Size and shape of Mott regions for fermionic atoms in a two-dimensional optical lattice

We investigate the harmonic-trap control of size and shape of Mott regions in the Fermi Hubbard model on a square optical lattice. The use of Lanczos diagonalization on clusters with twisted boundary conditions, followed by an average over 50-80 samples, drastically reduce finite-size effects in some ground state properties; calculations in the grand canonical ensemble together with a local-density approximation (LDA) allow us to simulate the radial density distribution. We have found that as the trap closes, the atomic cloud goes from a metallic state, to a Mott core, and to a Mott ring; the coverage of Mott atoms reaches a maximum at the core-ring transition. A `phase diagram' in terms of an effective density and the on-site repulsion is proposed, as a guide to maximize the Mott coverage. We also predict that the usual experimentally accessible quantities, the global compressibility and the average double occupancy (rather, its density derivative) display detectable signatures of the core-ring transition. Some spin correlation functions are also calculated, and predict the existence N\'eel ordering within Mott cores and rings.Comment: 5 pages, 6 figure

Highly resolved spectral functions of two-dimensional systems with neural quantum states

Spectral functions are central to link experimental probes to theoretical models in condensed matter physics. However, performing exact numerical calculations for interacting quantum matter has remained a key challenge especially beyond one spatial dimension. In this work, we develop a versatile approach using neural quantum states to obtain spectral properties based on simulations of the dynamics of excitations initially localized in real or momentum space. We apply this approach to compute the dynamical structure factor in the vicinity of quantum critical points (QCPs) of different two-dimensional quantum Ising models, including one that describes the complex density wave orders of Rydberg atom arrays. When combined with deep network architectures we find that our method reliably describes dynamical structure factors of arrays with up to $24\times24$ spins, including the diverging time scales at critical points. Our approach is broadly applicable to interacting quantum lattice models in two dimensions and consequently opens up a route to compute spectral properties of correlated quantum matter in yet inaccessible regimes.Comment: Published versio

Mean-field theories are simple for neural quantum states

The utilization of artificial neural networks for representing quantum many-body wave functions has garnered significant attention, with enormous recent progress for both ground states and non-equilibrium dynamics. However, quantifying state complexity within this neural quantum states framework remains elusive. In this study, we address this key open question from the complementary point of view: Which states are simple to represent with neural quantum states? Concretely, we show on a general level that ground states of mean-field theories with permutation symmetry only require a limited number of independent neural network parameters. We analytically establish that, in the thermodynamic limit, convergence to the ground state of the fully-connected transverse-field Ising model (TFIM), the mean-field Ising model, can be achieved with just one single parameter. Expanding our analysis, we explore the behavior of the 1-parameter ansatz under breaking of the permutation symmetry. For that purpose, we consider the TFIM with tunable long-range interactions, characterized by an interaction exponent $\alpha$. We show analytically that the 1-parameter ansatz for the neural quantum state still accurately captures the ground state for a whole range of values for $0\le \alpha \le 1$, implying a mean-field description of the model in this regime.Comment: 10 pages,3 figure

Diagnosing Potts criticality and two-stage melting in one-dimensional hard-boson models

We investigate a model of hard-core bosons with infinitely repulsive nearest- and next-nearest-neighbor interactions in one dimension, introduced by Fendley, Sengupta and Sachdev in Phys. Rev. B 69, 075106 (2004). Using a combination of exact diagonalization, tensor network, and quantum Monte Carlo simulations, we show how an intermediate incommensurate phase separates a crystalline and a disordered phase. We base our analysis on a variety of diagnostics, including entanglement measures, fidelity susceptibility, correlation functions, and spectral properties. According to theoretical expectations, the disordered-to-incommensurate-phase transition point is compatible with Berezinskii-Kosterlitz-Thouless universal behaviour. The second transition is instead non-relativistic, with dynamical critical exponent $z > 1$. For the sake of comparison, we illustrate how some of the techniques applied here work at the Potts critical point present in the phase diagram of the model for finite next-nearest-neighbor repulsion. This latter application also allows to quantitatively estimate which system sizes are needed to match the conformal field theory spectra with experiments performing level spectroscopy.Comment: 18 pages, 14 figure

Personalismo e confiança pessoal na vida econômica cotidiana: o caso do “Crediário da Casas Bahia” (1970-2005)

Nosso problema de pesquisa concerne às bases de confiança na vida econômica. Nosso objetivo central de investigação é identificar e analisar os modos de coexistência entre interesses próprios e generosidades nas relações pessoais em acordos e negociações de crédito e cobrança no chão das lojas da rede varejista Casas Bahia entre os anos de 1970 e 2005. Nosso enfoque de pesquisa é qualitativo. Como instrumentos de coleta de dados, mobilizamos entrevistas semiestruturadas com ex-funcionários da rede varejista da Casas Bahia e com frequentadores da loja; recopilação de documentos; consulta a arquivos de dados e análise secundária de bancos de dados e de materiais biográficos compilados. Este estudo mostra como a confiança em pessoas pode estar mediada, em determinados contextos da vida econômica cotidiana, por uma dimensão que não foge ao interesse próprio, mas não funciona puramente sob o princípio da troca. Propriedades intermediárias entre o puro interesse próprio e a pura generosidade poderão ser úteis a quem se interessa pela questão de como se gera confiança na vida econômica – no Brasil e em outras sociedades.Our research problem is about the foundations of trust in economic life. Our main research objective is to identify and analyze the modes of coexistence between self-interest and generosity in personal relations under agreements and negotiations of credit and charge at the retail chain Casas Bahia between 1970 and 2005. Our research approach is qualitative. To collect data we mobilized semi-structured interviews with former employees of Casas Bahia and with customers; compilation of documents; consultation of data files and secondary analysis of databases and biographical materials. This study shows how trust in people can be mediated, in certain contexts of everyday economic life, by a dimension that does not escape self-interest, but does not work exclusively under the principle of exchange. Intermediate properties between pure self-interest and pure generosity may be useful to those interested in the question of how trust is generated in economic life – in Brazil and in other societies

Redes ópticas passivas de operação estendida

Mestrado em Engenharia Electrónica e TelecomunicaçõesA crescente procura por serviços de banda larga, e diversificação de conteúdos e serviços disponibilizados pela rede tem contribuído fortemente para o que se designa “last mile bottleneck”. Como solução surgiram as redes óticas de acesso passivas, que oferecem elevada largura de banda e uma relação preço por número de clientes servidos muito competitiva. Ao longo deste documento são estudadas as redes passivas de acesso que atualmente têm maior taxa de implementação, EPON e GPON e é proposto um cenário de convivência entre as duas utilizando a mesma fibra de distribuição. Para implementação do cenário proposto, foi estudada e avaliada separadamente a convivência no sentido ONUOLT e OLT-ONU. Na comunicação ONU-OLT a viabilidade estatística da convivência foi comprovada através de simulação e seguidamente corroboração experimental. Na comunicação OLT-ONU diversos mecanismos de conversão de comprimento de onda exclusivamente óticos como modulação de ganho e fase cruzada, e rotação cruzada de polarização, foram avaliados por simulação e comprovados em laboratório.The increasing demand for broadband services and the diversification of content and services provided by the network has strongly contributed for the so-called "last mile bottleneck". As a solution, the passive optical networks (PON) emerged, offering high bandwidth and a very competitive cost per number of customers served. Throughout this document are studied the passive access networks that currently experience higher deployment rates, EPON and GPON and a coexistence scenario over the same distribution fiber is proposed. To implement the proposed scenario and due to incompatibility between the two referred standards, upstream and downstream communication directions were studied and evaluated separately. For upstream communication the statistical viability of the network was assessed using simulation and experimental corroboration. In the downstream communication several mechanisms for all-optical wavelength conversion such as cross gain and phase modulation, and also cross polarization rotation were evaluated and tested

Tipos habitacionais e estratégias de reabilitação do núcleo urbano antigo do Seixal

Os núcleos urbanos antigos são portadores de um património histórico e arquitectónico a proteger e a potenciar. Para tal, é necessário compreender a sua génese, tanto no plano do edifício como no plano do conjunto urbano. Deste modo, considera-se importante o estudo do edificado como apoio a intervenções de reabilitação qualificadas, conscientes e sustentáveis. Este artigo aborda o assunto dos núcleos urbanos antigos na perspectiva da análise e inventariação das características do edificado. O processo de tipificação apresenta-se como síntese das principais formas de construção presentes, baseado na inspecção (observação e levantamento) e registo, tomando como caso de estudo o Núcleo Urbano Antigo do Seixal

Enhanced Phylogenetic Diversity of Anuran Communities: A Result of Species Loss in an Agricultural Environment

The great environmental changes induced by anthropogenic activities reshaped many ecosystem processes. Thus, the previously natural landscapes have been turned into mosaics of natural and seminatural lands embedded into human-modified landscape. To understand effects of these landscape modifications on the anuran communities, we aimed to compare pond-breeding anuran communities in a well-preserved forest with communities in agricultural landscape. We tested the values of taxonomic, phylogenetic and functional diversity, by analyzing four data matrixes containing environmental, phylogenetic, morphological and abundance information from 15 anuran species along this environmental gradient. Our analysis showed that only the phylogenetic component of the diversity was linked to the changes on the gradient and that the loss of environmental quality generates enhanced phylogenetic diversity. We showed that the anthropogenic disturbance acts negatively on anuran communities, forcing the species to adapt and behave like generalist species in order to survive at the modified places now available, where there were well-preserved forests before the humans changed it

Architectonic and constructive characterisation of the old urban centre of Seixal, Portugal

This paper addresses the issue of the old urban centres from the perspective of the analysis and inventory of building features. The cataloguing process of the building typologies is presented herein as a synthesis of the main construction forms, with the old city centre of Seixal being used as a case study. Furthermore, and taking into account the relation between such two dimensions, a material and constructive characterisation of the buildings in the old urban centre of Seixal is presented. As marks of an historical and architectural heritage, old urban centres should be protected, safeguarded and potentiated. For such, it is fundamental to have a complete understanding of the genesis of both buildings and urban mesh. This fact is essential to the support of qualified, conscientious and sustainable rehabilitation interventions on the old building stock