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Inverse Statistical Mechanics: Probing the Limitations of Isotropic Pair Potentials to Produce Ground-State Structural Extremes
Inverse statistical-mechanical methods have recently been employed to design
optimized short-ranged radial (isotropic) pair potentials that robustly produce
novel targeted classical ground-state many-particle configurations. The target
structures considered in those studies were low-coordinated crystals with a
high degree of symmetry. In this paper, we further test the fundamental
limitations of radial pair potentials by targeting crystal structures with
appreciably less symmetry, including those in which the particles have
different local structural environments. These challenging target
configurations demanded that we modify previous inverse optimization
techniques. Using this modified optimization technique, we have designed
short-ranged radial pair potentials that stabilize the two-dimensional kagome
crystal, the rectangular kagome crystal, and rectangular lattices, as well as
the three-dimensional structure of CaF crystal inhabited by a single
particle species. We verify our results by cooling liquid configurations to
absolute zero temperature via simulated annealing and ensuring that such states
have stable phonon spectra. Except for the rectangular kagome structure, all of
the target structures can be stabilized with monotonic repulsive potentials.
Our work demonstrates that single-component systems with short-ranged radial
pair potentials can counterintuitively self-assemble into crystal ground states
with low symmetry and different local structural environments. Finally, we
present general principles that offer guidance in determining whether certain
target structures can be achieved as ground states by radial pair potentials
Spin Dynamics in the Second Subband of a Quasi Two Dimensional System Studied in a Single Barrier Heterostructure by Time Resolved Kerr Rotation
By biasing a single barrier heterostructure with a 500nm-thick GaAs layer as
the absorption layer, the spin dynamics for both of the first and second
subband near the AlAs barrier are examined. We find that when simultaneously
scanning the photon energy of both the probe and pump beams, a sign reversal of
the Kerr rotation (KR) takes place as long as the probe photons break away the
first subband and probe the second subband. This novel feature, while stemming
from the exchange interaction, has been used to unambiguously distinguish the
different spin dynamics ( and ) for the first and second
subbands under the different conditions by their KR signs (negative for
and positive for ). In the zero magnetic field, by scanning
the wavelength towards the short wavelength, decreases in accordance
with the D'yakonov-Perel' (DP) spin decoherence mechanism. At 803nm,
(450ps) becomes ten times longer than (50ps). However, the
value of at 803nm is roughly the same as the value of at
815nm. A new feature has been disclosed at the wavelength of 811nm under the
bias of -0.3V (807nm under the bias of -0.6V) that the spin coherence times
( and ) and the effective factors ( and
) all display a sudden change, due to the "resonant" spin exchange
coupling between two spin opposite bands.Comment: 9pages, 3 figure
Is perpendicular magnetic anisotropy essential to all-optical ultrafast spin reversal in ferromagnets?
All-optical spin reversal presents a new opportunity for spin manipulations,
free of a magnetic field. Most of all-optical-spin-reversal ferromagnets are
found to have a perpendicular magnetic anisotropy (PMA), but it has been
unknown whether PMA is necessary for the spin reversal. Here we theoretically
investigate magnetic thin films with either PMA or in-plane magnetic anisotropy
(IMA). Our results show that the spin reversal in IMA systems is possible, but
only with a longer laser pulse and within a narrow laser parameter region. The
spin reversal does not show a strong helicity dependence where the left- and
right-circularly polarized light lead to the identical results. By contrast,
the spin reversal in PMA systems is robust, provided both the spin angular
momentum and laser field are strong enough while the magnetic anisotropy itself
is not too strong. This explains why experimentally the majority of all-optical
spin-reversal samples are found to have strong PMA and why spins in Fe
nanoparticles only cant out of plane. It is the laser-induced spin-orbit torque
that plays a key role in the spin reversal. Surprisingly, the same spin-orbit
torque results in laser-induced spin rectification in spin-mixed configuration,
a prediction that can be tested experimentally. Our results clearly point out
that PMA is essential to the spin reversal, though there is an opportunity for
in-plane spin reversal.Comment: 20 pages, 4 figures and one tabl
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