1,392 research outputs found
Perturbations of time optimal control problems for a class of abstract parabolic systems
In this work we study the asymptotic behavior of the solutions of a class of
abstract parabolic time optimal control problems when the generators converge,
in an appropriate sense, to a given strictly negative operator. Our main
application to PDEs systems concerns the behavior of optimal time and of the
associated optimal controls for parabolic equations with highly oscillating
coefficients, as we encounter in homogenization theory. Our main results assert
that, provided that the target is a closed ball centered at the origin and of
positive radius, the solutions of the time optimal control problems for the
systems with oscillating coefficients converge, in the usual norms, to the
solution of the corresponding problem for the homogenized system. In order to
prove our main theorem, we provide several new results, which could be of a
broader interest, on time and norm optimal control problems
A Comparative Study on Spin-Orbit Torque Efficiencies from W/ferromagnetic and W/ferrimagnetic Heterostructures
It has been shown that W in its resistive form possesses the largest
spin-Hall ratio among all heavy transition metals, which makes it a good
candidate for generating efficient dampinglike spin-orbit torque (DL-SOT)
acting upon adjacent ferromagnetic or ferrimagnetic (FM) layer. Here we provide
a systematic study on the spin transport properties of W/FM magnetic
heterostructures with the FM layer being ferromagnetic
CoFeB or ferrimagnetic CoTb with
perpendicular magnetic anisotropy. The DL-SOT efficiency , which is
characterized by a current-induced hysteresis loop shift method, is found to be
correlated to the microstructure of W buffer layer in both
W/CoFeB and W/CoTb systems. Maximum values
of and are achieved when
the W layer is partially amorphous in the W/CoFeB and
W/CoTb heterostructures, respectively. Our results suggest that
the spin Hall effect from resistive phase of W can be utilized to effectively
control both ferromagnetic and ferrimagnetic layers through a DL-SOT mechanism
New class of 3D topological insulator in double perovskite
We predict a new class of three-dimensional topological insulators (TIs) in
which the spin-orbit coupling (SOC) can more effectively generate a large band
gap at point. The band gap of conventional TI such as BiSe is
mainly limited by two factors, the strength of SOC and, from electronic
structure perspective, the band gap when SOC is absent. While the former is an
atomic property, we find that the latter can be minimized in a generic
rock-salt lattice model in which a stable crossing of bands {\it at} the Fermi
level along with band character inversion occurs for a range of parameters in
the absence of SOC. Thus, large-gap TI's or TI's comprised of lighter elements
can be expected. In fact, we find by performing first-principle calculations
that the model applies to a class of double perovskites ABiXO (A = Ca,
Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, more
detailed calculations considering realistic surface structure indicate that the
Dirac cones are robust against the presence of dangling bond at the boundary
with a specific termination.Comment: submitted; title changed and new references added; see DOI for
published versio
Higher-order solutions to non-Markovian quantum dynamics via hierarchical functional derivative
Solving realistic quantum systems coupled to an environment is a challenging
task. Here we develop a hierarchical functional derivative (HFD) approach for
efficiently solving the non-Markovian quantum trajectories of an open quantum
system embedded in a bosonic bath. An explicit expression for arbitrary order
HFD equation is derived systematically. Moreover, it is found that for an
analytically solvable model, this hierarchical equation naturally terminates at
a given order and thus becomes exactly solvable. This HFD approach provides a
systematic method to study the non-Markovian quantum dynamics of an open system
coupled to a bosonic environment.Comment: 5 pages, 2 figure
Modeling Noise Coupling Between Package and PCB Power/Ground Planes with an Efficient 2-D FDTD/Lumped Element Method
An efficient numerical approach based on the 2-D finite-difference time-domain (FDTD) method is proposed to model the power/ground plane noise or simultaneously switching noise (SSN), including the interconnect effect between the package and the print circuit board (PCB). The space between the power and ground planes on the package and PCB are meshed with 2-D cells. The equivalent R-L-C circuits of the via and the solder balls connecting the package and PCB can be incorporated into a 2-D Yee cell based on a novel integral formulation in the time domain. An efficient recursive updating algorithm is proposed to fit the lumped networks into the Yee equations. A test sample of a ball grid array (BGA) package mounted on a PCB was fabricated. The power/ground noise coupling behavior was measured and compared with the simulation. The proposed method significantly reduces the computing time compared with other full-wave numerical approaches
Landau level crossing in a spin-orbit coupled two-dimensional electron gas
We have studied experimentally theᅠLandau levelᅠ(LL) spectrum of a two-dimensionalᅠelectron gasᅠ(2DEG)ᅠin an In0.53Ga0.47As/InPᅠquantum wellᅠstructure by means of low-temperature magneto-transport coincidence measurement in vectorᅠmagnetic fields.ᅠIt is well known that LL crossing occurs in tiltedᅠmagnetic fieldsᅠdue to a competition between cyclotron energy andᅠZeeman effect.ᅠRemarkably, here we observe an additional type of level-crossing resulting from a competition between Rashba andᅠZeeman splittingᅠin a smallᅠmagnetic field,ᅠconsistent with theᅠtheoreticalᅠprediction for strongly spin-orbit coupledᅠ2DEG
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