142 research outputs found
Magnon Current Generation by Dynamical Distortion
The interaction between spin and nanomechanical degrees of freedom attracts
interest from the viewpoint of basic science and device applications. We study
the magnon current induced by the torsional oscillation of ferromagnetic
nanomechanical cantilever. We find that a finite Dzyaloshinskii-Moriya (DM)
interaction emerges by the torsional oscillation, which is described by the
spin gauge field, and the DM interaction leads to the detectably-large magnon
current with frequency same as that of the torsional oscillation. Our theory
paves the way for studying torsional spin-nanomechanical phenomena by using the
spin gauge field.Comment: 5 pages, 4 figure
Renormalization of spin-rotation coupling
We predict the enhancement of the spin-rotation coupling due to the interband
mixing. The Bloch wavefunctions in the presence of mechanical rotation are
constructed with the generalized crystal momentum which includes a gauge
potential arising from the rotation. Using the eight- band Kane model, the
renormalized spin-rotation coupling is explicitly obtained. As a result of the
renormalization, the rotational Doppler shift in electron spin resonance and
the mechanical torque on an electron spin will be strongly modulated.Comment: 8 page
Theory of spin hydrodynamic generation
Spin-current generation by fluid motion is theoretically investigated. Based
on quantum kinetic theory, the spin-diffusion equation coupled with fluid
vorticity is derived. We show that spin currents are generated by the vorticity
gradient in both laminar and turbulent flows and that the generated spin
currents can be detected by the inverse spin Hall voltage measurements, which
are predicted to be proportional to the flow velocity in a laminar flow. In
contrast, the voltage in a turbulent flow is proportional to the square of the
flow velocity. This study will pave the way to fluid spintronics.Comment: 5 pages, 3 figure
Microscopic Theory of the Spin Hall Magnetoresistance
We consider a microscopic theory for the spin Hall magnetoresistance (SMR).
We generally formulate a spin conductance at an interface between a normal
metal and a magnetic insulator in terms of spin susceptibilities. We reveal
that SMR is composed of static and dynamic parts. The static part, which is
almost independent of the temperature, originates from spin flip caused by an
interfacial exchange coupling. However, the dynamic part, which is induced by
the creation or annihilation of magnons, has an opposite sign from the static
part. By the spin-wave approximation, we predict that the latter results in a
nontrivial sign change of the SMR signal at a finite temperature. In addition,
we derive the Onsager relation between spin conductance and thermal
spin-current noise.Comment: 10 pages, 3 figure
Word-Alignment-Based Segment-Level Machine Translation Evaluation using Word Embeddings
One of the most important problems in machine translation (MT) evaluation is
to evaluate the similarity between translation hypotheses with different
surface forms from the reference, especially at the segment level. We propose
to use word embeddings to perform word alignment for segment-level MT
evaluation. We performed experiments with three types of alignment methods
using word embeddings. We evaluated our proposed methods with various
translation datasets. Experimental results show that our proposed methods
outperform previous word embeddings-based methods.Comment: 5 page
Universal -suppression of magnonic shot noise in diffusive insulating magnets
Extending a Boltzmann-Langevin theory to magnons, we show a universality of
current-noise suppression in diffusive systems against the difference of
quantum-statistical properties of bosons and fermions. To this end, starting
from a quantum kinetic equation for magnons subjected to thermal gradient in
dilute impurities, we derive a bosonic counterpart of the semiclassical
Boltzmann-Langevin equation for electrons and evaluate a magnonic current-noise
in the diffusive insulating magnet. We theoretically discover that compared
with a Poissonian shot noise of magnons in an insulating ferromagnetic
junction, the magnonic shot noise is suppressed in the diffusive insulating
bulk magnet and noise-to-current ratio (Fano factor) at low temperatures
exhibits a universal behavior, i.e., the same suppression as the one for
electron transport in diffusive conductors, despite the difference of
quantum-statistical properties. Finally, we show that our predictions are
within experimental reach with current device and measurement technologies.Comment: 4+4 pages; the title has been changed following the suggestion from
the publishe
Spontaneous generation of spin current from the vacuum by strong electric fields
We discuss spontaneous spin current generation from the vacuum by strong
electric fields as a result of interplay between the Schwinger mechanism and a
spin-orbit coupling. By considering a homogeneous slow strong electric field
superimposed by a fast weak transverse electric field, we explicitly evaluate
the vacuum expectation value of a spin current (the Bargmann-Wigner spin
current) by numerically solving the Dirac equation. We show that a
non-vanishing spin current polarized in the direction perpendicular to the
electric fields flows mostly in the longitudinal direction. We also find that a
relativistic effect due to the helicity conservation affects
direction/polarization of spin current.Comment: v2: 16 pages, 5 figures; discussions and references adde
Determination of spin relaxation time and spin diffusion length by oscillation of spin pumping signal
We theoretically investigate a manipulation method of nonequilibrium spin
accumulation in the paramagnetic normal metal of a spin pumping system, by
using the spin precession motion combined with the spin diffusion transport. We
demonstrate based on the Bloch-Torrey equation that the direction of the
nonequilibrium spin accumulation is changed by applying an additional external
magnetic field, and consequently, the inverse spin Hall voltage in an adjacent
paramagnetic heavy metal changes its sign. We find that the spin relaxation
time and the spin diffusion length are simultaneously determined by changing
the magnitude of the external magnetic field and the thickness of the normal
metal in a commonly-used spin pumping system
Acoustic Rashba-Edelstein effect
We theoretically study the mechanical induction of the spin density via the
Rashba spin--orbit interaction (SOI). The spin density in the linear response
to lattice distortion dynamics is calculated based on the microscopic theory.
We reveal that there are two mechanisms of spin induction: one is the acoustic
Edelstein effect (AEE) from the acceleration of the lattice dynamics and the
other is caused by the Rashba spin--vorticity coupling (RSVC). We find that the
AEE induces a more efficient spin-to-charge conversion in comparison with the
conventional electric Edelstein effect. The induced spin density due to the
RSVC is expressed as a Berry curvature-like quantity; therefore, it can be
attributed to the spatial symmetry breaking due to the Rashba SOI. Our work
demonstrates high-efficiency spin generation in Rashba systems.Comment: 5 pages, 3 figure
Alternating Current-induced Interfacial Spin-transfer Torque
We investigate an interfacial spin-transfer torque and -term torque
with alternating current (AC) parallel to a magnetic interface. We find that
both torques are resonantly enhanced as the AC frequency approaches to the
exchange splitting energy. We show that this resonance allows us to estimate
directly the interfacial exchange interaction strength from the domain wall
motion. We also find that the -term includes an unconventional
contribution which is proportional to the time derivative of the current and
exists even in absence of any spin relaxation processes.Comment: 5 pages, 2 figure
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