1,703 research outputs found
Robust Intrinsic Ferromagnetism and Half Semiconductivity in Stable Two-Dimensional Single-Layer Chromium Trihalides
Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are crucial
to develop low-dimensional spintronic devices. Using density functional theory,
we show that single-layer chromium trihalides (SLCTs) (CrX,X=F, Cl, Br and
I) constitute a series of stable 2D intrinsic FM semiconductors. A
free-standing SLCT can be easily exfoliated from the bulk crystal, due to a low
cleavage energy and a high in-plane stiffness. Electronic structure
calculations using the HSE06 functional indicate that both bulk and
single-layer CrX are half semiconductors with indirect gaps and their
valence bands and conduction bands are fully spin-polarized in the same spin
direction. The energy gaps and absorption edges of CrBr and CrI are
found to be in the visible frequency range, which implies possible
opt-electronic applications. Furthermore, SLCTs are found to possess a large
magnetic moment of 3 per formula unit and a sizable magnetic anisotropy
energy. The magnetic exchange constants of SLCTs are then extracted using the
Heisenberg spin Hamiltonian and the microscopic origins of the various exchange
interactions are analyzed. A competition between a near 90 FM
superexchange and a direct antiferromagnetic (AFM) exchange results in a FM
nearest-neighbour exchange interaction. The next and third nearest-neighbour
exchange interactions are found to be FM and AFM respectively and this can be
understood by the angle-dependent extended Cr-X-X-Cr superexchange interaction.
Moreover, the Curie temperatures of SLCTs are also predicted using Monte Carlo
simulations and the values can further increase by applying a biaxial tensile
strain. The unique combination of robust intrinsic ferromagnetism, half
semiconductivity and large magnetic anisotropy energies renders the SLCTs as
promising candidates for next-generation semiconductor spintronic applications.Comment: 12 pages, 14 figures. published in J. Mater. Chem.
Cosmology-independent Estimate of the Fraction of Baryon Mass in the IGM from Fast Radio Burst Observations
The excessive dispersion measure (DM) of fast radio bursts (FRBs) has been proposed to be a powerful tool to study intergalactic medium (IGM) and to perform cosmography. One issue is that the fraction of baryons in the IGM, f IGM, is not properly constrained. Here, we propose a method of estimating f IGM using a putative sample of FRBs with the measurements of both DM and luminosity distance d L. The latter can be obtained if the FRB is associated with a distance indicator (e.g., a gamma-ray burst or a gravitational-wave event), or the redshift z of the FRB is measured and d L at the corresponding z is available from other distance indicators (e.g., SNe Ia) at the same redshift. As d L/DM essentially does not depend on cosmological parameters, our method can determine f IGM independent of cosmological parameters. We parameterize f IGM as a function of redshift and model the DM contribution from a host galaxy as a function of star formation rate. Assuming f IGM has a mild evolution with redshift with a functional form and by means of Monte Carlo simulations, we show that an unbiased and cosmology-independent estimate of the present value of f IGM with a ~12% uncertainty can be obtained with 50 joint measurements of d L and DM. In addition, such a method can also lead to a measurement of the mean value of DM contributed from the local host galaxy
Nonlinear control for a model-scaled helicopter with constraints on rotor thrust and fuselage attitude
A nonlinear control is proposed for trajectory tracking of a 6-DOF model-scaled helicopter with constraints on main rotor thrust and fuselage attitude. In the procedure of control design, the mathematical model of helicopter is simplified into three subsystems: altitude subsystem, longitudinal-lateral subsystem and attitude subsystem. The proposed control is developed by combining the sub-controls for the corresponding subsystems. The sub-controls for altitude subsystem and longitudinal-lateral subsystem are designed with hyperbolic tangent functions to satisfy the constraints; the sub-control for attitude subsystem is based on backstepping technique such that fuselage attitude tracks the virtual control for longitudinal-lateral subsystem. It is proved theoretically that tracking errors are ultimately bounded, and control constraints are satisfied. Performances of the proposed controller are demonstrated by simulation results.http://www.journals.elsevier.com/acta-automatica-sinica/hb201
3-D path-following control for a model-scaled autonomous helicopter
A 3-D path-following controller is proposed in
this brief for a 6-degrees-of-freedom model-scaled autonomous
helicopter. The reference path and path-following errors are
newly defined using implicit expressions. On the basis of geometric
analysis, a new speed error is designed for singularity
avoidance. The proposed control algorithm is designed using
command filtered backstepping, such that complicated solutions
for derivatives of virtual controls are circumvented. It is proved
that, with the proposed controller, path-following errors are
locally ultimately bounded. Theoretical results are demonstrated
by the numerical simulation.http://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=87hb201
Integrating Spatial Data Linkage and Analysis Services in a Geoportal for China Urban Research
Many geoportals are now evolving into online analytical environments, where large amounts of data and various analysis methods are integrated. These spatiotemporal data are often distributed in different databases and exist in heterogeneous forms, even when they refer to the same geospatial entities. Besides, existing open standards lack sufficient expression of the attribute semantics. Client applications or other services thus have to deal with unrelated preprocessing tasks, such as data transformation and attribute annotation, leading to potential inconsistencies. Furthermore, to build informative interfaces that guide users to quickly understand the analysis methods, an analysis service needs to explicitly model the method parameters, which are often interrelated and have rich auxiliary information. This work presents the design of the spatial data linkage and analysis services in a geoportal for China urban research. The spatial data linkage service aggregates multisource heterogeneous data into linked layers with flexible attribute mapping, providing client applications and services with a unified access as if querying a big table. The spatial analysis service incorporates parameter hierarchy and grouping by extending the standard WPS service, and data‐dependent validation in computation components. This platform can help researchers efficiently explore and analyze spatiotemporal data online.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110740/1/tgis12084.pd
Transforming Wikipedia into Augmented Data for Query-Focused Summarization
The manual construction of a query-focused summarization corpus is costly and
timeconsuming. The limited size of existing datasets renders training
data-driven summarization models challenging. In this paper, we use Wikipedia
to automatically collect a large query-focused summarization dataset (named as
WIKIREF) of more than 280,000 examples, which can serve as a means of data
augmentation. Moreover, we develop a query-focused summarization model based on
BERT to extract summaries from the documents. Experimental results on three DUC
benchmarks show that the model pre-trained on WIKIREF has already achieved
reasonable performance. After fine-tuning on the specific datasets, the model
with data augmentation outperforms the state of the art on the benchmarks
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