1,615 research outputs found
On Rastall gravity formulation as a and a theory
No full textRastall introduced a stress-energy tensor whose divergence is proportional to
the gradient of the Ricci scalar. This proposal leads to a change in the form
of the field equations of General Relativity, but it preserves the number of
degrees of freedom. Rastall's field equations can be either interpreted as GR
with a redefined SET, or it can imply different physical consequences inside
the matter sector. We investigate limits under which the Rastall field
equations can be directly derived from an action, in particular from two
-gravity extensions: and . We show that there
are similarities between these theories, but the Rastall SET cannot be fully
recovered from them, apart from certain particular cases here discussed. It is
remarkable that a simple, covariant and invertible redefinition of the SET, as
the one proposed by Rastall, is hard to be directly implemented in the action.Comment: 11 pages, accepted for publication in the European Physical Journal
Plu
Nonzero Skyrmion Hall Effect in Topologically Trivial Structures
No full textIt is widely believed that the skyrmion Hall effect, often disruptive for
device applications, vanishes for overall topologically trivial structures such
as (synthetic) antiferromagnetic skyrmions and skyrmioniums due to a
compensation of Magnus forces. In this manuscript, however, we report that in
contrast to the case of spin-transfer torque driven skyrmion motion, this
notion is generally false for spin-orbit torque driven objects. We show that
the skyrmion Hall angle is directly related to their helicity and imposes an
unexpected roadblock for developing faster and lower input racetrack memories
based on spin-orbit torques
Effective description of domain wall strings
No full textThe analysis of domain wall dynamics is often simplified to one-dimensional physics. For domain walls in thin films, more realistic approaches require the description as two-dimensional objects. This includes the study of vortices and curvatures along the domain walls as well as the influence of boundary effects. Here we provide a theory in terms of soft modes that allows us to analytically study the physics of extended domain walls and their stability. By considering irregularly shaped skyrmions as closed domain walls, we analyze their plasticity and compare their dynamics with those of circular skyrmions. Our theory directly provides an analytical description of the excitation modes of magnetic skyrmions, previously accessible only through sophisticated micromagnetic numerical calculations and spectral analysis. These analytical expressions provide the scaling behavior of the different physics on parameters that experiments can test
Facilitating domain wall injection in magnetic nanowires by electrical means
No full textWe investigate how to facilitate the injection of domain walls in chiral ferromagnetic nanowires by electrical means. We calculate the critical current density above which domain walls are injected into the nanowire depending on the material parameters and the source of interaction including spin-transfer torques as well as spin-orbit torques. We demonstrate that the Dzyaloshinskii-Moriya interaction can significantly reduce the required critical current to inject the types of domain walls favored by the Dzyaloshinskii-Moriya interaction. We find that in chiral magnets it is only possible to shed a single domain wall by means of spin-orbit torques, as they modify the ground state orientation of the system. In contrast, for spin-transfer torque-induced shedding of domain walls, we show that there exist two different critical current densities for the two different domain wall chiralities, respectively. Additionally, for the consecutive creation of domain walls by means of spin-transfer torques, we find that the interaction between the domain walls cannot be neglected and even may lead to the pairwise annihilation of consecutive domain walls with opposite chiralities
Current-induced H-shaped-skyrmion creation and their dynamics in the helical phase
No full textInevitable for the basic principles of skyrmion racetrack-like applications is not only their confined motion along one-dimensional channels but also their controlled creation and annihilation. Helical magnets have been suggested to naturally confine the motion of skyrmions along the tracks formed by the helices, which also allow for high-speed skyrmion motion. We propose a protocol to create topological magnetic structures in a helical background. We furthermore analyse the stability and current-driven motion of the skyrmions in a helical background with in-plane uniaxial anisotropy fixing the orientation of the helices
Transformações da cobertura vegetal do Parque Ecológico Municipal Professor João Davi Ferreira Lima e a sua importância como área verde para Florianópolis-SC
No full textTCC (graduação) - Universidade Federal de Santa Catarina, Centro de Filosofia e Ciências Humanas, Curso de Geografia.O presente trabalho tem por objetivo analisar as transformações da cobertura vegetal ocorridas no Parque Ecológico Municipal Professor João Davi Ferreira Lima ou simplesmente Parque Ecológico do Córrego Grande-PECG, a partir da década de 1930 até os dias atuais. O foco central deste trabalho consiste no estudo dos sucessivos processos de degradação ao longo da sua história e seu posterior redirecionamento para a conservação da biodiversidade: como a produção e plantio de mudas de espécies nativas, especialmente para a substituição das espécies exóticas do gênero Pinus.spp e Eucaliptus.spp, e na avaliação de como se encontra a regeneração após o repovoamento com as espécies nativas e seu atual estágio de sucessão. Outro enfoque não menos importante, é a importância do PECG como área verde de lazer, para a comunidade da região e daqueles que o frequentam; e também como área de relevante interesse ecológico, e de educação ambiental. A metodologia fundamenta-se em pesquisa bibliográfica; pesquisa de matérias em periódicos locais; fotointerpretação, que permitiu comparar e descrever a evolução da cobertura vegetal de cada período; entrevistas com antigos moradores da região e administradores antigos e atuais do Parque,que remete ao seu passado, traz parte da sua história e permite comparação com a atualidade. Também foram aplicados cinquenta questionários entre os frequentadores do parque, o que possibilitou saber qual a visão socioambiental desses; foram confeccionados mapas para delinear sua localização e estudos específicos, além de gráficos para se obter os percentuais equitativos dos questionários aplicados. Os resultados do trabalho foram significativos porque foi feito pela primeira vez um levantamento completo do histórico do PECG, ressaltando a importância da continuidade da recomposição da sua cobertura vegetal, que também reflete na volta da fauna à região, a necessidade de transformar-se em Unidade de Conservação e sua importância para a integração homem-natureza
Spin texture motion in antiferromagnetic and ferromagnetic nanowires
No full textWe propose a Hamiltonian dynamics formalism for the current and magnetic field driven dynamics of ferromagnetic and antiferromagnetic domain walls in one-dimensional systems. To demonstrate the power of this formalism, we derive Hamilton equations of motion via Poisson brackets based on the Landau-Lifshitz-Gilbert phenomenology, and add dissipative dynamics via the evolution of the energy. We use this approach to study current induced domain-wall motion and compute the drift velocity. For the antiferromagnetic case, we show that a nonzero magnetic moment is induced in the domain wall, which indicates that an additional application of a magnetic field would influence the antiferromagnetic domain-wall dynamics. We consider both cases of the magnetic field being parallel and transverse to the Néel field. Based on this formalism, we predict an orientation switch mechanism for antiferromagnetic domain walls which can be tested with the recently discovered Néel spin orbit torques
Rodrigues parameters on dual hyperbolic unit sphere H20
No full textRodrigues parameters depend on the tangent of the half rotation angle in Euclidean space but in Dual space, dual Rodrigues parameters contain both rotation angle and distance corresponding the shortest distance between the straight lines in R-3. In this paper, we give Cayley's formula for the dual hyperbolic spherical motion and explain 3x3 type L-Dual skew symmetric matrices by using properties of this formula. Then, we obtain Rodrigues parameters of dual Hyperbolic unit sphere and show that Rodrigues parameters contain the hyperbolic rotation angle which is being between timelike lines and distance which is the minimal Lorentzian distance between the timelike lines of R-1(3)TUBITAK (The Scientific and Technological Research Council of Turkey)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK)The first author would like to thank TUBITAK (The Scientific and Technological Research Council of Turkey) for their financial supports during her PhD studies
Dzyaloshinskii-Moriya Induced Spin-Transfer Torques in Kagome Antiferromagnets
No full textIn recent years antiferromagnets (AFMs) have become very promising for
nanoscale spintronic applications due to their unique properties such as THz
dynamics and absence of stray fields. Manipulating antiferromagnetic textures
is currently, however, limited to very few exceptional material symmetry
classes allowing for staggered torques on the magnetic sublattices. In this
work, we predict for kagome AFMs with broken mirror symmetry a new coupling
mechanism between antiferromagnetic domain walls (DWs) and spin currents,
produced by the relativistic Dzyaloshinskii-Moriya interaction (DMI). We
microscopically derive the DMI's free energy contribution for the kagome AFMs.
Unlike ferromagnets and collinear AFMs, the DMI does not lead to terms linear
in the spatial derivatives but instead renormalizes the spin-wave stiffness and
anisotropy energies. Importantly, we show that the DMI induces a highly
nontrivial twisted DW profile that is controllable via two linearly independent
components of the spin accumulation. This texture manipulation mechanism goes
beyond the concept of staggered torques and implies a higher degree of
tunability for the current-driven DW motion compared to conventional
ferromagnets and collinear AFMs
Sempre este homem fatal: Otto Lara Resende, personagem de Nelson Rodrigues/ Always Writing About this Fatal Man: Otto Lara Resende, a Nelson Rodrigues’s Character
No full textResumo: Nelson Rodrigues é conhecido por ser um escritor obsessivo, reiterando muitas vezes certos temas, certos procedimentos, certos personagens. Uma de suas maiores obsessões foi seu amigo Otto Lara Resende, inúmeras vezes retratado em seus trabalhos. Este artigo analisa como o escritor mineiro é apresentado nas obras de Nelson, que estratégias são utilizadas para isso e a que serve tratar constantemente da personalidade de Otto em textos nos quais ele surge meio abruptamente em meio a outros personagens, interagindo com eles com a máxima naturalidade, ou em que é citado como protagonista de histórias exemplares ou emissor de frases marcadas por extraordinário brilhantismo verbal.Palavras-chave: Nelson Rodrigues; Otto Lara Resende; obsessão; ficção e realidade.Abstract: Nelson Rodrigues is well-known for being an obsessive writer, often reiterating subjects, procedures, and characters. One of his strongest and most recurring obsessions was his friend Otto Lara Resende, a writer and journalist who was portrayed many times in his works. This article analyzes how Rodrigues shows Resende in his writings, the strategies used in order to do that and what purpose he has in mind when he constantly explores aspects of Resende’s personality in the midst of other characters, naturallyinteracting with them, or when Rodrigues tells stories in which Resende is the protagonist or he is the author of phrases that are expression of an extraordinary verbal talent.Keywords: Nelson Rodrigues; Otto Lara Resende; obsession; fiction and reality.Resumo: Nelson Rodrigues é conhecido por ser um escritor obsessivo, reiterando muitas vezes certos temas, certos procedimentos, certos personagens. Uma de suas maiores obsessões foi seu amigo Otto Lara Resende, inúmeras vezes retratado em seus trabalhos. Este artigo analisa como o escritor mineiro é apresentado nas obras de Nelson, que estratégias são utilizadas para isso e a que serve tratar constantemente da personalidade de Otto em textos nos quais ele surge meio abruptamente em meio a outros personagens, interagindo com eles com a máxima naturalidade, ou em que é citado como protagonista de histórias exemplares ou emissor de frases marcadas por extraordinário brilhantismo verbal.Palavras-chave: Nelson Rodrigues; Otto Lara Resende; obsessão; ficção e realidade.Abstract: Nelson Rodrigues is well-known for being an obsessive writer, often reiterating subjects, procedures, and characters. One of his strongest and most recurring obsessions was his friend Otto Lara Resende, a writer and journalist who was portrayed many times in his works. This article analyzes how Rodrigues shows Resende in his writings, the strategies used in order to do that and what purpose he has in mind when he constantly explores aspects of Resende’s personality in the midst of other characters, naturallyinteracting with them, or when Rodrigues tells stories in which Resende is the protagonist or he is the author of phrases that are expression of an extraordinary verbal talent.Keywords: Nelson Rodrigues; Otto Lara Resende; obsession; fiction and reality
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