245 research outputs found
Rashba spin-orbit interaction enhanced by graphene in-plane deformations
Graphene consists in a single-layer carbon crystal where 2 electrons
display a linear dispersion relation in the vicinity of the Fermi level,
conveniently described by a massless Dirac equation in spacetime.
Spin-orbit effects open a gap in the band structure and offer perspectives for
the manipulation of the conducting electrons spin. Ways to manipulate
spin-orbit couplings in graphene have been generally assessed by proximity
effects to metals that do not compromise the mobility of the unperturbed system
and are likely to induce strain in the graphene layer. In this work we explore
the gauge fields that result from the uniform
stretching of a graphene sheet under a perpendicular electric field.
Considering such deformations is particularly relevant due to the
counter-intuitive enhancement of the Rashba coupling between 30-50% for small
bond deformations well known from tight-binding and DFT calculations. We report
the accessible changes that can be operated in the band structure in the
vicinity of the K points as a function of the deformation strength and
direction.Comment: 10 pages, 7 figure
Bipolar spin filter in a quantum dot molecule
We show that the tunable hybridization between two lateral quantum dots
connected to non-magnetic current leads in a `hanging-dot' configuration that
can be used to implement a bipolar spin filter. The competition between Zeeman,
exchange interaction, and interdot tunneling (molecular hybridization) yields a
singlet-triplet transition of the double dot {\it ground state} that allows
spin filtering in Coulomb blockade experiments. Its generic nature should make
it broadly useful as a robust bidirectional spin polarizer.Comment: 5 pages, 3 figures (to appear in Appl. Phys. Lett.
From classical to quantum spintronics: Theory of coherent spin injection and spin valve phenomena
We present a theory of coherent quantum transport in ferromagnetic/
non-magnetic/ ferromagnetic heterojunctions. We predict quantum coherence to
give rise to a quantum spin valve effect that, unlike its familiar classical
analog, occurs even in the absence of a net spin current through the
heterostructure. Thus the relationship between spin and charge transport is
qualitatively different in the presence of quantum interference than in the
(semi)classical regime. This has important implications for the design of
quantum coherent spintronic devices and the interpretation of experiments.Comment: 5 pages, 2 figures. To appear in EP
Spin rotation for ballistic electron transmission induced by spin-orbit interaction
We study spin dependent electron transmission through one- and
two-dimensional curved waveguides and quantum dots with account of spin-orbit
interaction. We prove that for a transmission through arbitrary structure there
is no spin polarization provided that electron transmits in isolated energy
subband and only two leads are attached to the structure. In particular there
is no spin polarization in the one-dimensional wire for which spin dependent
solution is found analytically. The solution demonstrates spin evolution as
dependent on a length of wire. Numerical solution for transmission of electrons
through the two-dimensional curved waveguides coincides with the solution for
the one-dimensional wire if the energy of electron is within the first energy
subband. In the vicinity of edges of the energy subbands there are sharp
anomalies of spin flipping.Comment: 9 oages, 7 figure
Spin transport of electrons through quantum wires with spatially-modulated strength of the Rashba spin-orbit interaction
We study ballistic transport of spin-polarized electrons through quantum
wires in which the strength of the Rashba spin-orbit interaction (SOI) is
spatially modulated. Subband mixing, due to SOI, between the two lowest
subbands is taken into account. Simplified approximate expressions for the
transmission are obtained for electron energies close to the bottom of the
first subband and near the value for which anticrossing of the two lowest
subbands occurs. In structures with periodically varied SOI strength, {\it
square-wave} modulation on the spin transmission is found when only one subband
is occupied and its possible application to the spin transistor is discussed.
When two subbands are occupied the transmission is strongly affected by the
existence of SOI interfaces as well as by the subband mixing
A Multi-Dimensional Heat Transfer Model of a Tie-Tube and Hexagonal Fuel Element for Nuclear Thermal Propulsion
The Space Capable Cryogenic Thermal Engine (SCCTE) effort considers a nuclear thermal rocket design based around a Low-Enriched Uranium (LEU) design fission reactor. The reactor core is comprised of bundled hexagonal fuel elements that directly heat hydrogen for expansion in a thrust chamber and hexagonal tie-tubes that house zirconium hydride moderator mass for the purpose of thermalizing fast neutrons resulting from fission events. Created 3D steady state Hex fuel rod model with 1D flow channels. Hand Calculation were used to set up initial conditions for fluid flow. The Hex Fuel rod uses 1D flow paths to model the channels using empirical correlations for heat transfer in a pipe. Created a 2-D axisymmetric transient to steady state model using the CFD turbulent flow and Heat Transfer module in COMSOL. This model was developed to find and understand the hydrogen flow that might effect the thermal gradients axially and at the end of the tie tube where the flow turns and enters an annulus. The Hex fuel rod and Tie tube models were made based on requirements given to us by CSNR and the SCCTE team. The models helped simplify and understand the physics and assumptions. Using pipe correlations reduced the complexity of the 3-D fuel rod model and is numerically more stable and computationally more time-efficient compared to the CFD approach. The 2-D axisymmetric tie tube model can be used as a reference "Virtual test model" for comparing and improving 3-D Models
The electronic properties of bilayer graphene
We review the electronic properties of bilayer graphene, beginning with a
description of the tight-binding model of bilayer graphene and the derivation
of the effective Hamiltonian describing massive chiral quasiparticles in two
parabolic bands at low energy. We take into account five tight-binding
parameters of the Slonczewski-Weiss-McClure model of bulk graphite plus intra-
and interlayer asymmetry between atomic sites which induce band gaps in the
low-energy spectrum. The Hartree model of screening and band-gap opening due to
interlayer asymmetry in the presence of external gates is presented. The
tight-binding model is used to describe optical and transport properties
including the integer quantum Hall effect, and we also discuss orbital
magnetism, phonons and the influence of strain on electronic properties. We
conclude with an overview of electronic interaction effects.Comment: review, 31 pages, 15 figure
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