Local Gap Research Articles

Energy-Resolved Photoconductivity Mapping in a Monolayer-Bilayer WSe2 Lateral Heterostructure.

Vertical and lateral heterostructures of van der Waals materials provide tremendous flexibility for band-structure engineering. Because electronic bands are sensitively affected by defects, strain, and interlayer coupling, the edge and heterojunction of these two-dimensional (2D) systems may exhibit novel physical properties, which can be fully revealed only by spatially resolved probes. Here, we report the spatial mapping of photoconductivity in a monolayer-bilayer WSe2 lateral heterostructure under multiple excitation lasers. As the photon energy increases, the light-induced conductivity detected by microwave impedance microscopy first appears along the heterointerface and bilayer edge, then along the monolayer edge, inside the bilayer area, and finally in the interior of the monolayer region. The sequential emergence of mobile carriers in different sections of the sample is consistent with the theoretical calculation of local energy gaps. Quantitative analysis of the microscopy and transport data also reveals the linear dependence of photoconductivity on the laser intensity and the influence of interlayer coupling on carrier recombination. Combining theoretical modeling, atomic-scale imaging, mesoscale impedance microscopy, and device-level characterization, our work suggests an exciting perspective for controlling the intrinsic band gap variation in 2D heterostructures down to a regime of a few nanometers.

Saving Women's Lives in Latin America and the Caribbean: Improving Cervical Cancer Screening and Treatment Quality and Access

Background and context: It is the third leading cause of cancer deaths among females in Latin America and the Caribbean, and yet cervical cancer is almost entirely preventable and treatable. In a region where many lack even basic access to quality sexual and reproductive healthcare, screening and treatment services for HPV and cervical cancer are far from universally available. International Planned Parenthood Federation/Western Hemisphere Region (IPPF/WHR) and its member associations (MAs) are working to reverse this trend, identifying and bridging local gaps in access and services wherever possible. Aim: IPPF/WHR and its MAs seek to improve both providers' ability to deliver - and women's ability to access - quality cervical cancer screening and treatment through institutional capacity building, the introduction of new technologies, advocacy and community awareness efforts. Strategy/Tactics: 1) Increase cervical cancer services by training providers in the provision of low cost, high capacity screening and treatment methods, including visual inspection with acetic acid (VIA), HC2 and HPV DNA screening technologies, and a single-visit approach (SVA) to treatment using cryotherapy, thermocoagulation and LEEP. 2) Educate populations and increase demand for cervical cancer services by implementing public awareness campaigns and community information, education and communication (IEC) activities promoting the importance of early detection and treatment. 3) Improve and standardize clinical protocols and referral pathways by advocating among and collaborating with key decision-makers and local ministries of health. Program/Policy process: MAs are implementing small-scale pilot studies to incorporate VIA, HC2 and HPV DNA screening and new treatment to provide even greater numbers of women with potentially life-saving diagnostics and care in Belize, Honduras, El Salvador and Bolivia. As leading clinical experts and advocates, several MAs are also working with public sector counterparts to refine two-way referral pathways, standardize screening protocols and clinical guidelines, and to ensure data quality and collection. A range of Caribbean MAs have also been trained in the use of VIA, cryo and LEEP. Outcomes: From 2016 to 2017, MAs from Belize, Grenada, Suriname, and several additional Caribbean countries who received training in VIA and other screening and treatment techniques saw an average 7% increase in the number of direct cervical cancer services provided. MAs from Belize, Bolivia and Honduras also contributed to updated national cervical cancer protocols. What was learned: An effective national response to cervical cancer requires the support and collaboration of civil society organizations, which can deliver direct services and play a catalytic role in advancing technical recommendations and policy dialogue. Countries should continue to improve the quality of VIA services, until more advanced screening technologies become available and can be scaled up.

Complex spin texture of Dirac cones induced via spin-orbit proximity effect in graphene on metals

We use large-scale DFT calculations to investigate with unprecedented detail the so-called spin-orbit (SO) proximity effect in graphene adsorbed on the Pt(111) and Ni(111)/Au semi-infinite surfaces, previously studied via spin and angle resolved photoemission (SP-ARPES) experiments. The key finding is that, due to the hybridization with the metal's bands, the Dirac cones manifest an unexpectedly rich spin texture including out-of-plane and even radial in-plane spin components at (anti)crossings where local gap openings and deviations from linearity take place. Both the continuum character of the metallic bands and the back folding associated to the moir\'e patterns enhance the spin texture and induce sizable splittings which, nevertheless, only become giant (~100 meV) at anticrossing regions; that is, where electronic transport is suppressed. At the quasilinear regions the splitted bands typically disperse with different broadenings and tend to cross with their magnetization continuously changing in order to match that at the edges of the upper and lower gaps. As a result, both the splittings and spin direction become strongly k dependent. The SO manifests in an analogous way for the spin-polarized G/Au/Ni(111) system, although here the magnetic exchange interactions dominate inducing small splittings (~10 meV) in the $\pi$ bands while the SO mainly introduces a small Rashba splitting in the Dirac cones as their magnetization acquires a helical component. While revealing such complex spin texture seems challenging from the experimental side, our results provide an important reference for future SP-ARPES measurements of similar graphene based systems extensively investigated for applications in spintronics.

Self-recording and manipulation of fast long-range hydrogen diffusion in quasifree magnesium

Understanding diffusion of large solutes such as hydrogen and lithium in solids is of paramount importance for energy storage in metal hydrides and advanced batteries. Due to its high gravimetric and volumetric densities, magnesium is a material of great potential for solid-state hydrogen storage. However, the slow hydrogen diffusion kinetics and the deleterious blocking effect in magnesium have hampered its practical applications. Here, we demonstrate fast lateral hydrogen diffusion in quasifree magnesium films without the blocking effect. Massive concomitant lattice expansion leads to the formation of remarkable self-organized finger patterns extending over tens of micrometers. Detailed visualization of diffusion fronts reveals that the fingers in these patterns follow locally the direction of hydrogen diffusion. Thus, the streamlines of the diffusion process are self-recorded by means of the finger pattern. By inclusion of fast hydrogen diffusion objects or local gaps, the resulting streamlines exhibit a clear analogy to optical rays in geometric optics. The possibility to spatially manipulate hydrogen diffusion opens an avenue to build advanced hydrogen storage systems, cloaking and active plasmonic devices, as well as prototype systems for computational models.

Geometry, electronic structure, morphology, and photoluminescence emissions of BaW1-xMoxO4 (x = 0, 0.25, 0.50, 0.75, and 1) solid solutions: Theory and experiment in concert

The design of a solid solution with tunable electro-optical properties and multifunctionality is a promising strategy for developing novel materials. In this work, BaW1-xMoxO4 (x = 0, 0.25, 0.5, 0.75, and 1) solid solutions have been successfully prepared for the first time by a co-precipitation method. Their crystal structure and phase composition were determined by X-ray diffraction and Rietveld refinements. Fourier transform infrared and micro Raman spectroscopy in combination with field-emission scanning electron microscopy (FE-SEM) were used to describe the microstructures and chemical compositions of the synthesized materials. The influence of chemical composition on morphology and photoluminescence (PL) emission has been analyzed. The geometry, electronic structures, and morphologies of BaW1-xMoxO4 (x = 0, 0.25, 0.5, 0.75, and 1) solid solutions were investigated by first-principles quantum-mechanical calculations based on the density functional theory. By using Wulff construction and the values of the surface energies for the (1 1 2), (0 0 1), (1 1 0), (1 0 1), (1 0 0), and (1 1 1) crystal faces, a complete map of the available morphologies for the BaW1-xMoxO4 solid solutions was obtained. These results show a qualitative agreement between the experimental morphologies obtained using the FE-SEM images and the computational models. The substitution of W6+ by Mo6+ enhances the electron-transfer process due to a stronger Mo(4d)-O(2p) hybridization compared to W(5d)-O(2p) for the W/Mo-O superficial bonds, and is responsible for the change in morphology from BaWO4 to BaMoO4. Such a fundamental study, which combines multiple experimental methods and first-principles calculations, has provided valuable insight into obtaining a basic understanding of the local structures, bonding, morphologies, band gaps, and electronic and optical properties of the BaW1-xMoxO4 (x = 0, 0.25, 0.5, 0.75, and 1) solid solutions.

Investigating cumulative effects across ecological scales.

Species, habitats, and ecosystems are increasingly exposed to multiple anthropogenic stressors, fueling a rapidly expanding research program to understand the cumulative impacts of these environmental modifications. Since the 1970s, a growing set of methods has been developed through two parallel, sometimes connected, streams of research within the applied and academic realms to assess cumulative effects. Past reviews of cumulative effects assessment (CEA) methods focused on approaches used by practitioners. Academic research has developed several distinct and novel approaches to conducting CEA. Understanding the suite of methods that exist will help practitioners and academics better address various ecological foci (physiological responses, population impacts, ecosystem impacts) and ecological complexities (synergistic effects, impacts across space and time). We reviewed 6 categories of methods (experimental, meta-analysis, single-species modeling, mapping, qualitative modeling, and multispecies modeling) and examined the ability of those methods to address different levels of complexity. We focused on research gaps and emerging priorities. We found that no single method assessed impacts across the 4 ecological foci and 6 ecological complexities considered. We propose that methods can be used in combination to improve understanding such that multimodel inference can provide a suite of comparable outputs, mapping methods can help prioritize localized models or experimental gaps, and future experiments can be paired from the outset with models they will inform.

Topological interface modes in local resonant acoustic systems

Topological phononic crystals (PCs) are periodic artificial structures which can support nontrivial acoustic topological bands, and their topological properties are linked to the existence of topological edge modes. Most previous studies focused on the topological edge modes in Bragg gaps which are induced by lattice scatterings. While local resonant gaps would be of great use in subwavelength control of acoustic waves, whether it is possible to achieve topological interface states in local resonant gaps is a question. In this article, we study the topological bands near local resonant gaps in a time-reversal symmetric acoustic systems and elaborate the evolution of band structure using a spring-mass model. Our acoustic structure can produce three band gaps in subwavelength region: one originates from local resonance of unit cell and the other two stem from band folding. It is found that the topological interface states can only exist in the band folding induced band gaps but never appear in the local resonant band gap. The numerical simulation perfectly agrees with theoretical results. Our study provides an approach of localizing the subwavelength acoustic wave.

The combination of electroanatomic mapping and minielectrodes in a series of cases of redo procedures

BackgroundIn patients with supraventricular tachycardia, catheter ablation is an important treatment option. However, approximately one quarter of these patients remain symptomatic, so sustainable strategies for the treatment of those patients who do not benefit from the first catheter ablation are required. MethodsIn a series of redo procedures, we investigated the combined use of an electro-anatomic mapping system and an ablation catheter with mini-electrodes. ResultsCatheter ablation was successful in two patients with recurrent common type atrial flutter and one patient with recurrent ectopic atrial tachycardia. In a patient with recurrent perimitral flutter, the ablation procedure had to be stopped early, due to pericardial effusion. ConclusionThe combination of electro-anatomic mapping and mini-electrodes might be useful, especially in the treatment of ectopic atrial tachycardias, but also in redo procedures of CTI ablations, that require not only the visualization of the tachycardia, but also the detection of a local focus or a local gap. For an optimal use of the ME ablation catheter, the generator settings should be evaluated in further studies.

Electronic properties of a quantum ring perturbed with a quantum well in the presence of perpendicular magnetic flux

Here, we study the effects of the number of sites, quantum ring radius and potential well depth on the energy levels, persistent current, magnetic susceptibility and density of states (DOS) of a quantum ring with a quantum well within its circumstance in a magnetic flux perpendicular to its plane. We show that, for small radius quantum ring systems, there are periodic local gaps along the magnetic flux axis in the DOS plots and along the axis ‘energy’. For large radius quantum ring systems, a uniform gap along the energy axis exists and along the phi axis nothing changes. In quantum rings with a quantum well in their circumstance, by using the large confining potential, we can create uniform gaps in the Energy–phi plane. The energy eigenvalues, persistent current and magnetic susceptibility decrease by increasing the confining potential. A quantum ring even with a very small confining potential in its circumstance can sensibly decrease the persistent current and magnetic susceptibility, although it may do not change the energy eigenvalues and DOS maximum considerably. Thus, by using the abovementioned parameters, we are able to tune the DOS, persistent current, magnetic susceptibility and energy levels, desirably.

Gap-planar graphs

We introduce the family of k-gap-planar graphs for k≥0, i.e., graphs that have a drawing in which each crossing is assigned to one of the two involved edges and each edge is assigned at most k of its crossings. This definition is motivated by applications in edge casing, as a k-gap-planar graph can be drawn crossing-free after introducing at most k local gaps per edge. We present results on the maximum density of k-gap-planar graphs, their relationship to other classes of beyond-planar graphs, characterization of k-gap-planar complete graphs, and the computational complexity of recognizing k-gap-planar graphs.

Growth of two-dimensional decagonal colloidal quasicrystals

The growth of quasicrystals, i.e. structures with long-range positional order but no periodic translational symmetry, is more complex than the growth of periodic crystals. By employing Brownian dynamics simulations in two dimensions for colloidal particles that interact according to an isotropic pair potential with two incommensurate lengths, we study the growth of quasicrystalline structures by sequentially depositing particles at their surface. We quantify the occurrence of quasicrystalline order as a function of the temperature and the rate of added particles. In addition, we explore defects like local triangular order or gaps within the quasicrystalline structure. Furthermore, we analyze the shapes of the surfaces in grown structures which tend to build straight lines along the symmetry axes of the quasicrystal. Finally, we identify phasonic flips which are rearrangements of the particles due to additional degrees of freedom. The number of phasonic flips decreases with the distance to the surface.

Wave propagation in acoustic metamaterials with resonantly shunted cross-shape piezos

Cross-shape piezoelectric patches were originally proposed to improve the band-gap properties of acoustic metamaterials with shunting circuits. The dispersion curves are characterized through the application of finite element method. Also, the theoretical band-gap predictions are verified by simulation results obtained from COMSOL. The investigation results show that the proposed scheme distinguishes itself from the conventional square patches by broader band gaps, whose bandwidth is almost doubled. The inherent capacitance of the piezoelectric patch is strongly related to the boundary conditions, so the local resonant band gap is strongly affected by the shape of piezoelectric patches as well. As a result, the band-gap width and location of metamaterials with different shape patches are rather different, even with the same size patches. Also, negative modulus (NM) and Poisson’s ratio were observed around the resonant frequencies. The transmission properties of finite periods agree well with band-gap predictions. An obvious attenuation zone (AZ) is produced around the band-gap location, in which the wave propagation is decayed strongly. Similarly, the width of AZ of the proposed metamaterial is much larger than that of the conventional one. Hence, the proposed scheme demonstrates more advantages in the application to vibration isolation when compared with the conventional.

Strongly ergodic actions have local spectral gap

We show that an ergodic measure preserving action Γ ↷ ( X , μ ) \Gamma \curvearrowright (X,\mu ) of a discrete group Γ \Gamma on a σ \sigma -finite measure space ( X , μ ) (X,\mu ) satisfies the local spectral gap property introduced in Invent. Math. 208 (2017), 715–802, if and only if it is strongly ergodic. In fact, we prove a more general local spectral gap criterion in arbitrary von Neumann algebras. Using this criterion, we also obtain a short proof of Connes’ spectral gap theorem for full I I 1 \mathrm {II}_1 factors as well as its recent generalization to full type I I I \mathrm {III} factors.

Research on low-frequency band gap property of a hybrid phononic crystal

A hybrid phononic crystal has been investigated. The characteristic frequency of XY mode, transmission loss and displacement vector have been calculated by the finite element method. There are Bragg scattering band gap and local resonance band gap in the band structures. We studied the influence factors of band gap. There are many flat bands in the eigenfrequencies curve. There are many flat bands in the curve. The band gap covers a large range in low frequency. The band gaps cover more than 95% below 3000 Hz.

Allied health clinicians using translational research in action to develop a reliable stroke audit tool.

To design and establish reliability of a local stroke audit tool by engaging allied health clinicians within a privately funded hospital. Design: Two-stage study involving a modified Delphi process to inform stroke audit tool development and inter-tester reliability. Allied health clinicians. A modified Delphi process to select stroke guideline recommendations for inclusion in the audit tool. Reliability study: 1 allied health representative from each discipline audited 10 clinical records with sequential admissions to acute and rehabilitation services. Recommendations were admitted to the audit tool when 70% agreement was reached, with 50% set as the reserve agreement. Inter-tester reliability was determined using intra-class correlation coefficients (ICCs) across 10 clinical records. Twenty-two participants (92% female, 50% physiotherapists, 17% occupational therapists) completed the modified Delphi process. Across 6 voting rounds, 8 recommendations reached 70% agreement and 2 reached 50% agreement. Two recommendations (nutrition/hydration; goal setting) were added to ensure representation for all disciplines. Substantial consistency across raters was established for the audit tool applied in acute stroke (ICC .71; range .48 to .90) and rehabilitation (ICC.78; range .60 to .93) services. Allied health clinicians within a privately funded hospital generally agreed in an audit process to develop a reliable stroke audit tool. Allied health clinicians agreed on stroke guideline recommendations to inform a stroke audit tool. The stroke audit tool demonstrated substantial consistency supporting future use for service development. This process, which engages local clinicians, could be adopted by other facilities to design reliable audit tools to identify local service gaps to inform changes to clinical practice.

The razor’s edge: Social impact bonds and the financialization of early childhood services

ABSTRACT In a growing number of U.S. cities, social impact bonds (SIBs) introduce an experimental strategy into the politics of fiscal constraint. With limited political willpower and public funding, some have used SIBs to leverage new support for social programs. We argue that cities that engage in SIBs walk a razor’s edge between promoting public investment and the risk of deepening financialization in the social service sector. We explore efforts in 3 cities to expand early childhood services through SIBs: Salt Lake City, Utah; Chicago, Illinois; and Greenville, South Carolina. We test the balance between promise and risk through four foci: systemic change, performance metrics, cost structure, and social equity. We show that the context of political fiscal climate and strategic policy change matters in SIBs’ justification and impact; whereas Salt Lake City and Greenville scaled investment up to the state level, Chicago merely plugged short-term local budget gaps.

Atomic-Scale Electronic Characterization of Defects in Silicon Carbide Nanowires by Electron Energy-Loss Spectroscopy

The atomic-level resolution of scanning transmission electron microscopy (TEM) is used for structural characterization of nanomaterials, but the resolution afforded by TEM also enables electronic characterization of defects in these materials through electron energy-loss spectroscopy (EELS). Here, the power of EELS is harnessed to characterize the local band gap of inclusion defects in hexagonal silicon carbide nanowires with a high density of stacking faults. The band gaps we extract from the EELS data align within 0.1 eV of expected values for hexagonal silicon carbide and stacking faults within hexagonal silicon carbide. These experiments show that individual cubic phase inclusions in hexagonal silicon carbide significantly alter the local electronic structure, in particular, the band gap, in contrast to the polarizability tensor that retains the characteristic signature of the global hexagonal crystal structure.

Structural, morphological and optical characteristics of fusiform Co-doped CeO2 via a facile hydrothermal method

In this study, the fusiform Co-doped CeO2 particles were prepared by a hydrothermal method and characterized by X-ray diffraction (XRD), thermo-gravimetric differential thermal analysis (TG–DTA), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectrometer (EDX), X-ray photo-electron spectroscopy (XPS), Raman, photoluminescence (PL) and UV–Vis spectroscopy. The cubic fluorite structure of CeO2 were supported by XRD. Initially thermal gravimetric and differential thermal analysis were used to analyse the reaction mechanism and chemical process, which indicated a crystal temperature of the as-synthesized CeO2 particles at 600 °C. The reaction concentration of the solution were systematically researched. It is found that the reaction concentration is one key parameter for controlling the final micrographs. The optical properties were characterized by Raman, PL and UV–Vis spectroscopy. It has been demonstrated that oxygen vacancies and Ce3+ exist at the particle surfaces and boundaries. The appearance of luminescence peaks and emission peaks can be ascribed to the related oxygen defects. The appearance of Ce3+ and oxygen defects existing in fusiform Co-doped CeO2 products can cause the formation of several local band gap states, which is perhaps responsible for the red shift of band gap.

Doping and band gap control at poly(vinylidene fluoride)/graphene interface

Using the density-functional first-principles calculations, we investigate the electronic structures of poly(vinylidene fluoride) PVDF/graphene composite systems. The n- and p-doping of graphene can be flexibly switched by reversing the ferroelectric polarization of PVDF, without scarifying the intrinsic π-electron band dispersions of graphene that are usually undermined by chemical doping. The doping degree is also dependent on the thickness of PVDF layers, which will get saturated when PVDF is thick enough. In PVDF/bilayer graphene (BLG) heterostructure, the doping degree directly determines the local energy gap of the charged BLG. The sandwich structure of PVDF/BLG/PVDF can further enhance the local energy gap as well as keep the electric neutrality of BLG, which will be of great application potentials in graphene-based nanoelectronics.

Approximation Algorithms for Minimum-Load k -Facility Location

We consider a facility-location problem that abstracts settings where the cost of serving the clients assigned to a facility is incurred by the facility. Formally, we consider the minimum-load k-facility location (ML k FL) problem, which is defined as follows. We have a set F of facilities, a set C of clients, and an integer k ≥ 0. Assigning client j to a facility f incurs a connection cost d ( f , j ). The goal is to open a set F ⊆ F of k facilities and assign each client j to a facility f ( j )∈ F so as to minimize max f ∈ F ∑ j ∈ C : f ( j )= f d ( f , j ); we call ∑ j ∈ C : f ( j )= f d ( f , j ) the load of facility f . This problem was studied under the name of min-max star cover in References [3, 7], who (among other results) gave bicriteria approximation algorithms for ML k FL for when F = C . ML k FL is rather poorly understood, and only an O ( k )-approximation is currently known for ML k FL, even for line metrics . Our main result is the first polytime approximation scheme (PTAS) for ML k FL on line metrics (note that no non-trivial true approximation of any kind was known for this metric). Complementing this, we prove that ML k FL is strongly NP -hard on line metrics. We also devise a quasi-PTAS for ML k FL on tree metrics. ML k FL turns out to be surprisingly challenging even on line metrics and resilient to attack by a variety of techniques that have been successfully applied to facility-location problems. For instance, we show that (a) even a configuration-style LP-relaxation has a bad integrality gap and (b) a multi-swap k -median style local-search heuristic has a bad locality gap. Thus, we need to devise various novel techniques to attack ML k FL. Our PTAS for line metrics consists of two main ingredients. First, we prove that there always exists a near-optimal solution possessing some nice structural properties. A novel aspect of this proof is that we first move to a mixed-integer LP (MILP) encoding of the problem and argue that a MILP-solution minimizing a certain potential function possesses the desired structure and then use a rounding algorithm for the generalized-assignment problem to “transfer” this structure to the rounded integer solution. Complementing this, we show that these structural properties enable one to find such a structured solution via dynamic programming.