91 research outputs found
Switching of the vortex polarity in a magnetic nanodisk by a DC current
We study the dynamics of a vortex state nanodisk due to a dc spin current,
perpendicular to the disk plane. The irreversible switching of the vortex
polarity takes place above some threshold current. The detailed description of
these processes is obtained by spin-lattice simulations.Comment: REVTeX, 4 pages, 3 figure
Hole spin relaxation in -type (111) GaAs quantum wells
Hole spin relaxation in -type (111) GaAs quantum wells is investigated in
the case with only the lowest hole subband, which is heavy-hole like in (111)
GaAs/AlAs and light-hole like in (111) GaAs/InP quantum wells, being relevant.
The subband L\"{o}wdin perturbation method is applied to obtain the effective
Hamiltonian including the Dresselhaus and Rashba spin-orbit couplings. Under a
proper gate voltage, the total in-plane effective magnetic field in (111)
GaAs/AlAs quantum wells can be strongly suppressed in the whole momentum space,
while the one in (111) GaAs/InP quantum wells can be suppressed only on a
special momentum circle. The hole spin relaxation due to the D'yakonov-Perel'
and Elliott-Yafet mechanisms is calculated by means of the fully microscopic
kinetic spin Bloch equation approach with all the relevant scatterings
explicitly included. For (111) GaAs/AlAs quantum wells, extremely long
heavy-hole spin relaxation time (upto hundreds of nanoseconds) is predicted. In
addition, we predict a pronounced peak in the gate-voltage dependence of the
heavy-hole spin relaxation time due to the D'yakonov-Perel' mechanism. This
peak origins from the suppression of the unique inhomogeneous broadening in
(111) GaAs/AlAs quantum wells. Moreover, the Elliott-Yafet mechanism influences
the spin relaxation only around the peak area due to the small spin mixing
between the heavy and light holes in quantum wells with small well width. We
also show the anisotropy of the spin relaxation. In (111) GaAs/InP quantum
wells, a mild peak, similar to the case for electrons in (111) GaAs quantum
wells, is also predicted in the gate-voltage dependence of the light-hole spin
relaxation time. The contribution of the Elliott-Yafet mechanism is always
negligible in this case.Comment: 9 pages, 4 figure
Step-Wise Computational Synthesis of Fullerene C60 derivatives. 1.Fluorinated Fullerenes C60F2k
The reactions of fullerene C60 with atomic fluorine have been studied by
unrestricted broken spin-symmetry Hartree-Fock (UBS HF) approach implemented in
semiempirical codes based on AM1 technique. The calculations were focused on a
sequential addition of fluorine atom to the fullerene cage following indication
of the cage atom highest chemical susceptibility that is calculated at each
step. The effectively-non-paired-electron concept of the fullerene atoms
chemical susceptibility lays the foundation of the suggested computational
synthesis. The obtained results are analyzed from energetic, symmetry, and the
composition abundance viewpoints. A good fitting of the data to experimental
findings proves a creative role of the suggested synthesis methodology.Comment: 33 pages, 11 figures, 2 tables, 2 chart
Magnetic vortex-antivortex crystals generated by spin-polarized current
We study vortex pattern formation in thin ferromagnetic films under the
action of strong spin-polarized currents. Considering the currents which are
polarized along the normal of the film plane, we determine the critical current
above which the film goes to a saturated state with all magnetic moments being
perpendicular to the film plane. We show that stable square vortex-antivortex
superlattices (\emph{vortex crystals}) appears slightly below the critical
current. The melting of the vortex crystal occurs with current further
decreasing. A mechanism of current-induced periodic vortex-antivortex lattice
formation is proposed. Micromagnetic simulations confirm our analytical results
with a high accuracy.Comment: 12 pages, 11 figure
Continuous symmetry of C60 fullerene and its derivatives
Conventionally, the Ih symmetry of fullerene C60 is accepted which is
supported by numerous calculations. However, this conclusion results from the
consideration of the molecule electron system, of its odd electrons in
particular, in a close-shell approximation without taking the electron spin
into account. Passing to the open-shell approximation has lead to both the
energy and the symmetry lowering up to Ci. Seemingly contradicting to a
high-symmetry pattern of experimental recording, particularly concerning the
molecule electronic spectra, the finding is considered in the current paper
from the continuous symmetry viewpoint. Exploiting both continuous symmetry
measure and continuous symmetry content, was shown that formal Ci symmetry of
the molecule is by 99.99% Ih. A similar continuous symmetry analysis of the
fullerene monoderivatives gives a reasonable explanation of a large variety of
their optical spectra patterns within the framework of the same C1 formal
symmetry exhibiting a strong stability of the C60 skeleton.Comment: 11 pages. 5 figures. 6 table
Vortex polarity switching by a spin--polarized current
The spin-transfer effect is investigated for the vortex state of a magnetic
nanodot. A spin current is shown to act similarly to an effective magnetic
field perpendicular to the nanodot. Then a vortex with magnetization (polarity)
parallel to the current polarization is energetically favorable. Following a
simple energy analysis and using direct spin--lattice simulations, we predict
the polarity switching of a vortex. For magnetic storage devices, an electric
current is more effective to switch the polarity of a vortex in a nanodot than
the magnetic field
The Kramers problem:beyond quasi-stationarity
Noise-induced escape from a metastable potential is considered on time-scales preceding the formation of quasi-equilibrium within the metastable part of the potential. It is shown that the escape flux may then depend exponentially strongly, and in a complicated manner, on time and friction
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