Stability of Droplets for the Three Dimensional Stochastic Ising Model

In this paper we study the evolution of the finite volume stochastic Ising model with magnetic field at very low temperatures starting from regular droplets, namely configurations on which all spins are -1 except those inside a parallelepiped

Neel order in the two-dimensional S=1/2 Heisenberg Model

The existence of Neel order in the S=1/2 Heisenberg model on the square lattice at T=0 is shown using inequalities set up by Kennedy, Lieb and Shastry in combination with high precision Quantum Monte Carlo data.Comment: 4 pages, 1 figur

Low-temperature nucleation in a kinetic Ising model with soft stochastic dynamics

We study low-temperature nucleation in kinetic Ising models by analytical and simulational methods, confirming the general result for the average metastable lifetime, = A*exp(beta*Gamma) (beta = 1/kT) [E. Jordao Neves and R.H. Schonmann, Commun. Math. Phys. 137, 209 (1991)]. Contrary to common belief, we find that both A and Gamma depend significantly on the stochastic dynamic. In particular, for a ``soft'' dynamic, in which the effects of the interactions and the applied field factorize in the transition rates, Gamma does NOT simply equal the energy barrier against nucleation, as it does for the standard Glauber dynamic, which does not have this factorization property.Comment: 4 pages RevTex4, 2 figures. Phys. Rev. Lett., in pres

Scaling analysis of a divergent prefactor in the metastable lifetime of a square-lattice Ising ferromagnet at low temperatures

We examine a square-lattice nearest-neighbor Ising quantum ferromagnet coupled to $d$-dimensional phonon baths. Using the density-matrix equation, we calculate the transition rates between configurations, which determines the specific dynamic. Applying the calculated stochastic dynamic in Monte Carlo simulations, we measure the lifetimes of the metastable state. As the magnetic field approaches $|H|/J=2$ at low temperatures, the lifetime prefactor diverges because the transition rates between certain configurations approaches zero under these conditions. Near $|H|/J=2$ and zero temperature, the divergent prefactor shows scaling behavior as a function of the field, temperature, and the dimension of the phonon baths. With proper scaling, the simulation data at different temperatures and for different dimensions of the baths collapse well onto two master curves, one for $|H|/J>2$ and one for $|H|/J<2$.Comment: published versio

Estudo de adaptação e validação de uma escala de perceção de liderança ética para líderes portugueses

Os líderes influenciam o comportamento individual e organizacional, incluindo a dimensão ética, sendo igualmente influenciados pela suas expetativas, interpretações e interações com os outros (Glynn & Jamerson, 2006; Kellerman, 2004).Frequentemente intervêm em diferentes contextos éticos, o que torna difícil para as pessoas "boas" tomarem boas decisões em situações más" (Glynn & Jamerson, 2006, p. 154).A liderança ética pode ser considerada como a "demonstração de conduta normativamente adequada através de ações pessoais e das relações interpessoais, assim como a promoção de tal conduta para os liderados através de uma comunicação bidireccional, reforço e tomada de decisão" (Brown, Trevino & Harrison, 2005, 9.120).O estudo aqui apresentado teve como objetivo desenvolver uma escala de liderança ética (Hanges & Dickson, 2004) para líderes portugueses com base na adaptação e validação da escala realizada por De Hoog e Den Hartog (2008).Os resultados obtidos evidenciam qualidades psicométicas adequadas, com um valor elevado de consistência interna. As análises fatoriais exploratórias revelam uma estrutura que aponta para a existência de dois fatores para a escala definidos como Liderança Ètica e Liderança Despótica que integram as respetivas dimensões.Importa, futuramente realizar uma análise da estabilidade das escalas com outra amostra por forma a verificar-se a consistência do s valores obtidos

Monte Carlo with Absorbing Markov Chains: Fast Local Algorithms for Slow Dynamics

A class of Monte Carlo algorithms which incorporate absorbing Markov chains is presented. In a particular limit, the lowest-order of these algorithms reduces to the $n$-fold way algorithm. These algorithms are applied to study the escape from the metastable state in the two-dimensional square-lattice nearest-neighbor Ising ferromagnet in an unfavorable applied field, and the agreement with theoretical predictions is very good. It is demonstrated that the higher-order algorithms can be many orders of magnitude faster than either the traditional Monte Carlo or $n$-fold way algorithms.Comment: ReVTeX, Request 3 figures from [email protected]

Symmetry Breaking and Finite Size Effects in Quantum Many-Body Systems

We consider a quantum many-body system on a lattice with a continuous symmetry which exhibits a spontaneous symmetry breaking in its infinite volume ground states, but in which the order operator does not commute with the Hamiltonian. A typical example is the Heisenberg antiferromagnet with a Neel order. In the corresponding finite system, the symmetry breaking is usually "obscured" by "quantum fluctuation" and one gets a symmetric ground state with a long range order. In such a situation, we prove that there exist ever increasing numbers of low-lying eigenstates whose excitation energies are bounded by a constant times 1/N, where N denotes the number of sites. By forming linear combinations of these low-lying states and the (finite-volume) ground state, and by taking infinite volume limits, we construct infinite volume ground states with explicit symmetry breaking. Our general theorems do not only shed light on the nature ofsymmetry breaking in quantum many-body systems, but provide indispensable information for numerical approaches to these systems. We also discuss applications of our general results to a variety of examples. The present paper is intended to be accessible to the readers without background in mathematical approaches to quantum many-body systems.Comment: LaTeX, 58 pages, no figures. Notes about Bose-Einstein condenstaion are added after the publicatio

Dilemas da geração Z: até que ponto ir à procura de padrões?

The reflective critical descriptive article aims to highlight the dilemmas of the generation that is the generation that has a ton of information in the palm of their hand in fractions of seconds, through the Internet. The following categories were listed: invasive plastic surgery procedures; generation Z, self-acceptance and image and finally mental and aesthetic health. Concluding that the search for the standard and the perfect stereotype associated with the day-to-day access to social media that do not always show the real life of the “bloggers” triggers the desire and obsession for aesthetic changes, most of the time radical and unnecessary, whose ages are between 18 and 24 years. Obviously, in most cases, the lack of confidence, insecurity and the difficulty of self-acceptance, trigger psychic disorders that lead these people to never be satisfied, and in search of diverting frustrations in most invasive surgeries that end up generating the need constant change, according to the temporary standards of beauty established by the media, but in some cases it is extremely necessary, such as bariatric surgery patients.O artigo descritivo crítico reflexivo tem como objetivo destacar os dilemas da geração que se trata da geração que tem uma tonelada de informações na palma da mão em frações de segundos, através da Internet. Foram elencadas as seguintes categorias: procedimentos invasivos de cirurgia plástica; geração Z, autoaceitação e imagem e por fim saúde mental e estética. Concluindo que a busca pelo padrão e o estereótipo perfeito associados ao acesso dia a dia das mídias sociais que nem sempre mostram a vida real das “blogueirinhas” faz desencadear o desejo e a obsessão por mudanças estéticas, na maioria das vezes radicais e sem necessidade, cuja idades estão entre 18 e 24 anos. Obviamente que na maioria dos casos a falta de confiança, insegurança e a dificuldade de autoaceitação, desencadeiam transtornos psíquicos que levam essas pessoas a nunca estarem satisfeitas, e em busca de desviar as frustrações em cirurgias na maioria das vezes invasivas e que acabam gerando a necessidade constante de mudança, de acordo com os padrões temporários de beleza estabelecidos pela mídia, porém em alguns casos e extremamente necessário como os pacientes de cirurgia bariátrica