Evolution Of Gap Research Articles

Unexpected band gap evolution and high carrier mobility sparked by the orbital variation in two-dimensional GaGeX (X = S, Se, Te)

As elemental group material, germanium, enjoys a high status in the field of two-dimensional (2D) materials. However, 2D germanene is a gapless semiconductor, which greatly limits its application in optoelectronic devices. Herein, we found an interesting class of 2D materials, named GaGeX (X = S, Se, Te), consisting of two outer GaX layers and an inner germanene, a sandwich structure stacked together by interlayer Ga-Ge bonds. Compared with known 2D materials (such as MoX2), the band gaps gradually decrease in the order of sulfide, selenide, and telluride, the electronic calculations show that they have a unique band gaps reversely enlarging trend of 0.03, 0.46 and 0.72 eV from GaGeS to GaGeTe, respectively. Moreover, GaGeX is predicted to have superior mobility, especially with a hole mobility of up to 5510 cm2 V−1 s−1 in GaGeTe. These novel phenomena are found closely related to the orbital variation. Our findings render these 2D materials potential candidates for future optoelectronic devices.

Neutron Spectroscopy Evidence for a Possible Magnetic-Field-Induced Gapless Quantum-Spin-Liquid Phase in a Kitaev Material α-RuCl3

As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates, α-RuCl3 has received a great deal of attention. However, its ground state exhibits a long-range zigzag magnetic order, which defies the QSL phase. Nevertheless, the magnetic order is fragile and can be completely suppressed by applying an external magnetic field. Here, we explore the evolution of magnetic excitations of α-RuCl3 under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. Under zero field, there exist spin-wave excitations near the M point and a continuum near the Γ point, which are believed to be associated with the zigzag magnetic order and fractional excitations of the Kitaev QSL state, respectively. By increasing the magnetic field, the spin-wave excitations gradually give way to the continuous excitations. On the verge of the critical field μ 0 H c = 7.5 T, the former ones vanish and only the latter ones are left, indicating the emergence of a pure QSL state. By further increasing the field strength, the excitations near the Γ point become more intense. By following the gap evolution of the excitations near the Γ point, we are able to establish a phase diagram composed of three interesting phases, including a gapped zigzag order phase at low fields, possibly gapless QSL phase near μ 0 H c, and gapped partially polarized phase at high fields. These results demonstrate that an in-plane magnetic field can drive α-RuCl3 into a long-sought QSL state near the critical field.

Tunable Antibacterial Activity of a Polypropylene Fabric Coated with Bristling Ti3C2Tx MXene Flakes Coupling the Nanoblade Effect with ROS Generation

Polypropylene (PP) is a thermoplastic polymer widely used as a medical textile in healthcare applications due to its low cost and superior performance. However, it does not show antibacterial properties leading to the possibility of pathogen transmission. Herein, we have developed an antibacterial medical fabric by facile self-assembly of delaminated two-dimensional (2D) Ti3C2Tx MXene flakes bristling on the surface of PP fibers. The increasing amount of MXene in the coating solution from 1 up to 32 mg/mL allowed for edge-on assembly of MXene flakes on the PP surface and tracking the evolution of the band gap for a restacked structure. Characterization of the PP/Ti3C2Tx nanocomposite has proven that it exhibited highly effective antibacterial, robust coating, and chemically/thermally stable properties. The in vitro microbiological studies against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus have shown that PP/Ti3C2Tx reduced the bacterial viability up to 100%, as driven by synergistic membrane stress mediated by physical contact and light-induced reactive oxygen species (ROS) generation. Moreover, the use of l-ascorbic acid for MXene stabilization allowed for achieving excellent thermal stability of the PP/Ti3C2Tx nanocomposite upon accelerated thermal aging. Collectively, this work provides a facile surface engineering strategy for designing medical fabrics with outstanding functional performances. By demonstrating the exceptional performance of the stabilized MXene in a self-assembly nanocomposite structure, we are opening the door for MXenes to be applied in other biomedical fields.

Excitation Spectrum and Superfluid Gap of an Ultracold Fermi Gas.

Ultracold atomic gases are a powerful tool to experimentally study strongly correlated quantum many-body systems. In particular, ultracold Fermi gases with tunable interactions have allowed to realize the famous BEC-BCS crossover from a Bose-Einstein condensate (BEC) of molecules to a Bardeen-Cooper-Schrieffer (BCS) superfluid of weakly bound Cooper pairs. However, large parts of the excitation spectrum of fermionic superfluids in the BEC-BCS crossover are still unexplored. In this work, we use Bragg spectroscopy to measure the full momentum-resolved low-energy excitation spectrum of strongly interacting ultracold Fermi gases. This enables us to directly observe the smooth transformation from a bosonic to a fermionic superfluid that takes place in the BEC-BCS crossover. We also use our spectra to determine the evolution of the superfluid gap and find excellent agreement with previous experiments and self-consistent T-matrix calculations both in the BEC and crossover regime. However, toward the BCS regime a calculation that includes the effects of particle-hole correlations shows better agreement with our data.

Unified picture for the pressure-controlled band gap in inorganic halide perovskites: Role of strain-phonon and phonon-phonon couplings

Pressure in halide perovskites attracts extensive attention recently as an effective tool for band-gap engineering. Here, combining first-principles calculations and symmetry-mode analyses, we give a general insight into the role of pressure in inorganic halide perovskites and provide a complete and consistent description of the evolution of band gap that observed in high-pressure experiments. We reveal that strain-phonon and phonon-phonon couplings are the essential factors determining the band-gap evolution. The subtle interplay between strain-phonon and phonon-phonon couplings triggers the increase of out-of-phase tilt at a larger pressure, which results in the simultaneous increase of the band gap. Additionally, we point out that the bond lengths vary continuously, and their nonlinear behaviors originate from strain-phonon coupling instead of the stiffening of the volume. With this knowledge, we propose that epitaxial compressive strain continuously decreases the tilt distortion, and reduction of band gap of 0.5 eV is achieved in $\mathrm{Cs}\mathrm{Pb}{\mathrm{Br}}_{3}$ by 5% compressive strain, which may dramatically enhance the energy conversion efficiency.

Combining semilocal exchange with dynamical mean-field theory: Electronic structure and optical response of rare-earth sesquioxides

In rare-earth semiconductors, wide ligand $p$ and rare-earth 5$d$ bands coexist with localized, partially filled 4$f$ shells. A simultaneous description for both extended and localized states represents a significant challenge for first-principles theories. Here, we combine an {\it ab initio} dynamical mean-field theory approach to strong local correlations with a perturbative application of the semi-local modified Becke-Johnson exchange potential to correct the semiconducting gap. We apply this method to calculate the electronic structure and optical response of the light rare-earth sesquioxides RE$_2$O$_3$ (RE= La, Ce, Pr and Nd). Our calculations correctly capture a non-trivial evolution of the optical gap in RE$_2$O$_3$ due to a progressive lowering of the 4$f$ states along the series and their multiplet structure. 2$p$ $-$ 4$f$ hybridization is found to induce a substantial upward shift for the occupied 4$f$ states occurring within the $p-d$ gap, thus reducing the magnitude of the optical gap. We show that a characteristic plateau observed in the optical conductivity in the Pr and Nd sequioxides right above their absorption edge is a fingerprint of 4$f$ states located within the $p-d$ gap.

The Gender Wage Gap in Peru: Drivers, Evolution, and Heterogeneities

Despite the recent economic growth and gender equality improvement in educational attainment, important gender disparities remain in the Peruvian labour market. This article provides a comprehensive overview of the Peruvian gender wage gap evolution during 2007–2018 and identifies key elements that explain its patterns. First, the article shows that the raw wage gap showed an upward trend between 2007–2011, ranging from 6% to 12%, and remaining around that top bound ever since. Second, using Oaxaca‐Blinder decomposition we find that the unexplained wage gap has remained virtually unchanged at around 17% during the study period. Reductions in endowment differences between men and women coupled with a stagnant unexplained gap led to slightly larger raw wage gaps over time. Moreover, the stagnant unexplained gap suggests the presence of structural problems regarding social norms, gender stereotyping and potential discrimination that affects the wage gap. Third, we show that both at a national and regional level, gender wage gaps are larger within the lowest percentiles, and they mostly have a downward slope across the earnings distribution. Finally, after computing the raw and unexplained gap at the region‐year level, we show that smaller regional gender gaps are associated with (a) higher GDP, (b) lower levels of domestic physical violence against women, and (c) lower percentages of women as household heads.

Electronic and optical properties of SixGe1−x−ySny alloys lattice-matched to Ge

We present a combined experimental and theoretical analysis of the evolution of the near-band gap electronic and optical properties of Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ alloys lattice-matched to Ge and GaAs substrates. We perform photoreflectance (PR) and photoluminescence (PL) measurements on Si$_{x}$Ge$_{1-x-y}$Sn$_{y}$ epitaxial layers grown via chemical vapour deposition, for Si (Sn) compositions up to $x =$ 9.6% ($y =$ 2.5%). Our measurements indicate the presence of an indirect fundamental band gap, with PL observed $\approx$ 200-250 meV lower in energy than the direct $E_0$ transition identified by PR measurements. The measured PL is Ge-like, suggesting that the alloy conduction band (CB) edge is primarily derived from the Ge L-point CB minimum. Interpretation of the PR and PL measurements is supported by atomistic electronic structure calculations. Effective alloy band structures calculated via density functional theory confirm the presence of an indirect fundamental band gap, and reveal the origin of the observed inhomogeneous broadening of the measured optical spectra as being alloy-induced band hybridisation occurring close in energy to the CB edge. To analyze the evolution of the band gap, semi-empirical tight-binding (TB) calculations are employed to enable calculations for large supercell sizes. TB calculations reveal that the alloy CB edge is hybridized in nature, consisting at low Si and Sn compositions of an admixture of Ge L-, $\Gamma$- and X-point CB edge states, and confirm that the alloy CB edge retains primarily Ge L-point CB edge character. Our experimental measurements and theoretical calculations confirm a direct transition energy close to 1 eV in magnitude for Si and Sn compositions $x =$ 6.8 - 9.6% and $y =$ 1.6 - 2.2%.

Expanding the Gap: An Updated Look Into Sex Differences in Running Performance.

Males consistently outperform females in athletic endeavors, including running events of standard Olympic distances (100 m to Marathon). The magnitude of this percentage sex difference, i.e., the sex gap, has evolved over time. Two clear trends in sex gap evolution are evident; a narrowing of the gap during the 20th century, followed by a period of stability thereafter. However, an updated perspective on the average sex gap from top 20 athlete performances over the past two decades reveals nuanced trends over time, indicating the sex gap is not fixed. Additionally, the sex gap varies with performance level; the difference in absolute running performance between males and females is lowest for world record/world lead performances and increases in lower-ranked elite athletes. This observation of an increased sex gap with world rank is evident in events 400 m and longer and indicates a lower depth in female competitive standards. Explanations for the sex difference in absolute performance and competition depth include physical (physiological, anatomical, neuromuscular, biomechanical), sociocultural, psychological, and sport-specific factors. It is apparent that females are the disadvantaged sex in sport; therefore, measures should be taken to reduce this discrepancy and enable both sexes to reach their biological performance potential. There is scope to narrow the sex performance gap by addressing inequalities between the sexes in opportunities, provisions, incentives, attitudes/perceptions, research, and media representation.

Effects of the Phase, Morphology, Band Gap and Hydrogen Evolution of Vanadium Oxide with Reduced Graphene Oxide

Effects of the Phase, Morphology, Band Gap and Hydrogen Evolution of Vanadium Oxide with Reduced Graphene Oxide

Parenthood penalty and gender wage gap: Recent evidence from Thailand

This study first examines the evolution of gender wage gap in Thailand, using cross-sectional data from the Labor Force Survey (LFS) for 1985–2017. We find that education, occupation, and industry significantly contribute to gender wage gap convergence in Thailand. Furthermore, for females, the wage gap between mothers and non-mothers has increased over time, while for males, the changes are relatively small. Thereafter, we examine the gender wage gap associated with marriage and parental status, using panel data from the Socio-Economic Survey (SES) for 2005–2012, and find wage penalty for both motherhood and fatherhood in Thailand.

Density dependence of the excitation gaps in an undoped Si/SiGe double-quantum-well heterostructure

We report low-temperature magneto-transport measurements of an undoped Si/SiGe asymmetric double quantum well heterostructure. The density in both layers is tuned independently utilizing top and bottom gates, allowing the investigation of quantum wells at both imbalanced and matched densities. Integer quantum Hall states at total filling factor νT=1 and νT=2 are observed in both density regimes, and the evolution of their excitation gaps is reported as a function of the density. The νT=1 gap evolution departs from the behavior generally observed for valley splitting in the single layer regime. Furthermore, by comparing the νT=2 gap to the single particle tunneling energy, ΔSAS, obtained from Schrödinger–Poisson (SP) simulations, evidence for the onset of spontaneous interlayer coherence is observed for a relative filling fraction imbalance smaller than ∼50%.

Polarization-dependent optical responses in natural 2D layered mineral teallite

Multi-element layered materials enable the use of stoichiometric variation to engineer their optical responses at subwavelength scale. In this regard, naturally occurring van der Waals minerals allow us to harness a wide range of chemical compositions, crystal structures and lattice symmetries for layered materials under atomically thin limit. Recently, one type of naturally occurring sulfide mineral, ternary teallite has attained significant interest in the context of thermoelectric, optoelectronic, and photovoltaic applications, but understanding of light-matter interactions in such ternary teallite crystals is scarcely available. Herein, polarization-dependent linear and nonlinear optical responses in mechanically exfoliated teallite crystals are investigated including anisotropic Raman modes, wavelength-dependent linear dichroism, optical band gap evolution, and anisotropic third-harmonic generation (THG). Furthermore, the third-order nonlinear susceptibility of teallite crystal is estimated using the thickness-dependent THG emission process. We anticipate that our findings will open the avenue to a better understanding of the tailored light-matter interactions in complex multi-element layered materials and their implications in optical sensors, frequency modulators, integrated photonic circuits, and other nonlinear signal processing applications.

A thermal-stress modelling methodology in ABAQUS for fundamentally describing the die/casting interface behaviour in a cyclic permanent die casting process

This manuscript describes a novel method for thermal-stress modelling of a cyclic, permanent die casting process in the commercial software package ABAQUS. The key element in this method is that it updates the casting's geometry according to the deformed die at elevated temperature before filling the die cavity. Using this method, the evolution of gap and/or pressure at the die/casting interface has been predicted. This in turn has allowed for a more fundamental and less empirical description of the evolution in resistance to heat transfer at the interface during a casting cycle. The new methodology has been compared to a previously published methodology, and the comparison shows a noticeable difference between the two approaches in predicting the temperature field in the die and the casting and prediction of areas prone to shrinkage-based porosity. This methodology also predicts the stress distribution in the die in the cyclic steady-state operation, which may be of value in predicting die fatigue life.

Topological phase transition in chiral graphene nanoribbons: from edge bands to end states

Precise control over the size and shape of graphene nanostructures allows engineering spin-polarized edge and topological states, representing a novel source of non-conventional π-magnetism with promising applications in quantum spintronics. A prerequisite for their emergence is the existence of robust gapped phases, which are difficult to find in extended graphene systems. Here we show that semi-metallic chiral GNRs (chGNRs) narrowed down to nanometer widths undergo a topological phase transition. We fabricated atomically precise chGNRs of different chirality and size by on surface synthesis using predesigned molecular precursors. Combining scanning tunneling microscopy (STM) measurements and theory simulations, we follow the evolution of topological properties and bulk band gap depending on the width, length, and chirality of chGNRs. Our findings represent a new platform for producing topologically protected spin states and demonstrate the potential of connecting chiral edge and defect structure with band engineering.

Mechanochemistry for Energy Materials: Impact of High-Energy Milling on Chemical, Electric and Thermal Transport Properties of Chalcopyrite CuFeS2 Nanoparticles.

Chalcopyrite CuFeS2, a semiconductor with applications in chemical sector and energy conversion engineering, was synthetized in a planetary mill from elemental precursors. The synthesis is environmentally friendly, waste‐free and inexpensive. The synthesized nano‐powders were characterized by XRD, SEM, EDX, BET and UV/Vis techniques, tests of chemical reactivity and, namely, thermoelectric performance of sintered ceramics followed. The crystallite size of ∼13 nm and the strain of ∼17 were calculated for CuFeS2 powders milled for 60, 120, 180 and 240 min, respectively. The evolution of characteristic band gaps, Eg, and the rate constant of leaching, k, of nano‐powders are corroborated by the universal evolution of the parameter SBET/X (SBET ‐specific surface area, X‐crystallinity) introduced for complex characterization of mechanochemically activated solids in various fields such as chemical engineering and/or energy conversion. The focus on non‐doped semiconducting CuFeS2 enabled to assess the role of impurities, which critically and often negatively influence the thermoelectric properties.

Funding female entrepreneurs in North Africa: self-selection vs discrimination? MSMEs, the informal sector and the microfinance industry

PurposeThis paper aims to address the following research question: Is loan funding to female entrepreneurs in Egypt, Tunisia and Morocco affected by self-selection from borrowers or/and discrimination from lenders? This paper sheds light on empirical literature review, which displays mixed evidence.Design/methodology/approachThe authors use a pooled sample of 3,896 businesses in Egypt, Morocco and Tunisia drawn from the 2013 World Bank Enterprise Survey (WBES). Despite selection biases and overweighing, the sample provides descriptive statistics upon gender ownership and gender management (human capital characteristics and financial data). The authors design two regression logistic models with interaction to investigate loan demand and loan granting with respect to self-selection vs discrimination. Female management is disentangled from female ownership with respect to entrepreneurship.FindingsNeither self-selection nor discrimination affects female owners compared with their male counterparts, whereas female managers do self-select themselves. In as much as the WBES female subsample include several biases, the authors eventually emphasise the importance of the non-surveyed informal sector, which includes most (micro-)businesses, and loan funding provided by the microfinance industry to these female businesses. Microfinance fills the gap for working capital but not for fixed assets. The size of the business is a major factor explaining both self-selection and discrimination.Research limitations/implicationsFindings of this study have important policy implications for closing the gender gap in accessing finance. In addition to supply-side factors, demand-side factors should be addressed. Informality also needs to be addressed, as many micro and small enterprises owned or managed by women are informal entities without registration or/and social protection. One way to increase women's demand for financial services is to introduce financial products to meet their needs (e.g. social protection basic coverage). Governments can help develop these new products by strengthening the microfinance industry with a favourable regulatory and institutional framework. The authors also wonder about the extension of this study. Thus, a new cross-sectional analysis of the most recent surveys in the North African region would allow the authors to enlarge the overall sample and measure the evolution of the gender gap over time.Originality/valueSo far, funding female entrepreneurship remained little investigated in these North African countries. Several sampling biases in the WBES – small businesses underestimation and manufacturing industry overweighting, which have been overlooked so far, explain the absence of self-selection and discrimination. In contrast, size plays an important role. Hence, the focus on microenterprises (the informal sector) and the microfinance industry suggests indeed that female entrepreneurs operating in small businesses have to cope with both self-selection and discrimination.

H-BN nanosheets doped with transition metals for environmental remediation; a DFT approach and molecular docking analysis

This study compares the catalytic and antimicrobial potential of BN nanosheets doped with various transition metals-TMs (Co, Cu, Ni, Zr, and Bi). Evaluation of catalytic activity demonstrated that prepared products can be used as efficient nanocatalysts for wastewater management. TMs-doped BN depicted higher bactericidal efficacy against S. aureus compared to E. coli with molecular docking analysis. Density functional theory calculations were also performed to investigate the structural stability and electronic behavior of samples. It was found that the band gap evolution corroborates well with the experimental trends, exhibiting a diminution of the band gap value with substitutional TM atoms. Moreover, the adsorption energies of NaBH4 molecule on undoped and TMs doped BN nanosheets are investigated, in which the adsorption energy between the Co-doped BN monolayer and NaBH4 is greater compared with other doped nanosheets.

The Role of Canopy Cover Dynamics over a Decade of Changes in the Understory of an Atlantic Beech-Oak Forest

The understory of temperate forests harbour most of the plant species diversity present in these ecosystems. The maintenance of this diversity is strongly dependent on canopy gap formation, a disturbance naturally happening in non-managed forests, which promotes spatiotemporal heterogeneity in understory conditions. This, in turn, favours regeneration dynamics, functioning and structural complexity by allowing changes in light, moisture and nutrient availability. Our aim is to study how gap dynamics influence the stability of understory plant communities over a decade, particularly in their structure and function. The study was carried out in 102 permanent plots (sampled in 2006 and revisited in 2016) distributed throughout a 132 ha basin located in a non-managed temperate beech-oak forest (Bertiz Natural Park, Spain). We related changes in the taxonomical and functional composition and diversity of the understory vegetation to changes in canopy coverage. We found that gap dynamics influenced the species composition and richness of the understory through changes in light availability and leaf litter cover. Species with different strategies related to shade tolerance and dispersion established in the understory following the temporal evolution of gaps. However, changes in understory species composition in response to canopy dynamics occur at a slow speed in old-growth temperate forests, needing more than a decade to really be significant. The presence of gaps persisting more than ten years is essential for maintaining the heterogeneity and stability of understory vegetation in old-growth temperate forests.

Fingerprinting triangular-lattice antiferromagnet by excitation gaps

CeCd$_3$As$_3$ is a rare-earth triangular-lattice antiferromagnet with large inter-layer separation. Our field-dependent heat capacity measurements at dilution fridge temperatures allow us to trace the field-evolution of the spin-excitation gaps throughout the antiferromagnetic and paramagnetic regions. The distinct gap evolution places strong constraints on the microscopic pseudo-spin model, which, in return, yields a close {\it quantitative} description of the gap behavior. This analysis provides crucial insights into the nature of the magnetic state of CeCd$_3$As$_3$, with a certainty regarding its stripe order and low-energy model parameters that sets a compelling paradigm for exploring and understanding the rapidly growing family of the rare-earth-based triangular-lattice systems.