Band Gap Formation Research Articles

Bragg resonances in left-handed bigyrotropic media based on periodic ferromagnetic semiconductor

In this work, a theoretical model for a periodic bigyrotropic left-handed medium, which is a ferromagnetic semiconductor, is developed. The presence of periodicity in such a structure leads to the formation of Bragg band gaps in the spectrum of propagating waves, both in the microwave and terahertz ranges. Band gaps in the microwave range are formed in the frequency region with double negative effective parameters of the medium. The band gaps density decreases with increasing frequency in the microwave range and increases in the terahertz range.

Investigating the photocatalytic efficacy of a porous cuprous gallate (CuGa2O4) thick film

Abstract The study aimed to investigate the photocatalytic activity of porous copper gallate (CuGa2O4) thick film with significant wall dimensions. The thick film was meticulously fabricated using tape casting systems, with CuGa2O4 nanoparticles synthesized by Sol–Gel technique and subsequently transformed into gels for the tape casting process. Rietveld refinements were employed to elucidate the crystal structure and lattice parameters of the CuGa2O4 spinel oxide lattice. Moreover, electronic energy configurations and optical transmittance measurements were acquired through UV-visible spectrophotometry. The correlations between the crystal structure, band gap change and formation of electronic energy levels in relation to the photocatalytic performance of CuGa2O4 were explored. The comprehensive examination provides valuable insights into the complex interaction between material properties and photocatalytic behavior, offering a nuanced understanding of the potential applications of porous CuGa2O4 thick films in various technological fields.

Bandgap formation mechanism in tacticity inspired elastic mechanical metastructures

Tacticity is long known as a significant contributor in changing the chemical and mechanical properties of the polymers drastically. This study explores mechanism of bandgap formation in elastic mechanical metastructures designed with a focus on tacticity. We introduce metabeams, comprising a primary slender beam embedded with short secondary beams featuring end masses at their tips. The investigation delves into the numerically simulated vibration characteristics of metabeams using finite element analysis, with a subsequent comparison to experimental results for fabricated metabeams. Employing a unit-cell design approach that manipulates spatial and physical parameters, we explore a wide range of uniform and non-uniform metabeam configurations based on the distance between secondary beams and distribution of local resonators as per tacticity. Hence, drawing inspiration from tacticity, we extend our investigation to isotactic and syndiotactic metabeams, altering physical parameters (mass) within the unit cell for both configurations. The strategic distribution of end masses on attached secondary beams introduces unique characteristics to isotactic and syndiotactic metabeams, allowing for the modulation of bandgaps without altering the natural frequencies of the resonators in symmetric and anti-symmetric metabeam designs. Our research demonstrates, incorporating tacticity in metabeam design offers a novel and unconventional approach to modulate the bandgap formation mechanism.

Composite metastructure with tunable ultra-wide low-frequency three-dimensional band gaps for vibration and noise control

Low-frequency vibration and noise control present enduring engineering challenges that garner extensive research attention. Despite numerous active and passive control solutions, achieving multiple ultra-wide attenuation regions remains elusive. Addressing vibration and noise control across a multidirectional broad low-frequency spectrum, three-dimensional metastructures have emerged as innovative solutions. This study introduces a novel three-dimensional composite metastructure featuring multiple ultra-wide three-dimensional complete band gaps. The research emphasizes the design strategy of elastic ligaments to achieve multiple ultra-wide attenuation regions spanning from 0.7 to 40 kHz. The band structures are elucidated through modal analysis and further substantiated by an analytical model based on a spring-mass chain with an additional resonator. The underlying physical mechanism for the formation of multiple ultra-wide band gaps is revealed through novel vibration modes from finite element analyses. Furthermore, we demonstrate that the distribution and the relative width of the ultra-wide band gaps can be tuned by modifying the geometric parameters of the metastructure. Utilizing additive manufacturing, prototypes are fabricated, and low-amplitude vibration tests are conducted to evaluate real-time vibration attenuation properties. Consistency is observed among theoretical, numerical, and experimental results. The proposed structure shows significant potential for high-performance meta-devices aimed at controlling noise and vibration across an extremely wide low-frequency spectrum.

Structural defect regulation in corundum-type medium-entropy oxides for enhanced infrared emissivity performance at high temperature

With the increasing demand in industry and hypersonic vehicles, advanced materials with high-performance infrared radiation need to be developed. Herein, two corundum-type medium-entropy oxide ceramics M3 (Al1/3Cr1/3Fe1/3)2O3 and M4 (Al0.25Cr0.25Fe0.25Ga0.25)2O3 were prepared by solid-state reaction method at different temperatures. Their chemical structure and infrared emissivity performance were analyzed through systematic experiments and theoretical analysis. Compared to the M3, M4 prepared at 1773 K possessed more preeminent emissivity performance, exhibiting emissivity values of 0.917, 0.912 and 0.90 in the wavelength of 0.2–0.78 μm, 0.78–2.50 μm and 2.5–14 μm, respectively. Meanwhile, the infrared emissivity of M4 still could exceed 0.85 at 800 °C in the wavelength of 1.5–25 μm, displaying excellent emissivity and stability under high temperature. As a consequence of lower bandgap and defects formation energy, more oxygen vacancy defects were generated in M4, resulting in better infrared emissivity performance. The results demonstrates that M4 as coating has great potential to be applied in energy-related and thermal protection of hypersonic vehicles.

Indirect-to-direct bandgap transition in GaP semiconductors through quantum shell formation on ZnS nanocrystals

Although GaP, a III-V compound semiconductor, has been extensively utilized in the optoelectronic industry for decades as a traditional material, the inherent indirect bandgap nature of GaP limits its efficiency. Here, we demonstrate an indirect-to-direct bandgap transition of GaP through the formation of quantum shells on the surface of ZnS nanocrystals. The ZnS/GaP quantum shell with a reverse-type I heterojunction, consisting of a monolayer-thin GaP shell grown atop a ZnS core, exhibits a record-high photoluminescence quantum yield of 45.4% in the violet emission range (wavelength = 409 nm), validating its direct bandgap nature. Density functional theory calculations further reveal that ZnS nanocrystals, as the growth platform for GaP quantum shells, play a crucial role in the direct bandgap formation through hybridization of electronic states with GaP. These findings suggest potential for achieving direct bandgaps in compounds that are constrained by their inherent indirect energy gaps, offering a strategy for tailoring energy structures to significantly improve efficiencies in optoelectronics and photovoltaics.

Stealthy and hyperuniform isotropic photonic band gap structure in 3D.

In photonic crystals, the propagation of light is governed by their photonic band structure, an ensemble of propagating states grouped into bands, separated by photonic band gaps. Due to discrete symmetries in spatially strictly periodic dielectric structures their photonic band structure is intrinsically anisotropic. However, for many applications, such as manufacturing artificial structural color materials or developing photonic computing devices, but also for the fundamental understanding of light-matter interactions, it is of major interest to seek materials with long range nonperiodic dielectric structures which allow the formation of isotropic photonic band gaps. Here, we report the first ever 3D isotropic photonic band gap for an optimized disordered stealthy hyperuniform structure for microwaves. The transmission spectra are directly compared to a diamond pattern and an amorphous structure with similar node density. The band structure is measured experimentally for all three microwave structures, manufactured by 3D laser printing for metamaterials with refractive index up to . Results agree well with finite-difference-time-domain numerical investigations and a priori calculations of the band gap for the hyperuniform structure: the diamond structure shows gaps but being anisotropic as expected, the stealthy hyperuniform pattern shows an isotropic gap of very similar magnitude, while the amorphous structure does not show a gap at all. Since they are more easily manufactured, prototyping centimeter scaled microwave structures may help optimizing structures in the technologically very interesting region of infrared.

Theoretical investigation on vibration reduction characteristics of a novel foundation metaconcrete beam

The issue of low-frequency vibration problems in foundation beams is becoming increasingly serious. Therefore, it is imperative to find new methods for effectively reducing and controlling these low-frequency vibrations. This study proposes a novel foundation metaconcrete beam to address the challenge of low-frequency vibrations based on locally resonance theory. Additionally, an improved transfer matrix method (ITMM) is proposed to quickly and effectively calculate the bandgap of foundation metaconcrete beam. The validity of the ITMM is verified through the plane wave expansion method (PWEM), and transmission characteristics are fully analyzed using the spectral element method (SEM). Furthermore, the influences of geometric and material parameters of the foundation metaconcrete beam on band structures and transmission functions are investigated in detail. The results show that the proposed foundation metaconcrete beam exhibits multiple bandgaps, and can effectively attenuate low-frequency vibrations. These bandgaps can be tailored by appropriately adjusting relevant parameters. The foundation properties determine the formation of the first bandgap, the damping ratio of the resonator has double effects on band structures, the mass ratio of the resonator is crucial in adjusting these bandgaps, and the axial force can adjust the attenuation capability of the first bandgap.

Competent visible light driven photocatalytic removal of gaseous styrene over TiO2/g-C3N4: Characterization, parameters, kinetics, mechanism

BackgroundRemoval of volatile organic compounds (VOCs), including styrene, via photocatalytic oxidation is preferable. The photocatalytic activity of g-C3N4 and TiO2 are limited by its fast recombination rate and wide band gap, respectively. MethodThis study examined improvement in photocatalytic degradation of styrene using g-C3N4 after doping with various proportions of TiO2. The best photocatalyst were also tested under various conditions (retention time, relative humidity, styrene concentration, and temperature) prior to study of reaction kinetics. Significant findings20 wt% TiO2/g-C3N4 presented the best styrene conversion and supported by characterization result. Formation of narrow band gap reached to 2.68 eV enhanced the performance of doped g-C3N4. Langmuir Hinshelwood model 4 presented a well fitted result with the experimental data.

Investigation of the electronic properties of a topological semimetal LaAuPb under uniaxial strain and hydrostatic pressure: A DFT approach

In this study, we utilized first-principles calculations to investigate the structural, electronic, mechanical, and vibrational properties of the topological semimetal LaAuPb. Our research delves into how uniaxial strain and hydrostatic pressure influence the electronic band structure of LaAuPb. We discovered that LaAuPb exhibits ductile behavior and maintains dynamic stability under various conditions. Upon introducing compressive strains of −6%, we observed a significant separation of degenerate states at the gamma point, which consequently led to the formation of a band gap. Similarly, the application of hydrostatic pressure at 16 GPa also resulted in the creation of a band gap by altering the electronic band structure. These findings suggest that both compressive strain and hydrostatic pressure can be effective in modulating the electronic properties of LaAuPb, potentially making it useful for applications requiring tunable electronic behavior. Conversely, when subject to tensile strain, no band gap formation was observed. Instead, there was a notable gradual overlap between the valence and conduction bands, indicating that tensile strain does not induce the same electronic separations as compressive strain or hydrostatic pressure. This asymmetrical response to different types of strain underscores the complex nature of LaAuPb's electronic properties and provides valuable insights for its potential use in electronic and mechanical applications. Overall, our findings contribute to the understanding of how external mechanical forces can be harnessed to control the electronic properties of topological semimetals like LaAuPb.

Ultra-wide band gap and wave attenuation mechanism of a novel star-shaped chiral metamaterial

A novel hollow star-shaped chiral metamaterial (SCM) is proposed by incorporating chiral structural properties into the standard hollow star-shaped metamaterial, exhibiting a wide band gap over 1 500 Hz. To broaden the band gap, solid single-phase and two-phase SCMs are designed and simulated, which produce two ultra-wide band gaps (approximately 5 116 Hz and 6 027 Hz, respectively). The main reason for the formation of the ultra-wide band gap is that the rotational vibration of the concave star of two novel SCMs drains the energy of an elastic wave. The impacts of the concave angle of a single-phase SCM and the resonator radius of a two-phase SCM on the band gaps are studied. Decreasing the concave angle leads to an increase in the width of the widest band gap, and the width of the widest band gap increases as the resonator radius of the two-phase SCM increases. Additionally, the study on elastic wave propagation characteristics involves analyzing frequency dispersion surfaces, wave propagation directions, group velocities, and phase velocities. Ultimately, the analysis focuses on the transmission properties of finite periodic structures. The solid single-phase SCM achieves a maximum vibration attenuation over 800, while the width of the band gap is smaller than that of the two-phase SCM. Both metamaterials exhibit high vibration attenuation capabilities, which can be used in wideband vibration reduction to satisfy the requirement of ultra-wide frequencies.

A novel Star-4 honeycomb with the inclined ligaments for enhanced tunability of wave propagation behaviors

The dynamic mechanical properties of mechanical metamaterials based on the honeycomb structures are widely concerned, especially the wave propagation behaviors. In this study, a novel type of honeycomb is designed by combing the Star-4 structure with the inclined ligaments. A dynamic analysis model of the Star-4 honeycomb with the inclined ligaments (S4HILs) is established based on the periodic description by the Bloch’s theorem and the discretization by the finite element theory of Timoshenko beam. The band gap characteristics calculated by the proposed model are compared with the frequency responses obtained by the commercial software to provide the modeling validation. The boundary vibration modes of S4HILs are compared to reveal the formation and closure mechanism of band gap. Moreover, the effects of different types of geometrical features on the band gap distribution and the macro-Poisson’s ratio are systematically investigated. The group velocity is finally discussed according to the global dispersion relation to explore the directivity of wave propagation. The results indicate that the inclined ligament can significantly enhance the tunability of wave propagation behaviors, which is mainly reflected in the broadened regulation range of band structure, Poisson’s ratio, and group velocity. This work provides an innovative idea for the regulation of wave propagation behaviors though the connection design between mechanical metamaterial unit cells.

Unique electronic and optical properties of ABBA tetralayer graphene under external electric fields

In this study, we examine the electronic and optical properties of ABBA-stacked tetralayer graphene (4LG), emphasizing its unique characteristics in contrast to its counterparts. The full tight-binding model elucidates the significance of interlayer couplings in the system. We investigate the influence of gate voltage on the system and uncover alterations in the band structure, the emergence of edge states, and the formation of band gaps. The analysis of the density of states reveals the existence of van Hove singularities, which dynamically evolve with the changing gate voltage, resulting in a transition from semimetallic to semiconductor properties. The optical absorption spectrum demonstrates asymmetrical peaks and step-like structures, further affected by a potential difference. Under a finite electric field, optical excitations in gated 4LG reveal triangular isoenergy loops with significant interaction-induced distortions of up to 100 meV. This study anticipates the potential for tuning optical properties in ABBA-stacked 4LG through external electric fields, offering opportunities for experimental exploration.

Bandgap formation in topological metamaterials with spatially modulated resonators

Within the field of elastic metamaterials, topological metamaterials have recently received much attention due to their ability to host topologically robust edge states. Introducing local resonators to these metamaterials also opens the door for many applications such as energy harvesting and reconfigurable metamaterials. However, the interactions between phenomena from local resonance and modulation patterning are currently unknown. This work fills that gap by studying multiple cases of spatially modulated metamaterials with local resonators to reveal the mechanisms behind bandgap formation. Their dispersion relations are determined analytically for infinite chains and validated numerically using eigenvalue analysis. The inverse method is used to determine the imaginary wavenumber components from which each bandgap is characterized by its formation mechanism. The topological nature of the bandgaps is also explored through calculating the Chern number and integrated density of states. The band structures are obtained for various sources of modulation as well as multiple resonator parameters to illustrate how both local resonance and modulation patterning interact together to influence the band structure. Other unique features of these metamaterials are further demonstrated through the mode shapes obtained using the eigenvectors. The results reveal a complex band structure that is highly tunable, and the observations given here can be used to guide designers in choosing resonator parameters and patterning to fit a variety of applications.

Bandgap accuracy and characteristics of fluid-filled periodic pipelines utilizing precise parameters transfer matrix method

The phononic crystal theory provides a novel approach for effectively controlling the bending vibrations in fluid-filled pipelines. This paper innovatively proposes the precise parameters transfer matrix method to investigate the band calculation accuracy and bandgap characteristics of fluid-filled periodic pipelines with various beam types. Firstly, the differential equations for bending vibration of fluid-filled pipelines are established based on deformation and force analysis. The parameters of the system state are precisely represented by the structural form of multiplying the constitutive matrix with the derivative matrix. Combined with Bloch's theorem, the novel precise parameters transfer matrix method for calculating the band structure is proposed. Secondly, the validity of this method is verified through a comparison with finite element simulation results. A detailed analysis is provided regarding the mechanism of bandgap formation and the effect of fluid filling on the band structure. Then, the influence of shear deformation, moment of inertia, and their coupling on the band calculation accuracy for fluid-filled periodic pipelines is studied based on various beam theories. Finally, it delves into the bandgap characteristics of fluid-filled periodic pipelines under different material parameters, structural parameters, and excitation conditions. This research offers valuable insights for the structural design and vibration damping application in fluid-filled periodic pipelines, providing theoretical support for accurately determining their bandgaps.

Family behavior and Dirac bands in armchair nanoribbons with 4–8 defect lines

Bottom-up synthesis from molecular precursors is a powerful route for the creation of novel synthetic carbon-based low-dimensional materials, such as planar carbon lattices. The wealth of conceivable precursor molecules introduces a significant number of degrees-of-freedom for the design of materials with defined physical properties. In this context, a priori knowledge of the electronic, vibrational and optical properties provided by modern ab initio simulation methods can act as a valuable guide for the design of novel synthetic carbon-based building blocks. Using density functional theory, we performed simulations of the electronic properties of armchair-edged graphene nanoribbons (AGNR) with a bisecting 4–8 ring defect line. We show that the electronic structures of the defective nanoribbons of increasing width can be classified into three distinct families of semiconductors, similar to the case of pristine AGNR. In contrast to the latter, we find that every third nanoribbon is a zero-gap semiconductor with Dirac-type crossing of linear bands at the Fermi energy. By employing tight-binding models including interactions up to third-nearest neighbors, we show that the family behavior, the formation of direct and indirect band gaps and of linear band crossings in the defective nanoribbons is rooted in the electronic properties of the individual nanoribbon halves on either side of the defect lines, and can be effectively through introduction of additional ‘interhalf’ coupling terms.

Strain-induced multi-band spin-wave logic gate based on alligator-type magnonic crystal/PZT structure

Here, we report the results of strain-controlled spin-wave propagation regimes in a double-period multiferroic structure. It consists of an alligator-type magnonic crystal with a period of 250 μm and a piezoelectric layer, featuring a periodic counter-pin-type electrode system with a period of 125 μm. Employing microwave measurements, we acquired the transmission and dispersion of spin waves under various external electric field configurations applied to the piezoelectric layer. The formation of bandgaps in the magnon spectrum and the variation of the spin-wave transmission when altering the configurations of the external electric field are demonstrated. A finite element method reveals that the combination of the non-uniformity in the initial internal magnetic field of the magnonic crystal, which is caused by the presence of periodic alligator-type regions, together with elastic deformations, heightens the amplitude of the modulation of the internal magnetic field. Micromagnetic modeling has demonstrated that this modulation enhancement results in the variation of the spin-wave transmission at the frequency of the magnonic bandgap center of the magnonic crystal. The proposed design of the reconfigurable magnonic crystal creates a condition for the nucleation of the spin-wave bandgap, with further enhancement of the spin-wave reflection from the periodic grating induced by strain. We demonstrate the potential use of the proposed device as a multi-band NAND/NXOR spin-wave based logic gate.

Band gap formation in commensurate twisted bilayer graphene/hBN moiré lattices

Band gap formation in commensurate twisted bilayer graphene/hBN moiré lattices

Mesoscale modelling of metaconcrete containing rubber aggregates towards wave attenuation against impact loadings

This paper proposes a novel protective material that enhances the impact resistance of metaconcrete by integrating rubber aggregates. The study delves into the mesoscopic mechanisms of wave attenuation in this material and the regulation of its performance accordingly. It first explores the effect of various design parameters of engineering aggregates on bandgap formation and then further investigates the influence of material viscoelastic properties on its bandgap width. Mesoscale simulations of spalling tests demonstrate that incorporating rubber aggregates can effectively expand the bandgap range, enhance energy absorption, reduce damage to the mortar matrix, and ultimately improve the impact resistance of metaconcrete. Simulation results in the present study highlight the effectiveness of rubber aggregates in strengthening metaconcrete's impact resistance.

Rigid-Elastic Combined Metamaterial Beam With Tunable Band Gaps for Broadband Vibration Suppression

Abstract Extensive research efforts have been dedicated to exploring the application of metamaterial beams for vibration suppression. However, most existing designs primarily focused on utilizing the translational motion of local resonators to create band gaps. To address this limitation of employing solo motion to induce a relatively narrow band gap, this study proposes a novel design: a rigid-elastic combined metamaterial beam utilizing both translational and rotational motions of local resonators. Theoretical framework development involves extending the transfer matrix method to incorporate rigid bodies, with analytical results validated through finite element simulations and experimental data. Compared to conventional metamaterial beams, the proposed design exhibits an additional wide band gap in the low-frequency region that can be utilized for broadband vibration suppression. A parametric study elucidates the influences of geometric parameters on band gap formation, followed by an exploration of the tunability of the proposed meta-beam through a graded scheme and optimization strategy. In particular, a multiple-objective optimization approach is employed to enlarge the vibration suppression region and enhance vibration suppression ability. The optimized meta-beam demonstrates a remarkable 45% wider dominant suppression region and a 14% lower average transmittance compared to a uniform model.