Combined analytical and numerical approach to magnetization plateaux in one-dimensional spin tube antiferromagnets

In this paper, we investigate the properties of frustrated three-leg spin tubes under a magnetic field. We concentrate on two kind of geometries for these tubes, one of which is relevant for the compound $\mathrm{[(CuCl_2tachH)_3Cl]Cl_2}$. We combine an analytical path integral approach with a strong coupling approach, as well as large-scale Density Matrix Renormalization Groups (DMRG) simulations, to identify the presence of plateaux in the magnetization curve as a function of the value of spin $S$. We also investigate the issue of gapless non-magnetic excitations on some plateaux, dubbed chirality degrees of freedom for both tubes.Comment: 17 page

Three-sublattice Skyrmion crystal in the antiferromagnetic triangular lattice

The frustrated classical antiferromagnetic Heisenberg model with Dzyaloshinskii-Moriya (DM) interactions on the triangular lattice is studied under a magnetic field by means of semiclassical calculations and large-scale Monte Carlo simulations. We show that even a small DM interaction induces the formation of an Antiferromagnetic Skyrmion crystal (AF-SkX) state. Unlike what is observed in ferromagnetic materials, we show that the AF-SkX state consists of three interpenetrating Skyrmion crystals (one by sublattice), and most importantly, the AF-SkX state seems to survive in the limit of zero temperature. To characterize the phase diagram we compute the average of the topological order parameter which can be associated to the number of topological charges or Skyrmions. As the magnetic field increases this parameter presents a clear jump, indicating a discontinuous transition from a spiral phase into the AF-SkX phase, where multiple Bragg peaks coexist in the spin structure factor. For higher fields, a second (probably continuous) transition occurs into a featureless paramagnetic phase.Comment: 8 pages, 8 figure

Finite-temperature ordering in a two-dimensional highly frustrated spin model

We investigate the classical counterpart of an effective Hamiltonian for a strongly trimerized kagome lattice. Although the Hamiltonian only has a discrete symmetry, the classical groundstate manifold has a continuous global rotational symmetry. Two cases should be distinguished for the sign of the exchange constant. In one case, the groundstate has a 120^\circ spin structure. To determine the transition temperature, we perform Monte-Carlo simulations and measure specific heat, the order parameter as well as the associated Binder cumulant. In the other case, the classical groundstates are macroscopically degenerate. A thermal order-by-disorder mechanism is predicted to select another 120^\circ spin-structure. A finite but very small transition temperature is detected by Monte-Carlo simulations using the exchange method.Comment: 11 pages including 9 figures, uses IOP style files; to appear in J. Phys.: Condensed Matter (proceedings of HFM2006

Espacios verdes urbanos y diversidad vegetal a diferentes escalas espacio-temporales: el ejemplo de Beijing, China

Beijing, the capital city of China, is one of the largest and most rapidly-urbanizing cities in the world. In this work, we present the main results of one decade’s research on urban vegetation and plant diversity changes in different urban structural units. Urban vegetation/plant diversity has been studied at two different levels: at the landscape level (greening percentage, fragmentation degree) and at the plant species level (structure, composition, and origin). Finally, concerns with the ability to study Beijing’s plant urban ecology are discussed.Pekín, la capital de China, es una de las urbes más pobladas y a la vez con una de las tasas de expansión urbana más rápidas del mundo. En el presente trabajo, se presentan los principales resultados tras una década de estudio de los cambios en la vegetación y diversidad vegetal urbana a lo largo de las diferentes unidades estructurales urbanas. La vegetación y la diversidad vegetal urbana se han estudiado a dos niveles: a nivel de paisaje (porcentaje de zonas verdes, grado de fragmentación) y a nivel de especie (estructura, composición y origen). En último lugar se discuten algunos aspectos relacionados con la metodología de estudio de la ecología urbana en Pekín

A Theory for steady and self-sustained premixed combustion waves

Based on the compressible Navier – Stokes equations for reactive flow problems, an eigenvalue problem for the steady and self-sustained premixed combustion wave propagation is developed. The eigenvalue problem is analytically solved and a set of analytic formulae for description of the wave propagation is found out. The analytic formulae are actually the exact solution of the eigenvalue problem in the form of integration, based on which author develops an iterative and numerical algorithm for calculation of the steady and self-sustained premixed combustion wave propagation and its speed. In order to explore the mathematical model and test the computational method developed in this paper, three groups of combustion wave propagation modes are calculated. The computational results show that the non-trivial modes of the combustion wave propagation exist and their distribution is not continuous but discrete

Emergence of Global Preferential Attachment From Local Interaction

Global degree/strength based preferential attachment is widely used as an evolution mechanism of networks. But it is hard to believe that any individual can get global information and shape the network architecture based on it. In this paper, it is found that the global preferential attachment emerges from the local interaction models, including distance-dependent preferential attachment (DDPA) evolving model of weighted networks(M. Li et al, New Journal of Physics 8 (2006) 72), acquaintance network model(J. Davidsen et al, Phys. Rev. Lett. 88 (2002) 128701) and connecting nearest-neighbor(CNN) model(A. Vazquez, Phys. Rev. E 67 (2003) 056104). For DDPA model and CNN model, the attachment rate depends linearly on the degree or strength, while for acquaintance network model, the dependence follows a sublinear power law. It implies that for the evolution of social networks, local contact could be more fundamental than the presumed global preferential attachment. This is onsistent with the result observed in the evolution of empirical email networks.Comment: 9 pages, 5 figure

SARS- CoV-2 infection and oxidative stress in early-onset preeclampsia

SARS-CoV-2 causes coronavirus disease 2019 (COVID-19) also in pregnant women. Infection in pregnancy leads to maternal and placental functional alterations. Pregnant women with vascular defects such as preeclampsia show high susceptibility to SARS-CoV-2 infection by undefined mechanisms. Pregnant women infected with SARS-CoV-2 show higher rates of preterm birth and caesarean delivery, and their placentas show signs of vasculopathy and inflammation. It is still unclear whether the foetus is affected by the maternal infection with this virus and whether maternal infection associates with postnatal affections. The SARS-CoV-2 infection causes oxidative stress and activation of the immune system leading to cytokine storm and next tissue damage as seen in the lung. The angiotensin-converting-enzyme 2 expression is determinant for these alterations in the lung. Since this enzyme is expressed in the human placenta, SARS-CoV-2 could infect the placenta tissue, although reported to be of low frequency compared with maternal lung tissue. Early-onset preeclampsia (eoPE) shows higher expression of ADAM17 (a disintegrin and metalloproteinase 17) causing an imbalanced renin-angiotensin system and endothelial dysfunction. A similar mechanism seems to potentially account for SARS-CoV-2 infection. This review highlights the potentially common characteristics of pregnant women with eoPE with those with COVID-19. A better understanding of the mechanisms of SARS-CoV-2 infection and its impact on the placenta function is determinant since eoPE/COVID-19 association may result in maternal metabolic alterations that might lead to a potential worsening of the foetal programming of diseases in the neonate, young, and adult

Chiral phase transition and thermal Hall effect in an anisotropic spin model on the kagome lattice

We present a study of the thermal Hall effect in the extended Heisenberg model with $XXZ$ anisotropy in the kagome lattice. This model has the particularity that, in the classical case, and for a broad region in parameter space, an external magnetic field induces a chiral symmetry breaking: the ground state is a doubly degenerate $q=0$ order with either positive or negative net chirality. Here, we focus on the effect of this chiral phase transition in the thermal Hall conductivity using Linear-Spin-Waves theory. We explore the topology and calculate the Chern numbers of the magnonic bands, obtaining a variety of topological phase transitions. We also compute the magnonic effect to the critical temperature associated with the chiral phase transition ($T_c^{SW}$). Our main result is that, the thermal Hall conductivity, which is null for $T>T_c^{SW}$, becomes non-zero as a consequence of the spontaneous chiral symmetry breaking at low temperatures. Therefore, we present a simple model where it is possible to "switch" on/off the thermal transport properties introducing a magnetic field and heating or cooling the system.Comment: 9 pages, 6 figures, Accepted for publication in Phys. Rev.