394 research outputs found
Analytical description of ballistic spin currents and torques in magnetic tunnel junctions
In this work we demonstrate explicit analytical expressions for both charge
and spin currents which constitute the 2x2 spinor in magnetic tunnel junctions
with noncollinear magnetizations under applied voltage. The calculations have
been performed within the free electron model in the framework of the Keldysh
formalism and WKB approximation. We demonstrate that spin/charge currents and
spin transfer torques are all explicitly expressed through only three
irreducible quantities, without further approximations. The conditions and
mechanisms of deviation from the conventional sine angular dependence of both
spin currents and torques are shown and discussed. It is shown in the thick
barrier approximation that all tunneling transport quantities can be expressed
in an extremely simplified form via Slonczewski spin polarizations and our
effective spin averaged interfacial transmission probabilities and effective
out-of-plane polarizations at both interfaces. It is proven that the latter
plays a key role in the emergence of perpendicular spin torque as well as in
the angular dependence character of all spin and charge transport considered.
It is demonstrated directly also that for any applied voltage, the parallel
component of spin current at the FM/I interface is expressed via collinear
longitudinal spin current components. Finally, spin transfer torque behavior is
analyzed in a view of transverse characteristic length scales for spin
transport.Comment: 10 pages, 6 figure
Generation of Attosecond Pulses with Controllable Carrier-Envelope Phase via High-order Frequency Mixing
Advancing table-top attosecond sources in brightness and pulse duration is of
immense interest and importance for an expanding sphere of applications. Recent
theoretical studies [New J. Phys., 22 093030 (2020)] have found that high-order
frequency mixing (HFM) in a two-color laser field can be much more efficient
than high-order harmonic generation (HHG). Here we study the attosecond
properties of the coherent XUV generated via HFM analytically and numerically,
focusing on the practically important case when one of the fields has much
lower frequency and much lower intensity than the other one. We derive simple
analytical equations describing intensities and phase locking of the HFM
spectral components. We show that the duration of attosecond pulses generated
via HFM, while being very similar to that obtained via HHG in the plateau, is
shortened for the cut-off region. Moreover, our study demonstrates that the
carrier-envelope phase of the attopulses produced via HFM, in contrast to HHG,
can be easily controlled by the phases of the generating fields
Macroscopic effects in generation of attosecond XUV pulses via high-order frequency mixing in gases and plasma
We study the generation of attosecond XUV pulses via high-order frequency
mixing (HFM) of two intense generating fields, and compare this process with
the more common high-order harmonic generation (HHG) process. We calculate the
macroscopic XUV signal by numerically integrating the 1D propagation equation
coupled with the 3D time-dependent Schr\"odinger equation. We analytically find
the length scales which limit the quadratic growth of the HFM macroscopic
signal with propagation length. Compared to HHG these length scales are much
longer for a group of HFM components, with orders defined by the frequencies of
the generating fields. This results in a higher HFM macroscopic signal despite
the microscopic response being lower than for HHG. In our numerical
simulations, the intensity of the HFM signal is several times higher than that
for HHG in a gas, and it is up to three orders of magnitude higher for
generation in plasma; it is also higher for longer generating pulses. The HFM
provides very narrow XUV lines ()
with well-defined frequencies, thus allowing for a simple extension of optical
frequency standards to the XUV range. Finally, we show that the group of HFM
components effectively generated due to macroscopic effects provides a train of
attosecond pulses such that the carrier-envelope phase of an individual
attosecond pulse can be easily controlled by tuning the phase of one of the
generating fields.Comment: 14 pages, 7 figure
Temperature dependence of the probability of "small heating" and total losses of ucns on the surface of fomblin oils of different molecular mass
We measured the temperature dependence of the probability of small heating
and total losses of UCNs on the PFPE Fomblin Y surface with various molecular
masses Mw=2800, 3300, 6500 amu in the temperature range of 100-300 K. The
probability of small heating sharply decreases with increasing Mw and
decreasing temperature. The probability of total loss weakly decreases with
decreasing temperature and takes the minimum value at Mw=3300 amu. As this oil
provides a homogeneous surface with minimal probabilities of small heating and
total losses of UCNs, it is the preferred candidate for experiments on
measuring the neutron lifetime
Does Giant Magnetoresistance Survive in Presence of Superconducting Contact?
The giant magnetoresistance (GMR) of ferromagnetic bilayers with a
superconducting contact (F1/F2/S) is calculated in ballistic and diffusive
regimes. As in spin-valve, it is assumed that the magnetization in the two
ferromagnetic layers F1 and F2 can be changed from parallel to antiparallel. It
is shown that the GMR defined as the change of conductance between the two
magnetic configurations is an oscillatory function of the thickness of F2 layer
and tends to an asymptotic positive value at large thickness. This is due to
the formation of quantum well states in F2 induced by Andreev reflection at the
F2/S interface and reflection at F1/F2 interface in antiparallel configuration.
In the diffusive regime, if only spin-dependent scattering rates in the
magnetic layers are considered (no difference in Fermi wave-vectors between
spin up and down electrons) then the GMR is supressed due to the mixing of spin
up and down electron-hole channels by Andreev reflection.Comment: 7 pages, 4 figures, submitted to Phys.Rev.Let
Finite Element Modeling of Charge and Spin-currents in Magnetoresistive Pillars with Current Crowding Effects
The charge and spin diffusion equations taking into account spin-flip and
spin-transfer torque were numerically solved using a finite element method in
complex non-collinear geometry. This approach was used to study the
spin-dependent transport in giant magnetoresistance metallic pillars sandwiched
between extended electrodes as in magnetoresistive heads for hard disk drives.
The charge current crowding around the boundaries between the electrodes and
the pillar has a quite significant influence on the spin current.Comment: 11 pages, 4 figure
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