995 research outputs found
FMR and voltage induced transport in normal metal-ferromagnet-superconductor trilayers
We study the subgap spin and charge transport in normal
metal-ferromagnet-superconductor trilayers induced by bias voltage and/or
magnetization precession. Transport properties are discussed in terms of
time-dependent scattering theory. We assume the superconducting gap is small on
the energy scales set by the Fermi energy and the ferromagnetic exchange
splitting, and compute the non-equilibrium charge and spin current response to
first order in precession frequency, in the presence of a finite applied
voltage. We find that the voltage-induced instantaneous charge current and
longitudinal spin current are unaffected by the precessing magnetization, while
the pumped transverse spin current is determined by spin-dependent conductances
and details of the electron-hole scattering matrix. A simplified expression for
the transverse spin current is derived for structures where the ferromagnet is
longer than the transverse spin coherence length.Comment: 10 page
Nonequilibrium Kondo Effect in a Quantum Dot Coupled to Ferromagnetic Leads
We study the Kondo effect in the electron transport through a quantum dot
coupled to ferromagnetic leads, using a real-time diagrammatic technique which
provides a systematic description of the nonequilibrium dynamics of a system
with strong local electron correlations. We evaluate the theory in an extension
of the `resonant tunneling approximation', introduced earlier, by introducing
the self-energy of the off-diagonal component of the reduced propagator in spin
space. In this way we develop a charge and spin conserving approximation that
accounts not only for Kondo correlations but also for the spin splitting and
spin accumulation out of equilibrium. We show that the Kondo resonances, split
by the applied bias voltage, may be spin polarized. A left-right asymmetry in
the coupling strength and/or spin polarization of the electrodes significantly
affects both the spin accumulation and the weight of the split Kondo resonances
out of equilibrium. The effects are observable in the nonlinear differential
conductance. We also discuss the influence of decoherence on the Kondo
resonance in the frame of the real-time formulation.Comment: 13 pages, 13 figure
Spin effects in single-electron tunneling in magnetic junctions
Spin dependent single electron tunneling in ferromagnetic double junctions is
analysed theoretically in the limit of sequential tunneling. The influence of
discrete energy spectrum of the central electrode (island)on the spin
accumulation, spin fluctuations and tunnel magnetoresistance is analysed
numerically in the case of a nonmagnetic island. It is shown that spin
fluctuations are significant in magnetic as well as in nonmagnetic junctions.Comment: 14 pages, 3 eps-figures include
Frequency-Dependent Current Noise through Quantum-Dot Spin Valves
We study frequency-dependent current noise through a single-level quantum dot
connected to ferromagnetic leads with non-collinear magnetization. We propose
to use the frequency-dependent Fano factor as a tool to detect single-spin
dynamics in the quantum dot. Spin precession due to an external magnetic and/or
a many-body exchange field affects the Fano factor of the system in two ways.
First, the tendency towards spin-selective bunching of the transmitted
electrons is suppressed, which gives rise to a reduction of the low-frequency
noise. Second, the noise spectrum displays a resonance at the Larmor frequency,
whose lineshape depends on the relative angle of the leads' magnetizations.Comment: 12 pages, 15 figure
Interaction-driven spin precession in quantum-dot spin valves
We analyze spin-dependent transport through spin valves composed of an
interacting quantum dot coupled to two ferromagnetic leads. The spin on the
quantum dot and the linear conductance as a function of the relative angle
of the leads' magnetization directions is derived to lowest order in
the dot-lead coupling strength. Due to the applied bias voltage spin
accumulates on the quantum dot, which for finite charging energy experiences a
torque, resulting in spin precession. The latter leads to a non-trivial,
interaction-dependent, -dependence of the conductance. In particular,
we find that the spin-valve effect is reduced for all .Comment: 5 pages, 3 figures, version to be published in Phys. Rev. Let
Electric-field controlled spin reversal in a quantum dot with ferromagnetic contacts
Manipulation of the spin-states of a quantum dot by purely electrical means
is a highly desirable property of fundamental importance for the development of
spintronic devices such as spin-filters, spin-transistors and single-spin
memory as well as for solid-state qubits. An electrically gated quantum dot in
the Coulomb blockade regime can be tuned to hold a single unpaired spin-1/2,
which is routinely spin-polarized by an applied magnetic field. Using
ferromagnetic electrodes, however, the properties of the quantum dot become
directly spin-dependent and it has been demonstrated that the ferromagnetic
electrodes induce a local exchange-field which polarizes the localized spin in
the absence of any external fields. Here we report on the experimental
realization of this tunneling-induced spin-splitting in a carbon nanotube
quantum dot coupled to ferromagnetic nickel-electrodes. We study the
intermediate coupling regime in which single-electron states remain well
defined, but with sufficiently good tunnel-contacts to give rise to a sizable
exchange-field. Since charge transport in this regime is dominated by the
Kondo-effect, we can utilize this sharp many-body resonance to read off the
local spin-polarization from the measured bias-spectroscopy. We show that the
exchange-field can be compensated by an external magnetic field, thus restoring
a zero-bias Kondo-resonance, and we demonstrate that the exchange-field itself,
and hence the local spin-polarization, can be tuned and reversed merely by
tuning the gate-voltage. This demonstrates a very direct electrical control
over the spin-state of a quantum dot which, in contrast to an applied magnetic
field, allows for rapid spin-reversal with a very localized addressing.Comment: 19 pages, 11 figure
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