J. E. Padilha

Directional dependence of the electronic and transport properties of 2D borophene and borophane

Very recently two dimensional layers of boron atoms, so called borophene, have been successfully synthesized. It presents a metallic band structure, with a strong anisotropic character. Upon further hydrogen adsorption a new material is obtained, borophane; giving rise to a Dirac cone structure like the one in graphene. We have performed a first-principles study of the electronic and transport properties of borophene and borophane through the Landauer-Büttiker formalism. We find that borophene presents an electronic current two orders of magnitude larger than borophane. In addition we verified the direction dependence of the electronic current in two perpendicular directions, namely, Ix and Iy; where for both systems, we found a current ratio, η = Ix/Iy, of around 2. Aiming to control such a current anisotropy, η, we performed a study of its dependence with respect to an external strain. Where, by stretching the borophane sheet, η increases by 11% for a bias voltage of 50 mV.

Bilayer graphene dual-gate nanodevice: Anab initiosimulation

We study the electronic transport properties of a dual-gated bilayer graphene nanodevice via first principles calculations. We investigate the electric current as a function of gate length and temperature. Under the action of an external electrical field we show that even for gate lengths up 100 Ang., a non zero current is exhibited. The results can be explained by the presence of a tunneling regime due the remanescent states in the gap. We also discuss the conditions to reach the charge neutrality point in a system free of defects and extrinsic carrier doping.

A new class of large band gap quantum spin hall insulators: 2D fluorinated group-IV binary compounds.

We predict a new class of large band gap quantum spin Hall insulators, the fluorinated PbX (X = C, Si, Ge and Sn) compounds, that are mechanically stable two-dimensional materials. Based on first principles calculations we find that, while the PbX systems are not topological insulators, all fluorinated PbX (PbXF2) compounds are 2D topological insulators. The quantum spin Hall insulating phase was confirmed by the explicitly calculation of the Z2 invariant. In addition we performed a thorough investigation of the role played by the (i) fluorine saturation, (ii) crystal field, and (iii) spin-orbital coupling in PbXF2. By considering nanoribbon structures, we verify the appearance of a pair of topologically protected Dirac-like edge states connecting the conduction and valence bands. The insulating phase which is a result of the spin orbit interaction, reveals that this new class of two dimensional materials present exceptional nontrivial band gaps, reaching values up to 0.99 eV at the Γ point, and an indirect band gap of 0.77 eV. The topological phase is arisen without any external field, making this system promising for nanoscale applications, using topological properties.

Microscopic Description of the Ferroism in Lead-Free AlFeO 3

The microscopic origin of the ferroic and multiferroic properties of AlFeO3 have been carefully investigated. The maximum entropy method was applied to X-ray diffraction data and ab initio density functional theory calculations in order to obtain the electron density distributions and electric polarization. The study of chemical bonds shows that the bonds between Fe(3d) and O(2p) ions are anisotropic, leading to the configuration of shorter/longer and stronger/weaker bonds. This leads to electric polarization. Density of states calculations showed a magnetic polarization as a result of a weak ferromagnetic ordering. These results unambiguously show that AlFeO3 is a multiferroic material and exhibits a magnetoelectric coupling at room temperature, as has already been shown by experiments.

Electron density distribution and electronic structure as tools to study the origin of ferroic states in ferroelectric and magnetic materials

ABSTRACT In this paper, three well-known prototypic ferroic materials, BaTiO3, PbZr0.4Ti0.6O3 and Fe3O4, were experimentally and theoretically investigated to explore the origin of their respective ferroic state. Their structural properties were determined by X-ray powder diffraction and Rietveld refinement. The electron density distribution around cationic and anionic sites were obtained by using the Maximum Entropy Method on analyzing the X-ray diffraction results. Electron density distributions and the electronic band structures were determined by Density Functional Theory simulations using experimental and refined structural parameters as input data. The results show a strong correspondence for all the obtained properties and features, suggesting the important role of structural aspects and chemical bonding in the stabilization of the ferroic states in each studied system.