355 research outputs found
Hysteresis of static polarizability of a ballistic ring in presence of intense high-frequency field
A ballistic ring in presence of collinear dc and high-frequency electric
fields is considered. A possibility is demonstrated of the system switching
between two states by changing the strength of dc and/or high-frequency field.
It leads to hysteretic behavior of the ring dc dipole moment as a function of
the dc electric field.Comment: 3 pages, 1 figur
Parametric instability of a magnetic junction under modulated spin-polarized current
The stability is analyzed of the magnetic junction collinear configurations
against small fluctuations under amplitude-modulated current with CPP mode.
High spin injection is assumed. Under parametric resonance conditions, with the
modulation frequency twice the precession frequency, instability is possible of
one, or another, or both the collinear configurations. When the dc component of
the current density exceeds the instability threshold of the antiparallel
configuration, the parametric instability is suppressed by nonparametric one
which is induced by the dc current. The parametric instability manifests itself
as lowering the threshold of the dc current density in presence of the
high-frequency current, such an effect has been observed in experiments
repeatedly.Comment: 9 pages, 2 figure
Quasi-one-dimensional ballistic ring in the field of circularly polarized electromagnetic wave
Dynamics is studied of an electron in a quasi-one-dimensional ballistic ring
under circularly polarized electromagnetic field propagating along the normal
to the ring plane. The average emission intensity from the ring is calculated.
The value and direction of the electron average angular velocity in the ring
depend on the incident wave parameters. It is found that the ring average
dipole moment can remain constant under certain conditions. Possibility is
shown of higher harmonics enhancement in the ring radiation spectrum.Comment: 8 pages, 6 figure
Nonequilibrium phase transitions in a quasi-two-dimensional superlattice with parabolic miniband
Distribution function and current density in a one-dimensional superlattice
with parabolic miniband are calculated. The current dependence on the
temperature coincides with experimental data. Generalization is carried out to
quasi-two-dimensional superlattice with paraboloidal miniband. For a sample
opened in Y direction with dc current in X direction, a novel nonequilibrium
phase transition is found, namely, appearing a spontaneous transverse electric
field under temperature rising.Comment: 9 pages, 6 figure
Stochastic resonance in a quasi-two-dimensional superlattice. II
A fluctuation theory is presented for the nonequilibrium second order phase
transition in a quasi-two-dimensional electron gas. A transverse (with respect
to the current through the sample) spontaneous electric field as an order
parameter and a driving longitudinal field as a control parameter are used. In
addition to the earlier results, the intrawell dynamics is taken into account.
Non-monotonous behavior of the periodic signal gain as a function of the noise
power (stochastic resonance) is predicted.Comment: 9 pages, 2 figure
Phenomenological theory of current driven exchange switching in ferromagnetic nanojunctions
Phenomenological approach is developed in the theory of spin-valve type
ferromagnetic junctions to describe exchange switching by current flowing
perpendicular to interfaces. Forward and backward current switching effects are
described and they may be principally different in nature. Mobile electron
spins are considered as being free in all the contacting ferromagnetic layers.
Joint action of the following two current effects is investigated: the
nonequilibrium longitudinal spin-injection effective field and the transverse
spin-transfer surface torque. Dispersion relation for fluctuations is derived
and solved for a junction model having spatially localized spin transfer
torque: depth of the torque penetration into the free layer is assumed much
smaller than the total free layer thickness. Some critical value of the well
known Gilbert damping constant is established for the first time. Spin transfer
torque dominates in the instability threshold determination for small enough
damping constants, while the spin-injection effective field dominates for high
damping. Fine interplay between spin transfer torque and spin injection is
necessary to provide a hysteretic behavior of the resistance versus current
dependence. The state diagram building up shows the possibility of
non-stationary (time dependent) nonlinear states arising due to instability
development. Calculations lead to the instability rise time values of the order
of 0.1 ns. Spin wave resonance frequency spectrum softening occurs under the
current growing to the instability threshold. Magnetization fluctuations above
the threshold rise oscillating with time for low damping, but rise
aperiodically and much more rapid for high damping.Comment: 16 pages, 7 figures. Corrected typos. Submitted to Phys. Rev.
Quasi-one-dimensional ballistic ring in crossed high-frequency electric fields
We study electron dynamics in a quasi-one-dimensional ballistic ring driven
by two crossed high-frequency electric fields parallel to the ring plane. The
averaged dipole moment and emission intensity are calculated. The emission
polarization coincides with the direction of one of the fields. A possibility
is shown of the polarization switching to perpendicular direction under changes
in the field amplitudes and frequencies.Comment: 7 pages, 2 figure
Conductivity of a superlattice with parabolic miniband
The static and high-frequency differential conductivity of a one-dimensional
superlattice with parabolic miniband, in which the dispersion law is assumed to
be parabolic up to the Brillouin zone edge, are investigated theoretically.
Unlike the earlier published works, devoted to this problem, the novel formula
for the static current density contains temperature dependence, which leads to
the current maximum shift to the low field side with increasing temperature.
The high-frequency differential conductivity response properties including the
temperature dependence is examined and opportunities of creating a terahertz
oscillator on Bloch electron oscillations in such superlattices are discussed.
Analysis shows that superlattices with parabolic miniband dispersion law may be
used for generation and amplification of terahertz fields only at very low
temperatures.Comment: 10 pages, 5 figure
Disturbance of spin equilibrium by current through the interface of noncollinear ferromagnets
Boundary conditions are derived that determine the penetration of spin
current through an interface of two non-collinear ferromagnets with an
arbitrary angle between their magnetization vectors. We start from the
well-known transformation properties of an electron spin wave functions under
the rotation of a quantization axis. It allows directly find the connection
between partial electric current densities for different spin subbands of the
ferromagnets. No spin scattering is assumed in the near interface region, so
that spin conservation takes place when electron intersects the boundary. The
continuity conditions are found for partial chemical potential differences in
the situation. Spatial distribution of nonequilibrium electron magnetizations
is calculated under the spin current flowing through a contact of two
semi-infinite ferromagnets. The distribution describes the spin accumulation
effect by current and corresponding shift of the potential drop at the
interface. These effects appear strongly dependent on the relation between spin
contact resistances at the interface.Comment: 10 pages, 2 figures. Submitted to J. Magn. Magn. Mater. Corrected
typo
Tunnel magnetoresistance of magnetic junctions with cubic symmetry of the layers
A tunnel magnetic junction is considered with magnetic hard and magnetic soft
layers of cubic symmetry. The magnetic switching is analyzed of the layers by a
magnetic field perpendicular to the initial magnetizations. In such a
situation, an additional peak of the TMR ratio appears at the magnetic field
value lower substantially than the anisotropy energy of the soft layer.Comment: 7 pages, 3 figure
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