660 research outputs found
Ab-initio transport theory for digital ferromagnetic heterostructures
MnAs/GaAs superlattices, made by -doping GaAs with Mn, are known as
digital ferromagnetic heterostructures. Here we present a theoretical density
functional study of the electronic, magnetic and transport properties of such
heterostructures. In the absence of intrinsic donors these systems show an half
metallic density of states, with an exchange interaction much stronger than
that of a random alloy with the same Mn concentration. {\it Ab initio}
ballistic transport calculations show that the carriers with energies close to
the Fermi energy are strongly confined within a few monolayers around the MnAs
plane. This strong confinement is responsible for the large exchange coupling.
Therefore the system can be described as a two dimensional half metal with
large conductance in the MnAs plane and small conductance in the perpendicular
direction
Simulating STM transport in alkanes from first principles
Simulations of scanning tunneling microscopy measurements for molecules on
surfaces are traditionally based on a perturbative approach, most typically
employing the Tersoff-Hamann method. This assumes that the STM tip is far from
the sample so that the two do not interact with each other. However, when the
tip gets close to the molecule to perform measurements, the electrostatic
interplay between the tip and substrate may generate non-trivial potential
distribution, charge transfer and forces, all of which may alter the electronic
and physical structure of the molecule. These effects are investigated with the
ab initio quantum transport code SMEAGOL, combining non-equilibrium Green's
functions formalism with density functional theory. In particular, we
investigate alkanethiol molecules terminated with either CH3 or CF3 end-groups
on gold surfaces, for which recent experimental data are available. We discuss
the effects connected to the interaction between the STM tip and the molecule,
as well as the asymmetric charge transfer between the molecule and the
electrodes.Comment: 10 pages, 18 figure
Theoretical studies of spin-dependent electrical transport through carbon nanotbes
Spin-dependent coherent quantum transport through carbon nanotubes (CNT) is
studied theoretically within a tight-binding model and the Green's function
partitioning technique. End-contacted metal/nanotube/metal systems are modelled
and next studied in the magnetic context, i.e. either with ferromagnetic
electrodes or at external magnetic fields. The former case shows that quite a
substantial giant magnetoresistance (GMR) effect occurs () for
disorder-free CNTs. Anderson-disorder averaged GMR, in turn, is positive and
reduced down to several percent in the vicinity of the charge neutrality point.
At parallel magnetic fields, characteristic Aharonov-Bohm-type oscillations are
revealed with pronounced features due to a combined effect of:
length-to-perimeter ratio, unintentional electrode-induced doping, Zeeman
splitting, and energy-level broadening. In particular, a CNT is predicted to
lose its ability to serve as a magneto-electrical switch when its length and
perimeter become comparable. In case of perpendicular geometry, there are
conductance oscillations approaching asymptotically the upper theoretical limit
to the conductance, . Moreover in the ballistic transport regime,
initially the conductance increases only slightly with the magnetic field or
remains nearly constant because spin up- and spin down-contributions to the
total magnetoresistance partially compensate each other.Comment: 15 pages, 6 figures (to apppear in Semicond. Sci. Technol.
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