Analytical and numerical studies of disordered spin-1 Heisenberg chains with aperiodic couplings

We investigate the low-temperature properties of the one-dimensional spin-1 Heisenberg model with geometric fluctuations induced by aperiodic but deterministic coupling distributions, involving two parameters. We focus on two aperiodic sequences, the Fibonacci sequence and the 6-3 sequence. Our goal is to understand how these geometric fluctuations modify the physics of the (gapped) Haldane phase, which corresponds to the ground state of the uniform spin-1 chain. We make use of different adaptations of the strong-disorder renormalization-group (SDRG) scheme of Ma, Dasgupta and Hu, widely employed in the study of random spin chains, supplemented by quantum Monte Carlo and density-matrix renormalization-group numerical calculations, to study the nature of the ground state as the coupling modulation is increased. We find no phase transition for the Fibonacci chain, while we show that the 6-3 chain exhibits a phase transition to a gapless, aperiodicity-dominated phase similar to the one found for the aperiodic spin-1/2 XXZ chain. Contrary to what is verified for random spin-1 chains, we show that different adaptations of the SDRG scheme may lead to different qualitative conclusions about the nature of the ground state in the presence of aperiodic coupling modulations.Comment: Accepted for publication in Physical Review

The Anomalous Hall effect in re-entrant AuFe alloys and the real space Berry phase

The Hall effect has been studied in a series of AuFe samples in the re-entrant concentration range, as well as in the spin glass range. The data demonstrate that the degree of canting of the local spins strongly modifies the anomalous Hall effect, in agreement with theoretical predictions associating canting, chirality and a real space Berry phase. The canonical parametrization of the Hall signal for magnetic conductors becomes inappropriate when local spins are canted.Comment: 4 pages, 1 eps figur

Explorando...: a complexidade do corpo humano: guião didáctico para professores: 1º ciclo

O presente livro faz parte da Coleção “Ensino Experimental das Ciências”, um conjunto de textos concebidos para apoiar a formação de professores(as) com vista à generalização do ensino das Ciências no 1.º Ciclo do Ensino Básico (CEB). Trata-se de um livro de suporte à Educação em Ciências para os primeiros níveis de escolaridade desenvolvido tendo por base orientações recentes a nível nacional e internacional

Magnetic field dependence of the density of states in the multiband superconductor β\beta-Bi2_2Pd

We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $\beta$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We compare the upper critical field $H_{c2}(T)$ obtained in our sample with previous measurements and explain available data within multiband supercondutivity. We propose that $\beta$-Bi$_2$Pd is a single gap multiband superconductor. We anticipate that single gap multiband superconductivity can occur in other compounds with complex Fermi surfaces.Comment: 8 pages, 7 figure

Multilayered interface in Ti/Macor (R) machinable glass-ceramic joints

Macor® machinable glass-ceramic and commercially pure (cp.) titanium were joined by active metal brazing, using a 64Ag–34.5Cu–1.5Ti (wt.%) filler alloy. The influence of the brazing temperature and holding stage on the microstructure and hardness profile of the interface, as well as on the shear strength of the joint, were assessed. Brazing was performed in a high vacuum furnace at 850, 890 and 930°C for 10 and 30 min. The reaction between the braze alloy and both materials led to the formation of a multilayered interface. The interfacial microstructure was analysed in a scanning electronic microscope (SEM) and the composition of each reaction layer was investigated by energy dispersive X-ray scans (EDS). The interfacial hardness profile was determined by a series of microhardness tests on each reaction layer. The mechanical strength of the joint was assessed from shear tests conducted at room temperature. Brazing at 850°C with a 10 min holding stage produced stronger joints, with an average shear strength of more than 85% of the glass-ceramic bulk strength

Desenvolvimento de linhagens de soja adaptadas as condições de cultivo do Rio Grande do Sul.

bitstream/item/133861/1/ID10317-1984-1985sojaresultados-p9-13.pdfTrabalho apresentado na XIII Reunião de Pesquisa de Soja da Região Sul, Porto Alegre, 1984

Avaliação do comportamento das cultivares de soja recomendadas para semeadura no Rio Grande do Sul.

bitstream/item/133864/1/ID10309-1984-1985sojaresultados-p58-61.pdfTrabalho apresentado na XIII Reunião de Pesquisa de Soja da Região Sul, Porto Alegre, 1984