Odd Nonlinear Conductivity under Spatial Inversion in Chiral Tellurium.

Electrical transport in noncentrosymmetric materials departs from the well-established phenomenological Ohm's law. Instead of a linear relation between current and electric field, a nonlinear conductivity emerges along specific crystallographic directions. This nonlinear transport is fundamentally related to the lack of spatial inversion symmetry. However, the experimental implications of an inversion symmetry operation on the nonlinear conductivity remain to be explored. Here, we report on a large, nonlinear conductivity in chiral tellurium. By measuring samples with opposite handedness, we demonstrate that the nonlinear transport is odd under spatial inversion. Furthermore, by applying an electrostatic gate, we modulate the nonlinear output by a factor of 300, reaching the highest reported value excluding engineered heterostructures. Our results establish chiral tellurium as an ideal compound not just to study the fundamental interplay between crystal structure, symmetry operations and nonlinear transport; but also to develop wireless rectifiers and energy-harvesting chiral devices.

Wannier90 as a community code: new features and applications.

Wannier90 is an open-source computer program for calculating maximally-localised Wannier functions (MLWFs) from a set of Bloch states. It is interfaced to many widely used electronic-structure codes thanks to its independence from the basis sets representing these Bloch states. In the past few years the development of Wannier90 has transitioned to a community-driven model; this has resulted in a number of new developments that have been recently released in Wannier90 v3.0. In this article we describe these new functionalities, that include the implementation of new features for wannierisation and disentanglement (symmetry-adapted Wannier functions, selectively-localised Wannier functions, selected columns of the density matrix) and the ability to calculate new properties (shift currents and Berry-curvature dipole, and a new interface to many-body perturbation theory); performance improvements, including parallelisation of the core code; enhancements in functionality (support for spinor-valued Wannier functions, more accurate methods to interpolate quantities in the Brillouin zone); improved usability (improved plotting routines, integration with high-throughput automation frameworks), as well as the implementation of modern software engineering practices (unit testing, continuous integration, and automatic source-code documentation). These new features, capabilities, and code development model aim to further sustain and expand the community uptake and range of applicability, that nowadays spans complex and accurate dielectric, electronic, magnetic, optical, topological and transport properties of materials.

Gyrotropic Magnetic Effect and the Magnetic Moment on the Fermi Surface.

The current density j^{B} induced in a clean metal by a slowly-varying magnetic field B is formulated as the low-frequency limit of natural optical activity, or natural gyrotropy. Working with a multiband Pauli Hamiltonian, we obtain from the Kubo formula a simple expression for α_{ij}^{GME}=j_{i}^{B}/B_{j} in terms of the intrinsic magnetic moment (orbital plus spin) of the Bloch electrons on the Fermi surface. An alternate semiclassical derivation provides an intuitive picture of the effect, and takes into account the influence of scattering processes in dirty metals. This "gyrotropic magnetic effect" is fundamentally different from the chiral magnetic effect driven by the chiral anomaly and governed by the Berry curvature on the Fermi surface, and the two effects are compared for a minimal model of a Weyl semimetal. Like the Berry curvature, the intrinsic magnetic moment should be regarded as a basic ingredient in the Fermi-liquid description of transport in broken-symmetry metals.

Níveis de atividade física e estágios de mudança de comportamento de universitários da área de saúde / Level of physical activity and behavior change stages among undergraduate students of health sciences

O presente estudo teve como objetivo verificar o nível de atividade física e estágios de mudança de comportamento em universitários da área de saúde. A amostra foi composta por 416 indivíduos, com idade de 20,6 ± 2,8 anos, proveniente dos cursos de Educação Física (n=67), Enfermagem (n=87), Farmácia (n=79), Fisioterapia (n=99) e Nutrição (n=84). A massa corporal e a estatura foram reportadas e o índice de massa corporal (IMC) foi calculado pela equação de Quételet. Utilizou-se o Questionário Internacional de Atividade Física (IPAQ) - versão curta, para avaliar o nível de atividade física. Quanto ao estágio de mudança de comportamento, os indivíduos foram avaliados através do modelo transteorético de mudança de comportamento. Os resultados encontrados neste estudo indicaram que quase 30% dos acadêmicos da área de saúde foram considerados fisicamente inativos ou insuficientemente ativos, sendo 6% no curso de Educação Física, 34,5% em Enfermagem, 35,5% em Farmácia, 28,3 em Fisioterapia e 40,5 em Nutrição. Em relação ao estágio de mudança de comportamento, mais de 35% estavam se preparando para iniciar a prática de atividade física num futuro próximo. Conclui-se que, com exceção dos estudantes de Educação Física, há um número elevado de fisicamente inativos e insuficientemente ativos entre os acadêmicos dos cursos avaliados, mesmo se tratando de jovens e estudantes de cursos da área da saúde. Além disso, uma grande quantidade dos estudantes está propensa a mudar seu estilo de vida e terem uma vida mais ativa, o que deveria estimular o surgimento de mais oportunidades para a prática de atividade física.

This study aimed to verify the level of physical activity and behavior change stages among undergraduate students of health sciences. The sample was composed of 416 students majoring in Physical Education (n=67), Nursing (n=87), Pharmacy (n=79), Physiotherapy (n=99) and Nutrition (n=84). Weight and height were assessed in order to calculate the body mass index (BMI). The International Physical Activity Questionnaire (IPAQ) was used to evaluate the level of physical activity. In relation to the behavior change stage, the individuals were evaluated through a transtheoretical model of health behavior change (TTM). The results found in this study indicate that almost 30% of the undergraduate students of health sciences are considered physically inactive or insufficiently active (6% in Physical Education, 34,5% in Nursing, 35,5% in Pharmacy, 28,3% in Physiotherapy, and 40,5% in Nutrition). In the stage of behavior change, more than 35% intend to engage in physical activity in the near future. We conclude that, with the exception of Physical Education students, there are high numbers of physically inactive and insufficiently active undergraduate students, especially when taking into consideration that the individuals are young and students at a university health sector. Moreover, there are also a lot of subjects likely to change and have a more active lifestyle, which should stimulate the emergence of more opportunities for physical activity.

Anisotropy of spin relaxation and transverse transport in metals.

Using first-principles methods we explore the anisotropy of the spin relaxation and transverse transport properties in bulk metals with respect to the real-space direction of the spin-quantization axis in paramagnets or of the spontaneous magnetization in ferromagnets. Owing to the presence of the spin-orbit coupling the orbital and spin character of the Bloch states depends sensitively on the orientation of the spins relative to the crystal axes. This leads to drastic changes in quantities which rely on interband mixing induced by the spin-orbit interaction. The anisotropy is particularly striking for quantities which exhibit spiky and irregular distributions in the Brillouin zone, such as the spin-mixing parameter or the Berry curvature of the electronic states. We demonstrate this for three cases: (i) the Elliott-Yafet spin-relaxation mechanism in paramagnets with structural inversion symmetry; (ii) the intrinsic anomalous Hall effect in ferromagnets; and (iii) the spin Hall effect in paramagnets. We discuss the consequences of the pronounced anisotropic behavior displayed by these properties for spin-polarized transport applications.

Role of spin-flip transitions in the anomalous Hall effect of FePt alloy.

We carry out ab initio calculations which demonstrate the importance of the non-spin-conserving part of the spin-orbit interaction for the intrinsic anomalous Hall conductivity of ordered FePt alloys. The impact of this interaction is strongly reduced if Pt is replaced by the lighter isoelectronic element Pd. An analysis of the interband transitions responsible for the anomalous velocity reveals that spin-flip transitions occur not only at avoided band crossings near the Fermi level, but also between well-separated pairs of bands with similar dispersions. We also predict a strong anisotropy in the anomalous Hall conductivity of FePt caused entirely by low-frequency spin-flip transitions.

Orientation dependence of the intrinsic anomalous Hall effect in hcp Cobalt.

We carry out first-principles calculations of the dependence of the intrinsic anomalous Hall conductivity of hcp Co on the spin magnetization direction. The Hall conductivity drops from 481 to 116 S/cm as the magnetization is tilted from the easy axis (c axis) to the ab plane. These values agree reasonably well with measurements on single crystals, while the angular average of 226 S/cm is in excellent agreement with the value of 205 S/cm measured in polycrystalline films. The strong intrinsic anisotropy is shown to arise from quasidegeneracies near the Fermi level.

Electron-phonon interaction via electronic and lattice Wannier functions: superconductivity in boron-doped diamond reexamined.

We present a first-principles technique for investigating the electron-phonon interaction with millions of k points in the Brillouin zone, which exploits the spatial localization of electronic and lattice Wannier functions. We demonstrate the effectiveness of our technique by elucidating the phonon mechanism responsible for superconductivity in boron-doped diamond. Our calculated phonon self-energy and Eliashberg spectral function show that superconductivity cannot be explained without taking into account the finite-wave-vector Fourier components of the vibrational modes introduced by boron, as well as the breaking of the diamond crystal periodicity induced by doping.

First-principles approach to insulators in finite electric fields.

We describe a method for computing the response of an insulator to a static, homogeneous electric field. It consists of iteratively minimizing an electric enthalpy functional expressed in terms of occupied Bloch-like states on a uniform grid of k points. The functional has equivalent local minima below a critical field E(c) that depends inversely on the density of k points; the disappearance of the minima at E(c) signals the onset of Zener breakdown. We illustrate the procedure by computing the piezoelectric and nonlinear dielectric susceptibility tensors of III-V semiconductors.