43 research outputs found
Perspectives in spintronics: magnetic resonant tunneling, spin-orbit coupling, and GaMnAs
Spintronics has attracted wide attention by promising novel functionalities
derived from both the electron charge and spin. While branching into new areas
and creating new themes over the past years, the principal goals remain the
spin and magnetic control of the electrical properties, essentially the I-V
characteristics, and vice versa. There are great challenges ahead to meet these
goals. One challenge is to find niche applications for ferromagnetic
semiconductors, such as GaMnAs. Another is to develop further the science of
hybrid ferromagnetic metal/semiconductor heterostructures, as alternatives to
all-semiconductor room temperature spintronics. Here we present our
representative recent efiorts to address such challenges. We show how to make a
digital magnetoresistor by combining two magnetic resonant diodes, or how
introducing ferromagnetic semiconductors as active regions in resonant
tunneling diodes leads to novel efiects of digital magnetoresistance and of
magnetoelectric current oscillations. We also discuss the phenomenon of
tunneling anisotropic magnetoresistance in Fe/GaAs junctions by introducing the
concept of the spin-orbit coupling field, as an analog of such fields in
all-semiconductor junctions. Finally, we look at fundamental electronic and
optical properties of GaMnAs by employing reasonable tight-binding models to
study disorder efiects.Comment: 13 pages, 7 figures; in the Proceedings of the International
Conference on Theoretical Physics'DUBNA-NANO2008
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Dialing in single-site reactivity of a supported calixarene-protected tetrairidium cluster catalyst.
A closed Ir4 carbonyl cluster, 1, comprising a tetrahedral metal frame and three sterically bulky tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl) ligands at the basal plane, was characterized with variable-temperature 13C NMR spectroscopy, which show the absence of scrambling of the CO ligands at temperatures up to 313 K. This demonstration of distinct sites for the CO ligands was found to extend to the reactivity and catalytic properties, as shown by selective decarbonylation in a reaction with trimethylamine N-oxide (TMAO) as an oxidant, which, reacting in the presence of ethylene, leads to the selective bonding of an ethyl ligand at the apical Ir site. These clusters were supported intact on porous silica and found to catalyze ethylene hydrogenation, and a comparison of the kinetics of the single-hydrogenation reaction and steady-state hydrogenation catalysis demonstrates a unique single-site catalyst-with each site having the same catalytic activity. Reaction orders in the catalytic ethylene hydrogenation reaction of approximately 1/2 and 0 for H2 and C2H4, respectively, nearly match those for conventional noble-metal catalysts. In contrast to oxidative decarbonylation, thermal desorption of CO from silica-supported cluster 1 occurred exclusively at the basal plane, giving rise to sites that do not react with ethylene and are catalytically inactive for ethylene hydrogenation. The evidence of distinctive sites on the cluster catalyst leads to a model that links to hydrogen-transfer catalysis on metals-involving some surface sites that bond to both hydrocarbon and hydrogen and are catalytically engaged (so-called "*" sites) and others, at the basal plane, which bond hydrogen and CO but not hydrocarbon and are reservoir sites (so-called "S" sites)
Band-structure topologies of graphene: spin-orbit coupling effects from first principles
The electronic band structure of graphene in the presence of spin-orbit
coupling and transverse electric field is investigated from first principles
using the linearized augmented plane-wave method. The spin-orbit coupling opens
a gap at the -point of the magnitude of 24 eV (0.28 K). This
intrinsic splitting comes 96% from the usually neglected and higher
orbitals. The electric field induces an additional (extrinsic)
Bychkov-Rashba-type splitting of 10 eV (0.11 K) per V/nm, coming from the
- mixing. A 'mini-ripple' configuration with every other atom is
shifted out of the sheet by less than 1% differs little from the intrinsic
case.Comment: 4 pages, 4 figure
Electrical control of ferromagnetism in Mn-doped semiconductor heterostructures
The interplay of tunneling transport and carrier-mediated ferromagnetism in
narrow semiconductor multi-quantum well structures containing layers of GaMnAs
is investigated within a self-consistent Green's function approach, accounting
for disorder in the Mn--doped regions and unwanted spin-flips at
heterointerfaces on phenomenological ground. We find that the magnetization in
GaMnAs layers can be controlled by an external electric bias. The underlying
mechanism is identified as spin-selective hole tunneling in and out of the
Mn-doped quantum wells, whereby the applied bias determines both hole
population and spin polarization in these layers. In particular we predict
that, near resonance, ferromagnetic order in the Mn doped quantum wells is
destroyed. The interplay of both magnetic and transport properties combined
with structural design potentially leads to several interrelated physical
phenomena, such as dynamic spin filtering, electrical control of magnetization
in individual magnetic layers, and, under specific bias conditions, to
self-sustained current and magnetization oscillations
(magneticmulti-stability). Relevance to recent experimental results is
discussed.Comment: 10 pages, 8 figure
Spin-orbit effects in a graphene bipolar pn junction
A graphene junction is studied theoretically in the presence of both
intrinsic and Rashba spin-orbit couplings. We show that a crossover from
perfect reflection to perfect transmission is achieved at normal incidence by
tuning the perpendicular electric field. By further studying angular dependent
transmission, we demonstrate that perfect reflection at normal incidence can be
clearly distinguished from trivial band gap effects. We also investigate how
spin-orbit effects modify the conductance and the Fano factor associated with a
potential step in both and cases.Comment: 6 pages, 5 figures, conductance and Fano factor plots adde
Electronic properties of graphene and graphene nanoribbons with "pseudo-Rashba" spin-orbit coupling
We discuss the electronic properties of graphene and graphene nanoribbons
including "pseudo-Rashba" spin-orbit coupling. After summarizing the bulk
properties, we first analyze the scattering behavior close to an infinite mass
and zigzag boundary. For low energies, we observe strong deviations from the
usual spin-conserving behavior at high energies such as reflection acting as
spin polarizer or switch. This results in a spin polarization along the
direction of the boundary due to the appearance of evanescent modes in the case
of non-equilibrium or when there is no coherence between the two one-particle
branches. We then discuss the spin and density distribution of graphene
nanoribbons.Comment: 18 pages, 9 figures; section on nanoribbons adde
Spin Relaxation in Single Layer Graphene with Tunable Mobility
Graphene is an attractive material for spintronics due to theoretical
predictions of long spin lifetimes arising from low spin-orbit and hyperfine
couplings. In experiments, however, spin lifetimes in single layer graphene
(SLG) measured via Hanle effects are much shorter than expected theoretically.
Thus, the origin of spin relaxation in SLG is a major issue for graphene
spintronics. Despite extensive theoretical and experimental work addressing
this question, there is still little clarity on the microscopic origin of spin
relaxation. By using organic ligand-bound nanoparticles as charge reservoirs to
tune mobility between 2700 and 12000 cm2/Vs, we successfully isolate the effect
of charged impurity scattering on spin relaxation in SLG. Our results
demonstrate that while charged impurities can greatly affect mobility, the spin
lifetimes are not affected by charged impurity scattering.Comment: 13 pages, 5 figure
Electronic Spin Transport in Dual-Gated Bilayer Graphene
The elimination of extrinsic sources of spin relaxation is key in realizing
the exceptional intrinsic spin transport performance of graphene. Towards this,
we study charge and spin transport in bilayer graphene-based spin valve devices
fabricated in a new device architecture which allows us to make a comparative
study by separately investigating the roles of substrate and polymer residues
on spin relaxation. First, the comparison between spin valves fabricated on
SiO2 and BN substrates suggests that substrate-related charged impurities,
phonons and roughness do not limit the spin transport in current devices. Next,
the observation of a 5-fold enhancement in spin relaxation time in the
encapsulated device highlights the significance of polymer residues on spin
relaxation. We observe a spin relaxation length of ~ 10 um in the encapsulated
bilayer with a charge mobility of 24000 cm2/Vs. The carrier density dependence
of spin relaxation time has two distinct regimes; n<4 x 1012 cm-2, where spin
relaxation time decreases monotonically as carrier concentration increases, and
n>4 x 1012 cm-2, where spin relaxation time exhibits a sudden increase. The
sudden increase in the spin relaxation time with no corresponding signature in
the charge transport suggests the presence of a magnetic resonance close to the
charge neutrality point. We also demonstrate, for the first time, spin
transport across bipolar p-n junctions in our dual-gated device architecture
that fully integrates a sequence of encapsulated regions in its design. At low
temperatures, strong suppression of the spin signal was observed while a
transport gap was induced, which is interpreted as a novel manifestation of
impedance mismatch within the spin channel
A unified theory of the Elliott-Yafet and the D’yakonov-Perel’ spin-relaxation mechanisms
We present a unified treatment of the Elliott-Yafet (EY) and the D’yakonov-Perel’ (DP) spin-relaxation mechanisms
using the Mori-Kawasaki formula, which gives the spin-relaxation rate to lowest order in the spin-orbit
coupling (SOC) but to infinite order in the quasi-particle scattering rate