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We present experimental evidence of electronic and optical interlayer resonances in graphene van der Waals heterostructure interfaces. Using the spectroscopic mode of a low-energy electron microscope (LEEM), we characterized these interlayer resonant states up to 10 eV above the vacuum level. Compared with nontwisted, AB-stacked bilayer graphene (AB BLG), an $\ensuremath{\approx}0.2\text{ }\text{ }\AA{}$ increase was found in the interlayer spacing of 30\ifmmode^\circ\else\textdegree\fi{} twisted bilayer graphene (30\ifmmode^\circ\else\textdegree\fi{}-tBLG). In addition, we used Raman spectroscopy to probe the inelastic light-matter interactions. A unique type of Fano resonance was found around the D and G modes of the graphene lattice vibrations. This anomalous, robust Fano resonance is a direct result of quantum confinement and the interplay between discrete phonon states and the excitonic continuum.

Flat band electronic states are proposed to be a fundamental tool to achieve various quantum states of matter at higher temperatures due to the enhanced electronic correlations. However, materials with such peculiar electronic states are rare and often rely on subtle properties of the band structures. Here, by using angle-resolved photoemission spectroscopy, we show the emergent flat band in a VSe2 / Bi2Se3 heterostructure. Our photoemission study demonstrates that the flat band covers the entire Brillouin zone and exhibits 2D nature with a complex circular dichroism. In addition, the Dirac cone of Bi2Se3 is not reshaped by the flat band even though they overlap in proximity of the Dirac point. These features make this flat band distinguishable from the ones previously found. Thereby, the observation of a flat band in the VSe2 / Bi2Se3 heterostructure opens a promising pathway to realize strongly correlated quantum effects in topological materials. Dispersionless flat bands are often required to observe unusual quantum states of matter. Here, angle-resolved photoemission spectroscopy (ARPES) reveals a flat band electronic structure in a VSe2/Bi2Se3 heterostructure, and exhibits complex circular dichroism.

Recently, ultrathin metal-oxide thin film transistors (TFTs) have shown very high on-off ratio and ultra-sharp subthreshold swing, making them promising candidates for applications beyond conventional large-area electronics. While the on-off operation in typical TFTs results primarily from the modulation of charge carrier density by gate voltage, the high on-off ratio in ultrathin oxide TFTs can be associated with a large carrier mobility modulation, whose origin remains unknown. We investigate 3.5 nm-thick TiOx-based ultrathin TFTs exhibiting on-off ratio of ~106, predominantly driven by ~6-decade gate-induced mobility modulation. The power law behavior of the mobility features two regimes, with a very high exponent at low gate voltages, unprecedented for oxide TFTs. We find that this phenomenon is well explained by the presence of high-density tail states near the conduction band edge, which supports carrier transport via variable range hopping. The observed two-exponent regimes reflect the bi-exponential distribution of the density of band-tail states. This improved understanding would be significant in fabricating high-performance ultrathin oxide devices. The origin of large mobility modulation in ultrathin oxide transistors, promising for their high on-off ratio, remains mostly unknown. Here, a 106 gate-induced mobility modulation in 3.5 nm-thick TiOx transistors is explained by a high density of tail states, mediating variable range hopping of carriers.

We have used selected area low energy electron diffraction intensity-voltage (μLEED-IV) analysis to investigate the surface structure of crystalline 2H molybdenum disulfide (MoS2) and mechanically exfoliated and suspended monolayer MoS2. Our results show that the surface structure of bulk 2H-MoS2 is distinct from its bulk and that it has a slightly smaller surface relaxation at 320 K than previously reported at 95 K. We concluded that suspended monolayer MoS2 shows a large interlayer relaxation compared to the MoS2 sandwich layer terminating the bulk surface. The Debye temperature of MoS2 was concluded to be about 600 K, which agrees with a previous theoretical study. Our work has shown that the dynamical μLEED-IV analysis performed with a low energy electron microscope (LEEM) is a powerful technique for determination of the local atomic structures of currently extensively studied two-dimensional (2-D) materials.

α-RuCl3, a layered 2D material, was recently identified as a promising candidate for realizing a Kitaev quantum spin liquid. However, one fundamental property, the crystal structure, has not been well resolved yet due to difficulty of bulk diffraction techniques caused by layer stacking faults. Furthermore, the surface relaxation of monolayer-level thin films is completely unknown yet. In this report, surface sensitive low energy electron diffraction technique with μm selectivity (μ-LEED) combined with dynamical LEED analysis were used to reveal the detailed crystal structure of the surface monolayer of α-RuCl3. A surface structural distortion that breaks the inversion symmetry of the ideal bulk structure was revealed. To be specific, we found the surface Cl sub-lattice is buckled with one Cl atom approximately 0.16 A below the other two Cl atoms, in the unit cell. The Ru atomic layer shows an even larger buckling of approximately 0.31 A. Through density functional theory (DFT) calculations, we suggested that this surface distortion may be induced by Cl-vacancies that are inevitable in these samples. Inversion symmetry breaking in this material could have a significant impact on the 2D Kitaev interaction for both, interfaces with other 2D materials, such as graphene, and future monolayer devices.

The transition-metal dichalcogenide compounds ${\mathrm{MoTe}}_{2}$ and ${\mathrm{WTe}}_{2}$ are polymorphic with both semiconducting and metallic phases. The thermodynamically stable phase of ${\mathrm{WTe}}_{2}$ at room temperature is orthorhombic and metallic and displays a wide range of interesting phenomena including type-II Weyl fermions, titanic magnetoresistance and superconductivity in bulk, and quantum spin Hall insulator behavior in the monolayer case. On the other hand, the stable phase of ${\mathrm{MoTe}}_{2}$ at room temperature is a trigonal prismatic semiconductor that has a direct gap in the monolayer with strong spin-orbit coupling. The alloy series ${\mathrm{Mo}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{Te}}_{2}$ thus offers the possibility for tuning the structural and, consequently, electronic phases via tuning of the composition. Here, we report comprehensive studies of the electronic structure of ${\mathrm{Mo}}_{1\ensuremath{-}x}{\mathrm{W}}_{x}{\mathrm{Te}}_{2}$ alloys using angle-resolved photoemission spectroscopy and first-principles calculations as a function of composition. At room temperature, we find a sharp boundary between the orthorhombic and the trigonal prismatic phases at $x=0.10$ using structural characterization. We also show that by compositional tuning it is possible to control the band inversion in this series of compounds thus yielding important consequences for topological surface states.

The surface structure of black phosphorus materials is determined using surface-sensitive dynamical microspot low energy electron diffraction ($\ensuremath{\mu}\mathrm{LEED}$) analysis using a high spatial resolution low energy electron microscopy (LEEM) system. Samples of (i) crystalline cleaved black phosphorus (BP) at 300 K and (ii) exfoliated few-layer phosphorene (FLP) of about 10 nm thickness which were annealed at 573 K in vacuum were studied. In both samples, a significant surface buckling of 0.22 \AA{} and 0.30 \AA{}, respectively, is measured, which is one order of magnitude larger than previously reported. As direct evidence for large buckling, we observe a set of (for the flat surface forbidden) diffraction spots. Using first-principles calculations, we find that the presence of surface vacancies is responsible for the surface buckling in both BP and FLP, and is related to the intrinsic hole doping of phosphoresce materials previously reported.

Topological crystalline insulators have been recently predicted and observed in rock-salt structure SnSe {111} 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, is used to demonstrate that a rock-salt SnSe {111} thin film epitaxially grown on Bi_{2}Se_{3} 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 {111} 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 {111} thin film is shown to yield a high Fermi velocity, 0.50×10^{6} m/s, which suggests a potential mechanism of engineering the Dirac surface state of topological materials by tuning the surface configuration.

In order to explore the evaluation system of lights cape under forest in the urban park, an evaluation system of lights cape under forest in the urban park was constructed, which was composed of physical factors, psychological factors and spatial factors and the corresponding 12 indexes affecting the lights cape based on Grey Statistical Theory and Analytic Hierarchy Process. The results show that: 1) Psychological and physical factors are the dominant influencing indexes in the evaluation system, and the weight accounting of these factors is more than 70%. 2) The total weight of illumination uniformity is higher than other evaluation indexes. 3)The important indicators and sub-important indicators that affect the evaluation of lights cape under forest in the urban park are as follows: illumination Uniformity, satisfaction of lights cape, audio-visual coordination, sky View Factor, daylight factor under forests, perception of privacy, perception of security and clear bole height, etc. The evaluation system is expected to provide theoretical basis and effective methods for the future study in urban light environment.

PLEASE, the Python Low-energy Electron Analysis SuitE, provides an open source and cross-platform graphical user interface (GUI) for rapid analysis and visualization of low energy electron microscopy (LEEM) data sets. LEEM and the associated technique, selected area micro-spot low energy electron diffraction (μ-LEED), are powerful tools for analysis of the surface structure for many novel materials. Specifically, these tools are uniquely suited for the characterization of two-dimensional materials. PLEASE offers a user-friendly point-and-click method for extracting intensity-voltage curves from LEEM and LEED data sets. Analysis of these curves provides insight into the atomic structure of the target material surface with unparalleled resolution.

The cellular responses to polyhydroxyalkanoates (PHA) degradation products oligo-hydroxyalkanoates (OHAs) are very important factors that control the biocompatibility of these polymers when they are used in tissue-engineering applications. In this study, oligo(3-hydroxybutyrate) (OHB, Mn 2000), oligo(3-hydroxybutyrate-co-4-hydroxybutyrate) (O3HB4HB, Mn 2100, 6 mol% 4HB), oligo(3-hydroxybutyrate-co-3-hydroxyhexanoate) (OHBHHx, Mn 2800, 12 mol% 3HHx) and medium-chain-length oligo(3-hydroxyalkanoates) (OmclHAs, Mn 2300, 2 mol% 3-hydroxyhexanoate (3HHx), 25 mol% 3-hydroxyoctanoate, 71 mol% 3-hydroxydecanoate and 3 mol% 3-hydroxydodecanoate) were prepared as insoluble particles in aqueous solution. Liposomes were employed to encapsulate OHAs and facilitate their transfer into the cytosol. The methylthiazol tetrazolium (MTT) assay, a cell apoptosis study and the flow cytometry were used to evaluate cell viability of mouse fibroblast L929. OHAs in concentration lower than 20mg/l did not significantly affect cell viability, while OHAs over 40 mg/l reduced cell viability with more cell apoptosis, more cell death, delayed cell cycle and reduced cell proliferation. The cytotoxicity of OHAs decreased with increasing OHAs side chain length. It is proposed that medium-chain-length OHAs containing PHA, such as poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) and medium-chain-length PHA Poly(3-hydroxyalkanoates) (mcl PHA), are preferred for development and application as PHA-based tissue-engineering biomaterials.

The unidirectional transmission of acoustic waves is realized by a simple geometrically asymmetric steel grating structure. This exotic phenomenon stems from the one-way diffraction effect induced by the different periods of the slits on the both surfaces of the sample. And the frequency range of unidirectional transmission is simply determined by the structure periods. The experimental results agree well with the theoretical simulation. This remarkable effect is expected potential applications in ultrasonic devices, such as acoustic rectifiers and acoustic diodes.