Spin-glass behaviour on random lattices

The ground-state phase diagram of an Ising spin-glass model on a random graph with an arbitrary fraction $w$ of ferromagnetic interactions is analysed in the presence of an external field. Using the replica method, and performing an analysis of stability of the replica-symmetric solution, it is shown that $w=1/2$, correponding to an unbiased spin glass, is a singular point in the phase diagram, separating a region with a spin-glass phase ($w<1/2$) from a region with spin-glass, ferromagnetic, mixed, and paramagnetic phases ($w>1/2$)

Proposta de um padrão de implementação de sistemas de equações diferenciais ordinárias em linguagem R.

A linguagem R é utilizada para cálculos e análise de dados estatísticos e gráficos e, portanto, é utilizada em diversas áreas de pesquisas. Foi proposto um padrão de codificação para modelos matemáticos baseados em sistemas de equações diferenciais ordinárias, a fim de melhorar a legibilidade, facilitar o aprendizado, o reúso, a verificação e integração com eventuais projetos que possam aparecer. Este pensamento surgiu baseado nas experiências obtidas por meio da codificação de modelos matemáticos desenvolvidos para o projeto Pecus

Semiclassical Evolution of Dissipative Markovian Systems

A semiclassical approximation for an evolving density operator, driven by a "closed" hamiltonian operator and "open" markovian Lindblad operators, is obtained. The theory is based on the chord function, i.e. the Fourier transform of the Wigner function. It reduces to an exact solution of the Lindblad master equation if the hamiltonian operator is a quadratic function and the Lindblad operators are linear functions of positions and momenta. Initially, the semiclassical formulae for the case of hermitian Lindblad operators are reinterpreted in terms of a (real) double phase space, generated by an appropriate classical double Hamiltonian. An extra "open" term is added to the double Hamiltonian by the non-hermitian part of the Lindblad operators in the general case of dissipative markovian evolution. The particular case of generic hamiltonian operators, but linear dissipative Lindblad operators, is studied in more detail. A Liouville-type equivariance still holds for the corresponding classical evolution in double phase, but the centre subspace, which supports the Wigner function, is compressed, along with expansion of its conjugate subspace, which supports the chord function. Decoherence narrows the relevant region of double phase space to the neighborhood of a caustic for both the Wigner function and the chord function. This difficulty is avoided by a propagator in a mixed representation, so that a further "small-chord" approximation leads to a simple generalization of the quadratic theory for evolving Wigner functions.Comment: 33 pages - accepted to J. Phys.

Experimental Observation of Environment-induced Sudden Death of Entanglement

We demonstrate the difference between local, single-particle dynamics and global dynamics of entangled quantum systems coupled to independent environments. Using an all-optical experimental setup, we show that, while the environment-induced decay of each system is asymptotic, quantum entanglement may suddenly disappear. This "sudden death" constitutes yet another distinct and counter-intuitive trait of entanglement.Comment: 4 pages, 4 figure

Decoherence of Semiclassical Wigner Functions

The Lindblad equation governs general markovian evolution of the density operator in an open quantum system. An expression for the rate of change of the Wigner function as a sum of integrals is one of the forms of the Weyl representation for this equation. The semiclassical description of the Wigner function in terms of chords, each with its classically defined amplitude and phase, is thus inserted in the integrals, which leads to an explicit differential equation for the Wigner function. All the Lindblad operators are assumed to be represented by smooth phase space functions corresponding to classical variables. In the case that these are real, representing hermitian operators, the semiclassical Lindblad equation can be integrated. There results a simple extension of the unitary evolution of the semiclassical Wigner function, which does not affect the phase of each chord contribution, while dampening its amplitude. This decreases exponentially, as governed by the time integral of the square difference of the Lindblad functions along the classical trajectories of both tips of each chord. The decay of the amplitudes is shown to imply diffusion in energy for initial states that are nearly pure. Projecting the Wigner function onto an orthogonal position or momentum basis, the dampening of long chords emerges as the exponential decay of off-diagonal elements of the density matrix.Comment: 23 pg, 2 fi

Shrinking the plan. A middle-class wishful thinking in the outskirts of Lisbon

In the 1960s and 1970s, large-scale Portuguese architectural production was largely promoted by the State. This took further aspects after the 1974 revolution when housing construction became one of the pillars of the new state policy. It is in this context that Manuel Vicente develops the project for Quinta do Bacalhau in different times: 1– Before 1974 by EPUL as part of UNOR 26 of which M. Vicente was the coordinator, 2– From 1974 as a SAAL operation after the site was taken over by a residents' committee, 3– After 1976, as it was built outside the revolutionary period by a housing cooperative and financed by the state. Unlike most SAAL projects – small, contained and with low-row houses –, and despite the ideologically marked circumstances of this period, it presents somehow ostentatious. Volumes of broad and generous features, incorporation of commerce on the ground floors like a boulevard, contrast with the dominant model, thus raising some questions: – to what extent did a particular class position correspond to a certain typology, style, etc.? – wouldn't this imply an aspiration to the same “rights” as the middle-class, as far as architecture is concerned? M. Vicente remains throughout his life in an ideologically multifaceted position. Close to the Communist Party, he has a cosmopolitan experience still in the 1960s being in touch with the Western capitalism – in the United States and in Macao – and in close contact with the speculation and profit markets. It has thus a double folded stance regarding the state-sponsored housing and low standards one, as if responding to the residents were the same as responding to himself. In this communication we intend to analyse through drawings of the project how the notion of housing for “a middle-class” crossed through the project. This argument is revealed in a plan of a dwelling with dimensions close to middle-class, which is later reduced to smaller areas, but “keeping” architectural qualities.info:eu-repo/semantics/publishedVersio

Accuracy of a teleported trapped field state inside a single bimodal cavity

We propose a simplified scheme to teleport a superposition of coherent states from one mode to another of the same bimodal lossy cavity. Based on current experimental capabilities, we present a calculation of the fidelity that can be achieved, demonstrating accurate teleportation if the mean photon number of each mode is at most 1.5. Our scheme applies as well for teleportation of coherent states from one mode of a cavity to another mode of a second cavity, both cavities embedded in a common reservoir.Comment: 4 pages, 2 figures, in appreciation for publication in Physical Review

Diluted antiferromagnet in a ferromagnetic enviroment

The question of robustness of a network under random ``attacks'' is treated in the framework of critical phenomena. The persistence of spontaneous magnetization of a ferromagnetic system to the random inclusion of antiferromagnetic interactions is investigated. After examing the static properties of the quenched version (in respect to the random antiferromagnetic interactions) of the model, the persistence of the magnetization is analysed also in the annealed approximation, and the difference in the results are discussed

Nanometric pitch in modulated structures of twist-bend nematic liquid crystals

The extended Frank elastic energy density is used to investigate the existence of a stable periodically modulate structure that appears as a ground state exhibiting a twist-bend molecular arrangement. For an unbounded sample, we show that the twist-bend nematic phase $N_{TB}$ is characterized by a heliconical structure with a pitch in the nano-metric range, in agreement with experimental results. For a sample of finite thickness, we show that the wave vector of the stable periodic structure depends not only on the elastic parameters but also on the anchoring energy, easy axis direction, and the thickness of the sample.Comment: 11 page