31 research outputs found
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Electronic Structure and Surface Physics of Two-dimensional Material Molybdenum Disulfide
The interest in two-dimensional materials and materials physics has grown dramatically over the past decade. The family of two-dimensional materials, which includes graphene, transition metal dichalcogenides, phosphorene, hexagonal boron nitride, etc., can be fabricated into atomically thin films since the intralayer bonding arises from their strong covalent character, while the interlayer interaction is mediated by weak van der Waals forces. Among them, molybdenum disulfide (MoSâ‚‚) has attracted much interest for its potential applications in opto-electronic and valleytronics devices. Previously, much of the experimental studies have concentrated on optical and transport measurements while neglecting direct experimental determination of the electronic structure of MoSâ‚‚, which is crucial to the full understanding of its distinctive properties. In particular, like other atomically thin materials, the interactions with substrate impact the surface structure and morphology of MoSâ‚‚, and as a result, its structural and physical properties can be affected.
In this dissertation, the electronic structure and surface structure of MoSâ‚‚ are directly investigated using angle-resolved photoemission spectroscopy and cathode lens microscopy. Local-probe angle-resolved photoemission spectroscopy measurements of monolayer, bilayer, trilayer, and bulk MoSâ‚‚ directly demonstrate the indirect-to-direct bandgap transition due to quantum confinement as the MoSâ‚‚ thickness is decreased from multilayer to monolayer. The evolution of the interlayer coupling in this transition is also investigated using density functional theory calculations. Also, the thickness-dependent surface roughness is characterized using selected-area low energy electron diffraction (LEED) and the surface structural relaxation is investigated using LEED I-V measurements combined with dynamical LEED calculations. Finally, bandgap engineering is demonstrated via tuning of the interlayer interactions in van der Waals interfaces by twisting the relative orientation in bilayer-MoSâ‚‚ and graphene-MoSâ‚‚-heterostructure systems
Structural Behavior of Thin-Walled Concrete-Filled Steel Tube Used in Cable Tunnel: An Experimental and Numerical Investigation
One steel grid and five thin-walled concrete-filled steel tubes (CTST) used as the supports of tunnel were tested in site for investigating the mechanical behavior. The mechanical influences of thickness, node form, and concrete on CTST were gained and compared with the impacts on steel grid. It is indicated that high antideformation capacity of CTST improved the stability of surrounding rock in short time. The cementitious grouted sleeve connection exhibited superior flexibility when CTST was erected and built. Although the deformation of rock and soil in the tunnel was increasing, good compression resistance was observed by CTST with the new connection type. It was also seen that vault, tube foot, and connections were with larger absolute strain values. The finite element analysis (FEA) was carried out using ABAQUS program. The results were validated by comparison with experimental results. The FE model could be referred by similar projects
Neural Chinese Word Segmentation with Lexicon and Unlabeled Data via Posterior Regularization
Existing methods for CWS usually rely on a large number of labeled sentences
to train word segmentation models, which are expensive and time-consuming to
annotate. Luckily, the unlabeled data is usually easy to collect and many
high-quality Chinese lexicons are off-the-shelf, both of which can provide
useful information for CWS. In this paper, we propose a neural approach for
Chinese word segmentation which can exploit both lexicon and unlabeled data.
Our approach is based on a variant of posterior regularization algorithm, and
the unlabeled data and lexicon are incorporated into model training as indirect
supervision by regularizing the prediction space of CWS models. Extensive
experiments on multiple benchmark datasets in both in-domain and cross-domain
scenarios validate the effectiveness of our approach.Comment: 7 pages, 11 figures, accepted by the 2019 World Wide Web Conference
(WWW '19
Observation of oscillatory relaxation in the Sn-terminated surface of epitaxial rock-salt SnSe topological crystalline insulator
Topological crystalline insulators have been recently predicted and observed
in rock-salt structure SnSe thin films. Previous studies have
suggested that the Se-terminated surface of this thin film with hydrogen
passivation, has a reduced surface energy and is thus a preferred
configuration. In this paper, synchrotron-based angle-resolved photoemission
spectroscopy, along with density functional theory calculations, are used to
demonstrate conclusively that a rock-salt SnSe thin film
epitaxially-grown on \ce{Bi2Se3} has a stable Sn-terminated surface. These
observations are supported by low energy electron diffraction (LEED)
intensity-voltage measurements and dynamical LEED calculations, which further
show that the Sn-terminated SnSe thin film has undergone a surface
structural relaxation of the interlayer spacing between the Sn and Se atomic
planes. In sharp contrast to the Se-terminated counterpart, the observed Dirac
surface state in the Sn-terminated SnSe thin film is shown to yield a
high Fermi velocity, m/s, which suggests a potential mechanism
of engineering the Dirac surface state of topological materials by tuning the
surface configuration.Comment: 12 pages, 13 figures, supplementary materials include