Aggregation and magnetism of Cr, Mn, and Fe cations in GaN

A first-principles DFT-GGA+U study of the doping of GaN with Cr, Mn, or Fe confirms a strong tendency for the formation of embedded clusters occupying Ga sites of the wurtzite and zinc blende phases of GaN. Within the employed computation model, the tendency for aggregation is larger for Cr and Mn than for Fe. In contrast to previous DFT-GGA calculations, we predict a ferromagnetic ordering of the Cr and Mn clusters having more than two atoms while the Fe clusters are all antiferromagnetic. We have also investigated the magnetic ordering of nearest-neighboring ionic pairs that substitute gallium atoms at the (0001) wz-GaN surface. We find that Fe dopants tend to aggregate, whereas there is a repulsive interaction in the case of Cr and Mn. Nearest neighbor Mn and Fe pairs are coupled antiferromagnetically whereas the Cr pair is coupled ferromagnetically. The relevance of our finding to recent experimental findings is discussed.Comment: 8 pages, 5 figures, 2 table

The Symmetry of the Boron Buckyball and a Related Boron Nanotube

We investigate the symmetry of the boron buckyball and a related boron nanotube. Using large-scale ab-initio calculations up to second-order M{\o}ller Plesset perturbation theory, we have determined unambiguously the equilibrium geometry/symmetry of two structurally related boron clusters: the B80 fullerene and the finite-length (5,0) boron nanotube. The B80 cluster was found to have the same symmetry, Ih, as the C60 molecule since its 20 additional boron atoms are located exactly at the centers of the 20 hexagons. Additionally, we also show that the (5,0) boron nanotube does not suffer from atomic buckling and its symmetry is D5d instead of C5v as has been described by previous calculations. Therefore, we predict that all the boron nanotubes rolled from the \alpha -sheet will be free from structural distortions, which has a significant impact on their electronic properties.Comment: 4 pages, 3 figure

Boron Fullerenes: A First-Principles Study

A family of unusually stable boron cages was identified and examined using first-principles local density functional method. The structure of the fullerenes is similar to that of the B12 icosahedron and consists of six crossing double-rings. The energetically most stable fullerene is made up of 180 boron atoms. A connection between the fullerene family and its precursors, boron sheets, is made. We show that the most stable boron sheets are not necessarily precursors of very stable boron cages. Our finding is a step forward in the understanding of the structure of the recently produced boron nanotubes.Comment: 10 pages, 4 figures, 1 tabl

B12Hn and B12Fn: planar vs icosahedral structures

Using density functional theory and quantum Monte Carlo calculations, we show that B12Hn and B12Fn (n = 0 to 4) quasi-planar structures are energetically more favorable than the corresponding icosahedral clusters. Moreover, we show that the fully planar B12F6 cluster is more stable than the three-dimensional counterpart. These results open up the possibility of designing larger boron-based nanostructures starting from quasi-planar or fully planar building blocks

The Fe-Mg interplay and the effect of deposition mode in (Ga,Fe)N doped with Mg

The effect of Mg codoping and its deposition mode on the Fe distribution in (Ga,Fe)N layers grown by metalorganic vapor phase epitaxy is investigated. Both homogeneously- and digitally-Mg codoped samples are considered and contrasted to the case of (Ga,Fe)N layers obtained without any codoping by shallow impurities. The structural analysis of the layers by high-resolution transmission electron microscopy and by high-resolution- and synchrotron x-ray diffraction gives evidence of the fact that in the case of homogenous-Mg doping, Mg and Fe competitively occupy the Ga-substitutional cation sites, reducing the efficiency of Fe incorporation. Accordingly, the character of the magnetization is modified from ferromagnetic-like in the non-codoped films to paramagnetic in the case of homogeneous Mg codoping. The findings are discussed vis-`a-vis theoretical results obtained by ab initio computations, showing only a weak effect of codoping on the pairing energy of two Fe cations in bulk GaN. However, according to these computations, codoping reverses the sign of the paring energy of Fe cations at the Ga-rich surface, substantiating the view that the Fe aggregation occurs at the growth surface. In contrast to the homogenous deposition mode, the digital one is found to remarkably promote the aggregation of the magnetic ions. The Fe-rich nanocrystals formed in this way are distributed non-uniformly, giving reason for the observed deviation from a standard superparamagnetic behavior.Comment: 13 pages, 14 figure

2D BxC1−xB_xC_{1-x} Layers as Predicted by the Cluster-Expansion Approach

In this work, the cluster-expansion method combined with extensive first-principles calculations is used for predicting the most stable 2D $B_xC_{1-x}$ (x ≤0.5) layers. For concentrations of B up to ≈ 38%, the honeycomb structure of the boron-carbon compound is preserved, whereas for larger concentrations, the boron atoms tend to form 2D clusters and/or ribbons that are fragments of a triangular boron sheet. Our studies indicate that the incorporation of boron into graphene is energetically unfavorable even for low concentrations of B, however, the graphene-like structure of the $B_xC_{1-x}$ layer may be stabilized by a metallic substrate

Quantum Monte Carlo vs. Density Functional Methods for the Prediction of Relative Energies of Small Si-C Clusters

In the present paper, we assess the accuracy of popular and widely used approaches based on density functional theory by relating them to the most accurate at present quantum Monte Carlo calculations. As the test case, we consider the relative stability of small $Si_{n}C_{m}$ isomers. We find out that none of the studied DFT approaches employing local, semilocal, or even hybrid functionals are able to predict correctly the relative stability of the isomers