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Register a new userElectronic and magnetic properties of bimetallic L1$_0$ cuboctahedral clusters by means of a fully relativistic density functional based calculations
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By means of density functional theory (DFT) and the generalized gradient approximation (GGA) we present a structural, electronic and magnetic study of FePt, CoPt, FeAu and FePd based L1$_0$ ordered cuboctahedral nanoparticles, with total numbers of atoms, N$_{tot}$ = 13, 55, 147. After a conjugate gradient relaxation, the nanoparticles retain their L1$_0$ symmetry, but the small displacements of the atomic positions tune the electronic and magnetic properties. The value of the total magnetic moment stabilizes as the size increases. We also show that the Magnetic Anisotropy Energy (MAE) depends on the size as well as the position of the Fe-atomic planes in the clusters. We address the influence on the MAE of the surface shape, finding a small in-plane MAE for (Fe,Co)$_{24}$Pt$_{31}$ nanoparticles.
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Symmetry and magnitude of spin-orbit torques (SOT), i.e., current-induced torques on the magnetization of systems lacking inversion symmetry, are investigated in a fully relativistic linear response framework based on the Kubo formalism. By applying all space-time symmetry operations contained in the magnetic point group of a solid to the relevant response coefficient, the torkance expressed as torque-current correlation function, restrictions to the shape of the direct and inverse response tensors are obtained. These are shown to apply to the corresponding thermal analogues as well, namely the direct and inverse thermal SOT in response to a temperature gradient or heat current. Using an implementation of the Kubo-Bastin formula for the torkance into a first-principles multiple-scattering Greens function framework and accounting for disorder effects via the so-called coherent potential approximation (CPA), all contributions to the SOT in pure systems, dilute as well as concentrated alloys can be treated on equal footing. This way, material specific values for all torkance tensor elements in the fcc (111) trilayer alloy system Pt | Fe$_x$Co$_{1-x}$ | Cu are obtained over a wide concentration range and discussed in comparison to results for electrical and spin conductivity, as well as to previous work - in particular concerning symmetry w.r.t. magnetization reversal and the nature of the various contributions.
Ab initio calculations using the local spin density approximation and also including the Hubbard $U$ have been performed for three low energy configurations of the interface between LaAlO$_3$ and TiO$_2$-anatase. Two types of interfaces have been considered: LaO/TiO$_2$ and AlO$_2$/TiO, the latter with Ti-termination and therefore a missing oxygen. A slab-geometry calculation was carried out and all the atoms were allowed to relax in the direction normal to the interface. In all the cases considered, the interfacial Ti atom acquires a local magnetic moment and its formal valence is less than +4. When there are oxygen vacancies, this valence decreases abruptly inside the anatase slab while in the LaO/TiO$_2$ interface the changes are more gradual.
A systemically theoretical study has been presented to explored the crystal structures and electronic characteristics of polycyclic aromatic hydrocarbons (PAHs), such as solid phenanthrene, picene, 1,2;8,9-dibenzopentacene, and 7-phenacenes, since these PAHs exhibited the superconductivity when potassium doping into. For tripotassium-doped phenanthrene and picene, we demonstrate the K atomic positions to fit the experimental lattice parameters, and analyze the distinction between the stablest configuration and the fitted experimental one. Based on the first-principles calculations, for the first time, we predict the possible crystal configurations of pristine and tripotassium-doped 1,2;8,9-dibenzopentacene and 7-phenacenes, respectively. For these four PAHs, the electronic structures after doping are investigated in details. The results show that the electronic characters near the Fermi level are high sensitive to structure. Because of the change of the benzene rings arrangement, the 1,2;8,9-dibenzopentacene exhibits visibly different band structures from other three PAHs. In these metallic PAHs, two bands cross the Fermi level which results in the complicated multiband feature of Fermi surfaces. Fascinatingly, we find that the electronic states of potassium contribute to the Fermi surfaces especially for K-3$d$ electrons, which improves a way to understand this superconductivity. As a result, we suggest that the rigid-band picture is invalidated due to the hybridization between K atoms and PAH molecules as well as the rearrangement and distortion of PAH molecules.
We present results of a study of small stoichiometric $Cd_{n}Te_{n}$ ($1{leq}n{leq}6$) clusters and few medium sized non-stoichiometric $Cd_{m}Te_{n}$ [($m,n= 13, 16, 19$); ($m{ eq}n$)] clusters using the Density Functional formalism and projector augmented wave method within the generalized gradient approximation. Structural properties {it viz.} geometry, bond length, symmetry and electronic properties like HOMO-LUMO gap, binding energy, ionization potential and nature of bonding {it etc.} have been analyzed. Medium sized non-stoichiometric clusters were considered as fragments of the bulk with T{$_{d}$} symmetry. It was observed that upon relaxation, the symmetry changes for the Cd rich clusters whereas the Te rich clusters retain their symmetry. The Cd rich clusters develop a HOMO-LUMO gap due to relaxation whereas there is no change in the HOMO-LUMO gap of the Te rich clusters. Thus, the symmetry of a cluster seems to be an important factor in determining the HOMO-LUMO gap.
Composition, atomic structure, and electronic properties of TM$_x$Mg$_y$O$_z$ clusters (TM = Cr, Ni, Fe, Co, $x+y leq 3$) at realistic temperature $T$ and partial oxygen pressure $p_{textrm{O}_2}$ conditions are explored using the {em ab initio} atomistic thermodynamics approach. The low-energy isomers of the different clusters are identified using a massively parallel cascade genetic algorithm at the hybrid density-functional level of theory. On analyzing a large set of data, we find that the fundamental gap E$_textrm{g}$ of the thermodynamically stable clusters are strongly affected by the presence of Mg-coordinated O$_2$ moieties. In contrast, the nature of the transition metal does not play a significant role in determining E$_textrm{g}$. Using E$_textrm{g}$ of a cluster as a descriptor of its redox properties, our finding is against the conventional belief that the transition metal plays the key role in determining the electronic and therefore chemical properties of the clusters. High reactivity may be correlated more strongly with oxygen content in the cluster than with any specific TM type.
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