Two-body bound and edge states in the extended SSH Bose-Hubbard model

We study the bosonic two-body problem in a Su-Schrieffer-Heeger dimerized chain with on-site and nearest-neighbor interactions. We find two classes of bound states. The first, similar to the one induced by on-site interactions, has its center of mass on the strong link, whereas the second, existing only thanks to nearest-neighbors interactions, is centered on the weak link. We identify energy crossings between these states and analyse them using exact diagonalization and perturbation theory. In the presence of open boundary conditions, novel strongly-localized edge-bound states appear in the spectrum as a consequence of the interplay between lattice geometry, on-site and nearest-neighbor interactions. Contrary to the case of purely on-site interactions, such EBS persist even in the strongly interacting regime.Comment: 12 pages, 8 figures; Submitted to EPJ Special Topics, Quantum Gases and Quantum Coherenc

Turbulent heat transfer in spacer-filled channels: Experimental and computational study and selection of turbulence models

Heat transfer in spacer-filled channels of the kind used in Membrane Distillation was studied in the Reynolds number range 100–2000, encompassing both steady laminar and early-turbulent flow conditions. Experimental data, including distributions of the local heat transfer coefficient h, were obtained by Liquid Crystal Thermography and Digital Image Processing. Alternative turbulence models, both of first order (k-ε, RNG k-ε, k-ω, BSL k-ω, SST k-ω) and of second order (LRR RS, SSG RS, ω RS, BSL RS), were tested for their ability to predict measured distributions and mean values of h. The best agreement with the experimental results was provided by first-order ω-based models able to resolve the viscous/conductive sublayer, while all other models, and particularly ε-based models using wall functions, yielded disappointing predictions

Particle-hole character of the Higgs and Goldstone modes in strongly-interacting lattice bosons

We study the low-energy excitations of the Bose-Hubbard model in the strongly-interacting superfluid phase using a Gutzwiller approach and extract the single-particle and single-hole excitation amplitudes for each mode. We report emergent mode-dependent particle-hole symmetry on specific arc-shaped lines in the phase diagram connecting the well-known Lorentz-invariant limits of the Bose-Hubbard model. By tracking the in-phase particle-hole symmetric oscillations of the order parameter, we provide an answer to the long-standing question about the fate of the pure amplitude Higgs mode away from the integer-density critical point. Furthermore, we point out that out-of-phase oscillations are responsible for a full suppression of the condensate density oscillations of the gapless Goldstone mode. Possible detection protocols are also discussed.Comment: 6 pages, 3 figure

Finite-momentum Bose-Einstein condensates in shaken 2D square optical lattices

We consider ultracold bosons in a 2D square optical lattice described by the Bose-Hubbard model. In addition, an external time-dependent sinusoidal force is applied to the system, which shakes the lattice along one of the diagonals. The effect of the shaking is to renormalize the nearest-neighbor hopping coefficients, which can be arbitrarily reduced, can vanish, or can even change sign, depending on the shaking parameter. It is therefore necessary to account for higher-order hopping terms, which are renormalized differently by the shaking, and introduce anisotropy into the problem. We show that the competition between these different hopping terms leads to finite-momentum condensates, with a momentum that may be tuned via the strength of the shaking. We calculate the boundaries between the Mott-insulator and the different superfluid phases, and present the time-of-flight images expected to be observed experimentally. Our results open up new possibilities for the realization of bosonic analogs of the FFLO phase describing inhomogeneous superconductivity.Comment: 7 pages, 7 figure

Optimization of net power density in Reverse Electrodialysis

Reverse Electrodialysis (RED) extracts electrical energy from the salinity difference between two solutions using selective ion exchange membranes. In RED, conditions yielding a large net power density (NPD) are generally desired, due to the still large cost of the membranes. NPD depends on a large number of physical and geometric parameters. Some of these, for example the inlet concentrations of concentrate and diluate, can be regarded as “scenario” variables, imposed by external constraints (e.g., availability) or chosen by different criteria than NPD maximization. Others, namely the thicknesses HCONC, HDIL and the velocities UCONC, UDIL in the concentrate and diluate channels, can be regarded as free design parameters and can be chosen so as to maximize NPD. In the present study, a simplified model of a RED stack was coupled with an optimization algorithm in order to determine the conditions of maximum NPD in the space of the variables HCONC, HDIL,UCONC, UDIL for different sets of “scenario” variables. The study shows that an optimal choice of the free design parameters for any given scenario, as opposed to the adoption of standard fixed values for the same parameters, may provide significant improvements in NPD

Experimental investigation of two-side heat transfer in spacer-filled channels

In Membrane Distillation (MD), spacers support the membranes and promote mixing, thus reducing temperature polarization. Their efficient design requires a knowledge of the distribution of the local heat transfer coefficient h and of its dependence on Reynolds number, spacer geometry and flow-spacer relative orientation. In previous work, we applied Thermochromic Liquid Crystals (TLC) and digital image processing to the measurement of h distributions for different spacer configurations; data were used to validate CFD simulations and select turbulence models. For constructive reasons, the test section allowed only one-side heat transfer, while in most MD configurations (e.g. spiral-wound modules) heat transfer occurs from both sides of the feed water channels. Analytical and numerical solutions show that changing from one-side to two-side heat transfer deeply affects h values. This motivated the design and construction of an improved test section in which a hot channel is sandwiched between two cold channels, and twin cameras and lighting equipment allow the simultaneous acquisition of TLC images on both walls. This paper describes this new test section and the experimental technique, discusses measurement uncertainty, and presents preliminary results

Nuevas consideraciones sobre medición y evaluación de la vellosidad de los hilados.

Mediante el presente trabajo se hace un estudio de la vellosidad de determinados tipos de hilados. Los vellosímetros utilizados corresponden al aparato "Zweigle G 565" y al "Uster Tester 3". Para complementar el estudio, también se han realizado mediciones complementarias con el aparato "Digital ITQT". Los experimentos realizados con estos dos primeros aparatos pusieron de manifiesto que para determinados tipos de hilados existe una correlación elevada y fiable entre los resultados. En otros tipos de hilados de aspecto voluminoso, en los que no existía una textura nítida a ras del cuerpo del hilado, se observó, por el contrario, la presencia de anomalías que llevaron a tener que distinguir entre una vellosidad "corta" y una vellosidad "larga". Debido a la importancia que tiene ambos tipos de vellosidad, se aconseja que los aparatos comercializados evolucionen en el sentido de expresar la vellosidad en dos índices en vez de sólo uno, siguiendo el principio utilizado para el aparato digital "ITQT".This paper studies the hairiness of some spun yarns. The hair-meters used are the "Zwigle G 565" and "Uster Tester 3". Additional measures have been also made with the "Digital ITQT" device. The experiments carried out with the former devices revealed the existence of high and reliable correlation in the results found for some types of spun yarns. On the contrary in other types of bulky-looking spun yarns, with no smooth hand on the texture, the presence of some irregularities were observed so that a distinction behiveen "short hairiness" ad "long hairiness" had to be made. Because of the importance of both types of hairiness, the commercial devices should be modernized so as to be able to distinguish them with different indexes, following the principie used in the digital "ITQT" device.Moyennant le présent travail on fait un étude de la pilosité de certains types de filés. Les appareils pour la mesure de la pilosité utilisés concernent l'appareil "Zweigle G 565" et I'"Uster Tester 3". Pour compléter l'étude, on a réalisé, aussi, des mesures complémentaires avec l'appareil "Digital ITQT". Les expériences réalisées avec les deux premiers appareils ont mis de manifeste que pour certains types de filés il existe une corrélation élevée et fiable entre les résultats. Dans d'autres types de filés à aspect volumineux, où il n'existait pas de texture nette à ras du corps du filé, on observé, pas contre, la présence d'anomalies, qui ont mené au fait d'avoir à distinguer entre une pilosité "courte" et una pilosité "longue". A cause de l'importance de ces deux types de pilosité, on conseille que les appareils commercialisés évolutionnent dans le sens d'exprimer la pilosité en deux indices au lieu d'un seul, suivant le principe utilisé pour l'appareil digital "ITQT".Peer Reviewe

Human capital and convergence:Ttheory, estimation and applications.

In growth theory, convergence analysis tries to answer three fundamental questions "Are poor countries catching up with richer ones How quickly And what are the determinants of this process" This thesis deals with issues that are relevant to all these questions. It begins by setting out the key theoretical contributions to the analysis of the role of human capital in growth and convergence. Secondly, attention is turned to the way that convergence is estimated from data. The econometric techniques used in the convergence literature usually assume that shocks are uncorrelated across countries. We claim that this is unlikely for most data sets and investigate the use of an estimator so far ignored, namely the annual panel estimator where shocks are allowed to be correlated. Our analysis indicates that this estimator is more efficient than conventional ones for plausible values of cross-country error correlation. The study then turns to the analysis of the third question. Although differences in human capital endowments and rates of investment have long been recognised as crucial elements for explaining observed GDP gaps, nevertheless, human capital proxies are rarely significant in growth regressions. In this study some possible solutions to this puzzle are explored. We estimate aggregate returns to education in Italy and Spain, and compare our results with the predictions of competing theoretical frameworks. In general, our empirical analysis identifies a positive role for human capital, and stresses the relevance of theoretical models in which human capital has a fundamental but indirect role in the catching up process. The final part of the thesis proposes a new methodology designed to estimate technology levels and to test whether part of observed convergence is due to technology convergence. The results seem to confirm the existence of technology catch-up among regions