18 research outputs found
Extrinsic Entwined with Intrinsic Spin Hall Effect in Disordered Mesoscopic Bars
We show that pure spin Hall current, flowing out of a four-terminal
phase-coherent two-dimensional electron gas (2DEG) within inversion asymmetric
semiconductor heterostructure, contains contributions from both the extrinsic
mechanisms (spin-orbit dependent scattering off impurities) and the intrinsic
ones (due to the Rashba coupling). While the extrinsic contribution vanishes in
the weakly and strongly disordered limits, and the intrinsic one dominates in
the quasiballistic limit, in the crossover transport regime the spin Hall
conductance, exhibiting sample-to-sample large fluctuations and sign change, is
not simply reducible to either of the two mechanisms, which can be relevant for
interpretation of experiments on dirty 2DEGs [V. Sih et al., Nature Phys. 1, 31
(2005)].Comment: 5 pages, 3 color EPS figure
Uniform current in graphene strip with zigzag edges
Graphene exhibits zero-gap massless-Dirac fermion and zero density of states
at E = 0. These particles form localized states called edge states on finite
width strip with zigzag edges at E = 0. Naively thinking, one may expect that
current is also concentrated at the edge, but Zarbo and Nikolic numerically
obtained a result that the current density shows maximum at the center of the
strip. We derive a rigorous relation for the current density, and clarify the
reason why the current density of edge state has a maximum at the center.Comment: 5 pages, 3 figures; added references and corrected typos, to be
published in J. Phys. Soc. Jpn. Vol.78 No.
Spatial distribution of local currents of massless Dirac fermions in quantum transport through graphene nanoribbons
We employ the formalism of bond currents, expressed in terms of the
nonequilibrium Green functions, to image the charge flow between two sites of
the honeycomb lattice of graphene ribbons of few nanometers width. In sharp
contrast to nonrelativistic electrons, current density profiles of quantum
transport at energies close to the Dirac point in clean zigzag graphene
nanoribbons (ZGNR) differs markedly from the profiles of charge density peaked
at the edges due to zero-energy localized edge states. For transport through
the lowest propagating mode induced by these edge states, edge vacancies do not
affect current density peaked in the center of ZGNR. The long-range potential
of a single impurity acts to reduce local current around it while concurrently
increasing the current density along the zigzag edge, so that ZGNR conductance
remains perfect .Comment: 5 pages, 5 figure
The effect of magnetic field and disorders on the electronic transport in graphene nanoribbons
We developed a unified mesoscopic transport model for graphene nanoribbons,
which combines the non-equilibrium Green's function (NEGF) formalism with the
real-space {\pi}-orbital model. Based on this model, we probe the spatial
distributions of electrons under a magnetic field, in order to obtain insights
into the various signature Hall effects in disordered armchair graphene
nanoribbons (AGNR). In the presence of a uniform perpendicular magnetic field
(B\perp-field), a perfect AGNR shows three distinct spatial current profiles at
equilibrium, depending on its width. Under non-equilibrium conditions (i.e. in
the presence of an applied bias), the net electron flow is restricted to the
edges and occurs in opposite directions depending on whether the Fermi level
lies within the valence or conduction band. For electrons at energy level below
the conduction window, the B\perp-field gives rise to local electron flux
circulation, although the global flux is zero. Our study also reveals the
suppression of electron backscattering as a result of the edge transport which
is induced by the B\perp-field. This phenomenon can potentially mitigate the
undesired effects of disorders, such as the bulk and edge vacancies, on the
transport properties of AGNR. Lastly, we show that the effect of B\perp-field
on electronic transport is less significant in the multimode compared to the
single mode electron transport.Comment: 21 pages, 4 figure
Spin and Charge Shot Noise in Mesoscopic Spin Hall Systems
Injection of unpolarized charge current through the longitudinal leads of a
four-terminal two-dimensional electron gas with the Rashba spin-orbit (SO)
coupling and/or SO scattering off extrinsic impurities is responsible not only
for the pure spin Hall current in the transverse leads, but also for random
time-dependent current fluctuations. We employ the scattering approach to
current-current correlations in multiterminal nanoscale conductors to analyze
the shot noise of transverse pure spin Hall and zero charge current, or
transverse spin current and non-zero charge Hall current, driven by unpolarized
or spin-polarized longitudinal current, respectively. Since any spin-flip acts
as an additional source of noise, we argue that these shot noises offer a
unique tool to differentiate between intrinsic and extrinsic SO mechanisms
underlying the spin Hall effect in paramagnetic devices.Comment: 5 pages, 2 figures (5 embedded EPS files
Imaging mesoscopic spin Hall flow: Spatial distribution of local spin currents and spin densities in and out of multiterminal spin-orbit coupled semiconductor nanostructures
We introduce the concept of bond spin current, which describes the spin
transport between two sites of the lattice model of a multiterminal spin-orbit
(SO) coupled semiconductor nanostructure, and express it in terms of the
spin-dependent nonequilibrium (Landauer-Keldysh) Green functions of the device.
This formalism is applied to obtain the spatial distribution of microscopic
spin currents in {\em clean} phase-coherent two-dimensional electron gas with
the Rashba-type of SO coupling attached to four external leads. Together with
the corresponding profiles of the stationary spin density, such visualization
of the phase-coherent spin flow allow us to resolve several key issues for the
understanding of mechanisms which generate pure spin Hall currents in the
transverse leads of ballistic devices due to the flow of unpolarized charge
current through their longitudinal leads. The local spin current profiles
crucially depend on whether the sample is smaller or greater than the spin
precession length, thereby demonstrating its essential role as the
characteristic mesoscale for the spin Hall effect in ballistic multiterminal
semiconductor nanostructures. Although static spin-independent disorder reduces
the magnitude of the total spin current in the leads, the bond spin currents
continue to flow through the whole diffusive 2DEG sample, without being
localized as edge spin currents around any of its boundaries.Comment: 17 pages, 8 color EPS figures; preprint with much higher resolution
figures is available from
http://www.physics.udel.edu/~bnikolic/QTTG/publications.ht
Transverse Spin-Orbit Force in the Spin Hall Effect in Ballistic Semiconductor Wires
We introduce the spin and momentum dependent {\em force operator} which is
defined by the Hamiltonian of a {\em clean} semiconductor quantum wire with
homogeneous Rashba spin-orbit (SO) coupling attached to two ideal (i.e., free
of spin and charge interactions) leads. Its expectation value in the
spin-polarized electronic wave packet injected through the leads explains why
the center of the packet gets deflected in the transverse direction. Moreover,
the corresponding {\em spin density} will be dragged along the transverse
direction to generate an out-of-plane spin accumulation of opposite signs on
the lateral edges of the wire, as expected in the phenomenology of the spin
Hall effect, when spin- and spin- polarized packets
(mimicking the injection of conventional unpolarized charge current) propagate
simultaneously through the wire. We also demonstrate that spin coherence of the
injected spin-polarized wave packet will gradually diminish (thereby
diminishing the ``force'') along the SO coupled wire due to the entanglement of
spin and orbital degrees of freedom of a single electron, even in the absence
of any impurity scattering.Comment: 5 pages, 4 color EPS figures; 2 new figures and expanded discussion
on the sign of spin Hall quantities. To appear in Phys. Rev. B 72 (2005
Spin-injection Hall effect in a planar photovoltaic cell
Successful incorporation of the spin degree of freedom in semiconductor
technology requires the development of a new paradigm allowing for a scalable,
non-destructive electrical detection of the spin-polarization of injected
charge carriers as they propagate along the semiconducting channel. In this
paper we report the observation of a spin-injection Hall effect (SIHE) which
exploits the quantum-relativistic nature of spin-charge transport and which
meets all these key requirements on the spin detection. The two-dimensional
electron-hole gas photo-voltaic cell we designed to observe the SIHE allows us
to develop a quantitative microscopic theory of the phenomenon and to
demonstrate its direct application in optoelectronics. We report an
experimental realization of a non-magnetic spin-photovoltaic effect via the
SIHE, rendering our device an electrical polarimeter which directly converts
the degree of circular polarization of light to a voltage signal.Comment: 14 pages, 4 figure
DNA nucleotide-specific modulation of \mu A transverse edge currents through a metallic graphene nanoribbon with a nanopore
We propose two-terminal devices for DNA sequencing which consist of a
metallic graphene nanoribbon with zigzag edges (ZGNR) and a nanopore in its
interior through which the DNA molecule is translocated. Using the
nonequilibrium Green functions combined with density functional theory, we
demonstrate that each of the four DNA nucleotides inserted into the nanopore,
whose edge carbon atoms are passivated by either hydrogen or nitrogen, will
lead to a unique change in the device conductance. Unlike other recent
biosensors based on transverse electronic transport through DNA nucleotides,
which utilize small (of the order of pA) tunneling current across a nanogap or
a nanopore yielding a poor signal-to-noise ratio, our device concept relies on
the fact that in ZGNRs local current density is peaked around the edges so that
drilling a nanopore away from the edges will not diminish the conductance.
Inserting a DNA nucleotide into the nanopore affects the charge density in the
surrounding area, thereby modulating edge conduction currents whose magnitude
is of the order of \mu A at bias voltage ~ 0.1 V. The proposed biosensor is not
limited to ZGNRs and it could be realized with other nanowires supporting
transverse edge currents, such as chiral GNRs or wires made of two-dimensional
topological insulators.Comment: 6 pages, 6 figures, PDFLaTe
An antidamping spin–orbit torque originating from the Berry curvature
Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin–orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin–orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic paramagnetic heterostructures,eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin–orbit torque as well as its microscopic modelling. We expect the Berry curvature spin–orbit torque to be of comparable strength to the spin-Hall effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect