Gap Edge Research Articles

A systematic evaluation of gap size and within-gap position effects on seedling regeneration in a temperate secondary forest, Northeast China

Forest gaps play an important role in tree regeneration and forest restoration in modern silviculture. Many previous studies examined gap effects on seedling survival, growth, or both but lacked a systemic assessment incorporating morphological and physiological responses to varying environmental gradients along gap size and within-gap position, which limits foresters to understand the general gap impacts on the early stage of regeneration. Here, we systemically evaluated gap impacts on over 20 regeneration-related variables from two dominant tree species (Manchurian walnut [Juglans mandshurica Maxim.] and Korean spruce [Picea koraiensis Nakai]) planted along the gap-understory gradients based on our previous studies of survival and growth of these two species. The factor analysis of mixed data indicated that seedling specific leaf area, biomass allocation, chlorophyll content, and NSC content in gaps were largely determined by interspecific differences, but seedling survival, growth, and biomass accumulation were mainly related to gap size and within-gap position. Specifically, the net photosynthetic rates of Manchurian walnut were higher in large and medium gaps (or gap centers and transitions) than those in small gaps and forest understory (or gap edges). The root starch content of seedlings in gaps was almost twice as much as that in the forest understory, which also reflected species divergence in gaps. By contrast, Korean spruce exhibited less variation in net photosynthesis and root starch content and fewer differences for most other examined variables, which indicated that it had broad ecological niches and a relatively small gap would promote its regeneration during the seedling stage. Our findings provide evidence of gap partitioning on explaining regeneration patterns of light-demanding species. Therefore, gap size and within-gap position can compensate for each other during gap-based silviculture practices. Timely selective logging can extend an existing gap in a specific direction to change the relative position of target tree species for better light conditions. Appropriate selection and match of tree species to gap size and within-gap position during enrichment planting can improve gap space utilization.

Constructing a novel metal-free g-C3N4/g-CN vdW heterostructure with enhanced visible-light-driven photocatalytic activity for water splitting

With the increasing environmental pollution and energy crisis, developing and designing environmentally friendly, low-cost, metal-free and efficient photocatalyst for water splitting is vitally important. Herein, a novel g-C3N4/g-CN van der Waals (vdW) heterostructure is proposed and its structural, electronic and optical properties are systematically investigated based on first-principles method. The results show that the g-C3N4/g-CN vdW heterostructure is a type-II semiconductor, which satisfies band gap and band edge requirements of the redox potential of water splitting. In addition, the oxygen evolution reaction (OER) on g-C3N4 side and the hydrogen evolution reaction (HER) on g-CN side can proceed spontaneously in thermodynamics. Most notably, the electronic, thermodynamic and optical properties of the g-C3N4/g-CN vdW heterostructure can be modulated by applying biaxial strain to achieve the purpose of better catalytic water splitting. Also, the good visible-light absorption intensity of the g-C3N4/g-CN vdW heterostructure can also be exhibited without strain and it is much better under strain of −2%. Consequently, these findings suggest that the g-C3N4/g-CN vdW heterostructure has a promising potential as a photocatalyst for water splitting.

Band Gap Engineering in Solvochromic 2D Covalent Organic Framework Photocatalysts for Visible Light-Driven Enhanced Solar Fuel Production from Carbon Dioxide.

Solar light-driven fuel production from carbon dioxide using organic photocatalysts is a promising technique for sustainable energy sources. Band gap engineering in sustainable organic photocatalysts for improving efficiency and fulfilling the requirements is highly anticipated. Here, we present a new strategy to engineer the band gap in covalent organic framework (COF) photocatalysts by varying the push-pull electronic effect. To implement this strategy, we have designed and synthesized four different COFs using a tripodal amine 4,4',4″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1'-biphenyl]-4-amine)) [Ttba] with 1,3,5-triformylbenzene (COF-1), 2,4,6-triformylphloroglucinol (COF-2), 2,4,6-triformylphenol (COF-3), and 2,4,6-triformylresorcinol (COF-4). On varying the number of hydroxyl units in the aldehyde precursor, the resulting COFs allow the fine-tuning of their band gap and band edge positions and result in different morphologies with varying surface areas. The enhanced optical properties of COF-3 and COF-4 with very suitable band gaps of 2.02 and 1.95 eV, respectively, enable them to demonstrate a high-efficiency photobiocatalytic system for NADH photoregeneration and enhanced visible light-driven formic acid production at a rate of 226.3 μmol g-1 in 90 min. The triazine core enables efficient charge separation, while the hydroxyl groups induce an electronic push-pull effect, regulating their photocatalytic efficiency. The results demonstrated the morphology-guided enhanced surface area and dual keto-enol tautomerism-induced push-pull effect in asymmetrical charge distribution as key features in the fine-tuning of the photocatalysts.

On the spectral asymptotics of waves in periodic media with Dirichlet or Neumann exclusions

Summary We consider homogenization of the scalar wave equation in periodic media at finite wavenumbers and frequencies, with the focus on continua characterized by: (a) arbitrary Bravais lattice in $\mathbb{R}^d$, $d \geqslant 2$, and (b) exclusions, that is, ‘voids’ that are subject to homogeneous (Neumann or Dirichlet) boundary conditions. Making use of the Bloch-wave expansion, we pursue this goal via asymptotic ansatz featuring the ‘spectral distance’ from a given wavenumber-eigenfrequency pair (situated anywhere within the first Brillouin zone) as the perturbation parameter. We then introduce the effective wave motion via projection(s) of the scalar wavefield onto the Bloch eigenfunction(s) for the unit cell of periodicity, evaluated at the origin of a spectral neighborhood. For generality, we account for the presence of the source term in the wave equation and we consider—at a given wavenumber—generic cases of isolated, repeated, and nearby eigenvalues. In this way, we obtain a palette of effective models, featuring both wave- and Dirac-type behaviors, whose applicability is controlled by the local band structure and eigenfunction basis. In all spectral regimes, we pursue the homogenized description up to at least first order of expansion, featuring asymptotic corrections of the homogenized Bloch-wave operator and the homogenized source term. Inherently, such framework provides a convenient platform for the synthesis of a wide range of intriguing wave phenomena, including negative refraction and topologically protected states in metamaterials and phononic crystals. The proposed homogenization framework is illustrated by approximating asymptotically the dispersion relationships for (i) Kagome lattice featuring hexagonal Neumann exclusions and (ii) square lattice of circular Dirichlet exclusions. We complete the numerical portrayal of analytical developments by studying the response of a Kagome lattice due to a dipole-like source term acting near the edge of a band gap.

Two-dimensional overdamped fluctuations of the soft perovskite lattice in CsPbBr3.

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr3 crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties.

Changes in carbon, nitrogen and phosphorus stoichiometry in decaying logs with gap positions in a subalpine forest

Abstract Aims The redistribution of forest gaps based on solar radiation and precipitation (mainly rainfall and snowfall) can create heterogenous microenvironments inside and outside the gaps. Here, we investigated the effects of forest gaps on carbon (C), nitrogen (N) and phosphorus (P) stoichiometry in decaying logs by altering the microenvironments. Methods Minjiang fir (Abies faxoniana) logs of decay classes I–V were incubated in situ on the floor under the gap center (GC), gap edge (GE) and closed canopy (CC) in a subalpine forest in the eastern Qinghai-Tibet Plateau. The C, N and P concentrations in decaying bark, sapwood and heartwood were measured in August from 2013 to 2016. Important Findings Our results showed that the N concentration in bark decreased from the CC to the GC, while the opposite trend was found in sapwood and heartwood. Additionally, the C/N ratio in sapwood and heartwood of decay classes IV and V decreased from the CC to the GC, and the N/P ratio in heartwood of decay class V increased from the CC to the GC, implying that forest gaps have strong effects on C/N/P stoichiometry in highly decayed logs. Briefly, highly decayed logs are more susceptible to the microenvironment, and the proportion of highly decayed logs under the CC or the GC should be appropriately adjusted if necessary.

Recent developments and challenges in zeolite-based composite photocatalysts for environmental applications

• The latest progresses in the application of zeolite-based composites are summarized. • Pollutant adsorption and photocatalysis by zeolite-based photocatalysts are analyzed. • The effects of the characteristics of the zeolite-based composites are discussed. • The perspectives for the research of the zeolite-based composites are presented. Aluminosilicate zeolites have been found to be effective supports of semiconductor photocatalysts for the removal of organic compounds in wastewater and air. In this review, we summarize the progress in the application of zeolite materials such as Y, ZSM-5, mordenite, beta and clinoptilolite for improving the performance of semiconductor photocatalysts in degradation of environmental pollutants. The main synthesis strategies and the mechanisms of organic compounds adsorption and photocatalysis by zeolite-based photocatalysts are explicated in detail. Meanwhile, various types of zeolite-based photocatalytic composites are also thoroughly summarized. The more reliable zeolite-based photocatalysts with high photocatalytic activity under visible or sunlight irradiation should be extensively investigated. Additionally, we highlight the advancements for improving performance of different zeolite-based composites in photocatalytic degradation organic contaminants in wastewater and air. The achieved progress demonstrates that the photocatalytic enhancement of zeolite-based composites is predominantly dependent on the enhanced adsorption performance, as well as the high separation and transfer efficiency of photogenerated charge carriers. Hence, the characteristics of the zeolite-based composites (surface area, pHpzc value, hydrophobic, band gap and band edge), the nature of organic contaminants and other experiment factors need to be taken into account in applying to photodegradation process involving this catalyst. Finally, the current challenges and perspectives for the future research of these photocatalysts are presented.

Jumping the Gap: Searching for LIGO’s Biggest Black Holes

Abstract Gravitational wave (GW) detections of binary black holes (BBHs) have shown evidence for a dearth of component black holes with masses above ∼50M ⊙. This is consistent with expectations of a mass gap due to the existence of pair-instability supernovae (PISN). We argue that ground-based GW detectors will be sensitive to BBHs with masses above this gap, ≳120 M ⊙. With no detections, 2 yr at upgraded sensitivity (A+) would constrain the local merger rate of these BBHs on the “far side” of the PISN gap to be lower than 0.01 yr−1Gpc−3. Alternatively, with a few tens of events we could constrain the location of the upper edge of the gap to the percent level. We consider the potential impact of “interloper” black holes within the PISN mass gap on this measurement. Far side BBHs would also be observed by future instruments such as Cosmic Explorer (CE), Einstein Telescope (ET) and LISA, and may dominate the fraction of multiband events. We show that by comparing observations from ground and space it is possible to constrain the merger rate history. Moreover, we find that the upper edge of the PISN mass gap leaves an imprint on the spectral shape of the stochastic background of unresolved binaries, which may be accessible with A+ sensitivity. Finally, we show that by exploiting the upper edge of the gap, these high-mass BBHs can be used as standard sirens to constrain the cosmic expansion at redshifts of ∼0.4, 0.8, and 1.5 with LISA, LIGO-Virgo, and CE/ET, respectively. These far-side binaries would be the most massive BBHs LIGO-Virgo could detect.

Type‐II C2N/ZnTe Van Der Waals Heterostructure: A Promising Photocatalyst for Water Splitting

AbstractBased on density functional theory calculations under the framework of first‐principles, the electronic and optical characteristics of C2N/ZnTe van der Waals (vdW) heterostructure are investigated in detail. An inherent type‐II band alignment arises in the heterostructure, which possesses a 1.94 eV direct band gap. With light irradiation, the photogenerated electron‐hole pairs of heterostructure are separated on different monolayers. This helps to extending the lifetime of carriers. Additionally, the lower effective carrier mass is owned for the heterostructure. The heterostructure has suitable band gap and band edge positions, as well as, enhanced visible light absorption capacity, which can facilitate photocatalytic water splitting in acidic conditions. More interestingly, the redox reaction of photocatalytic water splitting can still proceed on the heterostructure after exerting a biaxial tensile strain of 2–6%. These results suggest that C2N/ZnTe vdW heterostructure will be a promising water splitting photocatalyst.

Enabled/VASP is required to mediate proper sealing of opposing cardioblasts during Drosophila dorsal vessel formation.

The Drosophila dorsal vessel (DV) is comprised of two opposing rows of cardioblasts (CBs) that migrate toward the dorsal midline during development. While approaching the midline, CBs change shape, enabling dorsal and ventral attachments with their contralateral partners to create a linear tube with a central lumen. We previously demonstrated DV closure occurs via a "buttoning" mechanism where specific CBs advance ahead of their lateral neighbors, and attach creating transient holes, which eventually seal. Here, we investigate the role of the actin-regulatory protein enabled (Ena) in DV closure. Loss of Ena results in DV cell shape and alignment defects. Live analysis of DV formation in ena mutants shows a reduction in CB leading edge protrusion length and gaps in the DV between contralateral CB pairs. These gaps occur primarily between a specific genetic subtype of CBs, which express the transcription factor seven-up (Svp) and form the ostia inflow tracts of the heart. In WT embryos these gaps between Svp+ CBs are observed transiently during the final stages of DV closure. Our data suggest that Ena modulates the actin cytoskeleton in order to facilitate the complete sealing of the DV during the final stages of cardiac tube formation.

One-step synthesis of two dimensional Bi2VxW1−xO6−δ solid solution for photocatalytic hydrogen evolution

Photocatalytic H2 evolution through water splitting on the semiconductor catalysts is a promising and efficient solution to convert inexhaustible solar energy into chemical fuel. In this report, the robust two-dimensional BWV-x (0 ≤ x ≤ 0.5) solid solution were synthesized via a facile one-step strategy for photocatalytic H2 evolution. The band gaps of BWV-x could be tuned easily in a wide range from 3.0 to 2.2 eV according to their chemical composition. When the value of x was 0.33, the BWV-0.33 displayed a high H2 generation rate of 747 μmol.h−1 gcat−1, which was about 5.2-fold that of pristine Bi2WO6. Mott-Schottky and UV–Vis absorbance spectroscopy studies of BWV-x revealed that the appropriate band gap and high conduction band edge position of BWV-0.33 led to a significant improvement in photocatalytic performance. The charge transfer mechanism for H2 production over the BWV-0.33 was further proposed. This work shows the potential of band gap engineering by constructing solid solutions for efficient semiconductor photocatalysis.

Constraining protoplanetary disc accretion and young planets using ALMA kinematic observations

ABSTRACT Recent ALMA molecular line observations have revealed 3D gas velocity structure in protoplanetary discs, shedding light on mechanisms of disc accretion and structure formation. (1) By carrying out viscous simulations, we confirm that the disc’s velocity structure differs dramatically using vertical stress profiles from different accretion mechanisms. Thus, kinematic observations tracing flows at different disc heights can potentially distinguish different accretion mechanisms. On the other hand, the disc surface density evolution is mostly determined by the vertically integrated stress. The sharp disc outer edge constrained by recent kinematic observations can be caused by a radially varying α in the disc. (2) We also study kinematic signatures of a young planet by carrying out 3D planet–disc simulations. The relationship between the planet mass and the ‘kink’ velocity is derived, showing a linear relationship with little dependence on disc viscosity, but some dependence on disc height when the planet is massive (e.g. 10MJ). We predict the ‘kink’ velocities for the potential planets in DSHARP discs. At the gap edge, the azimuthally averaged velocities at different disc heights deviate from the Keplerian velocity at similar amplitudes, and its relationship with the planet mass is consistent with that in 2D simulations. After removing the planet, the azimuthally averaged velocity barely changes within the viscous time-scale, and thus the azimuthally averaged velocity structure at the gap edge is due to the gap itself and not directly caused to the planet. Combining both axisymmetric kinematic observations and the residual ‘kink’ velocity is needed to probe young planets in protoplanetary discs.

Migration of Jupiter-mass planets in low-viscosity discs

Context. Type-II migration of giant planets has a speed proportional to the disc’s viscosity for values of the α viscosity parameter larger than 10−4. Previous studies based on two-dimensional simulations, have shown that, at even lower viscosities, migration can be very chaotic and is often characterised by phases of fast migration. The reason is that vortices appear in low-viscosity discs due to the Rossby-wave instability at the edges of the gap opened by the planet. Migration is then determined by vortex-planet interactions. Aims. Our goal is to study giant planet migration in low-viscosity discs with 3D simulations. In 3D, vortices are more complex than the simple vertical extension of their 2D counterparts; their impact on planet migration is therefore not obvious. Methods. We performed numerical simulations using two grid-based codes: FARGOCA for three-dimensional simulations and FARGO-ADSG for the two dimensional case. Two-dimensional simulations were used mainly for preliminary tests to check the impact of self-gravity on vortex formation and on vortex-disc dynamics. After selecting disc masses for which self-gravity is not important at the planet location, three-dimensional simulations without self-gravity can be safely used. We have considered an adiabatic equation of state with exponential damping of temperature perturbations in order to avoid the development of the vertical shear instability. In our nominal simulation, we set α = 0 so that only numerical viscosity is present. We then performed simulations with non-zero α values to assess the threshold of prescribed viscosity below which the new migration processes appear. Results. We show that for α ≲ 10−5 two migration modes are possible, which differ from classical Type-II migration in the sense that they are not proportional to the disc’s viscosity. The first occurs when the gap opened by the planet is not very deep. This occurs in 3D simulations and/or when a big vortex forms at the outer edge of the planetary gap, diffusing material into the gap. The de-saturation of co-orbital and co-rotation resonances keeps the planet’s eccentricity low. Inward planet migration then occurs as long as the disc can refill the gap left behind by the migrating planet, either due to diffusion caused by the presence of the vortex or to the inward migration of the vortex itself due to its interaction with the disc. We call this type of migration ‘vortex-driven migration’, which differs from ‘vortex-induced’ migration described in Lin & Papaloizou (2010, MNRAS, 405, 1473, and 2011a, MNRAS, 415, 1445). This migration is very slow and cannot continue indefinitely because eventually the vortex dissolves. The second migration mode occurs when the gap is deep so that the planet’s eccentricity grows to a value e ~ 0.2 due to inefficient eccentricity damping by co-rotation resonances. Once the planet is on an eccentric orbit, gas can pass through the gap and planet migration unlocks from the disc’s viscous evolution. This second, faster migration mode appears to be typical of two-dimensional models in discs with slower damping of temperature perturbations. Conclusions. Vortex-driven migration in low-viscosity discs can be very slow and eventually reverses and stops, offering an interesting mechanism to explain the existence of the cold-Jupiter population, even if these planets originally started growing at the disc’s snowline.

Electronic and Thermophysical Properties of Gas Hydrates: Ab Initio Simulation Results

Results of ab initio molecular-dynamic investigation of the electronic and thermophysical properties of methane hydrate with cubic structure sI are represented. The simulation results for specific heat at constant volume and density are in good agreement with experimental data. The temperature dependences of the electronic properties of methane hydrate (including Fermi energy level and width and edges of the band gap) are determined based on analysis of the density of electronic states. For an empty hydrate framework (aqueous clathrate framework), the spectra of electron energy E(k) along the M–X, X–Γ, Γ–M, and Γ–R directions are calculated. It is established that the presence of CH4 molecules in the aqueous clathrate increases the hydrate Fermi energy from 2.4 to 3.0 eV.

First-principles investigations of optoelectronic properties of ZnO$$\left( {11\overline{2}0} \right)$$ and ZnO(0001) monolayers

Devising the two-dimensional (2D) structures of low-cost and non-toxic semiconductors for nanoscale technological applications has attracted substantial interest since the past decade. In this work, we design two types of ZnO monolayers derived from polar 0001-plane and nonpolar $$11\overline{2}0$$ -plane of the wurtzite structure, and explore their physical properties using the first-principles approach. Both ZnO $$\left( {11\overline{2}0} \right)$$ and ZnO(0001) monolayers exhibited cohesive and formation energies comparable to that of the stable wurtzite-structured ZnO. However, both monolayers exhibited substantially different electronic structures of band gaps 1.56 eV for single-layered ZnO $$\left( {11\overline{2}0} \right)$$ and 0.71 eV for ZnO(0001) monolayer. The edges of the valence and conduction bands of ZnO $$\left( {11\overline{2}0} \right)$$ monolayer are formed by parabolic bands, whereas almost flat band gap edges have been seen for ZnO(0001) surface. As a result, charge carriers associated with ZnO $$\left( {11\overline{2}0} \right)$$ monolayer exhibited relatively lighter effective mass than ZnO(0001) monolayer. The ZnO(0001) monolayer exhibited symmetrical bond lengths and subsequently isotropic optical spectra, whereas asymmetrical bond lengths and anisotropic subsequent optical spectra have been recorded for ZnO $$\left( {11\overline{2}0} \right)$$ monolayer. The optical absorption recorded for the designed monolayers has been found higher than their bulk counterpart. The refraction spectra indicated these monolayers of transparent behavior over a significant range of the electromagnetic spectrum. These fascinating features of ZnO $$\left( {11\overline{2}0} \right)$$ and ZnO(0001) monolayers suggest them suitable for applications in electronic and optoelectronic devices.

Gap Size in Hyrcanian Forest Affects the Lignin and N Concentrations of the Oriental Beech (Fagus orientalis Lipsky) Fine Roots but Does Not Change Their Morphological Traits in the Medium Term

Research Highlights: Fine roots play an important role in plant growth as well as in carbon (C) and nutrient cycling in terrestrial ecosystems. Gaining a wider knowledge of their dynamics under forest gap opening would improve our understanding of soil carbon input and below-ground carbon stock accumulation. Single-tree selection is increasingly recognized as an alternative regime of selection cutting sustaining biodiversity and carbon stock, along with timber production, among ecosystem functions. However, the fine root response in terms of morphological and chemical composition to the resulting harvest-created gaps remains unclear. Background and Objectives: This paper investigates the effect in the medium term (i.e., 6 years after logging) of differently sized harvest-created gaps on fine root dynamics and chemical composition. Materials and Methods: A total of 15 differently sized gaps (86.05–350.7 m2) and the adjacent 20 m distant closed canopies (control) were selected in a temperate Fagus orientalis forest (Hyrcanian region, Iran). Eight soil cores were collected at the cardinal points of the gap edge, including four facing the gap area—the same at the adjacent intact forest. Results: For the selected edge trees, the different size of gaps, the core position, and the tree orientation did not affect the investigated morphological traits, except for the slightly higher specific root length (SRL) for the larger fine root fraction (1–2 mm) in the side facing the gap area. Differently, the investigated chemical traits such as N concentration and cellulose:lignin ratio significantly increased with increasing gap size, the opposite for C:N ratio and lignin. Moreover, N concentration and C:N significantly decreased and increased with the fine root diameter, respectively. Conclusions: This work highlighted that, in the medium term and within the adopted size range, artificial gap opening derived from single-tree selection practice affected the chemistry rather than the biomass and morphology of gap-facing fine roots of edge trees. The medium term of six years after gap creation might have been long enough for the recovery of the fine root standing biomass to the pre-harvest condition, particularly near the stem of edge trees. A clear size threshold did not come out; nevertheless, 300 m2 may be considered a possible cut-off determining a marked change in the responses of fine roots.

Seasonal dynamics of organic components in fresh foliar litters at different gap positions in an alpine forest on the eastern Tibetan Plateau

The organic components in fresh litters are important chemical traits for evaluating the initial quality that controls decomposition of plant litter. This study aimed to assess the seasonal dynamics and drivers of organic components in four dominant fresh litters and their variations at different gap positions in an alpine forest on the eastern Tibetan Plateau. We measured the water- (WSE), organic- (OSE), acid-soluble extractables (ASE) and acid-unhydrolyzable residues (AUR) in fresh litters of fir (Abies faxoniana), cypress (Sabina saltuaria), azalea (Rhododendron lapponicum), and cherry (Cerasus duclouxii) at the gap center, gap edge, and under the closed canopy over 1 year in an alpine forest. The contents of organic components significantly varied among litter species and collecting time, and the ASE content also showed significant variations among gap positions. Manganese (Mn) was negatively related to the OSE content, and dissolved organic carbon (DOC) was the most significant factor affecting WSE and AUR contents. Magnesium (Mg) was the factor of great magnitude that significantly influenced the ASE content, and the correlation coefficient between Mg and ASE in the gap center was higher than that in other gap positions. The contents of organic components were markedly controlled by litter species over time, and ASE content varied significantly after gap formation. The seasonal dynamics of these organic components were strongly mediated by litter nutrients, such as Mg, Mn, calcium (Ca), and nitrogen (N), while DOC was a significant factor controlling WSE and AUR contents in fresh litters in this alpine forest.

Negative Schoch displacement of two-dimensional acoustic metamaterials with near-zero refractive index

In this paper, the Schoch displacement at the interface between different two-dimensional triangular phononic crystal metamaterial and natural material is studied by using finite element software. As is well known, the Schoch displacement is highly dependent on the surface wave and leakage wave excited at the interface between different materials. So, the negative Schoch displacement can be more easily obtained by adding a suitable thickness of covering layer at the interface between metamaterial material and natural material. The numerical results show that when the negative Schoch displacement happens, the effective parameters of metamaterials are close to zero. It means that the effective refraction index is near to zero and the reduced frequency of the incident acoustic wave is correlated with the reduced frequency of the band gap. It is also found from the results that the reduced frequency of the incident acoustic wave is located at the edge of the band gap when the negative Schoch displacement occurs. The maximum of the metamaterial effective impedance and the maximum of the reflection coefficient are almost at the same frequency. The phase of the imaginary part of the reflection coefficient has a phase mutation in π rad at the corresponding frequency. The frequency of negative Schoch displacement is located in the first band gap of <i>MK</i> direction and near the upper boundary. The Schoch displacement at the interface between conventional materials is usually positive and negligible in previous reports. In this paper, the negative Schoch displacement is obtained by using the near-zero refraction index metamaterials. This not only enriches the physics contents of Schoch effect but also provides a theoretical reference for designing the acoustic devices based on acoustic wave displacement at the interface.

Acapulco earthquake struck the edge of a seismic gap

Acapulco earthquake struck the edge of a seismic gap

The mechanism of enhanced photocatalytic activity for water-splitting of ReS2 by strain and electric field engineering.

To enhance the photocatalytic water splitting performance of 2D ReS2, we theoretically propose a feasible strategy to engineer its band structure by applying strain or an electric field. Our calculated results show that the strains greatly tune the electronic structure of ReS2 especially band gap and band edge positions, because the strains significantly alter the crystal structure and then cause rearrangement of the surface charge. However, electric fields have little influence on band gap but obviously affect the band edge positions. This is because the electric fields have little effect on the crystal structure of ReS2 but easily produce an in-plane electric dipole moment. The shifts in band edge position mainly arise from competition between the surface charge and the in-plane electric dipole. For an applied strain, the shifts are dominated by rearrangement of surface charge; for an applied electric field, the shifts are determined by an induced electric dipole moment. Importantly, functionalized ReS2 with a bi-axial strain of −4% or an electronic field of −0.1 V Å−1 may be good candidates for water-splitting photocatalysts owing to their suitable band edge positions for water splitting, ideal band gaps, good stability, reduced electron–hole recombination and high carrier mobility. We hope our findings will stimulate experimental efforts to develop new photocatalysts based on functionalized ReS2.