Increase In Domain Size Research Articles

Understanding Process-Structure Relationships during Lamination of Halide Perovskite Interfaces.

Fabrication of halide perovskite (HP) solar cells typically involves the sequential deposition of multiple layers to create a device stack, which is limited by the thermal and chemical incompatibility of top contact layers with the underlying HP semiconductor. One emerging strategy to overcome these restrictions on material selection and processing conditions is lamination, where two half-stacks are independently processed and then diffusion bonded to complete the device. Lamination reduces the processing constraints on the top side of the solar cell to allow new device designs, expanded use of deposition methods, and self-encapsulation of devices. While laminated perovskite solar cells with high efficiencies and novel interlayer combinations have been demonstrated, there is a limited understanding of how the lamination process parameters affect the diffusion-bond quality and material properties of the resulting HP layer. In this study, we systematically vary temperature, pressure, and time during lamination and quantify the resulting impacts on bonded area, grain domain size, and photoluminescence. A design of experiments is performed, and statistical analysis of the experimental results is used to quantitatively evaluate the resulting process-structure-property relationships. The lamination temperature is found to be the key parameter controlling these properties. A temperature of 150 °C enables successful bonding over 95% of the substrate area and also results in increases in apparent grain domain size and photoluminescence intensity. Based on these insights, the lamination temperature of functional perovskite solar cell devices is varied, demonstrating the importance of the resulting bond quality on device performance metrics.

Dynamical decoding of the competition between charge density waves in a kagome superconductor

The kagome superconductor CsV3Sb5 hosts a variety of charge density wave (CDW) phases, which play a fundamental role in the formation of other exotic electronic instabilities. However, identifying the precise structure of these CDW phases and their intricate relationships remain the subject of intense debate, due to the lack of static probes that can distinguish the CDW phases with identical spatial periodicity. Here, we unveil the out-of-equilibrium competition between two coexisting 2 × 2 × 2 CDWs in CsV3Sb5 harnessing time-resolved X-ray diffraction. By analyzing the light-induced changes in the intensity of CDW superlattice peaks, we demonstrate the presence of both phases, each displaying a significantly different amount of melting upon excitation. The anomalous light-induced sharpening of peak width further shows that the phase that is more resistant to photo-excitation exhibits an increase in domain size at the expense of the other, thereby showcasing a hallmark of phase competition. Our results not only shed light on the interplay between the multiple CDW phases in CsV3Sb5, but also establish a non-equilibrium framework for comprehending complex phase relationships that are challenging to disentangle using static techniques.

Spatially-resolved in-situ/operando structural study of screen-printed BaTiO3/P(VDF-TrFE) flexible piezoelectric device

The incorporation of lead-free BaTiO3 particles into P(VDF-TrFE) provides a versatile way for tuning dielectric and piezoelectric properties. Screen-printing enables the easy production of flexible capacitors. To prevent particle aggregation, the BaTiO3 particles are functionalized with a surfactant. Initially, 60 % vol. BaTiO3 particles are deposited in the second layer out of three layers. After annealing and cooling, the particle distribution is successfully homogeneous in the whole film. After poling at 80 °C, the relative permittivity and the piezoelectric coefficient d33 significantly increase up to 159 and 8 pC/N respectively. In situ/operando spatially-resolved X-ray techniques allow exploring the structural evolution of the composite device during annealing and during an applied electric field. The application of an electric field leads to the quasi-disappearance of the P(VDF-TrFE) ferroelectric state. As a matter of fact, the interactions between the P(VDF-TrFE) and the surfactant may limit the rotation of P(VDF-TrFE) chains near the particles. Hence, paraelectric polymers could be chosen on a mechanical or economic basis for the matrix in piezoelectric composite devices. Regarding BaTiO3, the lattice strain evolves from an extrinsic strain at RT to an intrinsic strain after annealing and cooling. At the cooled state, the tetragonality c/a is higher than 1.01, leading to improved piezoelectric properties. The increase of the average domain size leads to the increase of the composite permittivity. Finally this knowledge facilitates the development of tailored flexible piezoelectric composite devices.

Structure and reactivity of surface vanadia sites in bi-layered supported VOx/AlOx/SiO2 catalysts via solid-state NMR, first-principles calculations, and catalytic studies

By combining multinuclear 1H, 29Si, 27Al and 51V solid-state NMR experimental measurements with reactivity and selectivity data, as well as first-principles calculations, for ternary bi-layered supported VOx/Al2O3/SiO2 catalysts, the structure of the dehydrated surface vanadia species was determined for each combination of supported VOx and AlOx active components. The specific structure of the surface vanadia sites in the ternary bi-layered supported VOx/Al2O3/SiO2 catalyst system have been investigated for the first time.The following vanadia sites are proposed based on the current findings:At low vanadia content, surface VO(OH)(OSi)2 sites form on the alumina-free surface of the SiO2 support in the bi-layered supported VOx/Al2O3/SiO2 catalyst system. In addition, two types of VO(OSi)3 surface species are likely present in this system. On small alumina clusters, weakly bonded surface VO(OH)(OAl)2 sites also form, along with the sites containing mixed Si/Al pods VO(OH)(OAl)(OSi). As the size of alumina clusters increases, strongly bonded VO(OAl)3 centers form, alongside the surface sites containing mixed pods VO(OAl)2(OSi) and VO(OAl)(OSi)2. The content of these sites increases with the loading of vanadia. Comparing supported VOx/SiO2, VOx/Al2O3, and bi-layered VOx/Al2O3/SiO2 catalyst systems revealed differences in catalytic activity due to the presence of new sites with different pods, with OH groups as well as the influence of Al-O-Si bonds on the electronic structure of surface vanadia sites on the alumina clusters. During the formation of dimethyl ether (DME), the turnover frequency (TOFDME) of the catalysts decreases with increasing domain size of the alumina clusters on SiO2. Addition of surface VOx sites, however, slightly decreases the TOFDME with the higher vanadia content weakening the activity of the alumina clusters. When surface VOx exists predominantly as V/Al1 and V/Al2, TOFredox depend on the vanadia content with lower vanadia content resulting in higher the TOFredox values. The 51V NMR spectra show that this decrease in redox TOF value correlates with the appearance of vanadia sites associated with silica pods. The activity of the catalysts was tested in methanol conversion reactions.

Design and characterization of novel Zr-Al-Fe-Y metallic glasses with nanoscale phase separation

We present a new phase-separated Zr-based metallic glass (MG) composition, free of toxic elements (Ni and Cu) prepared in a glass-forming system by alloying addition. The substitution of Y in Zr70-xAl12.5Fe17.5Yx (x = 0–15 at.%) results in phase separation, giving rise to nano-amorphous domains in a glassy matrix. A homogeneous MG feature is observed for low concentrations of Y up to x = 5. Microstructure and thermal property analysis indicate that the glasses with x = 10 and 15 at.% Y exhibit a typical liquid phase separation-induced two-glassy phase (Zr-rich and Y-rich) with droplet-like microstructures (nano-amorphous domains). With increasing Y addition, the domain size increases. The microstructural evolution arising due to phase separation in these alloys is confirmed by a combination of transmission electron microscopy (TEM) and atom probe tomography (APT) analysis. The formation of nanometer-size Y-enriched clusters is confirmed by APT for x = 15.

Enhancing the piezoelectric properties and thermal stability of PMN-PMS-PSZT high-power piezoelectric ceramics through defect engineering

The development of high-power piezoelectric ceramics with superior piezoelectric properties and broad temperature usage ranges is vital for the thriving progress of electromechanical applications. However, achieving high piezoelectricity, high mechanical quality factor, and temperature stability often presents a trade-off. In this study, we present an advanced ceramic material with excellent high-power piezoelectric properties and superior temperature stability. The piezoelectric coefficient d33 reaches 348 pC N−1, the electromechanical coupling factor kp is 54%, the mechanical quality factor Qm is 1501, the dielectric loss tanδ is 0.35%, and the d33 variation is less than 10% within 25–210 °C in 0.05Pb(Mg1/3Nb2/3)O3-0.05Pb(Mn1/3Sb2/3)O3-0.9Pb0.95Sr0.05(Zr0.48Ti0.52)O3 ceramic. The excellent piezoelectricity is attributed to the enhancement of intrinsic ionic displacement and reversible domains wall motion because of symmetry-conforming short-range distribution of oxygen vacancy, while the high Qm is a result of increased domain size and oxygen vacancy. The outstanding temperature stability is related to the stable non-180°domains structure due to the oxygen vacancy pinning effect. These properties hold great potential for advanced applications, particularly for piezoelectric high-power applications requiring constant d33 over a broad temperature range.

Tailoring Magnetic Anisotropy in Ultrathin Cobalt by Surface Carbon Chemistry

AbstractThe ability to manipulate magnetic anisotropy is essential for magnetic sensing and storage tools. Surface carbon species offer cost‐effective alternatives to metal‐oxide and noble metal capping layers, inducing perpendicular magnetic anisotropy in ultrathin ferromagnetic films. Here, the different mechanisms by which the magnetism in a few‐layer‐thick Co thin film is modified upon adsorption of carbon monoxide (CO), dispersed carbon, and graphene are elucidated. Using X‐ray microscopy with chemical and magnetic sensitivity, the in‐plane to out‐of‐plane spin reorientation transition in cobalt is monitored during the accumulation of surface carbon up to the formation of graphene. Complementary magneto‐optical measurements show weak perpendicular magnetic anisotropy (PMA) at room temperature for dispersed carbon on Co, while graphene‐covered cobalt exhibits a significant out‐of‐plane coercive field. Density‐functional theory (DFT) calculations show that going from CO/Co to C/Co and to graphene/Co, the magnetocrystalline and magnetostatic anisotropies combined promote out‐of‐plane magnetization. Anisotropy energies weakly depend on carbidic species coverage. Instead, the evolution of the carbon chemical state from carbidic to graphitic is accompanied by an exponential increase in the characteristic domain size, controlled by the magnetic anisotropy energy. Beyond providing a basic understanding of the carbon‐ferromagnet interfaces, this study presents a sustainable approach to tailor magnetic anisotropy in ultrathin ferromagnetic films.

The effect of Li/Ce co‐doping on structural and electrical properties of Sr2−x(Li, Ce)x/2Nb2O7 piezoelectric ceramics

AbstractAs a typical perovskite‐like layered structure (PLS) ceramic, Sr2Nb2O7 (SNO) possesses a high Curie temperature TC value above 1300°C, which is expected to work in higher temperature environment. However, pure SNO presents a piezoelectric constant d33 value of ∼ 1 pC/N. Hence, (Li, Ce) ions are doped in SNO ceramics, which increases the distortion of oxygen octahedrons and enhances the intrinsic contribution of piezoelectric properties. The increased domain size with large proportion of high response areas means that the extrinsic contribution is also improved. The introduction of (Li, Ce) ions can also promote ceramic's grain growth and reduce oxygen vacancies, leading to the improvement of electrical properties. The optimal component Sr1.96(Li, Ce)0.02Nb2O7 ceramics exhibit a d33 value of 3.1 pC/N, a higher resistivity of 1.65×106 Ω·cm at 700°C and an excellent thermal stability (a d33 value of 3.0 pC/N after annealing at 1200°C for 2 h). This work provides an effective approach with simple process and low cost for the application of PLS materials in high‐temperature environments.

Revealing the mechanism of competing A‐site doping ions in (K, Na)NbO3‐based ceramics

AbstractAlthough past studies have shown a competitive relationship between bismuth ion (Bi3+) and rare earth ions in potassium sodium niobate {(K, Na)NbO3, KNN}‐based ceramics, a comprehensive investigation of this competition has been relatively scarce. Herein, we conducted an in‐depth exploration of this competition by substituting Bi3+ with praseodymium ions (Pr3+) in KNN‐based ceramics possessing high piezoelectric properties. The substitution of Bi3+ with Pr3+ results in a decrease in the extent of multi‐phase coexistence at room temperature, an increase in domain size and grain size, and the enhancement of ferroelectricity. However, it also leads to a deterioration in piezoelectricity. Through a combination of experimental findings and first‐principles calculations, we analyzed the alterations in phase structure, ferroelectric domains, and electrical properties, attributing these changes to the competitive nature of Bi‐O and Pr‐O bonds with their distinct ionic/covalent characteristics. Consequently, this study not only unveils the underlying physical mechanism behind the competition between Bi3+ and Pr3+ ions in KNN‐based piezoceramics but also presents a novel approach to tailor the phase transition temperature, thereby facilitating the overall performance improvement of KNN‐based ceramics.

Engineering internal nanostructure in 3D‐printed materials via polymer molecular weight distribution

AbstractThe distribution of molecular weights in polymers, known as the molecular weight distribution (MWD), plays a significant role in dictating the behavior of polymer self‐assembly and influencing the characteristics of the resulting materials. This study investigates how MWD of macromolecular chain‐transfer agents (macroCTAs) impact internal nanostructures in materials prepared by polymerization‐induced microphase separation (PIMS) 3D printing. In the aim of elucidating this relationship, the study initially harnessed the precision offered by narrow‐MWD macroCTAs, which provide precise control over phase separation, as assessed by atomic force microscopy (AFM) and small‐angle X‐ray scattering (SAXS) measurements. Through systematic variation of macroCTA molecular weights, the dimensions of the distinct domains were precisely tuned from 10 to 90 nanometers and a decrease of materials stiffness was observed with increased domain size. In contrast, the utilization of a broader MWD, achieved by blending two distinct macroCTAs, resulted in increased domain size dispersity and reduced interface sharpness, without significantly affecting the mechanical properties of the 3D‐printed materials. Overall, this approach expands the strategies for manipulating the nanoscale architecture of 3D‐printed PIMS materials, opening new possibilities for printing advanced engineering materials with tailorable properties.

Spontaneous autophoretic motion of isotropic disks

It is theoretically known that an isotropic chemically active particle in an unbounded solution undergoes symmetry breaking when the intrinsic Péclet number ${{Pe}}$ exceeds a finite critical value (Michelin et al., Phys. Fluids, vol. 25, 2013, 061701). At that value, a transition takes place from a stationary state to spontaneous motion. In two dimensions, where no stationary state is possible in an unbounded domain, a linear stability analysis in a large bounded domain (Hu et al., Phys. Rev. Lett., vol. 123, 2019, 238004) reveals that the critical ${{Pe}}$ value slowly diminishes as the domain size increases. Motivated by these findings, we here consider an unbounded domain from the outset, addressing the two-dimensional problem of steady self-propulsion with a focus on the limit ${{Pe}}\ll 1$ . This singular limit is handled using matched asymptotic expansions, conceptually decomposing the fluid domain into a particle-scale region, where the leading-order solute transport is diffusive, and a remote region, where diffusion and advection are comparable. The expansion parameter is provided by the product of ${{Pe}}$ and $U$ , the unknown particle speed (normalised by the standard autophoretic scale). The problem is unconventional in that the scaling of $U$ with ${{Pe}}$ must be determined in the course of the perturbation analysis. The resulting approximation, $U=4\exp ({-2/{Pe}-\gamma _{E}-1})/{{Pe}}$ ( $\gamma _{E}$ being the Euler–Mascheroni constant), is in remarkable agreement with the numerical predictions of Hu et al. in the common interval of validity.

Quantitative image analysis of influence of polysaccharides on protein network formation in GDL-acidified milk gels

Exopolysaccharides (EPS) are commonly used to improve the texture of yogurt. These polysaccharides interact with casein micelles, the major protein in milk, via electrostatic and depletion mechanisms during fermentation by lactic acid bacteria (LAB). However, the relationship between the physicochemical properties and monosaccharide composition of EPS and their impact on yogurt texture is not yet fully understood. To address this knowledge gap, we studied the effects of polysaccharides commonly used as food additives on acid-induced milk protein networks. Confocal laser scanning microscopy (CLSM) was used to image the network microstructures. Image analysis, including Fourier transform, autocorrelation, and binarization-based techniques, was applied to quantify key structural features of the mixed milk protein/polysaccharide gels. These parameters were then related to the macroscopic properties of the model food matrices, such as elastic and viscous moduli and yield point. We found that the addition of neutral polysaccharides resulted in a concentration-dependent increase in structure factor, protein domain size, and pore fraction. In contrast, the presence of charged polysaccharides led to an increase in protein domain size, a decrease in pore fraction, and a decrease in elastic and viscous moduli. These results demonstrate the use of a quantitative image analysis method for selecting LAB with favorable EPS properties to improve yogurt texture.

Endothermic Dehydrogenation-Driven Preventive Magnesiation of Sio by Magnesium Hydride for High-Performance Sio-Based Lithium Storage Materials

Silicon monoxide (SiO)-based materials have gained much attention as high-capacity lithium storage materials because of their superior cycle performance. However, low initial Coulombic efficiency (ICE) of SiO derived from the irreversible electrochemical reaction of the amorphous SiO2 phase in SiO makes wide use of the SiO-based anode materials in lithium-ion batteries challenging. Magnesiation of SiO is one of the promising solutions to improve ICE of SiO-based anode materials. Here, we propose an endothermic dehydrogenation-driven magnesiation of SiO by employing MgH2 to improve both ICE and cycle performance. The resulting Si/Mg2SiO4 nanocomposite material showed much-improved ICE of 89.5% and long-term cycle performance for over 300 cycles compared to the homologue prepared by the magnesiation of SiO using Mg and pristine SiO. High-resolution transmission electron microscopy with thermogravimetry−differential scanning calorimetry revealed that the endothermic dehydrogenation of MgH2 mitigates the rapid temperature rise during magnesiation of SiO, thereby inhibiting the increase of Si domain size in the resulting Si/Mg2SiO4 nanocomposite. Detailed material characterization and electrochemical analysis of Si/Mg2SiO4 nanocomposite materials will be discussed in this presentation.

Microstructure and microwave dielectric properties of nonstoichiometric 1:1 ordered Nd(Zn1/2Ti1/2+x)O3+2x perovskite ceramics with Ti4+ self-doping

The grain size, B-site 1:1 cation ordering, chemical defects and microwave dielectric properties of the non-stoichiometric Nd(Zn1/2Ti1/2+x)O3+2x (x = −0.02–0.02) complex perovskite ceramics were significantly tuned by self-doping methods. The increase of Ti4+ self-doping significantly increases the grain size of Nd(Zn1/2Ti1/2+x)O3+2x ceramics which could still maintain a monoclinic structure with B-site 1:1 cation ordering (space group P21/n) in the case of deviation from stoichiometry. However, the degree of 1:1 B-site cation ordering is tuned to be negatively correlated with the mutual occupancy of Ti4+ and Zn2+ by Ti4+ self-doping according to the results of crystal structure refinement. With the increase of Ti4+ component, the degree of 1:1 cation ordering first increased and then decreased, reaching the maximum value (S = 0.81) at x = 0.01. Non-stoichiometry can tune cation ordering because it modulates the effective radius difference and charge difference of B-site ions. The increase Ti4+ component promotes the formation of ordered domains with uniformly distributed domain boundaries and increasing domain size up to x = 0.01. With further increase of Ti4+ component to x = 0.02, ferroelastic domains appear due to the oxygen octahedral torsion. The B-site 1:1 cation ordering significantly reduces the dielectric loss of the ceramics, which makes the Q × f value exhibit the same variation rule as the degree of B-site 1:1 cation ordering. High Q × f value of 58000 GHz is achieved in the non-stoichiometric samples (x = 0.01), and it is 45% higher than those in traditional stoichiometric samples (x = 0). In this work, the microstructure and properties of the ceramics are effectively regulated by non-stoichiometry, which provides an effective way to tune the properties of 1:1 ordered ceramics.

Large-Area Ultrathin Moiré Chiral Metamaterials by Thermal-Tape-Transfer Printing

Moiré chiral metamaterials (MCMs) consisting of stacked plasmonic nanohole arrays with twist angles exhibit strong chiroptical response, thereby opening doors to ultrathin chiroptical sensors, switches, and detectors. The existing fabrication of MCMs on the basis of colloidal lithography results in significant inhomogeneity within the structure, which impairs the responsiveness and tunability of its optical chirality. Here, we develop thermal-tape-transfer printing to enable the fabrication of large-scale and homogeneous MCMs with arbitrary twist angles and tunable optical chirality. As a demonstration, large-scale (100 × 100 μm2) gold MCMs with an ultrathin thickness (∼40 nm) were fabricated, which marked a 10-fold increase in single domain size over the colloidal lithographic method, while showing spatially uniform and strong chiroptical response. With the ultrathin thickness and high tunability, the MCMs developed by our fabrication method will advance a variety of biological, photonic, and optoelectronic applications.

Editorial: Celebrating the contributions of Professor Guieppe Marrucci to rheology and non-Newtonian fluid mechanics

Direct numerical simulations (DNS) of generalized Newtonian fluids in a rectangular channel at a friction velocity of 180 are performed using OpenFOAM. The simulations are performed for a Newtonian fluid, a power-law fluid, and a Herschel-Bulkley (HB) fluid. Two-point correlations of the streamwise velocity reveal that a significant increase in the computational domain size is required to avoid statistical correlation due to the shear-thinning and yield stress. The analogous Kolmogorov scale shows that the dissipative structures may be orders of magnitude larger with these two non-Newtonian behaviors. This is demonstrated to be linked to the significant increase in the viscosity. The mean streamwise velocity field integrates to a larger bulk velocity with increased shear-thinning as compared with the Newtonian fluid flow. The addition of the yield stress does the same with respect to the power-law fluid flow. The difference in the velocity profiles is more pronounced in the outer layer. It, however, is observed that the rheology does not play a significant role in the outer layer for the mean streamwise velocity. It is shown that an HB fluid flow does not necessarily show an increase in the fluctuating streamwise velocity with respect to a power-law fluid flow. It is also shown that low power index and high Hedstrom number lead to strong turbulence anisotropy due to the weakening of turbulence. The turbulent kinetic energy (TKE) budget reveals that the studied non-Newtonian behavior mainly affects the inner layer of the flow. The yield stress is also seen to show small variations in the TKE budget in the outer layer. This is attributed to the weakening turbulence as a result of the increased viscosity near the center. It is also demonstrated that the studied HB fluid flow results can be successfully reproduced by under-resolved DNS.

Particle Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection* *Released January 10, 2023.

Magnetic reconnection in the relativistic regime has been proposed as an important process for the efficient production of nonthermal particles and high-energy emission. Using fully kinetic particle-in-cell simulations, we investigate how the guide-field strength and domain size affect the characteristic spectral features and acceleration processes. We study two stages of acceleration: energization up until the injection energy γ inj and further acceleration that generates a power-law spectrum. Stronger guide fields increase the power-law index and γ inj, which suppresses acceleration efficiency. These quantities seemingly converge with increasing domain size, suggesting that our findings can be extended to large-scale systems. We find that three distinct mechanisms contribute to acceleration during injection: particle streaming along the parallel electric field, Fermi reflection, and the pickup process. The Fermi and pickup processes, related to the electric field perpendicular to the magnetic field, govern the injection for weak guide fields and larger domains. Meanwhile, parallel electric fields are important for injection in the strong guide-field regime. In the post-injection stage, we find that perpendicular electric fields dominate particle acceleration in the weak guide-field regime, whereas parallel electric fields control acceleration for strong guide fields. These findings will help explain the nonthermal acceleration and emission in high-energy astrophysics, including black hole jets and pulsar wind nebulae.

Persistent modification of magnetotransport properties in LaAlO3/SrTiO3 by gate-induced modification of structural domains

We investigate the influence of large electric fields on the transport properties of nanostructures patterned into the electron gas at the interface between ${\mathrm{LaAlO}}_{3}$ and ${\mathrm{SrTiO}}_{3}$ (100). In these nanostructures, the transport is largely dominated by domain walls between structural domains in the ${\mathrm{SrTiO}}_{3}$ appearing below a structural phase transition temperature. We find that both positive and negative gate voltages applied in a side-gate configuration can induce persistent changes in the sample that are only reversed by warming through the phase transition. These changes include a resistance increase and a change in magnetoresistance in magnitude and sign. Furthermore, a resistance anomaly during warm-up that has been observed in the past can be further increased by briefly applying a side-gate voltage. These effects are typically observed in nanostructures below a certain size limit. The electric fields also increase this size limit from a few-hundred nm to more than 1 $\ensuremath{\mu}\mathrm{m}$. All these observations can be consistently explained by a field-induced removal of specific domain-wall types accompanied by an increasing domain size and a reduced number of domain walls in the structures. The results not only show that under certain conditions domain walls can dominate the transport properties even of micron-sized structures, but they also provide an additional tuning knob to induce nonvolatile changes in the transport properties of ${\mathrm{LaAlO}}_{3}\text{/}{\mathrm{SrTiO}}_{3}$ interfaces at low temperatures.

Standing Sausage Perturbations in Solar Coronal Slabs with Continuous Transverse Density Profiles: Cutoff Wavenumbers, Evanescent Eigenmodes, and Oscillatory Continuum

The lack of observed sausage perturbations in solar active region loops is customarily attributed to the relevance of cutoff axial wavenumbers and the consequent absence of trapped modes (called “evanescent eigenmodes” here). However, some recent eigenvalue problem studies suggest that cutoff wavenumbers may disappear for those equilibria where the external density varies sufficiently slowly, thereby casting doubt on the rarity of candidate sausage perturbations. We examine the responses of straight, transversely structured coronal slabs to small-amplitude sausage-type perturbations that excite axial fundamentals, by solving the pertinent initial value problem with eigensolutions for a closed domain. The density variation in the slab exterior is dictated by some steepness parameter μ, and cutoff wavenumbers are theoretically expected to be present (absent) when μ ≥ 2 (μ < 2). However, our numerical results show no qualitative difference in the system evolution when μ varies, despite the differences in the modal behavior. Only oscillatory eigenmodes are permitted when μ ≥ 2. Our discrete eigenspectrum becomes increasingly closely spaced when the domain broadens, and an oscillatory continuum results for a truly open system. Oscillatory eigenmodes remain allowed and dominate the system evolution when μ < 2. We show that the irrelevance of cutoff wavenumbers does not mean that all fast waves are evanescent. Rather, it means that an increasing number of evanescent eigenmodes emerge when the domain size increases. We conclude that sausage perturbations remain difficult to detect, even for the waveguide formulated here.

Nucleation and coalescence of tungsten disulfide layers grown by metalorganic chemical vapor deposition

The nucleation of tungsten disulfide WS2 crystallites and coalescence behaviour of monolayer WS2 films grown on C-plane sapphire have been studied in a commercial AIXTRON close coupled showerhead metalorganic chemical vapor deposition (MOCVD) reactor using tungsten hexacarbonyl and di-tert-butyl sulfide as precursors. The influence of sapphire substrates offcut (both orientation and angle), as well as the substrate annealing conditions and growth temperature, on the nucleation and size of the WS2 domains has been investigated. A higher nucleation temperature combined with increased substrate annealing temperatures was found to promote an increase in the WS2 domain size. The dependence of surface coverage and coalescence behaviour of the WS2 domains on the growth duration has also been determined, allowing monolayer WS2 films with < 10 % bilayer to be obtained. Crystalline quality of the coalesced WS2 films has been assessed by grazing incidence XRD and misorientation mapping using 4D-scanning transmission electron microscopy, showing a slight in-plane misorientation of the crystallites in the WS2 monolayer. The electrical performance of the coalesced WS2 films was assessed using backgated TLM structures demonstrating mobility values of up to 5 cm2/Vs and a high Ion/Ioff ratio greater than 108.