we thermally average it in order to obtain the actual, mechanical, surface tension $\tau$ of the vesicle. Both closed and poked vesicles are considered. We recover our results for $\tau$ by differentiating the free-energy with respect to the proper projected area. We show that $\tau$ may become negative well before the transition to oblate shapes and that it may reach quite large negative values in the case of small vesicles. This implies that spherical vesicles may have an inner pressure lower than the outer one.Comment: To appear in Eur. Phys. J. E, revised versio

Localization Effect in a 2D Superconducting Network without Disorder

The superconducting properties of a two-dimensional superconducting wire network with a new geometry have been measured as a function of the external magnetic field. The extreme localization effect recently predicted for this periodic lattice is revealed as a suppression of the critical current when the applied magnetic field corresponds to half a flux quantum per unit cell. For this particular magnetic field, the observed vortex state configuration is highly disordered.Comment: 6 pages, 2 eps figures, submitted to Physica C. Title change

Determination of the interactions in confined macroscopic Wigner islands: theory and experiments

Macroscopic Wigner islands present an interesting complementary approach to explore the properties of two-dimensional confined particles systems. In this work, we characterize theoretically and experimentally the interaction between their basic components, viz., conducting spheres lying on the bottom electrode of a plane condenser. We show that the interaction energy can be approximately described by a decaying exponential as well as by a modified Bessel function of the second kind. In particular, this implies that the interactions in this system, whose characteristics are easily controllable, are the same as those between vortices in type-II superconductors.Comment: 8 pages, 8 figure

Bi-defects of Nematic Surfactant Bilayers

We consider the effects of the coupling between the orientational order of the two monolayers in flat nematic bilayers. We show that the presence of a topological defect on one bilayer generates a nontrivial orientational texture on both monolayers. Therefore, one cannot consider isolated defects on one monolayer, but rather associated pairs of defects on either monolayer, which we call bi-defects. Bi-defects generally produce walls, such that the textures of the two monolayers are identical outside the walls, and different in their interior. We suggest some experimental conditions in which these structures could be observed.Comment: RevTeX, 4 pages, 3 figure

The Fe XXII I(11.92 A)/I(11.77 A) Density Diagnostic Applied to the Chandra High Energy Transmission Grating Spectrum of EX Hydrae

Using the Livermore X-ray Spectral Synthesizer, which calculates spectral models of highly charged ions based primarily on HULLAC atomic data, we investigate the temperature, density, and photoexcitation dependence of the I(11.92 A)/I(11.77 A) line ratio of Fe XXII. We find that this line ratio has a critical density n_c \approx 5x10^13 cm^-3, is approximately 0.3 at low densities and 1.5 at high densities, and is very insensitive to temperature and photoexcitation, so is a useful density diagnostic for sources like magnetic cataclysmic variables in which the plasma densities are high and the efficacy of the He-like ion density diagnostic is compromised by the presence of a bright ultraviolet continuum. Applying this diagnostic to the Chandra High Energy Transmission Grating spectrum of the intermediate polar EX Hya, we find that the electron density of its T_e \approx 12 MK plasma is n_e = 1.0^{+2.0}_{-0.5} x 10^14 cm^-3, orders of magnitude greater than that typically observed in the Sun or other late-type stars.Comment: 11 pages including 3 encapsulated postscript figures; LaTeX format, uses aastex.cls; accepted on 2003 April 3 for publication in The Astrophysical Journa

Nematic-Wetted Colloids in the Isotropic Phase: Pairwise Interaction, Biaxiality and Defects

We calculate the interaction between two spherical colloidal particles embedded in the isotropic phase of a nematogenic liquid. The surface of the particles induces wetting nematic coronas that mediate an elastic interaction. In the weak wetting regime, we obtain exact results for the interaction energy and the texture, showing that defects and biaxiality arise, although they are not topologically required. We evidence rich behaviors, including the possibility of reversible colloidal aggregation and dispersion. Complex anisotropic self-assembled phases might be formed in dense suspensions.Comment: 4 pages, 6 figure

Finitely presented left orderable monsters

A left orderable monster is a finitely generated left orderable group all of whose fixpoint-free actions on the line are proximal: the action is semiconjugate to a minimal action so that for every bounded interval $I$ and open interval $J$, there is a group element that sends $I$ into $J$. In his 2018 ICM address, Navas asked about the existence of left orderable monsters. By now there are several examples, all of which are finitely generated but not finitely presentable. We provide the first examples of left orderable monsters that are finitely presentable, and even of type $F_\infty$. The construction itself is elementary, and these groups satisfy several additional properties separating them from the previous examples: they are not simple, they act minimally on the circle, and they have an infinite-dimensional space of homogeneous quasimorphisms. Our construction is flexible enough that it produces infinitely many isomorphism classes of finitely presented (and type $F_{\infty}$) left orderable monsters.Comment: 12 pages. v2: Final version, to appear in Ergodic Theory and Dynamical System