Ferromagnetism and orbital order in a topological ferroelectric

We explore via density functional calculations the magnetic doping of a topological ferroelectric as an unconventional route to multiferroicity. Vanadium doping of the layered perovskite La$_{2}$Ti$_{2}$O$_{7}$ largely preserves electric polarization and produces robust ferromagnetic order, hence proper multiferroicity. The marked tendency of dopants to cluster into chains results in an insulating character at generic doping. Ferromagnetism stems from the symmetry breaking of the multi-orbital V system via an unusual "antiferro"-orbital order, and from the host's low-symmetry layered structure.Comment: 4 pages, 3 figures; Physical Review Letters 109, in print (2012

First-principles study of III-V semiconductors/oxide interfaces.

This work addresses the key issues of the passivation of the III-V compound semiconductors surfaces and the quality of the III-V/oxide interfaces, constituting crucial aspects in the development and the implementation of next generation MOSFETs, based on the use of innovative and alternative materials other than the traditional silicon, such as III-V compounds.We have carried out a theoretical investigation on physical systems of relevance to this purpose, such as the oxidation of GaAs and the properties of the resulting GaAs/oxide interface. The methodology used is based on the state-of-the-art first principles modeling techniques.We first investigated the adsorption of molecular oxygen on the GaAs(001)-ß2(2×4) surface and provided both the dynamical picture of the reaction on the surface and the analysis of the energetics and the electronic properties of the adorbates. The results clearly indicated that the most stable type of O bonding, the bridging Ga-O-As bonds, do not show any signature of correlated electronic states in the forbidden gap. The importance of this study lies with the finding that it rules out the scenario of a Fermi level pinning of extrinsic nature, that is, a mechanism in which the onset of electronic defects in the gap could be directly associated to the incorporation on the surface of the adsorbing foreign chemical species. The conclusions rather suggest that defect states in the band gap are generated by the process of adsorption itself, disrupting the morphology of the reconstructed GaAs(001) surface, with attendant generation of structural defects on the surface.We next provided a detailed description at atomic scale of the oxidation of the GaAs(001) surface, through the modeling of the direct oxidation, based on ab initio molecular dynamics simulations. As a result of this study, we could depict an atomistic model of the surface oxidation in its early stages. Specifically, it allowed us to realize the important role played by the stress accumulating at the interface during the oxidation and, more importantly, to identify a key mechanism of stress release, namely, Ga atom ejection into the amorphous oxide atop. As a matter of fact, this latter results naturally in the generation and accumulation of native defects at the interface between the crystalline substrate and the amorphous oxide layer.We then provided a thorough description of the structural and electronic properties of the atomistic models describing the GaAs/oxide interface with a thin oxide film. We found a peaked feature of defect states in the gap for the GaAs/oxide interface models, in agreement with the results reported in the literature of electrical characterizations of this interface. Specifically, the interface models showed: a) an isolated As defect - an As dangling bond - that gives electronic states close to the valence band; b) a sharp feature of midgap states, which can be attributed to a cluster of interacting As dangling bonds; c) and a wider distribution of defect states was also found, located close to the conduction band and correlated with defective (undercoordinated) Ga atoms.We next extended the study to two other III-V semiconductors, InAs and In0.5Ga0.5As, and investigated the properties of the interfaces formed with their native oxides. We generated a set of InAs/oxide and In0.5Ga0.5As/oxide interfaces models by using the GaAs ones as template structures.The analysis of the structural properties highlighted two main differences among the III-V/oxide models investigated: a) an increase of the coordination number of the cation atoms in the amorphous oxide with growing In content; and b) a considerable elongation of the length of the In-O bond compared to the Ga-O one. Another interesting finding is the observation of a clear trend in the structural changes occurring upon the introduction of indium, comprising a densification of the amorphous oxide film. These results suggest that the incorporation of indium, with a higher atomic radius than gallium, promotes the formation of chemical bonds in the amorphous oxide and at the interface that result locally into an increase of the atomic coordination number, and overall, in a densification of the amorphous film.We finally reviewed the electronic properties of the generated III-V/oxide interfaces models. We observed two main differences for the InAs case as compared to GaAs: a) a shift of the defect states distribution towards the edges of the band gap, due to the band gap shrinking associated with the presence of In in the semiconductor, and b) a reduction in the density of the interface states distribution with increasing In content.As to the origin of these improved electronic properties of the III-V/oxide interfaces with In content, a major role is played by the structural properties of indium in the oxide. The latter favors a structural local rearrangement in both the amorphous layer and at the interface, leading to a partial saturation of the dangling bonds.status: publishe

Magnetoelectric coupling and spin-induced electrical polarization in metal–organic magnetic chains

We report first-principles predictions of magnetoelectric coupling in organic magnetic helices and clarify the microscopic mechanism of spin-induced electric polarization.</p

Oxidation of the GaAs(001) surface: Insights from first-principles calculations

We performed a detailed investigation of the oxidation of the technologically relevant GaAs(001)-beta 2(2x4) surface via density functional calculations. The purpose is to gain insights on the atomistic mechanisms and local bondings that underlie the degradation of the surface properties once exposed to oxygen. The study comprises the adsorption of single O atoms, through the sampling of several adsorption sites, and the subsequent formation of the O adsorbate at increasing coverage by taking into account multiple-atom adsorption. Based on the evaluation of the energetics and the structural properties of the atomistic models generated, the results here reported delineate a consistent picture of the initial stage of the surface oxidation: (i) at low coverage, in the limit of single O insertions, oxygen is incorporated on the surface forming a twofold-bridging Ga-O-As bond; (ii) at increasing coverage, as multiple O atoms are involved, this is accompanied by the formation of a threefold-coordinated bond (with two Ga and one As atoms); (iii) the latter has important implications regarding the electronic properties of the adsorbate since this O bonding may result in the formation of As dangling bonds. Moreover, a clear trend of increased energy gain for the incorporation of neighboring O atoms compared to single O insertions indicates that the formation of oxide clusters is favored over a regime of uniform oxidation. Our findings provide a detailed description of the O bonding and stress the importance of modeling the adsorption of multiple O atoms for an accurate description of the surface oxidation.status: publishe