213 research outputs found
Chemical and magnetic impurity effects on electronic properties of semiconductor quantum wires
We present a theoretical study of electronic states in magnetic and
nonmagnetic semiconductor quantum wires. The effects of chemical and magnetic
disorder at paramagnetic temperatures are investigated in single-site coherent
potential approximation. It is shown that the nonmagnetic impurity shifts the
band of carriers and suppresses the van Hove singularities of the local density
of states (LDOS) depending on the value of impurity concentration. The magnetic
impurity, however, broadens the band which depends on the strength of exchange
coupling, and in the high impurity concentration, the van Hove singularities in
the LDOS can completely disappear and the curves become smooth.Comment: 7 pages, 6 figure
The effect of vacancy-induced magnetism on electronic transport in armchair carbon nanotubes
The influence of local magnetic moment formation around three kinds of
vacancies on the electron conduction through metallic single-wall carbon
nanotubes is studied by use of the Landauer formalism within the coherent
regime. The method is based on the single-band tight-binding Hamiltonian, a
surface Green's function calculation, and the mean-field Hubbard model. The
numerical results show that the electronic transport is spin-polarized due to
the localized magnetic moments and it is strongly dependent on the geometry of
the vacancies. For all kinds of vacancies, by including the effects of local
magnetic moments, the electron scattering increases with respect to the
nonmagnetic vacancies case and hence, the current-voltage characteristic of the
system changes. In addition, a high value for the electron-spin polarization
can be obtained by applying a suitable gate voltage.Comment: 6 pages, 6 figure
The effects of a magnetic barrier and a nonmagnetic spacer in tunnel structures
The spin-polarized transport is investigated in a new type of magnetic tunnel
junction which consists of two ferromagnetic electrodes separated by a magnetic
barrier and a nonmagnetic metallic spacer. Based on the transfer matrix method
and the nearly-free-electron-approximation the dependence of the tunnel
magnetoresistance (TMR) and electron-spin polarization on the nonmagnetic layer
thickness and the applied bias voltage are studied theoretically. The TMR and
spin polarization show an oscillatory behavior as a function of the spacer
thickness and the bias voltage. The oscillations originate from the quantum
well states in the spacer, while the existence of the magnetic barrier gives
rise to a strong spin polarization and high values of the TMR. Our results may
be useful for the development of spin electronic devices based on coherent
transport.Comment: 15 pages, 5 figure
Optical absorption spectrum in disordered semiconductor multilayers
The effects of chemical disorder on the electronic and optical properties of
semiconductor alloy multilayers are studied based on the tight-binding theory
and single-site coherent potential approximation. Due to the quantum
confinement of the system, the electronic spectrum breaks into a set of
subbands and the electronic density of states and hence the optical absorption
spectrum become layer-dependent. We find that, the values of absorption depend
on the alloy concentration, the strength of disorder, and the layer number. The
absorption spectrum in all layers is broadened because of the influence of
disorder and in the case of strong disorder regime, two optical absorption
bands appear. In the process of absorption, most of the photon energy is
absorbed by the interior layers of the system. The results may be useful for
the development of optoelectronic nanodevices.Comment: 6 pages, 6 EPS figures, revised versio
Modeling of gas adsorption on graphene nanoribbons
We present a theory to study gas molecules adsorption on armchair graphene
nanoribbons (AGNRs) by applying the results of \emph{ab} \emph{initio}
calculations to the single-band tight-binding approximation. In addition, the
effect of edge states on the electronic properties of AGNR is included in the
calculations. Under the assumption that the gas molecules adsorb on the ribbon
sites with uniform probability distribution, the applicability of the method is
examined for finite concentrations of adsorption of several simple gas
molecules (CO, NO, CO, NH) on 10-AGNR. We show that the states
contributed by the adsorbed CO and NO molecules are quite localized near the
center of original band gap and suggest that the charge transport in such
systems cannot be enhanced considerably, while CO and NH molecules
adsorption acts as acceptor and donor, respectively. The results of this theory
at low gas concentration are in good agreement with those obtained by
density-functional theory calculations.Comment: 7 pages, 6 figure
Exchange coupling between two ferromagnetic electrodes separated by a graphene nanoribbon
In this study, based on the self-energy method and the total energy
calculation, the indirect exchange coupling between two semi-infinite
ferromagnetic strips (FM electrodes) separated by metallic graphene nanoribbons
(GNRs) is investigated. In order to form a FM/GNR/FM junction, a graphitic
region of finite length is coupled to the FM electrodes along graphitic zigzag
or armchair interfaces of width . The numerical results show that, the
exchange coupling strength which can be obtained from the difference between
the total energies of electrons in the ferromagnetic and antiferromagnetic
couplings, has an oscillatory behavior, and depends on the Fermi energy and the
length of the central region.Comment: 4 pages, 6 figures, International Conference on Theoretical Physics
'Dubna-Nano2008
Spin-dependent resonant tunneling in ZnSe/ZnMnSe heterostructures
Using the transfer matrix method and the effective-mass approximation, the
effect of resonant states on spin transport is studied in
ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric
and magnetic fields. The numerical results show that the ZnMnSe layers, which
act as spin filters, polarize the electric currents. Variation of thickness of
the central ZnSe layer shifts the resonant levels and exhibits an oscillatory
behavior in spin current densities. It is also shown that the spin polarization
of the tunneling current in geometrical asymmetry of the heterostructure where
two ZnMnSe layers have different Mn concentrations, depends strongly on the
thickness and the applied bias.Comment: 13 pages, 6 figure
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