1,136 research outputs found
Enhanced Ferromagnetic Stability in Cu Doped Passivated GaN Nanowires
Density functional calculations are performed to investigate the room
temperature ferromagnetism in GaN:Cu nanowires (NWs). Our results indicate that
two Cu dopants are most stable when they are near each other. Compared to bulk
GaN:Cu, we find that magnetization and ferromagnetism in Cu doped NWs is
strongly enhanced because the band width of the Cu td band is reduced due to
the 1D nature of the NW. The surface passivation is shown to be crucial to
sustain the ferromagnetism in GaN:Cu NWs. These findings are in good agreement
with experimental observations and indicate that ferromagnetism in this type of
systems can be tuned by controlling the size or shape of the host materials.Comment: Nano Lett., ASAP Article, 10.1021/nl080261
Controlling doping in graphene through a SiC substrate: A first-principles study
Controlling the type and density of charge carriers by doping is the key step
for developing graphene electronics. However, direct doping of graphene is
rather a challenge. Based on first-principles calculations, a concept of
overcoming doping difficulty in graphene via substrate is reported.We find that
doping could be strongly enhanced in epitaxial graphene grown on silicon
carbide substrate. Compared to free-standing graphene, the formation energies
of the dopants can decrease by as much as 8 eV. The type and density of the
charge carriers of epitaxial graphene layer can be effectively manipulated by
suitable dopants and surface passivation. More importantly, contrasting to the
direct doping of graphene, the charge carriers in epitaxial graphene layer are
weakly scattered by dopants due to the spatial separation between dopants and
the conducting channel. Finally, we show that a similar idea can also be used
to control magnetic properties, for example, induce a half-metallic state in
the epitaxial graphene without magnetic impurity doping
Method of forming variable cross-sectional shaped three-dimensional fabrics
Method of weaving a variable cross-sectional shaped three-dimensional fabric which utilizes different weft yarn insertion from at least one side of the warp layers for selectively inserting weft yarns into different portions of the fabric cross-sectional profile defined by the warp yarn layers during the weaving process. If inserted from both sides of the warp yarn layers, the weft yarns may be inserted simultaneously or alternately from each side of the warp yarn layers. The vertical yarn is then inserted into the fabric by reciprocation of a plurality of harnesses which separate the vertical yarn into a plurality of vertical yarn systems as required by the shape of the three-dimensional fabric being formed
Spin-Orbit Coupling and Ion Displacements in Multiferroic TbMnO3
The electronic and magnetic properties of TbMnO3 leading to its ferroelectric
(FE) polarization were investigated on the basis of relativistic density
functional theory (DFT) calculations. In agreement with experiment, we show
that the spin-spiral plane of TbMnO3 can be either the bc- or ab-plane, but not
the ac-plane. As for the mechanism of FE polarization, our work reveals that
the "pure electronic" model by Katsura, Nagaosa and Balatsky (KNB) is
inadequate in predicting the absolute direction of FE polarization. For the
ab-plane spin-spiral state of TbMnO3, the direction of FE polarization
predicted by the KNB model is opposite to that predicted by DFT calculations.
In determining the magnitude and the absolute direction of FE polarization in
spin-spiral states, it is found crucial to consider the displacements of the
ions from their ecntrosymmetric positions
First-principles study on the effective masses of zinc-blend-derived Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se)
The electron and hole effective masses of kesterite (KS) and stannite (ST)
structured Cu_2Zn-IV-VI_4 (IV = Sn, Ge, Si and VI = S, Se) semiconductors are
systematically studied using first-principles calculations. We find that the
electron effective masses are almost isotropic, while strong anisotropy is
observed for the hole effective mass. The electron effective masses are
typically much smaller than the hole effective masses for all studied
compounds. The ordering of the topmost three valence bands and the
corresponding hole effective masses of the KS and ST structures are different
due to the different sign of the crystal-field splitting. The electron and hole
effective masses of Se-based compounds are significantly smaller compared to
the corresponding S-based compounds. They also decrease as the atomic number of
the group IV elements (Si, Ge, Sn) increases, but the decrease is less notable
than that caused by the substitution of S by Se.Comment: 14 pages, 6 figures, 2 table
Semaphorin3D regulates invasion of cardiac neural crest cells into the primary heart field
AbstractThe primary heart field in all vertebrates is thought to be derived exclusively from lateral plate mesoderm (LPM), which gives rise to a cardiac tube shortly after gastrulation. The heart tube then begins looping and additional cells are added from other embryonic regions, including the secondary heart field, cardiac neural crest and the proepicardial organ. Here we show in zebrafish that neural crest cells invade and contribute cardiac myosin light chain2 (cmlc2)-positive cardiomyocytes to the primary heart field. Knockdown of semaphorin3D, which is expressed in the neural crest but apparently not in LPM, reduces the size of the primary heart field and the number of cardiomyocytes in the primary heart field by 20% before formation of the primary heart tube. Sema3D morphants have subsequent complex congenital heart defects, including hypertrophic cardiomyocytes, decreased ventricular size and defects in trabeculation and in atrioventricular (AV) valve development. Neuropilin1A, a semaphorin receptor, is expressed in LPM but apparently not in the neural crest, and nrp1A morphants have cardiac development defects. We propose that a population of sema3D-dependent neural crest cells follow a novel migratory pathway, perhaps toward nrp1A-expressing LPM, and serve as an important early source of cardiomyocytes in the primary heart field
Electronic Control of Spin Alignment in pi-Conjugated Molecular Magnets
Intramolecular spin alignment in pi-conjugated molecules is studied
theoretically in a model of a Peierls-Hubbard chain coupled with two localized
spins. By means of the exact diagonalization technique, we demonstrate that a
spin singlet (S=0) to quartet (S=3/2) transition can be induced by electronic
doping, depending on the chain length, the positions of the localized spins,
and the sign of the electron-spin coupling. The calculated results provides a
theoretical basis for understanding the mechanism of spin alignment recently
observed in a diradical donor molecule.Comment: 4 pages, 4 figures, Physical Review Letters (in press
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