New results for the t-J model in ladders: Changes in the spin liquid state with applied magnetic field. Implications for the cuprates

Exact Diagonalization calculations are presented for the t-J model in the presence of a uniform magnetic field. Results for 2xL ladders (L=8,10,12) and 4x4 square clusters with 1 and 2 holes indicate that the diamagnetic response to a perpendicular magnetic field tends to induce a spin liquid state in the spin background. The zero-field spin liquid state of a two-leg ladder is reinforced by the magnetic field: a considerable increase of rung antiferromagnetic correlations is observed for J/t up to 0.6, for 1 and 2 holes. Pair-breaking is also clearly observed in the ladders and seems to be associated in part with changes promoted by the field in the spin correlations around the zero-field pair. In the 4x4 cluster, the numerical results seem to indicate that the field-induced spin liquid state competes with the zero-field antiferromagnetic short-range-order, the spin liquid state being favored by higher doping and smaller values of J/t. It is interesting to note that the field-effect can also be observed in a 2x2 plaquette with 1 and 2 holes. This opens up the possibility of gaining a qualitative understanding of the effect.Comment: 16 pages, 7 figures, latex New results adde

Determinação de extrato etéreo em amostras vegetais com uso de solvente sob alta pressão: avaliação do equipamento semiautomático ANKOM XT15.

O presente trabalho foi conduzido com o objetivo de avaliar a acurácia do método AOCS Am 5-04 para determinação de extrato etéreo em amostras vegetais (teor máximo de extrato etéreo de 5%). Adicionalmente, a substituição dos sacos de filtro ANKOM XT4 por sacos confeccionados em poliéster foi avaliada, com vistas à redução do custo analítico.bitstream/item/124947/1/cnpc-2015-Cot142.pd

Social Effects in Science: Modelling Agents for a Better Scientific Practice

Science is a fundamental human activity and we trust its results because it has several error-correcting mechanisms. Its is subject to experimental tests that are replicated by independent parts. Given the huge amount of information available, scientists have to rely on the reports of others. This makes it possible for social effects to influence the scientific community. Here, an Opinion Dynamics agent model is proposed to describe this situation. The influence of Nature through experiments is described as an external field that acts on the experimental agents. We will see that the retirement of old scientists can be fundamental in the acceptance of a new theory. We will also investigate the interplay between social influence and observations. This will allow us to gain insight in the problem of when social effects can have negligible effects in the conclusions of a scientific community and when we should worry about them.Comment: 14 pages, 5 figure

On the Stability of Fundamental Couplings in the Galaxy

Astrophysical tests of the stability of Nature's fundamental couplings are a key probe of the standard paradigms in fundamental physics and cosmology. In this report we discuss updated constraints on the stability of the fine-structure constant $\alpha$ and the proton-to-electron mass ratio $\mu=m_p/m_e$ within the Galaxy. We revisit and improve upon the analysis by Truppe {\it et al.} by allowing for the possibility of simultaneous variations of both couplings and also by combining them with the recent measurements by Levshakov {\it et al.} By considering representative unification scenarios we find no evidence for variations of $\alpha$ at the 0.4 ppm level, and of $\mu$ at the 0.6 ppm level; if one uses the Levshakov bound on $\mu$ as a prior, the$\alpha$ bound is improved to 0.1 ppm. We also highlight how these measurements can constrain (and discriminate among) several fundamental physics paradigms.Comment: 7 pages, 1 figur

Vorton Formation

In this paper we present the first analytic model for vorton formation. We start by deriving the microscopic string equations of motion in Witten's superconducting model, and show that in the relevant chiral limit these coincide with the ones obtained from the supersonic elastic models of Carter and Peter. We then numerically study a number of solutions of these equations of motion and thereby suggest criteria for deciding whether a given superconducting loop configuration can form a vorton. Finally, using a recently developed model for the evolution of currents in superconducting strings we conjecture, by comparison with these criteria, that string networks formed at the GUT phase transition should produce no vortons. On the other hand, a network formed at the electroweak scale can produce vortons accounting for up to 6% of the critical density. Some consequences of our results are discussed.Comment: 41 pages; color figures 3-6 not included, but available from authors. To appear in Phys. Rev.

Qualitative understanding of the sign of t' asymmetry in the extended t-J Model and relevance for pairing properties

Numerical calculations illustrate the effect of the sign of the next nearest-neighbor hopping term t' on the 2-hole properties of the t-t'-J model. Working mainly on 2-leg ladders, in the -1.0 < t'/t < 1.0 regime, it is shown that introducing t' in the t-J model is equivalent to effectively renormalizing J, namely t' negative (positive) is equivalent to an effective t-J model with smaller (bigger) J. This effect is present even at the level of a 2x2 plaquette toy model, and was observed also in calculations on small square clusters. Analyzing the transition probabilities of a hole-pair in the plaquette toy model, it is argued that the coherent propagation of such hole-pair is enhanced by a constructive interference between both t and t' for t'>0. This interference is destructive for t'<0.Comment: 5 pages, 4 figures, to appear in PRB as a Rapid Communicatio

Topological Defects in Contracting Universes

We study the behaviour and consequences of cosmic string networks in contracting universes. They approximately behave during the collapse phase as a radiation fluids. Scaling solutions describing this are derived and tested against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation it generates, can affect the dynamics of the universe both locally and globally, and be an important source of radiation, entropy and inhomogeneity. We discuss possible implications for bouncing and cyclic models.Comment: Shorter version of astro-ph/0206287. To appear in Phys. Rev. Let

Non-commutative fermion mass matrix and gravity

The first part is an introductory description of a small cross-section of the literature on algebraic methods in non-perturbative quantum gravity with a specific focus on viewing algebra as a laboratory in which to deepen understanding of the nature of geometry. This helps to set the context for the second part, in which we describe a new algebraic characterisation of the Dirac operator in non-commutative geometry and then use it in a calculation on the form of the fermion mass matrix. Assimilating and building on the various ideas described in the first part, the final part consists of an outline of a speculative perspective on (non-commutative) quantum spectral gravity. This is the second of a pair of papers so far on this project.Comment: To appear in Int. J. Mod. Phys. A Previous title: An outlook on quantum gravity from an algebraic perspective. 39 pages, 1 xy-pic figure, LaTex Reasons for new version: added references, change of title and some comments more up-to-dat

Symptom appraisal and help-seeking in men with symptoms of possible prostate cancer: a qualitative study with an ethnically diverse sample in London

This is the final version. Available on open access from the Royal College of General Practitioners via the DOI in this recordBACKGROUND: Prostate cancer mortality in Black males is disproportionately high. This problem may be overcome by reducing delays in the pathway to diagnosis, particularly those occurring before initial medical help seeking. A greater understanding of symptom appraisal and help seeking could support the development of targeted interventions for improving early presentation among Black males. AIM: To provide an in-depth understanding of males' pre-consultation experiences following the onset of symptoms of possible prostate cancer, identifying both general trends as well as potential differences that may exist between Black and White males. DESIGN AND SETTING: Qualitative study of 18 males (nine Black, nine White) in London, UK, who had recently seen their GP with urinary symptoms, erectile dysfunction, or haematuria. METHOD: Semi-structured interviews from a previous multi-methods study of primary care use by males with symptoms of possible prostate cancer were analysed using thematic framework analysis. RESULTS: Symptoms were often interpreted by patients as unimportant. Most delays occurred due to the absence of reasons to seek help, which, in Black males, often stemmed from poor awareness of prostate cancer. This lack of awareness could have been a consequence of their reluctance to seek health information and discuss health issues with others in their social network. Friends and relatives played an important role in symptom appraisal and help seeking. CONCLUSION: Cognitive biases, cultural stigmas, and everyday interpersonal interactions should be important areas at which to target strategies seeking to reduce delays and improve early presentation in males with possible prostate cancer, particularly Black males.Cancer Research UKNational Institute for Health and Care Research (NIHR