arXiv subjects

Hongwei Wang

Publications and source records attributed to Hongwei Wang.

187 records · Page 11Linked to original sources

arXiv2014

Structural and electronic origin of the magnetic structures in hexagonal LuFeO$_3$

Using combined theoretical and experimental approaches, we studied the structural and electronic origin of the magnetic structure in hexagonal LuFeO$_3$. Besides showing the strong exchange coupling that is consistent with the high magnetic ordering temperature, the previously observed spin reorientation transition is explained by the theoretically calculated magnetic phase diagram. The structural origin of this spin reorientation that is responsible for the appearance of spontaneous magnetization, is identified by theory and verified by x-ray diffraction and absorption experiments.

cond-mat.mtrl-sci↗

arXiv2013

Finite-State Markov Modeling of Tunnel Channels in Communication-based Train Control (CBTC) Systems

Communication-based train control (CBTC) is gradually adopted in urban rail transit systems, as it can significantly enhance railway network efficiency, safety and capacity. Since CBTC systems are mostly deployed in underground tunnels and trains move in high speed, building a train-ground wireless communication system for CBTC is a challenging task. Modeling the tunnel channels is very important to design and evaluate the performance of CBTC systems. Most of existing works on channel modeling do not consider the unique characteristics in CBTC systems, such as high mobility speed, deterministic moving direction, and accurate train location information. In this paper, we develop a finite state Markov channel (FSMC) model for tunnel channels in CBTC systems. The proposed FSMC model is based on real field CBTC channel measurements obtained from a business operating subway line. Unlike most existing channel models, which are not related to specific locations, the proposed FSMC channel model takes train locations into account to have a more accurate channel model. The distance between the transmitter and the receiver is divided into intervals, and an FSMC model is applied in each interval. The accuracy of the proposed FSMC model is illustrated by the simulation results generated from the model and the real field measurement results.

cs.DM↗

arXiv2013

Finite-State Markov Modeling of Leaky Waveguide Channels in Communication-based Train Control (CBTC) Systems

Leaky waveguide has been adopted in communication based train control (CBTC) systems, as it can significantly enhance railway network efficiency, safety and capacity. Since CBTC systems have high requirements for the train ground communications, modeling the leaky waveguide channels is very important to design the wireless networks and evaluate the performance of CBTC systems. In the letter, we develop a finite-state Markov channel (FSMC) model for leaky waveguide channels in CBTC systems based on real field channel measurements obtained from a business operating subway line. The proposed FSMC channel model takes train locations into account to have a more accurate channel model. The overall leaky waveguide is divided into intervals, and an FSMC model is applied in each interval. The accuracy of the proposed FSMC model is illustrated by the simulation results generated from the model and the real field measurement results.

cs.DM↗

arXiv2013

Room temperature multiferroism in CaTcO$_3$ by interface engineering

The structural instabilities of ATcO$_3$(A= Ca, Sr, Ba) are investigated by first-principles calculations. In addiction to the large octahedral rotation instability, a weak ferroelectric tendency is identified in CaTcO$_3$. We show that the ferroelectricity in CaTcO$_3$ can be recovered by interface engineering based on CaTcO$_3$/BaTcO$_3$ superlattices, where the octahedral rotation is largely suppressed. The N{é}el temperature is found to be $\sim$ 816 K, indicating that CaTcO$_3$ can be engineered into a new room temperature multiferroic material.

cond-mat.mtrl-sci↗

arXiv2012

Cyrstal field splitting and optical band gap of hexagonal LuFeO$_3$ films

Hexagonal LuFeO$_3$ films have been studied using x-ray absorption and optical spectroscopy. The crystal splittings of Fe$^{3+}$ are extracted as $E_{e'}-E_{e"}$=0.7 eV and $E_{a_1'}-E_{e'}$=0.9 eV and a 2.0 eV optical band gap is determined assuming a direct gap. First-principles calculations confirm the experiments that the relative energies of crystal field splitting states do follow $E_{a_1'}>E_{e'}>E_{e"}$ with slightly underestimated values and a band gap of 1.35 eV.

cond-mat.mtrl-sci↗

arXiv2011

Interface induced giant magnetoelectric coupling in multiferroic superlattices

The electric and magnetic properties of (BaTiO$_3$)$_n$/(CaMnO$_3$)$_n$ short-period superlattices are studied by the first-principles calculations. The local electric polarizations in the CaMnO$_3$ layers are significant, comparable to that in the BaTiO$_3$ layers. Remarkably, the electric polarization is almost doubled when the spin configuration changes from antiferromagnetic to ferromagnetic in the superlattices, indicating a giant magnetoelectric coupling. This enhancement of the magnetoelectric coupling is due to the suppression of the antiferrodistortive mode in the CaMnO$_3$ layers at the interfaces.

cond-mat.mtrl-sci↗

arXiv2010

Global solutions for the generalized Boussinesq equation in low-order Sobolev spaces

We show that the Cauchy problem for the defocusing generalized Boussinesq equation $u_{tt}-u_{xx}+u_{xxxx}-(|u|^{2k}u)_{xx}=0$, $k\geq1$, on the real line is globally well-posed in $H^{s}(\R)$ for $s>1-({1}/{3k})$. We use the "$I$-method" to define a modification of the energy functional that is "almost conserved" in time. Our result extends the previous one obtained by Farah and Linares (2010 \textit{J. London Math. Soc.} \textbf{81} 241-254) when $k=1$.

math.AP↗