Gap Edge Research Articles

Direct numerical simulation of fluid flow in a 5x5 square rod bundle

Characterization of parallel flow through rod bundles is of key importance in assessing the performance and safety of several engineering systems, including a majority of nuclear reactor concepts. Inhomogeneities in the bundle cross-section can present complex flow phenomena, including varying local conditions of turbulence. With the ever-increasing capabilities of high-performance computing, Direct Numerical Simulation (DNS) of turbulent flows is becoming more feasible. Through resolving all scales of turbulence, DNS can serve as a “numerical experiment,” and can provide substantial insight into flow physics, but at considerable computational cost. Thus to date, the DNS in open literature for rod bundle flows is relatively scarce, and largely limited to unit-cell domains. Since wall effects are important in rod bundle flows, a multiple-pin DNS study can expand understanding of rod bundle flows while providing valuable reference data for evaluating reduced-resolution techniques. In this work, DNS of a 5x5 square bare rod bundle representative of typical light water reactor fuel dimensions was performed using the spectral element code Nek5000. Turbulent microscales based on an advanced Reynolds-Averaged Navier–Stokes model were used to establish the required DNS resolution. Velocity and Reynolds stress fields are analyzed in detail, and invariant analysis is used for further investigation into flow physics. The results show stark changes in the structure of turbulence in the edge gaps, suggesting the presence of gap vortices in these regions. In addition, turbulent kinetic energy budgets are presented to more fully illustrate the various turbulent processes. These data can prove useful for rigorous evaluation of lower-fidelity turbulence modeling approaches.

In situ thermal, optical, and structural investigations on reversible thermochromism in bismuth molybdate

Reversible thermochromic inorganic materials show stable and perceptible color change with changing environmental temperatures. This property makes them excellent materials for fabricating temperature indicators, military camouflage materials, and thermal warning devices. However, the mechanism of thermochromism has not been fully understood. Here, we prepared novel bismuth molybdate materials by calcination of Bi2O3 and MoO3 mixtures. The materials exhibit reversible thermochromism, gradually changing color from milky white at 25 °C to bright yellow at 500 °C. The phase composition of the product is tunable by adjusting the ratio of Bi2O3 and MoO3 in the starting materials, producing either single phase of α-Bi2Mo3O12 or mixed phases of α-Bi2Mo3O12,β-Bi2Mo2O9 and γ-Bi2MoO6. The mechanism of reversible thermochromism was studied by in situ UV/Vis diffuse reflectance spectroscopy (UV/Vis DRS), in situ synchrotron radiation powder X-ray diffraction (SRPXRD), and differential scanning calorimetry (DSC). In situ SRPXRD revealed no phase transition during heating and cooling, which agrees with DSC analysis with no thermal event detected until the melting temperature of α-Bi2Mo3O12 at 661.6 °C. With increasing temperature, in situ SRPXRD also revealed anisotropic expansion of the α-Bi2Mo3O12 lattice parameters, while in situ UV/Vis DRS showed a continuous red shift in the absorption edge and gradual decrease of band gap, suggesting that lattice expansion and shrinking upon heating and cooling is the main reason for the thermochromic behavior in bismuth molybdate materials.

Design principles for controlling soot deposition location in resistive PM sensors

Resistive soot sensors remain attractive due to their simplicity but tightening diesel particulate filter monitoring regulations necessitate more sensitive soot measurement. We studied the influence of inter-digital electrode (IDE) pattern design on deposition location control as a means to concentrate soot and improve resistive soot sensor response. Sensors with five electrode angles from 0° to 90° relative to flow as well as three electrode widths from 68 μm to 585 μm are included in our design study. We observe deposition primarily around the periphery of the IDE pattern on narrow electrode width samples, with increasing coverage as width increases. The flow-aligned electrode fingers are an exception to this behavior, exhibiting dendrites across much of the IDE pattern area. Samples with electrode fingers angled relative to flow primarily show deposition on the downstream side of IDE fingers, near the inter-electrode gap edge. Despite this control over how much of the IDE pattern collects signal-generating dendrites and control over the proximity of soot deposition trajectories to the inter-electrode gap, we do not observe dramatically improved sensitivity.

Reusable Capillary Flow-Based Wax Switch Valve for Centrifugal Microfluidics

Introduction: This paper presents a new repeatable Open-Close switch valve (SV) for the CD microfluidic platforms. The SV is designed by using a combination of controlled phase change and capillary flow of Ferrowax. The phase change of the wax is controlled using a laser. When the wax is illuminated with a laser, the energy of the laser is absorbed by the Ferro particles embedded inside the wax, causing the wax to melt. The melted wax can spontaneously wick through capillary channels due to its low contact angle. The Open-Close nature of the valve is attained by rationally designing the capillary flow path of the wax. The capillary flow path design is such a way that the melted wax is directed to the open fluidic path (Normally open configuration) to block the channel (Close configuration). The reversal of the blocking is achieved by remelting the wax and driving it to the overflow chamber downstream. A unique wax flow path is also designed to enhance the capillary flow and the response time of the valve. Background: Centrifugal platform-based CD fluidics has created a new paradigm of inexpensive point-of-care diagnostics[1]. It is now widely used in applications like polymerase chain reaction assays, blood plasma separation, etc.[2,3] Since CD fluidics has presented an innovation in point-of-care diagnostics, it is essential to have a proper valving system for reliable opening/closing of the channels. Currently, there are two main types of valves in the CD platform: laser valves and z-axis wax valves. Both of these valves are good for one-time use only and have complex fabrication routes. Therefore, there is a need for simple, reusable SV in CD fluidic platforms. Theory: The capillary pressure drives the capillary flow in a channel. The capillary pressure difference between atmosphere and the liquid meniscus inside a rectangular microchannel (Pc) is given by Young-Laplace’s equation:-Pc = 2γ(cosθ)(1/w+1/h) (1)where γ is the surface tension, θ is the contact angle with the channel material, w is the width of the channel, and h is the height of the channel. In this design, the resistance offered by the trailing liquid is given by the equation:Rhyd = 12μL/(h3w*(1-0.63(h/w)) (2)For capillary flow, the flow rate (Q) can be obtained byQ = wh(dL/dt) = -Pc/Rhyd (3)For a shallow channel (h<<w), we can neglect the terms containing 1/w, giving the following simplified equation for the flow velocitydL/dt = γ(cosθ)h/(6μL) (4)Upon integration, the fluid length at an instant t is obtainedL = √(γht(cosθ)/3μ) = W√t (5)where W is the Washburn’s constant. We use a new channel design and a flexible adhesive film, and the modified channel shows significant improvement in the capillary flow velocity. A modified empirical model capturing this effect is currently being developed. Materials and Method: The channels and the chambers are milled on an acrylic sheet of thickness 3 mm using Tormac PCNC CNC milling machine (Fig 1a). The Ferro-wax is prepared by mixing Sigma Aldrich Paraffin wax (MP 53 - 37oC) with Ferrofluid (Ferrotec EFHI 60 cc) at the ratio of 2:1 and stirring the mixture at 65oC for 12 hours. The mixture is brought down to room temperature and placed inside the designated chamber. Then, the CD is laminated using a single side pressure-sensitive adhesive (Fig 1a). To achieve Close configuration (Fig 1b), the wax inside the CD is heated up for 5 minutes at 65oC. The melted wax flows to the reagent flow path and solidifies to block the channel. Remelting the wax and directing the melted wax to the overflow chamber brings the valve back to the Open configuration (Fig 1b). The unique channel edges for enhanced capillary flow are fabricated by applying two layers of tape (Fig 1a). Experimental Results:- Experimental characterization of SV was performed by fabricating channels having cross-section areas ranging from 0.05 to 0.625 mm2. A 2.32 W laser placed at a distance of 3 mm from the top surface of the CD is used for heating the Ferrowax. The Normally Close configuration (Fig 1b) is achieved by heating the wax inlet chamber for 5 minutes at 65oC. The wax is driven to the reagent channel by moving along the edge gaps of the wax chamber. The solidified wax prevents the fluid from moving downstream. The sequential release of the reagent is achieved using SV as shown (Fig 1c, 5). A ‘teeth’ design is used at the bottom of the wax inlet to avoid bubble formation inside the chamber. This teeth design also ensures a large number of Open-Close operations for a given amount of wax. Finally, we demonstrate the fidelity of the SV closure (Close configuration) on the CD platform on upwards of 7000 RPM. A parametric study of the air gap is being carried out to determine an optimized design for a faster response.

Lanthanide ions doped ZnO based photocatalysts

ZnO is a versatile semiconductor that finds prominence in the area of photocatalysis due to their compatible crystal structure and band gap edge potentials that are vital for the pollutant degradation reaction. Thus, fabrication of ZnO based nanomaterials with tailored properties remains as the trending topic in recent times. In this context, doping with lanthanide ion finds interesting because of their unique 4f electronic configuration which offers stupendous opportunity to modulate the optical, luminescent and surface acid-base properties of ZnO. This article provides concise overview about the influence of lanthanide ions dopant on the structure-optical properties of ZnO. The performance of Ln-ZnO towards the degradation of organic pollutants is discussed encompassing both the materials properties and photocatalytic reaction conditions under UV/visible and solar light. The co-doping of Ln-ZnO with non-metal/transition metal ion and modification of Ln-ZnO with carbon materials is also emphasized to underscore the recent advancements in this field. We anticipate that our review article provides adequate information into the rational design and development of Ln-ZnO for various applications.

Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs—excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta2NiSe5 disentangling it from the lattice phonons. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to noncritical lattice modes is shown to enhance the transition temperature. On cooling, we observe the gradual emergence of coherent superpositions of band states at the correlated insulator gap edge, with strong departures from mean-field theory predictions. Our results demonstrate the realization of a strongly correlated excitonic state in an equilibrium bulk material.

Frequency Conversion of Lasers in a Dynamic Plasma Grating

When a dynamic medium in which the laser propagates changes its refractive index in time, the laser changes its frequency while keeping its wavevector unchanged to fullfil the dispersion relation. This is usually applied to upshift the laser frequency with ionizing plasma. We propose an alternative technique to modify light frequency. A transient plasma grating can be generated by two identical counterpropagating laser pulses via strongly coupled stimulated Brillouin scattering (SC SBS). The rapid evolution of the plasma grating affects the wave-dispersion relation and a band gap develops around the laser frequency, dependent on the grating amplitude. As a result, the lasers convert their frequency downward to the low edge of the band gap, while a free-traveling laser converts its frequency to both the upper and lower edge of the band gap. Depending on the considered setup, practical applications of this technique include either laser-frequency downshift or spectral splitting can be exploited. The former can be used for Raman amplification in plasma and the latter for dual-color x-ray generation by Thomson and/or Compton scattering.

Microscopic calculation of spin torques in textured antiferromagnets

A microscopic calculation is presented for the spin-transfer torques (STT) and damping torques in metallic antiferromagnets (AF). It is found that the sign of the STT is opposite to that in ferromagnets because of the AF transport character, and the current-to-STT conversion factor is enhanced near the AF gap edge. The dissipative torque parameter $\beta_n$ and the damping parameter $\alpha_n$ for the N\'eel vector arise from spin relaxation of electrons. Physical consequences are demonstrated for the AF domain wall motion using collective coordinates, and some similarities to the ferromagnetic case are pointed out such as intrinsic pinning and the specialty of $\alpha_n = \beta_n$. A recent experiment on a ferrimagnetic GdFeCo near its angular-momentum compensation temperature is discussed.

Within-gap position shapes fifty years of forest dynamics in a temperate hardwood forest in Connecticut, USA

Predictions of forest succession have a tremendous influence on forest management decisions, strategies, and policies. Although generalized forest succession models are intensively studied, there is a distinct lack of long-term studies confirming these models due to the logistical constraints of tracking a forest as it develops over decades or centuries. Here we use a long-term dataset spanning >50 years of third-growth forest development in experimental forest gaps within a second-growth northern temperate hardwood forest to examine the influence of gap position on the structure and composition of regeneration and confirm past models of forest succession. We document key stand development patterns in even-aged mixed-species stands, including vertical stratification and shifts in diameter distributions. While the stands have shifted into a classic inverse-J diameter distribution, these shifts are due to a combination of unimodal distributions of the shade-intolerant and intermediate guilds that grow in mean diameter over time with consistently high densities of small individuals from the shade-tolerant guild. The greatest vertical stratification between the shade-intolerant and intermediate guilds occurred 30 years after clearing but then disappeared as the intermediate guild increased in height. In contrast, the separation in strata between the shade-tolerant species and the more intolerant guilds only increased over time. Finally, we found that gap position had a significant effect on productivity. The basal area, height, and species richness (in this case, an indicator of canopy stratification) were best modeled with negative quadratic regressions with highest values in gap centers and lower values towards the edges of gaps. This pattern aligns more with expected moisture and nutrient availability within gaps than expected solar radiation in the northern hemisphere, suggesting that moisture and nutrient availability may play a greater role than is often acknowledged in temperate forest types. Furthermore, these relationships were readily apparent only after 30 years yet became more accentuated with time, highlighting the importance of long-term research on successional dynamics for management. Ultimately our results document a clear successional trajectory in these gaps that follow expectations for even-aged mixed-species stand development where a full suite of species leads to rolling periods of dominance based on species functional guilds, with early successional species replaced by late-successional species in the progression of succession.

Experimental Evidence of Chiral Symmetry Breaking in Kekulé-Ordered Graphene.

The low-energy excitations of graphene are relativistic massless Dirac fermions with opposite chiralities at valleys K and K^{'}. Breaking the chiral symmetry could lead to gap opening in analogy to dynamical mass generation in particle physics. Here we report direct experimental evidences of chiral symmetry breaking (CSB) from both microscopic and spectroscopic measurements in a Li-intercalated graphene. The CSB is evidenced by gap opening at the Dirac point, Kekulé-O type modulation, and chirality mixing near the gap edge. Our work opens up opportunities for investigating CSB related physics in a Kekulé-ordered graphene.

Efficient utilization of light energy based on MoS&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-XS (X=Al, B, Ga) heterostructures

<p indent=0mm>In recent years, two-dimensional transition metal sulfide materials have received widespread attention due to their unique physical and chemical properties, and have been successfully applied in many fields such as microelectronic devices, photovoltaic technology and hydrogen evolution catalysis. However, high recombination rate, low mobility, and low cycle stability all limit the further improvement of the related properties of this kind of material to varying degrees. In our work, two-dimensional materials are composited to optimize the band structure and physical/chemical properties. This kind of approach has been proved to be feasible in many experiments. Not long ago, Demirci et al. studied the III-VI two-dimensional single-layer hexagonal structure MX (M = B, Al, Ga, In; X = O, S, Se, Te). A series of analyses show that all the above binary compounds are structurally stable. They believe that the III-VI group of binary monolayer materials can be widely used for high-performance optoelectronic devices. Based on density functional theory, this work proposes a new heterostructure form of MoS₂-XS (X=Al, B, Ga). Both MoS₂ and XS (M = B, Al, Ga) have the same hexagonal crystal structure (P-6m2), and there are reasons to believe that the MoS₂-XS heterogeneous structures will be fabricated under certain experimental condition. Therefore, it is meaningful to understand how the single-layer MoS₂ and XS form a stable two-dimensional heterostructure. At present, MoS₂-XS based structural characteristics and related physical/chemical properties of the heterostructures have not been reported. Therefore, the purpose of this research is to grasp the basic characteristics and potential application fields of the MoS₂-XS as far as possible. Based on the periodic boundary conditions of the two-dimensional structure, the establishment of a heterostructure requires us to strictly control the mismatch between MoS₂ and XS. We have obtained three highly reliable heterostructures, which maintain the lowest binding energies as well as the smallest mismatches after a series of preliminary calculations. The VBM and CBM of four pristine materials are composed of S atomic states to varying degrees, especially MoS₂ and AlS. Almost all of their band edges are derived from S atoms, which will greatly enhance the recombination of carriers. Therefore, the electrons and holes that can be actually used in the reaction are significantly reduced. How to suppress the recombination of carriers will be one of main goals in this work. The electronic properties of MoS₂-XS will be significantly affected by external strain. We conducted a detailed analysis and discussion on how the strain affects the band gap and band edges. From variable interlayer spacing, electric field to strain, MoS₂-AlS exhibits good catalytic activity, and has an ideal type-II band alignment. Therefore, we believe that MoS₂-AlS is the most ideal choice in terms of photocatalytic water splitting. It is proved that MoS₂-BS is unsuitable for the application of water splitting, but it has excellent efficiency in solar cell applications and its PCE can be as high as ~20.4%. It is worth noting that this value is very close to or even better than that of the reported heterostructure solar cells. In short, we believe that this work provides a meaningful reference for how to efficiently use light energy in two-dimensional composite materials.

Einfluss von Randspalt und Randabweiser auf die Fluiddynamik einer Messzelle zur HETP‐Bestimmung

AbstractDer Scale‐up strukturierter Packungskolonnen ist ein zeit‐ und kostenintensiver Prozess. Um den Aufwand zur Ermittlung packungsspezifischer Kennzahlen zu reduzieren, wurde eine Messzelle für strukturierte Packungen entwickelt. Diese ermöglicht es charakteristische Bestandteile einer Packungskolonne im Labor‐Maßstab abzubilden. Im Rahmen dieses Beitrages wird der Einfluss der bei Packungskolonnen üblichen Randabweiser und des Abstands zwischen Wand und Packung auf die Fluiddynamik in der Messzelle untersucht.

Puffed-up Edges of Planet-opened Gaps in Protoplanetary Disks. I. Hydrodynamic Simulations

Abstract Dust gaps and rings appear ubiquitous in bright protoplanetary disks. Disk–planet interaction with dust trapping at the edges of planet-induced gaps is one plausible explanation. However, the sharpness of some observed dust rings indicate that sub-millimeter-sized dust grains have settled to a thin layer in some systems. We test whether or not such dust around gas gaps opened by planets can remain settled by performing three-dimensional, dust-plus-gas simulations of protoplanetary disks with an embedded planet. We find planets massive enough to open gas gaps stir small, sub-millimeter-sized dust grains to high disk elevations at the gap edges, where the dust scale height can reach ∼70% of the gas scale height. We attribute this dust “puff up” to the planet-induced meridional gas flows previously identified by Fung &amp; Chiang and others. We thus emphasize the importance of explicit 3D simulations to obtain the vertical distribution of sub-millimeter-sized grains around gas gaps opened by massive planets. We caution that the gas-gap-opening planet interpretation of well-defined dust rings is only self-consistent with large grains exceeding millimeter size.

Fault‐Valve Behavior Estimated From Intensive Foreshocks and Aftershocks of the 2017 M 5.3 Kagoshima Bay Earthquake Sequence, Kyushu, Southern Japan

AbstractDetermining fluid migration and pore pressure change within the Earth is key to understand earthquake occurrences. We investigated the spatiotemporal characteristics of the intense foreshocks and aftershocks of the 2017 ML 5.3 earthquake in Kagoshima Bay, Kyushu, southern Japan, to examine the physical processes governing this earthquake sequence. Our relocated hypocenters show the foreshocks moved on a sharply defined plane with a steep dip. The mainshock rupture initiated at the edge of the foreshock seismic gap. The size of the foreshock seismic gap is comparable to that of the mainshock estimated from the source corner frequency, suggesting this seismic gap corresponds to the large slip region of the mainshock. The aftershocks migrated upward along several steeply dipped planes with a seismicity pattern that deviated from the typical mainshock–aftershock type. This deviation of seismicity pattern, together with the hypocenter migrations, suggests aseismic processes, such as pore pressure migration and aseismic slip, affected this earthquake sequence. We established the following hypothesis. First, fluids originating from the subducting slab migrated upward and intruded into the fault plane, reducing the fault strength and causing the foreshock sequence and potentially aseismic slip. Second, the mainshock rupture occurred due to the decreased fault strength and the increased shear stress in an area with relatively high strength. Third, pore pressure increase associated with post‐failure fluid discharge caused the upward aftershock migration. These observations are consistent with the fault‐valve model and show the importance of fluid movement at depth not only in earthquake swarms but also in foreshock–mainshock–aftershock sequences.

Assessing the effects of forest gaps on beech (Fagus orientalis L.) trees traits in the logged temperate broad-leaf forest

Silvicultural operations, including single-tree selection create gaps in forest canopies and these gaps impact on the forests structure. This study examines the influences of different harvest-created gap sizes on the oriental beech (Fagus orientalis L.) trees traits in temperate Hyrcanian forest, northern Iran. We selected three gap sizes (small, medium and large) and the adjacent unlogged closed canopies with five replications for each and measured some features of beech trees in overstory at the gaps-edge and closed stands. The results showed that gap size significantly influenced on crown length, crown surface area, crown ratio, bole, height/diameter (H/D) and minimum radius / maximum radius (CA) ratios of beech trees six years following gap creation (P < 0.05). Many beech tree traits in adjacent closed forest had no significant difference with gaps, except for crown asymmetric (P < 0.01).Statistically significant differences were observed in beech trees regarding to the clear bole, crown ratio, and CA among the four cardinal points of gaps. A significantly positive correlation was observed between crown ratio with crown length, crown width, crown area, crown volume, and DBH for trees. We found that crown ratio increased significantly with decreasing the bole and total height of beech trees. Results indicated that H/D ratio and total height, DBH, crown length and bole for beech trees were significantly negatively correlated. The CA ratio increased significantly with increasing the bole of trees. The crown radii of border beech trees towards gap center were significantly larger than those of forest-facing side of trees (P < 0.01). This study demonstrates that beech trees in overstory and at the edge of gaps respond differently to artificial gaps after 6 years of formation in oriental beech stand in temperate Hyrcanian forest.

Role of Berry curvature in the generation of spin currents in Rashba systems

We study the background (equilibrium), linear and nonlinear spin currents in 2D Rashba spin-orbit coupled systems with Zeeman splitting and in 3D noncentrosymmetric metals using modified spin current operator by inclusion of the anomalous velocity. The linear spin Hall current arises due to the anomalous velocity of charge carriers induced by the Berry curvature. The nonlinear spin current occurs due to the band velocity and/or the anomalous velocity. For 2D Rashba systems, the background spin current saturates at high Fermi energy (independent of the Zeeman coupling), linear spin current exhibits a plateau at the Zeeman gap and nonlinear spin currents are peaked at the gap edges. The magnitude of the nonlinear spin current peaks enhances with the strength of Zeeman interaction. The linear spin current is polarized out of plane, while the nonlinear ones are polarized in-plane. We witness pure anomalous nonlinear spin current with spin polarization along the direction of propagation. In 3D noncentrosymmetric metals, background and linear spin currents are monotonically increasing functions of Fermi energy, while nonlinear spin currents vary non-monotonically as a function of Fermi energy and are independent of the Berry curvature. These findings may provide useful information to manipulate spin currents in Rashba spin-orbit coupled systems.

The fate of planetesimals formed at planetary gap edges

The presence of rings and gaps in protoplanetary disks are often ascribed to planet–disk interactions, where dust and pebbles are trapped at the edges of planetary-induced gas gaps. Recent works have shown that these are likely sites for planetesimal formation via the streaming instability. Given the large amount of planetesimals that potentially form at gap edges, we address the question of their fate and their ability to radially transport solids in protoplanetary disks. We performed a series ofN-body simulations of planetesimal orbits, taking into account the effect of gas drag and mass loss via ablation. We considered two planetary systems: one that is akin to the young Solar System and another inspired by the structures observed in the protoplanetary disk around HL Tau. In both systems, the proximity to the gap-opening planets results in large orbital excitations, causing the planetesimals to leave their birth locations and spread out across the disk soon after formation. We find that collisions between pairs of planetesimals are rare and should not affect the outcome of our simulations. Collisions with planets occur for ~1% of the planetesimals in the Solar System and for ~20% of the planetesimals in the HL Tau system. Planetesimals that end up on eccentric orbits interior of ~10 au experience efficient ablation and lose all mass before they reach the innermost disk region. In our nominal Solar System simulation, with a stellar gas accretion rate ofṀ0= 10−7M⊙yr−1andα= 10−2, we find that 70% of the initial planetesimal mass has been ablated after 500 kyr. Since the protoplanets are located further away from the star in the HL Tau system, the ablation rate is lower and only 11% of the initial planetesimal mass has been ablated after 1 Myr using the same disk parameters. The ablated material consist of a mixture of solid grains and vaporized ices, where a large fraction of the vaporized ices re-condense to form solid ice. Assuming that the solid grains and ices grow to pebbles in the disk midplane, this results in a pebble flux of ~10−100M⊕Myr−1through the inner disk. This occurred in the Solar System at a time so early in its evolution that there is not likely to be any record of it. Our results demonstrate that scattered planetesimals can carry a significant flux of solids past planetary-induced gaps in young and massive protoplanetary disks.

Topological phenomena demonstrated in photorefractive photonic lattices [Invited

Topological photonics has attracted widespread research attention in the past decade due to its fundamental interest and unique manner in controlling light propagation for advanced applications. Paradigmatic approaches have been proposed to achieve topological phases including topological insulators in a variety of photonic systems. In particular, photonic lattices composed of evanescently coupled waveguide arrays have been employed conveniently to explore and investigate topological physics. In this article, we review our recent work on the demonstration of topological phenomena in reconfigurable photonic lattices established by site-to-site cw-laser-writing or multiple-beam optical induction in photorefractive nonlinear crystals. We focus on the study of topological states realized in the celebrated one-dimensional Su-Schrieffer-Heeger lattices, including nonlinear topological edge states and gap solitons, nonlinearity-induced coupling to topological edge states, and nonlinear control of non-Hermitian topological states. In the two-dimensional case, we discuss two typical examples: universal mapping of momentum-space topological singularities through Dirac-like photonic lattices and realization of real-space nontrivial loop states in flatband photonic lattices. Our work illustrates how photorefractive materials can be employed conveniently to build up various synthetic photonic microstructures for topological studies, which may prove relevant and inspiring for the exploration of fundamental phenomena in topological systems beyond photonics.

Optical Properties and Conductivity of PVA–H3PO4 (Polyvinyl Alcohol–Phosphoric Acid) Film Blend Irradiated by γ-Rays

This study assesses the optical properties and conductivity of PVA–H3PO4 (polyvinyl alcohol–phosphoric acid) polymer film blend irradiated by gamma (γ) rays. The PVA–H3PO4 polymer film blend was prepared by the solvent-casting method at H3PO4 concentrations of 75 v% and 85 v%, and then irradiated up to 25 kGy using γ-rays from the Cobalt-60 isotope source. The optical absorption spectrum was measured using an ultraviolet–visible spectrophotometer over a wavelength range of 200 to 700 nm. It was found that the absorption peaks are in three regions, namely two peaks in the ultraviolet region (310 and 350 nm) and one peak in the visible region (550 nm). The presence of an absorption peak after being exposed to hυ energy indicates a transition of electrons from HOMO to LUMO within the polymer chain. The study of optical absorption shows that the energy band gap (energy gap) depends on the radiation dose and the concentration of H3PO4 in the polymer film blend. The optical absorption, absorption edge, and energy gap decrease with increasing H3PO4 concentration and radiation dose. The interaction between PVA and H3PO4 blend led to an increase in the conductivity of the resulting polymer blend film.

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.