248 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
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
Ab initio calculations of structures and stabilities of (NaI)_nNa+ and (CsI)_nCs+ cluster ions
Ab initio calculations using the Perturbed Ion model, with correlation
contributions included, are presented for nonstoichiometric (NaI)_nNa+ and
(CsI)_nCs+ (n=1-14) cluster ions. The ground state and several low-lying
isomers are identified and described. Rocksalt ground states are common and
appear at cluster sizes lower than in the corresponding neutral systems. The
most salient features of the measured mobilities seem to be explained by
arguments related to the changes of the compactness of the clusters as a
function of size. The stability of the cluster ions against evaporation of a
single alkali halide molecule shows variations that explain the enhanced
stabilities found experimentally for cluster sizes n=4, 6, 9, and 13. Finally,
the ionization energies and the orbital eigenvalue spectrum of two (NaI)_13Na+
isomers are calculated and shown to be a fingerprint of the structure.Comment: 8 pages plus 13 postscript figures, LaTeX. Accepted for publication
in Phys, Rev. B; minor changes including a more complete comparison to pair
potential result
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
Electron Confinement Induced by Diluted Hydrogen-like Ad-atoms in Graphene Ribbons
We report the electronic properties of two-dimensional systems made of
graphene nanoribbons which are patterned with ad-atoms in two separated
regions. Due to the extra electronic confinement induced by the presence of the
impurities, we find resonant levels, quasi-bound and impurity-induced localized
states, which determine the transport properties of the system. Regardless of
the ad-atom distribution in the system, we apply band-folding procedures to
simple models and predict the energies and the spatial distribution of those
impurity-induced states. We take into account two different scenarios: gapped
graphene and the presence of randomly distributed ad-atoms in a low dilution
regime. In both cases the defect-induced resonances are still detected. Our
findings would encourage experimentalist to synthesize these systems and
characterize their quasi-localized states employing, for instance, scanning
tunneling spectroscopy (STS). Additionally, the resonant transport features
could be used in electronic applications and molecular sensor devices.Comment: 12 pages, 11 figures, submitted (minor changes
Magnetism of Substitutional Co Impurities in Graphene: Realization of Single -Vacancies
We report {\it ab initio} calculations of the structural, electronic and
magnetic properties of a graphene monolayer substitutionally doped with Co
(Co) atoms. We focus in Co because among traditional ferromagnetic
elements (Fe, Co and Ni), only Co atoms induce spin-polarization in
graphene. Our results show the complex magnetism of Co substitutional impurites
in graphene, which is mapped into simple models such as the -vacancy and
Heisenberg model. The links established in our work can be used to bring into
contact the engineering of nanostructures with the results of -models in
defective graphene. In principle, the structures considered here can be
fabricated using electron irradiation or Ar ion bombardment to create
defects and depositing Co at the same time
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