621 research outputs found
Divacancy-induced Ferromagnetism in Graphene Nanoribbons
Zigzag graphene nanoribb ons have spin-polarized edges,
anti-ferromagnetically coupled in the ground state with total spin zero.
Customarily, these ribbons are made ferromagnetic by producing an imbalance
between the two sublattices. Here we show that zigzag ribbons can be
ferromagnetic due to the presence of reconstructed divacancies near one edge.
This effect takes place despite the divacancies are produced by removing two
atoms from opposite sublattices, being balanced before reconstruction to 5-8-5
defects. We demonstrate that there is a strong interaction between the
defect-localized and edge bands which mix and split away from the Fermi level.
This splitting is asymmetric, yielding a net edge spin-polarization. Therefore,
the formation of reconstructed divacancies close to the edges of the
nanoribbons can be a practical way to make them partially ferromagnetic
Substitutional 4d and 5d Impurities in Graphene
We describe the structural and electronic properties of graphene doped with
substitutional impurities of 4d and 5d transition metals. The binding energy
and distances for 4d and 5d metals in graphene show similar trends for the
later groups in the periodic table, which is also well-known characteristic of
3d elements. However, along earlier groups the 4d impurities in graphene show
very similar binding energies, distances and magnetic moments to 5d ones, which
can be related to the influence of the 4d and 5d lanthanide contraction.
Surprisingly, within the manganese group, the total magnetic moment of
3 for manganese is reduced to 1 for technetium and rhenium.
We find that with compared with 3d elements, the larger size of the 4d and 5d
elements causes a high degree hybridization with the neighbouring carbon atoms,
reducing spin splitting in the d levels. It seems that the magnetic adjustment
of graphene could be significantly different is 4d or 5d impurities are used
instead of 3d impurities.Comment: 16 pages, 4 figure
Optical spin control in nanocrystalline magnetic nanoswitches
We investigate the optical properties of (Cd,Mn)Te quantum dots (QDs) by
looking at the excitons as a function of the Mn impurities positions and their
magnetic alignments. When doped with two Mn impurities, the Mn spins, aligned
initially antiparallel in the ground state, have lower energy in the parallel
configuration for the optically active spin-up exciton. Hence, the
photoexcitation of the QD ground state with antiparallel Mn spins induces one
of them to flip and they align parallel. This suggests that (Cd,Mn)Te QDs are
suitable for spin-based operations handled by light
First-principles calculations of the magnetic properties of (Cd,Mn)Te nanocrystals
We investigate the electronic and magnetic properties of Mn-doped CdTe
nanocrystals (NCs) with 2 nm in diameter which can be experimentally
synthesized with Mn atoms inside. Using the density-functional theory, we
consider two doping cases: NCs containing one or two Mn impurities. Although
the Mn d peaks carry five up electrons in the dot, the local magnetic moment on
the Mn site is 4.65 mu_B. It is smaller than 5 mu_B because of the sp-d
hybridization between the localized 3d electrons of the Mn atoms and the s- and
p-type valence states of the host compound. The sp-d hybridization induces
small magnetic moments on the Mnnearest- neighbor Te sites, antiparallel to the
Mn moment affecting the p-type valence states of the undoped dot, as usual for
a kinetic-mediated exchange magnetic coupling. Furthermore, we calculate the
parameters standing for the sp-d exchange interactions. Conduction N0\alpha and
valence N0\beta are close to the experimental bulk values when the Mn
impurities occupy bulklike NCs' central positions, and they tend to zero close
to the surface. This behavior is further explained by an analysis of
valence-band-edge states showing that symmetry breaking splits the states and
in consequence reduces the exchange. For two Mn atoms in several positions, the
valence edge states show a further departure from an interpretation based in a
perturbative treatment. We also calculate the d-d exchange interactions |Jdd|
between Mn spins. The largest |Jdd| value is also for Mn atoms on bulklike
central sites; in comparison with the experimental d-d exchange constant in
bulk Cd0.95Mn0.05Te, it is four times smaller
Antiferromagnetic order in (Ga,Mn)N nanocrystals: A density functional theory study
We investigate the electronic and magnetic properties of (Ga,Mn)N
nanocrystals using the density functional theory. We study both wurtzite and
zinc-blende structures doped with one or two substitutional Mn impurities. For
a single Mn dopant placed close to surface, the behavior of the empty
Mn-induced state, hereafter referred to as "Mn hole", is different from bulk
(Ga,Mn)N. The energy level corresponding to this off-center Mn hole lies within
the nanocrystal gap near the conduction edge. For two Mn dopants, the most
stable magnetic configuration is antiferromagnetic, and this was unexpected
since (Ga,Mn)N bulk shows ferromagnetism in the ground state. The surprising
antiferromagnetic alignment of two Mn spins is ascribed also to the holes
linked to the Mn impurities located close to surface. Unlike Mn holes in
(Ga,Mn)N bulk, these Mn holes in confined (Ga,Mn)N nanostructures do not
contribute to the ferromagnetic alignment of the two Mn spins
Topologically confined states at corrugations of gated bilayer graphene
We investigate the electronic and transport properties of gated bilayer
graphene with one corrugated layer, which results in a stacking AB/BA boundary.
When a gate voltage is applied to one layer, topologically protected gap states
appear at the corrugation, which reveal as robust transport channels along the
stacking boundary. With increasing size of the corrugation, more localized,
quantum-well-like states emerge. These finite-size states are also conductive
along the fold, but in contrast to the stacking boundary states, which are
gapless, they present a gap. We have also studied periodic corrugations in
bilayer graphene; our findings show that such corrugations between AB- and
BA-stacked regions behave as conducting channels that can be easily identified
by their shape
An Array of Layers in Silicon Sulfides: Chain-like and Ground State Structures
While much is known about isoelectronic materials related to carbon
nanostructures, such as boron nitride layers and nanotubes, rather less is
known about equivalent silicon based materials. Following the recent discovery
of phosphorene, we herein discuss isoelectronic silicon monosulfide monolayers.
We describe a set of anisotropic ground state structures that clearly have a
high stability with respect to the near isotropic silicon monosulfide
monolayers. The source of the layer anisotropy is related to the presence of
Si-S double chains linked by some Si-Si covalent bonds, which lye at the core
of the increased stability, together with a remarkable spd hybridization on Si.
The involvement of d orbitals brings more variety to silicon-sulfide based
nanostructures that are isoelectronic to phosphorene, which could be relevant
for future applications, adding extra degrees of freedom.Comment: 16 pages, 6 figure
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