Size shrinking of composite bosons for increasing density in the BCS to Bose-Einstein crossover

We consider a system of fermions in the continuum case at zero temperature, in the strong-coupling limit of a short-range attraction when composite bosons form as bound-fermion pairs. We examine the density dependence of the size of the composite bosons at leading order in the density ("dilute limit"), and show on general physical grounds that this size should decrease with increasing density, both in three and two dimensions. We then compare with the analytic zero-temperature mean-field solution, which indeed exhibits the size shrinking of the composite bosons both in three and two dimensions. We argue, nonetheless, that the two-dimensional mean-field solution is not consistent with our general result in the "dilute limit", to the extent that mean field treats the scattering between composite bosons in the Born approximation which is known to break down at low energy in two dimensions.Comment: Revised version to be published on Eur. Phys. Jour. B, 7 pages, 1 figur

Ali in flusso subsonico e transonico: problematiche nella valutazione CFD

Negli ultimi anni, grazie anche agli enormi progressi delle tecnologie informatiche, la fluidodinamica computazionale, CFD, ha ottenuto notevoli successi, sia in ambito accademico che industriale; in questo panorama, il presente lavoro di tesi si propone di individuare un modello, efficace ed efficiente, per lo studio delle ali in flussi subsonici e transonici, attraverso il software Star-CCM+; analizzando, in particolare, l’evoluzione dei risultati al variare dell’ifittimento della griglia di calcolo. Si è partiti dai dati sperimentali, individuando alcune configurazioni notevoli; quindi si è provveduto alla generazione della geometria al CAD. Si è successivamente creato un modello fisico rappresentante il più possibile le condizioni delle prove sperimentali e lo si è verificato, confrontando nel frattempo più generi di mesh. Scelta la griglia ritenuta più efficiente è stato apportato un progressivo aumento del numero di celle, andando quindi ad analizzare l’evoluzione dei risultati ottenuti (in termini di Cp, CL, CM, CD). Si è ottenuto un buon accordo con i dati sperimentali, in termini di pressioni e forze di portanza, al netto degli errori attesi; per quanto concerne le forze di resistenza si è osservata la possibilità di aumentare l’accuratezza ottenibile e si sono indicate diverse vie di sviluppo in questo senso

Linear response theory around a localized impurity in the pseudogap regime of an anisotropic superconductor: precursor pairing vs the d-density-wave scenario

We derive the polarizability of an electron system in (i) the superconducting phase, with d-wave symmetry, (ii) the pseudogap regime, within the precursor pairing scenario, and (iii) the d-density-wave (dDW) state, characterized by a d-wave hidden order parameter, but no pairing. Such a calculation is motivated by the recent proposals that imaging the effects of an isolated impurity may distinguish between precursor pairing and dDW order in the pseudogap regime of the high-Tc superconductors. In all three cases, the wave-vector dependence of the polarizability is characterized by an azymuthal modulation, consistent with the d-wave symmetry of the underlying state. However, only the dDW result shows the fingerprints of nesting, with nesting wave-vector Q=(pi,pi), albeit imperfect, due to a nonzero value of the hopping ratio t'/t in the band dispersion relation. As a consequence of nesting, the presence of hole pockets is also exhibited by the (q,omega) dependence of the retarded polarizability.Comment: accepted in Phys. Rev.

Internal structure of preformed Cooper pairs

In order to obtain information about the internal structure of the preformed pairs in the pseudogap state of high $T_c$ superconductors, we calculate the propagator of a singlet pair with center of mass coordinate $\mathbf{r}$, and relative distance $\pmb{\rho}$, by solving the Bethe-Salpeter equation, representing the sum over repeated two-particle scattering events due to a distance dependent attraction. We define then a ``pair structure function'' $g_{P}(\mathbf{P},\pmb{\rho})$ that depends on the internal distance $\pmb{\rho}$ between the partners and on the momentum $\mathbf{P}$ of the pair. We calculate this function both for a local potential and $s-$wave symmetry of the order parameter and for a separable potential and $d-$wave symmetry of the order parameter. The influence of the center of mass momentum, strenght of the interaction, temperature, density of particles and of the pseudogap in the one-electron spectrum is studied for both cases.Comment: 12 pages, REVTeX4, 8 EPS figure

Size-shrinking of deuterons in very dilute superfluid nuclear matter

It is shown within the strong-coupling BCS approach that, starting from the zero-density limit of superfluid nuclear matter, with increasing density deuterons first shrink before they start expanding.Comment: 2 pages, Latex, 1 figure included, submitted to Phys. Rev.

Internal structure of fluctuating Cooper pairs

Abstract.: In order to obtain information about the internal structure of fluctuating Cooper pairs in the pseudogap state and below the transition temperature of high Tc superconductors, we solve the Bethe-Salpeter equation for the two-electron propagator in order to calculate a "pair structure function” $g_{P}({\mathbf{P}},\pmb{\rho})$ that depends on the internal distance $\pmb{\rho}$ between the partners and on the center of mass momentum P of the pair. We use an attractive Hubbard model with a local potential for s-wave and a separable potential for d-wave symmetry. The amplitude of gP for small ρ depends on temperature, chemical potential and interaction symmetry, but the ρ dependence itself is rather insensitive to the interaction strength. Asymptotically gP decreases as an inverse power of ρ for weak coupling, but exponentially when a pseudogap develops for stronger interaction. Some possibilities of observing the pair structure experimentally are mentione

Binary De Bruijn sequences for DS-CDMA systems: analysis and results

Abstract Code division multiple access (CDMA) using direct sequence (DS) spread spectrum modulation provides multiple access capability essentially thanks to the adoption of proper sequences as spreading codes. The ability of a DS-CDMA receiver to detect the desired signal relies to a great extent on the auto-correlation properties of the spreading code associated to each user; on the other hand, multi-user interference rejection depends on the cross-correlation properties of all the spreading codes in the considered set. As a consequence, the analysis of new families of spreading codes to be adopted in DS-CDMA is of great interest. This article provides results about the evaluation of specific full-length binary sequences, the De Bruijn ones, when applied as spreading codes in DS-CDMA schemes, and compares their performance to other families of spreading codes commonly used, such as m-sequences, Gold, OVSF, and Kasami sequences. While the latter sets of sequences have been specifically designed for application in multi-user communication contexts, De Bruijn sequences come from combinatorial mathematics, and have been applied in completely different scenarios. Considering the similarity of De Bruijn sequences to random sequences, we investigate the performance resulting by applying them as spreading codes. The results herein presented suggest that binary De Bruijn sequences, when properly selected, may compete with more consolidated options, and encourage further investigation activities, specifically focused on the generation of longer sequences, and the definition of correlation-based selection criteria

Algunas reflexiones sobre el lugar de la asistencia social en la política socialmoderna y la evolución de la noción de riesgo, a partir de la experiencia latinoamericana

Creo que la política asistencial es el conjunto de instrumentos con los que el Estado moderno intenta conjurar estos tres tipos de riesgos. Muchos están en un Ministerio de Desarrollo Social, ámbito especializado en estos temas. Pero otros están en el modo en que se regula el empleo informal o el no calificado, fuera de la seguridad social y de las relaciones Estado-sindicatos. Otros están en el sistema de Justicia -en los códigos, en las prácticas de policía, en los tribunales y las prisiones. Me propongo dedicar las próximas líneas a reflexionar sobre estas políticas, sus principios ordenadores y sus consecuencias en las fallas de la integración social que intentan conjurar.Eje 4: Balances y perspectivas sobre las políticas públicas en la región. Sus impactos en la desigualdad, la construcción de derechos y constitución de sujetos.Facultad de Trabajo Socia

Longitudinal Power Monitoring Performance with Subcarrier Multiplexing Transmission

We investigate the performance of Longitudinal Power Monitoring (LPM) algorithms at the small symbol rates achieved with subcarrier multiplexing. We show that at those symbol rates LPM is still effective, albeit with a noisier estimation profile

The role of transposable elements activity in aging and their possible involvement in laminopathic diseases

Abstract Eukaryotic genomes contain a large number of transposable elements, part of which are still active and able to transpose in the host genome. Mobile element activation is repressed to avoid deleterious effects, such as gene mutations or chromosome rearrangements. Control of transposable elements includes a variety of mechanisms comprising silencing pathways, which are based on the production of small non-coding RNAs. Silencing can occur either through transposable element RNA degradation or through the targeting of DNA sequences by heterochromatin formation and consequent transcriptional inhibition. Since the important role of the heterochromatin silencing, the gradual loss of heterochromatin marks in constitutive heterochromatin regions during the aging process promotes derepression of transposable elements, which is considered a cause of the progressive increase in genomic instability and of the activation of inflammatory responses. This review provides an overview of the effects of heterochromatin loss on the activity of transposable elements during the aging process and the possible impact on genome function. In this context, we discuss the possible role of the nuclear lamina, a major player in heterochromatin dynamics, in the regulation of transposable element activity and potential implications in laminopathic diseases