Examinando por Autor "Chico, Leonor"

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Charge-spin interconversion in graphene-based systems from density functional theory
(American Physical Society, 2021-12-22) Rassekh, Maedeh; Santos, Hernán; Latgé, Andrea; Chico, Leonor; Farjami Shayesteh, Saber; Palacios, Juan José
We present a methodology to address, from first principles, charge-spin interconversion in two-dimensional materials with spin-orbit coupling. Our study relies on an implementation of density functional theory based quantum transport formalism adapted to such purpose. We show how an analysis of the k-resolved spin polarization gives the necessary insight to understand the different charge-spin interconversion mechanisms. We have tested it in the simplest scenario of isolated graphene in a perpendicular electric field where effective tight-binding models are available to compare with. Our results show that the flow of an unpolarized current across a single layer of graphene produces, as expected, a spin separation perpendicular to the current for two of the three spin components (out-of-plane and longitudinal), which is the signature of the spin Hall effect. Additionally, it also yields an overall spin accumulation for the third spin component (perpendicular to the current), which is the signature of the Rashba-Edelstein effect. Even in this simple example, our results reveal an unexpected competition between the Rashba and the intrinsic spin-orbit coupling. Remarkably, the sign of the accumulated spin density does not depend on the electron or hole nature of the injected current for realistic values of the Rashba coupling.
Spin-polarized currents in corrugated graphene nanoribbons
(Elsevier, 2020-06-27) Santos, Hernán; Latgé, Andrea; Brey, Luis; Chico, Leonor
We investigate the production of spin-polarized currents in corrugated graphene nanoribbons. Such corrugations are modeled as multiple regions with Rashba spin-orbit interactions, where concave and convex curvatures are treated as Rashba regions with opposite signs. Numerical examples for different separated Rashba-zone geometries calculated within the tight-binding approximation are provided. Remarkably, the spin-polarized current in a system with several Rashba areas can be enhanced with respect to the case with a single Rashba part of the same total area. The enhancement is larger for configurations with multiple regions with the same Rashba sign. This indicates that the increase of the spin polarization is due to the scattering of the electrons traversing regions with and without Rashba interaction. Additionally, we relate the appearance of the spin-polarized currents to novel symmetry relations between the spin-dependent conductances. These symmetries turn out to be a combination of different symmetry operations in real and spin spaces, as those occurring in non-planar systems like carbon nanotubes. Our results show that two-dimensional devices with Rashba spin-orbit interaction can be used as excellent spintronic devices in an all-electrical or mechanical setup.
Transport in graphene nanoribbon-based systems
(IOP Publishing Ltd, 2019-11-12) Chico, Leonor; González, Jhon W.; Pelc, Marta; Santos, Hernán
The transport properties of quasi-one-dimensional systems based on graphene nanoribbons are reviewed. We focus on those with metallic behavior, especially relevant for transport applications. First, we analyze the role of nanoribbons as valley filters for carbon nanotubes, as an example of those occurring in partially unzipped carbon nanotubes. Later, we describe devices made of overlapping ribbons or flakes deposited on ribbons, where quantum effects modulate their conductance. Moiré bilayer nanoribbon systems are additionally considered, for which transport is governed by the interplay of edge and stacking regions. Finally, the generation of spin-polarized currents by Rashba spin–orbit interactions in graphene nanoribbons is studied, where symmetries play an important role.