Seminário do Grupo de Óptica: “Entanglement and exotic superfluidity”
Exotic superfluids are characterized by the coexistence of superconductivity and magnetism. In fermionic systems superfluidity is achieved by pairing of particles while magnetization is induced by an imbalanced population of two species of fermions. This exotic coexistence, first investigated by Fulde and Ferrell and Larkin and Ovchinnikov (FFLO), is predicted to show distinct properties in comparison to a conventional BCS superfluid. Experimentally, the FFLOstate has been investigated in cold atoms experiments by several groups (Ketterle, Hulet, Törmä). From the theoretical point of view, the Hubbard model has been used to investigate the FFLO properties and to predict the regimes where the FFLO state appears. In this talk we will discuss theoretical results for the 1D Hubbard lattice with spinimbalanced populations. In particular, we have obtained an analytical expression for the critical polarization below which the FFLO state emerges and depicted the phase diagram as a function of interaction and particle density. We have also investigated i) the effects of the experimental harmonic trap, ii) the local magnetization, iii)the behavior of entanglement, and iv) the spinflip channels which favor the pairing protection. We find that beyond the FFLOstate regime the spinflip processes trigger a breaking pairs avalanche, leading the system to the normal (non superfluid) polarized phase.
The contact process model of nonequilibrium phase transitions into absorbing states are widely used to study turbulent liquid crystals, driven suspensions, superconducting vortex dynamics, bacteria colony biofilms among other systems. We study this transition in the presence of a time-varying environmental noise and show that such temporal disorder gives rise to a distinct class of exotic “infinite-noise” critical points at which the effective noise amplitude diverges on long time scales. This leads to enormous density fluctuations characterized by an infinitely broad probability distribution at criticality.
In standard characterizations, science is self-correcting. Scientists examine each other’s work skeptically, try to replicate important discoveries, and thereby expose latent errors. Thus, while science is tentative, it also seems to have a system for correcting whatever mistakes arise. It powerfully explains and justifies the authority of science. Self-correction thus often serves emblematically in promoting science as a superior form of knowledge. But errors can and do occur. Some errors remain uncorrected for long periods. I present five sets of historical observations that indicate a need to rethink the widespread mythos of self-correction. First, some errors persist for decades, wholly undetected. Second, some corrections to erroneous theories have themselves been rejected when initially presented. Third, many errors seem corrected fortuitously, by independent happenstance, not by any methodical appraisal. Fourth, some errors have fostered further serious errors without the first error being noticed. Finally, some errors have been “corrected” in a cascade of successive errors that did not effectively remedy the ultimate source of the error. In all these cases, scientists failed to identify and correct the errors in a timely manner, or according to any uniform self-correcting mechanism. These historical perspectives underscore that error correction in science requires epistemic work. An understanding of errors forms an important type of negative knowledge.
Os modelos mais realistas dos fenômenos naturais envolvem equações diferenciais com termos não lineares, dispersivos e dissipativos. Em geral, essas contribuições tornam o estudo desses modelos muito mais difícil. Entretanto, existem combinações muito específicas dessas propriedades que produzem um tipo de solução conhecida como soliton.
The beauty of soap bubbles has long fascinated children and adults, and a quantitative model for explaining their burst was first developed by Lord Rayleigh in 1891. Slow-motion imaging of the rupture of soap bubbles generally shows the edges of liquid films retracting at a constant speed (known as the Taylor-Culick velocity), which is determined by momentum balance for the liquid rim collected at the film edge. In this talk we will report on our investigation[1] of soap bubbles formed from simple solutions of a cationic surfactant (cetyltrimethylammonium bromide – CTAB) and sodium salicylate, which have a viscolestic behavior due to the formation of “polymer-like” surfactant aggregates known as wormlike micelles. We demonstrate that these elastic bubbles collapse at a velocity up to thirty times higher than the Taylor-Culick limit, which has never been surpassed. This can be explained by a simple model that includes the elastic energy stored in the liquid film while the bubble is inflated, which is released when the film is ruptured, yielding an additional driving force for film retraction (besides surface tension). This new mechanism for the bursting of elastic liquid bubbles may have important implications to the breakup of viscoelastic sprays in industrial applications.
A proposta do programa é reunir estudantes e cientistas interessados em discutir temas relevantes na física de partículas, cosmologia e astrofísica de partículas.
Studying the high energy regime of QCD is a problem that can be tackled by perturbation theory. This is a consequence of asymptotic freedom, which states that the coupling constant of QCD decreases when increasing the energy. The low energy regime (often called the “nonperturbative regime”) is much more difficult to access, mainly because the standard perturbation theory is not applicable anymore.