Introduction to topological defects: from liquid crystals to particle physics

Liquid crystals are assemblies of rod-like molecules which self-organize to form mesophases, in-between ordinary liquids and anisotropic crystals. At each point, the molecules collectively orient themselves along a privileged direction, which locally defines an orientational order. Sometimes, this order is broken and singularities appear in the form of topological defects. This tutorial article is dedicated to the geometry, topology and physics of these defects. We introduce the main models used to describe the nematic phase and discuss the isotropic-nematic phase transition. Then, we present the different families of defects in nematics and examine some of their physical outcomes. Finally, we show that topological defects are universal patterns of nature, appearing not only in soft matter, but also in biology, cosmology, geology and even particle physics.Comment: 40 pages, 9 figures, review pape

Estudio del rol del factor UBF en la expresión de poblaciones de rDNA variantes durante la adaptación estacional de Cyprinus carpio.

Tesis (Doctor en Biociencias Moleculares)RESUMEN: La biogénesis ribosomal es uno de los procesos cruciales a nivel celular. En la regulación de este proceso participan dos mecanismos principales: uno que involucra los factores de transcripción clásicos de la RNA polimerasa I (RNA PolI), y un segundo que consiste en el despliegue de mecanismos epigenéticos. Actualmente, existe conocimiento de que ambas vías se encontrarían íntimamente relacionadas. En este contexto el pez Cyprinus carpio (carpa), presenta características especiales en cuanto a la regulación de la biogénesis ribosomal. Previamente, se ha reportado que durante el ciclo estacional este organismo debe implementar un proceso de adaptación a nivel molecular para adecuar el proceso fundamental de síntesis de ribosomas. La carpa durante la estación invernal sufre una segregación de sus componentes nucleolares, acompañada de un reordenamiento de la cromatina nucleolar, lo cual finalmente se traduce en una disminución de la expresión de los genes ribosomales. Este fenotipo celular es revertido al llegar la estación veraniega, continuando un proceso cíclico genéticamente programado denominado aclimatización estacional. Estos antecedentes han permitido postular que este organismo es capaz de generar mecanismos moleculares de adaptación compensatorios frente a los cambios en su entorno. Hasta nuestro conocimiento, la carpa constituye el único organismo en donde se ha descrito y estudiado que el efecto de estímulos medioambientales es capaz de controlar la transcripción de los genes ribosomales a lo largo del ciclo estacional. Por esta razón, C. carpio es un extraordinario modelo de estudio tanto para el control de la biogénesis ribosomal, como para profundizar el conocimiento sobre el efecto los estímulos medioambientales en la reprogramación génica de un organismo. En este trabajo de tesis hemos centrado nuestro estudio en la proteína UBF (Upstream Binding Factor). Esta proteína ha sido caracterizada como un factor de transcripción de la RNA PolI, que interviene en la formación del complejo de pre-iniciación sobre el promotor de los genes ribosomales. Sin embargo, recientemente se ha descrito que este factor también interviene ayudando a mantener la transcripción ribosomal activa. Así, otros estudios han demostrado que la proteína UBF no sólo se une al promotor del cistrón ribosomal, sino que también interactúa con gran parte de la secuencia del cistron ribosomal. En este caso, UBF desempeñaría un rol fundamental en la expresión de las diferentes poblaciones de estos genes,..ABSTRACT: Ribosomal biogenesis is a critical process at a cellular level. Two main mechanisms participate in the regulation of this process: one involving typical transcription factors of RNA polymerase I (RNA Pol I) and a second consisting of a display of epigenetic mechanisms. Nowadays, studies have proved that both pathways are closely related. Cyprinus carpio (carp) has special features concerning ribosomal biogenesis regulation. It has been previously reported that during each seasonal cycle, this organism must go through an adaptation process at a molecular level in order to adjust the fundamental process of ribosomal synthesis. During winter time, the carp undergoes a segregation process of the nucleolar components along with a rearrangement of the nucleolar chromatin, which results in a decreased expression of ribosomal genes. This cellular phenotype is reverted in the summer season, thus creating a genetically programmed cyclical process known as seasonal acclimatization. This suggests that this organism could generate compensatory molecular adaptation mechanisms when its environment changes. To our knowledge, modulation of ribosomal genes at a transcriptional level in response to environmental changes has been described only in carp. For these reasons, C. carpio represents an excellent model to study the mechanisms controlling ribosomal biogenesis, allowing us at the same time to deepen our knowledge about the effects of environmental stimuli over this and other processes. This thesis is focused on the protein UBF (Upstream Binding Factor), one of the several factors involved in ribosomal regulation. This protein has been characterized as a transcription factor for RNA Pol I, directly intervening in the formation of the pre-initiation complex in the ribosomal gene. However, it has been recently reported that this factor also acts as a maintainer of ribosomal transcription. Other studies have shown that UBF not only binds to the promoter of the ribosomal cistron but also interacts with a large part of the ribosomal gene sequence. If that is the case, UBF might have a significant role in the expression of ribosomal genes, keeping them in a structurally active state. Overall, these allow us to suggest UBF as a central factor in the regulation of ribosomal gene transcription, and our hypothesis is that UBF differentially controls the expression of rRNA during seasonal acclimation on C. Carpio by controlling the activity of a variant forms of rDNA (v-rDNA)..

Using torsion to manipulate spin currents

We address the problem of quantum particles moving on a manifold characterised by the presence of torsion along a preferential axis. In fact, such a torsion may be taylored by the presence of a single screw dislocation, whose Burgers vector measures the torsion amplitude. The problem, first treated in the relativistic limit describing fermions that couple minimally to torsion, is then analysed in the Pauli limit We show that torsion induces a geometric potential and also that it couples generically to the phase of the wave function, giving rise to the possibility of using torsion to manipulate spin currents in the case of spinor wave functions. These results emerge as an alternative strategy for using screw dislocations in the design of spintronic-based devices

Geometric theory of topological defects: methodological developments and new trends

Liquid crystals generally support orientational singularities of the director field known as topological defects. These latter modifiy transport properties in their vicinity as if the geometry was non-Euclidean. We present a state of the art of the differential geometry of nematic liquid crystals, with a special emphasis on linear defects. We then discuss unexpected but deep connections with cosmology and high-energy-physics, and conclude with a review on defect engineering for transport phenomena

On the possibility of classical vacuum polarization and magnetization

It is common practice to take for granted the equality (up to the constant $\varepsilon_0$) of the electric displacement ($\bf{D}$) and electric ($\bf{E}$) field vectors in vacuum. The same happens with the magnetic field ($\bf{H}$) and the magnetic flux density ($\bf{B}$) vectors (up to the constant $\mu_0^{-1}$). The fact that gravity may change this by effectively inducing dielectric or magnetic responses to the primary fields is commonly overlooked. It is the purpose of this communication to call attention to classical polarization or magnetization of the vacuum due to the concomitant presence of gravitational and electromagnetic sources. The formalism of differential forms (exterior calculus) is used since it provides a clear-cut way to achieve this. This work offers new routes for possible detection of various spacetime geometries via their electromagnetic manifestations and the way they influence light propagation