Depletion interaction between spheres of unequal size and demixing in binary mixtures of colloids

The possibility to induce demixing in a colloidal mixture by adding small polymers, or other equivalent depletant agents, is theoretically investigated. By use of Mean Field Theory, suitably generalized to deal with short range effective interactions, the phase diagram of a binary mixture ofcolloidal particles (modelled as hard spheres) in a solvent is determined as a function of the polymer concentration on the basis of the Asakura-Oosawa model.The topology of the phase diagram changes when the relative size of the colloidal particles is reduced: the critical line connecting the liquid-vapour critical points of the two pure fluids breaks and the branch starting from the critical point of the bigger particles bends to higher volume fractions, where concentration fluctuations drive the transition. The effects of a softer colloid-polymer interaction is also investigated: Even the presence of a small repulsive tail in the potential gives rise to a significant lowering of the stability threshold. In this case, phase transitions may take place by adding just a few percent of depletant in volume fraction. These results may be relevant for the interpretation of recent experiments of solidification kinetics in colloidal mixtures.Comment: To be published in Molecular Physic

Solvent mediated forces in critical fluids

The effective interaction between two planar walls immersed in a fluid is investigated by use of Density Functional Theory in the super-critical region of the phase diagram. A hard core Yukawa model of fluid is studied with special attention to the critical region. To achieve this goal a new formulation of the Weighted Density Approximation coupled with the Hierarchical Reference Theory, able to deal with critical long wavelength fluctuations, is put forward and compared with other approaches. The effective interaction between the walls is seen to change character on lowering the temperature: The strong oscillations induced by layering of the molecules, typical of the depletion mechanism in hard core systems, are gradually smoothed and, close to the critical point, a long range attractive tail emerges leading to a scaling form which agrees with the expectations based on the critical Casimir effect. Strong corrections to scaling are seen to affect the results up to very small reduced temperatures. By use of Derjaguin approximation, this investigation has natural implications for the aggregation of colloidal particles in critical solvents.Comment: 15 pages, 16 figure

Competitive density waves in quasi-one-dimensional electron systems

We investigate the nature of the ground state of the one-dimensional t-J model coupled to adiabatic phonons by use of the Lanczos technique at quarter filling. Due to the interplay between electron-electron and electron-phonon interactions, the model undergoes instabilities toward the formation of lattice and charge modulations. Moderate on-site and intra-site electron-phonon couplings lead to a competition of different spin-Peierls and dimerized states. In the former case two electrons belong to the unit cell and we expect a paramagnetic band insulator state, while lattice dimerization leads to a Mott insulating state with quasi long range antiferromagnetic order. The zero temperature phase diagram is obtained as a function of intra-site and inter-site electron-phonon couplings, analytically in the $J\to 0$ limit and numerically at finite J/t.Comment: 7 pages, 7 figures, to be published in Phys. Rev.

Educación en medios en acción : modelos, herramientas y ejemplos de buenas prácticas

Actualmente la educación en medios (EM) ya no puede ser considerada como un campo de estudio reservado a investigadores de la comunicación, ni como una práctica privilegiada de algunos profesores. Por un lado, la EM ahora es parte de la agenda de las organizaciones internacionales, que consideran el desarrollo de las competencias mediáticas como requisito necesario para ejercer plenamente la ciudadanía en la sociedad actual. Por otro lado, las prácticas de EM están cada vez más generalizadas en las escuelas e involucran a un número creciente de profesores. No obstante, la enseñanza de los medios de comunicación aún parece ser una tarea bastante solipsista, donde "todo está bien". De hecho, hay una tremenda falta de investigación sobre la calidad y la eficacia de las prácticas educativas. En su dimensión pedagógica, el proyecto On Air intenta hacer frente a estos problemas proporcionando un conjunto de herramientas para diseñar, desarrollar y evaluar las actividades de EM en las escuelas. El presente trabajo presenta y analiza sus resultados poniendo atención en la estructura general de la investigación y sus conclusiones principales.Currently, Media Education (ME) can no longer be considered as a field of study reserved for communication researchers, or as a privileged practice of some teachers. On the one hand, ME is now part of the agenda of international organizations, which considers the development of media skills as a prerequisite to exercise full citizenship in today's society. On the other hand, ME practices are becoming more widespread in schools and involve an increasing number of teachers. However, the teaching of the media still seems to be a fairly solipsistic, where "everything is fine". In fact, there is a tremendous lack of research on the quality and effectiveness of educational practices. In its educational dimension, the On Air project attempts to address these problems by providing a set of tools to design, develop and evaluate ME activities in schools. This paper presents and analyzes the results paying attention to the overall structure of the research and its main findings

Nonlinear unbalanced Bessel beams in the collapse of Gaussian beams arrested by nonlinear losses

Collapse of a Gaussian beam in self-focusing Kerr media arrested by nonlinear losses may lead to the spontaneous formation of a quasi-stationary nonlinear unbalanced Bessel beam with finite energy, which can propagate without significant distortion over tens of diffraction lengths, and without peak intensity attenuation while the beam power is drastically diminishing.Comment: 4 pages, 5 figure

The Ising phase in the J1-J2 Heisenberg Model

The two dimensional Heisenberg antiferromagnet on the square lattice with nearest (J1) and next-nearest (J2) neighbor couplings is investigated in the strong frustration regime (J2/J1>1/2). A new effective field theory describing the long wavelength physics of the model is derived from the quantum hamiltonian. The structure of the resulting non linear sigma model allows to recover the known spin wave results in the collinear regime, supports the presence of an Ising phase transition at finite temperature and suggests the possible occurrence of a non-magnetic ground state breaking rotational symmetry. By means of Lanczos diagonalizations we investigate the spin system at T=0, focusing our attention on the region where the collinear order parameter is strongly suppressed by quantum fluctuations and a transition to a non-magnetic state occurs. Correlation functions display a remarkable size independence and allow to identify the transition between the magnetic and non-magnetic region of the phase diagram. The numerical results support the presence of a non-magnetic phase with orientational ordering.Comment: 6 pages, 4 figures, to be published in PR

Comment on "Phase Diagram of an Asymmetric Spin Ladder."

A comment to the paper by S. Chen, H. B\"uttner, and J. Voit, [Phys. Rev. Lett. {\bf 87}, 087205 (2001)].Comment: 1 page, 1 figure, to appear in Physical Review Letter

Exotic gapless spectrum induced by frustration in quantum antiferromagnets

We show strong numerical evidence in favor of an unexpected virtually gapless spectrum, with edge states localized at the boundaries, in frustrated spin-1/2 antiferromagnetic ladders with an odd number of legs. These features can be accurately reproduced by using a projected BCS wave function with a non-trivial pairing, that mixes even and odd reflection symmetries. This approach gives the correct classification of the excitations and provides a simple and very appealing picture of an unconventional spin-liquid phase stabilized by frustration.Comment: 4 pages and 4 figure

An investigation of the SCOZA for narrow square-well potentials and in the sticky limit

We present a study of the self consistent Ornstein-Zernike approximation (SCOZA) for square-well (SW) potentials of narrow width delta. The main purpose of this investigation is to elucidate whether in the limit delta --> 0, the SCOZA predicts a finite value for the second virial coefficient at the critical temperature B2(Tc), and whether this theory can lead to an improvement of the approximate Percus-Yevick solution of the sticky hard-sphere (SHS) model due to Baxter [R. J. Baxter, J. Chem. Phys. 49, 2770 (1968)]. For SW of non vanishing delta, the difficulties due to the influence of the boundary condition at high density already encountered in an earlier investigation [E. Schoell-Paschinger, A. L. Benavides, and R. Castaneda-Priego, J. Chem. Phys. 123, 234513 (2005)] prevented us from obtaining reliable results for delta < 0.1. In the sticky limit this difficulty can be circumvented, but then the SCOZA fails to predict a liquid-vapor transition. The picture that emerges from this study is that for delta --> 0, the SCOZA does not fulfill the expected prediction of a constant B2(Tc) [M. G. Noro and D. Frenkel, J. Chem. Phys. 113, 2941 (2000)], and that for thermodynamic consistency to be usefully exploited in this regime, one should probably go beyond the Ornstein-Zernike ansatz.Comment: 40 pages, 13 figures. Previous Sec. 2 on the Yukawa potential has been removed. Only the square-well potential is considered in this versio