7,630 research outputs found
Positive exchange bias in ferromagnetic La0.67Sr0.33MnO3 / SrRuO3 bilayers
Epitaxial La0.67Sr0.33MnO3 (LSMO)/ SrRuO3 (SRO) ferromagnetic bilayers have
been grown on (001) SrTiO3 (STO) substrates by pulsed laser deposition with
atomic layer control. We observe a shift in the magnetic hysteresis loop of the
LSMO layer in the same direction as the applied biasing field (positive
exchange bias). The effect is not present above the Curie temperature of the
SRO layer (), and its magnitude increases rapidly as the temperature is lowered
below . The direction of the shift is consistent with an antiferromagnetic
exchange coupling between the ferromagnetic LSMO layer and the ferromagnetic
SRO layer. We propose that atomic layer charge transfer modifies the electronic
state at the interface, resulting in the observed antiferromagnetic interfacial
exchange coupling.Comment: accepted to Applied Physics Letter
Morphological evolution of a 3D CME cloud reconstructed from three viewpoints
The propagation properties of coronal mass ejections (CMEs) are crucial to
predict its geomagnetic effect. A newly developed three dimensional (3D) mask
fitting reconstruction method using coronagraph images from three viewpoints
has been described and applied to the CME ejected on August 7, 2010. The CME's
3D localisation, real shape and morphological evolution are presented. Due to
its interaction with the ambient solar wind, the morphology of this CME changed
significantly in the early phase of evolution. Two hours after its initiation,
it was expanding almost self-similarly. CME's 3D localisation is quite helpful
to link remote sensing observations to in situ measurements. The investigated
CME was propagating to Venus with its flank just touching STEREO B. Its
corresponding ICME in the interplanetary space shows a possible signature of a
magnetic cloud with a preceding shock in VEX observations, while from STEREO B
only a shock is observed. We have calculated three principle axes for the
reconstructed 3D CME cloud. The orientation of the major axis is in general
consistent with the orientation of a filament (polarity inversion line)
observed by SDO/AIA and SDO/HMI. The flux rope axis derived by the MVA analysis
from VEX indicates a radial-directed axis orientation. It might be that locally
only the leg of the flux rope passed through VEX. The height and speed profiles
from the Sun to Venus are obtained. We find that the CME speed possibly had
been adjusted to the speed of the ambient solar wind flow after leaving COR2
field of view and before arriving Venus. A southward deflection of the CME from
the source region is found from the trajectory of the CME geometric center. We
attribute it to the influence of the coronal hole where the fast solar wind
emanated from.Comment: ApJ, accepte
Frequency variations of gravity waves interacting with a time-varying tide
Using a nonlinear, 2-D time-dependent numerical model, we simulate the
propagation of gravity waves (GWs) in a time-varying tide. Our simulations
show that when a GW packet propagates in a time-varying tidal-wind
environment, not only its intrinsic frequency but also its ground-based
frequency would change significantly. The tidal horizontal-wind acceleration
dominates the GW frequency variation. Positive (negative) accelerations
induce frequency increases (decreases) with time. More interestingly,
tidal-wind acceleration near the critical layers always causes the GW
frequency to increase, which may partially explain the observations that
high-frequency GW components are more dominant in the middle and upper
atmosphere than in the lower atmosphere. The combination of the increased
ground-based frequency of propagating GWs in a time-varying tidal-wind field
and the transient nature of the critical layer induced by a time-varying
tidal zonal wind creates favorable conditions for GWs to penetrate their
originally expected critical layers. Consequently, GWs have an impact on the
background atmosphere at much higher altitudes than expected, which indicates
that the dynamical effects of tidal–GW interactions are more complicated
than usually taken into account by GW parameterizations in global models
Natural SQL: Making SQL Easier to Infer from Natural Language Specifications
Addressing the mismatch between natural language descriptions and the corresponding SQL queries is a key challenge for text-to-SQL translation. To bridge this gap, we propose an SQL intermediate representation (IR) called Natural SQL (NatSQL). Specifically, NatSQL preserves the core functionalities of SQL, while it simplifies the queries as follows: (1) dispensing with operators and keywords such as GROUP BY, HAVING, FROM, JOIN ON, which are usually hard to find counterparts for in the text descriptions; (2) removing the need for nested subqueries and set operators; and (3) making schema linking easier by reducing the required number of schema items. On Spider, a challenging text-to-SQL benchmark that contains complex and nested SQL queries, we demonstrate that NatSQL outperforms other IRs, and significantly improves the performance of several previous SOTA models. Furthermore, for existing models that do not support executable SQL generation, NatSQL easily enables them to generate executable SQL queries, and achieves the new state-of-the-art execution accuracy
Lattice Boltzmann study on Kelvin-Helmholtz instability: the roles of velocity and density gradients
A two-dimensional lattice Boltzmann model with 19 discrete velocities for
compressible Euler equations is proposed (D2V19-LBM). The fifth-order Weighted
Essentially Non-Oscillatory (5th-WENO) finite difference scheme is employed to
calculate the convection term of the lattice Boltzmann equation. The validity
of the model is verified by comparing simulation results of the Sod shock tube
with its corresponding analytical solutions. The velocity and density gradient
effects on the Kelvin-Helmholtz instability (KHI) are investigated using the
proposed model. Sharp density contours are obtained in our simulations. It is
found that, the linear growth rate for the KHI decreases with
increasing the width of velocity transition layer but increases with
increasing the width of density transition layer . After the
initial transient period and before the vortex has been well formed, the linear
growth rates, and , vary with and
approximately in the following way, and
, where , ,
and are fitting parameters and is the effective
interaction width of density transition layer. When
the linear growth rate does not vary significantly any more.
One can use the hybrid effects of velocity and density transition layers to
stabilize the KHI. Our numerical simulation results are in general agreement
with the analytical results [L. F. Wang, \emph{et al.}, Phys. Plasma
\textbf{17}, 042103 (2010)].Comment: Accepted for publication in PR
Adaptive feature extraction in EEG-based motor imagery BCI: tracking mental fatigue
Objective: Electroencephelogram (EEG) signals are non-stationary. This could be due to the internal fluctuation of brain states such as fatigue, frustration, etc. This necessitates the development of adaptive Brain-Computer Interfaces (BCI) whose performance does not deteriorate significantly with the adversary change in the cognitive state. In this paper, we put forward an unsupervised adaptive scheme to adapt the feature extractor of motor imagery (MI) BCIs by tracking the fatigue level of the user. Approach: Eleven subjects participated in the study during which they accomplished MI tasks while self-reporting their perceived levels of mental fatigue. Out of the 11 subjects, only 6 completed the whole experiment, while the others quit in the middle because of experiencing high fatigue. The adaptive feature extractor is attained through the adaptation of the common spatial patterns (CSP), one of the most popular feature extraction algorithms in EEG-based BCIs. The proposed method was analyzed in two ways: oine and in near real-time. The separability of the MI EEG features extracted by the proposed adaptive CSP (ADCSP) has been compared with that by the conventional CSP (C-CSP) and another CSP based adaptive method (ACSP) in terms of: Davies Bouldin Index (DBI), Fisher Score (FS) and Dunn's Index (DI). Results: Experimental results show significant improvement in the separability of MI EEG features extracted by ADCSP as compared to that by C-CSP and ACSP. Signficance: Collectively, the results of the experiments in this study suggest that adapting CSP based on mental fatigue can improve the class separability of MI EEG features
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