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

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

Interfaces of high-k dielectrics on GaAs: Their common features and the relationship with Fermi level pinning

Numerous metal oxides have been studied worldwide as possible high-k gate dielectric candidates for MOS devices on alternative semiconductor materials (Ge, III/V compounds). We will discuss thermal and plasma-enhanced atomic layer deposition (ALD) of a few materials, HfO2 and Al2O3, We will spend some attention to characteristic features of the growth process and specific growth precursors as this is known to influence strongly the quality of the layer bulk as well as the interface. Detailed electrical characterization of MOS capacitors build on such dielectric layers, before and after forming gas anneals, shows that these interface modifications can lead to a marked decrease of the smaller interface state peaks close to the edges of the bandgap, whereas the larger mid-gap peaks are barely touched. The results of atomistic modeling of the oxidation of a GaAs surface help to understand the origin of these mid-gap electronic states, which are responsible for the apparent pinning of the Fermi level. (C) 2009 Elsevier B.V. All rights reserved.status: publishe

Magnetic structures of heterometallic M(II)-M(III) formate compounds

A study of the magnetic structure of the [NH2(CH3)2]n[FeIIIMII(HCOO)6]n niccolite-like compounds, with MII = CoII (2) and MnII (3) ions, has been carried out using neutron diffraction and compared with the previously reported FeII-containing compound (1). The inclusion of two different metallic atoms into the niccolite-like structure framework leads to the formation of isostructural compounds with very different magnetic behaviors due to the compensation or not of the different spins involved in each lattice. Below TN, the magnetic order in these compounds varies from ferrimagnetic behavior for 1 and 2 to an antiferromagnetic behavior with a weak spin canting for 3. Structure refinements of 2 and 3 at low temperature (45 K) have been carried out combining synchrotron X-ray and high-resolution neutron diffraction in a multipattern approach. The magnetic structures have been determined from the difference patterns between the neutron data in the paramagnetic and the magnetically ordered regions. These difference patterns have been analyzed using a simulated annealing protocol and symmetry analysis techniques. The obtained magnetic structures have been further rationalized by means of ab initio DFT calculations. The direction of the magnetic moment of each compound has been determined. The easy axis of the MII for compound 1 (FeII) is along the c axis; for compound 2 (CoII), the moments are mainly within the ab plane; finally, for compound 3 (MnII), the calculations show that the moments have components both in the ab plane and along the c axis.Partial funding for this work is provided by the Ministerio Español de Ciencia e Innovacion through projects MAT2011-27233-C02-02 and MAT2011-25991 and Centro Universitario de la Defensa de Zaragoza through CUD 2013-17. J.A.R.V. acknowledges CSIC for a JAEdoc contract.Peer Reviewe

On The Density Functional Theory Treatment of Lanthanide Coordination Compounds: A Comparative Study in a Series of Cu–Ln (Ln = Gd, Tb, Lu) Binuclear Complexes

The nontrivial aspects of electron structure in lanthanide complexes, considering ligand field (LF) and exchange coupling effects, have been investigated by means of density functional theory (DFT) calculations, taking as a prototypic case study a series of binuclear complexes [LCu­(O₂COMe)­Ln­(thd)₂], where L^2– = N,N′-2,2-dimethyl-propylene-di­(3-methoxy-salicylidene-iminato) and Ln = Tb, Lu, and Gd. Particular attention has been devoted to the Cu–Tb complex, which shows a quasi-degenerate nonrelativistic ground state. Challenging the limits of density functional theory (DFT), we devised a practical route to obtain different convergent solutions, permuting the starting guess orbitals in a manner resembling the run of the β electron formally originating from the f⁸ configuration of the Tb­(III) over seven molecular orbitals (MOs) with predominant f-type character. Although the obtained states cannot be claimed as the DFT computed split of the ⁷F multiplet, the results are yet interesting numeric experiments, relevant for the ligand field effects. We also performed broken symmetry (BS) DFT estimation of exchange coupling in the Cu–Gd system, using different settings, with Gaussian-type and plane-wave bases, finding a good match with the coupling parameter from experimental data. We also caught BS-type states for each of the mentioned series of different states emulated for the Cu–Tb complex, finding almost equal exchange coupling parameters throughout the seven LF-like configurations, the magnitude of the <i>J</i> parameter being comparable with those of the Cu–Gd system

Magnetic Structures of Heterometallic M(II)–M(III) Formate Compounds

A study of the magnetic structure of the [NH₂(CH₃)₂]_<i>n</i>[Fe^IIIM^II(HCOO)₆]_<i>n</i> niccolite-like compounds, with M^II = Co^II (<b>2</b>) and Mn^II (<b>3</b>) ions, has been carried out using neutron diffraction and compared with the previously reported Fe^II-containing compound (<b>1</b>). The inclusion of two different metallic atoms into the niccolite-like structure framework leads to the formation of isostructural compounds with very different magnetic behaviors due to the compensation or not of the different spins involved in each lattice. Below <i>T</i>_N, the magnetic order in these compounds varies from ferrimagnetic behavior for <b>1</b> and <b>2</b> to an antiferromagnetic behavior with a weak spin canting for <b>3</b>. Structure refinements of <b>2</b> and <b>3</b> at low temperature (45 K) have been carried out combining synchrotron X-ray and high-resolution neutron diffraction in a multipattern approach. The magnetic structures have been determined from the difference patterns between the neutron data in the paramagnetic and the magnetically ordered regions. These difference patterns have been analyzed using a simulated annealing protocol and symmetry analysis techniques. The obtained magnetic structures have been further rationalized by means of ab initio DFT calculations. The direction of the magnetic moment of each compound has been determined. The easy axis of the M^II for compound <b>1</b> (Fe^II) is along the <i>c</i> axis; for compound <b>2</b> (Co^II), the moments are mainly within the <i>ab</i> plane; finally, for compound <b>3</b> (Mn^II), the calculations show that the moments have components both in the <i>ab</i> plane and along the <i>c</i> axis