Study of the bilinear biquadratic Heisenberg model on a honeycomb lattice via Schwinger bosons

We analyze the biquadratic bilinear Heisenberg magnet on a honeycomb lattice via Schwinger boson formalism. Due to their vulnerability to quantum fluctuations, non conventional lattices (kagome, triangular and honeycomb for example) have been cited as candidates to support spin liquid states. Such states without long range order at zero temperature are known in one-dimensional spin models but their existence in higher dimensional systems is still under debate. Biquadratic interaction is responsible for various possibilities and phases as it is well-founded for one-dimensional systems. Here we have used a bosonic representation to study the properties at zero and finite low temperatures of the biquadratic term in the two-dimensional hexagonal honeycomb lattice. The results show a ordered state at zero temperature but much more fragile than that of a square lattice; the behavior at finite low temperatures is in accordance with expectations.Comment: 14 pages, 5 figure

Energy in an Expanding Universe in the Teleparallel Geometry

The main purpose of this paper is to explicitly verify the consistency of the energy-momentum and angular momentum tensor of the gravitational field established in the Hamiltonian structure of the Teleparallel Equivalent of General Relativity (TEGR). In order to reach these objectives, we obtained the total energy and angular momentum (matter plus gravitational field) of the closed universe of the Friedmann-Lemaitre-Robertson-Walker (FLRW). The result is compared with those obtained from the pseudotensors of Einstein and Landau-Lifshitz. We also applied the field equations (TEGR) in an expanding FLRW universe. Considering the stress energy-momentum tensor for a perfect fluid, we found a teleparallel equivalent of Friedmann equations of General Relativity (GR).Comment: 19 pages, no figures. Revised in view of Referee's comments. Version to appear in the Brazilian Journal of Physic

Magnetization reversals in a disk-shaped small magnet with an interface

We consider a nanodisk possessing two coupled materials with different ferromagnetic exchange constant. The common border line of the two media passes at the disk center dividing the system exactly in two similar half-disks. The vortex core motion crossing the interface is investigated with a simple description based on a two-dimensional model which mimics a very thin real material with such a line defect. The main result of this study is that, depending on the magnetic coupling which connects the media, the vortex core can be dramatically and repeatedly flipped from up to down and vice versa by the interface. This phenomenon produces burst-like emission of spin waves each time the switching process takes place.Comment: 11 pages, 10 figure

Bound-states and polarized charged zero modes in three-dimensional topological insulators induced by a magnetic vortex

By coating a three-dimensional topological insulator (TI) with a ferromagnetic film supporting an in-plane magnetic vortex, one breaks the time-reversal symmetry (TRS) without generating a mass gap. It rather yields electronic states bound to the vortex center which have different probabilities associated with each spin mode. In addition, its associate current (around the vortex center) is partially polarized with an energy gap separating the most excited bound state from the scattered ones. Charged zero-modes also appear as fully polarized modes localized near the vortex center. From the magnetic point of view, the observation of such a special current in a TI-magnet sandwich comes about as an alternative technique for detecting magnetic vortices in magnetic thin films.Comment: 8 pages, 3 figures, new version with more discussions and results accepted for publication in The European Physical Journal

Pandemic context and implications for Portuguese SMEs’ internationalization

Considering that internationalization has such a big impact in SMEs and world’s economy, it is important to evaluate the consequences of a pandemic as it affects the functionality of the firms' operations and consequently the survival of economies. The purpose of this study is to perceive the repercussions for SMEs and their initiatives to overcome this recession period. Many questions emerge regarding this topic, namely: What are the main impacts caused by the pandemic? Did the internationalization strategy change after this? Did the firm stop its transnational operations? Which firm resources have proven to be essential to endure it? How did companies cope with it? Which approaches worked and which failed? Were there advantages for certain firms? Thus, the goal is to have a deep analysis of the factors influencing the internationalization of Portuguese SMEs using as a frame of reference a significant determinant, the pandemic.Considerando que a internacionalização tem um grande impacto nas PMEs e na economia do mundo, é importante avaliar as consequências de uma pandemia, uma vez que a mesma afeta as operações das empresas e consequentemente a sobrevivência das economias. O propósito deste estudo é perceber as repercussões para as PMEs e as suas iniciativas para superar este período de recessão. Surgem várias questões relacionadas com este tópico, nomeadamente: Quais são os principais impactos causados pela pandemia? A estratégia de Internacionalização teve de ser adaptada ou mudada? A empresa necessitou de interromper as suas operações transfronteiriças? Quais foram os recursos da empresa considerados essenciais para superar este período? Como é que as empresas lidaram com a pandemia? O que funcionou e o que falhou? Houve empresas que beneficiaram com a situação imposta? Assim, o objetivo é alcançar uma análise profunda dos fatores que influenciam a internacionalização das PMEs Portuguesas usando como referência um determinante significativo, a pandemia