Band Gap Closure Research Articles

Exploring bandgap tuning and vibration isolation performance of innovative negative stiffness metastructures

Negative stiffness metamaterials with tunable bandgap properties present promising applications in vibration control and mechanical property enhancement. This study proposes and compares two metastructures with unit cells exhibiting negative stiffness, achieved through the snap-through behavior of a top beam under displacement. The vibration isolation performance of these models is investigated through theoretical and finite element analyses, with experimental verification of transmission results. Both structures demonstrate significant band gaps, and mode shape analysis of the unit cells reveals the mechanisms underlying bandgap formation. A parametric study examines the effects on bandgap characteristics, including opening and closure, as well as transmission behavior in arrayed configurations. Results indicate that both metamaterial models, with their negative stiffness properties, can create multiple band gaps and achieve excellent vibration isolation, making them strong candidates for vibration isolation applications in engineering structures.

Vibron Softening of Solid Hydrogen under Nanoconfinement.

The vibron behavior of hydrogen bears significant importance for understanding the phases of solid hydrogen under high pressure. In this work, we reveal an unusual high-pressure behavior of hydrogen confined within nanopores through a combination of experimental measurements and theoretical calculations. The nanoconfined hydrogen molecules retain an hcp lattice up to 170 GPa, yet significant deviations from the vibrational characteristics of bulk hydrogen are observed in the primary vibrons of both Raman and infrared spectra. This lowered vibron peak is linked to the disorder of the hydrogen molecules with longer bonds and enhanced intermolecular interactions at the interface. Further investigation reveals that this nanoscale confinement leads to a considerable decrease in the band gap of solid hydrogen, potentially facilitating band gap closure at considerably lower pressures. Our findings provide crucial insights into the behavior of solid hydrogen under spatial nanoconfinement, paving the way for novel explorations into hydrogen metallization.

Electrically Tunable Spin‐Orbit Coupled Photonic Lattice in a Liquid Crystal Microcavity

AbstractA 1D photonic crystal is created with strong polarization dependence and tunable by an applied electric field. This is accomplished in a planar microcavity by embedding a cholesteric liquid crystal (LC), which spontaneously forms a uniform lying helix (ULH). The applied voltage controls the orientation of the LC molecules and, consequently, the strength of a polarization‐dependent periodic potential. It leads to opening or closing of photonic bandgaps in the dispersion of the massive photons in the microcavity. In addition, when the ULH structure possesses a molecular tilt, it induces a spin‐orbit coupling between the lattice bands of different parity. This interband spin‐orbit coupling (ISOC) is analogous to optical activity and can be treated as a synthetic non‐Abelian gauge potential. Finally, it is showed that doping the LC with dyes allows us to achieve lasing that inherits all the above‐mentioned tunable properties of LC microcavity, including dual and circularly‐polarized lasing.

Broken phase of parity-time symmetry enables efficient superluminal pulse transmission

Parity-time (PT) symmetric Bragg gratings (PTBGs) exhibit unique band characteristics compared to their traditional counterparts. Notably, when the PT symmetry is broken, the initial bandgap closes, and the upper and lower branches coalesce. We demonstrate that this believed to be novel band dispersion supports fast light, also known as the optical superluminality. A light pulse can propagate through a fiber PTBG with broken PT symmetry, achieving high transmission efficiency (comparable to, and even exceeding, unity) while maintaining its Gaussian shape. This effect offers a significant advantage over superluminal tunneling, where the transmission coefficient is typically very small. We also analyze the transmission of optical precursors and show that they cannot be superluminal, consistent with the principle of causality. This work presents a mechanism for realizing superluminality with some possible applications and underscores the vast potential of non-Hermitian optics.

A novel approach for observing band gap crossings using the SIMS technique in Pb1−x Sn x Te

This paper introduces a pioneering application of secondary ion mass spectrometry (SIMS) for estimating the electronic properties of Pb1−x Sn x Te, a compound categorized as a topological crystalline insulator. The proposed approach marks the first application of SIMS for such estimations and focuses on investigating variations in ionization probabilities and shifts in the energy distribution of secondary ions. The ionization probabilities are influenced by pivotal parameters such as the material's work function and electron affinity. The derivation of these parameters hinges upon the energy gap's positioning relative to the vacuum level for varying values of within the Pb1−x Sn x Te compound. The findings elucidate noteworthy alterations in SIMS signals, particularly near the critical point of band-gap closing.

Pressure-Dependent Thermal and Mechanical Behaviour of a Molecular Crystal of Bromine.

This study investigates the pressure-dependent thermal and mechanical properties of solid bromine through density functional theory (DFT) calculations used in conjunction with the quasi-harmonic approximation (QHA). At ambient pressure, bromine crystallizes as a molecular crystal of Cmca symmetry. Previous studies have indicated that upon compression, this polymorph should undergo a bandgap closure at 80 GPa followed by a phase transition to a nonmolecular phase at 90 GPa. By employing QHA, we model the lattice vibrations and calculate the free energy, thermal expansion, and specific heat capacities of solid molecular bromine over a temperature range from 0 to 1000 K and pressures up to 90 GPa. Furthermore, mechanical properties such as bulk modulus and elastic constants are also analyzed. The results reveal the significant impact that pressure has on the thermal properties, mechanical stability, and dynamical stability of a molecular crystal. These findings contribute to a deeper understanding of such systems under extreme conditions, potentially guiding future experimental and theoretical investigations.

Nonlinear pendulum metamaterial to realize an ultra-low-frequency field effect bandgap

A field effect transistor (FET) is an extensively employed component in large-scale integrated circuits to regulate circuit on or off by modulating its gate voltage. The mechanical counterpart of a FET is of great significance for advanced regulations of acoustic or elastic waves. Recently proposed active metamaterials are promising scenarios of FET based on the opening and closing of frequency bandgaps through active elements, but hindered by the structural complexity in practice. In this study, we propose and experimentally realize a nonlinear pendulum metamaterial (NPMM) as a mechanical ultra-low-frequency FET. By employing the perturbation method to investigate its dispersion relationship, we show that the NPMM possess a field effect bandgap. Namely, the bandgap can be opened or closed by controlling the excitation amplitude of the impinging wave, which is largely affected by the relative mass of the inner pendulum unit. As a verification, the transmission behavior of the NPMM is numerically studied and experimentally tested. It shows that, once the amplitude of the impinging wave exceeds a certain threshold, the disappearance of a bandgap will be observed with a sudden amplification of the output wave. The central frequency of the field effect bandgap is as low as 4 Hz, whereas the characteristic size of the unit cell is only 4 cm. We expect that the present work opens a new avenue for low-frequency wave regulation.

Pressure-induced photocurrent enhancement and metallization in van der Waals compound SiTe2

Layered van der Waals (vdW) dichalcogenides are distinguished by their unique crystal structures and high structural tunability, rendering them suitable for applications in optics and optoelectronics. Despite significant processes, some fundamental questions remain in two-dimensional (2D) vdW dichalcogenides, such as clarifying detailed structure–property relationship and further improving the optoelectronic performance. Herein, by applying pressure to tune the crystal structure in 2D vdW dichalcogenide SiTe2, we realized a five orders of magnitude boost in photocurrent at 8 GPa. Such an enhancement is attributed to bandgap narrowing and an increased carrier concentration. Furthermore, bandgap closing and metallization were observed at 15.4 GPa, further suggesting the significant change of electronic structure upon compression. This study not only elucidates the intriguing pressure-induced behavior of SiTe2 but also paves the way for harnessing the unique pressure-responsive properties of 2D vdW dichalcogenides in advanced optoelectronic systems.

Twisting Between Topological Phases in 1D Conjugated Polymers via a Multiradical Transition State

AbstractIn recent years, it has become possible via on‐surface bottom‐up synthesis to engineer the topological character of carbon nanostructures. Graphene nanoribbons and 1D conjugated polymers (1DCPs) have thus tailored so as to host either topologically trivial or non‐trivial phases. Molecular design is the primary means to set the topological class of these nanomaterials. However, external control over topology is also demonstrated via electric fields or top‐down hydrogenation. Inspired by the connection between topology and π‐conjugation, here it is demonstrated via first‐principles calculations that aryl ring twist angles also serve as topological knobs. Focusing on rationally designed 1DCPs composed of triarylmethyl (TAM) units, it is shown that rotation of certain aryl rings enables a transition from the trivial to the topologically non‐trivial phase. Accordingly, fixing a particular twist angle configuration (e.g., via chemical functionalization) is equivalent to robustly setting a targeted topological phase. It is also found that in considered 1DCPs, the quantum phase transition occurs without the electronic band gap closing, due to a multiradical antiferromagnetic phase emerging at the transition point. All in all, this study highlights the potential of aryl ring twisting for engineering topological properties in carbon nanomaterials and establishes TAM 1DCPs as exotic topological 1D systems.

Pressure-Induced Irreversible Metallization Accompanying Phase Transition of Chalcopyrite Cu(In0.7Ga0.3)Se2.

Chalcopyrite copper-indium-gallium diselenides (CIGS) have emerged as promising materials with remarkable electronic properties and potential applicability to high-efficiency solar cells. The crystal and electronic structures of CIGS can be continuously tuned from their initial states under pressure. Although pressure-induced band gap closure in CIGS has been predicted in extensive theoretical studies, it has not been supported by experimental evidence. Here, we comprehensively investigate the pressure-dependent optical, electronic, and structural properties of Cu(In0.7Ga0.3)Se2 up to 42.6 GPa. Our experimental results reveal an irreversible electronic transition from the semiconducting to the metallic state at 14.3 GPa. Under compression, the Cu(In0.7Ga0.3)Se2 structure evolves from a tetragonal I4̅2d phase to an orthorhombic Pna21 phase, which has not been previously reported in chalcopyrite. More intriguingly, the Pna21 phase is irreversible and possesses smaller Cu-Se and In/Ga-Se bond lengths and a smaller Cu-Se-Cu bond angle than the I4̅2d phase. Density functional theory calculations indicate a lower enthalpy of the Pna21 phase than that of the I4̅2d phase at pressures above 10.6 GPa. Meanwhile, density of states calculations illustrate that metallization arises from the overlap of the Se p and Cu d orbitals as the bond length reduces. This pressure-induced behavior could facilitate the development of novel devices with various phenomena involving strong coupling of the mechanical, electrical, and optical properties of chalcopyrite.

Topological design of soft substrate acoustic metamaterial for mechanical tuning of sound propagation

The design of tunable acoustic waves is crucial in phononic crystals (PnCs), as acoustic waves exhibit continuous variation across diverse frequency ranges in practical applications. While there have been research efforts on tunable PnCs, existing design strategies predominantly rely on predetermined topology and scatterer shapes based on empirical experience. This reliance poses challenges in ensuring customized and reversible tuning of the bandgap. In this work, we present a customized tunable PnC design model and solution strategy based on soft substrates, which enables reversible tuning of a specific frequency/order bandgap. The designed structure consists of a soft substrate and crystalline columns dispersed in the soft substrate and air. The relative positions of the scatterers in the air are changed by stretching the substrate, thereby realizing the modulation of sound propagation. Since soft materials have both material nonlinearities and complex geometrical variations that are difficult to obtain design sensitivity information, the material-field series expansion topology optimization approach is employed to achieve custom tunable design of the bandgap. The paper presents simulation-based analyses and experimental verification of the customized bandgap opening and closing. The results demonstrate a close alignment between theoretical predictions of propagation curves and experimental findings. This concurrence serves as evidence that the soft-substrate PnC, derived through topology optimization, effectively facilitates the adjustment of bandgaps of arbitrary orders and enables switching between various acoustic functions.

Modulating Carrier Dynamics in PdSe2: The Role of Pressure in Electronic and Phononic Interactions.

PdSe2 is a puckered transition metal dichalcogenide that has been reported to undergo a two-dimensional to three-dimensional structural transition under pressure. Here, we investigated the electronic and phononic evolution of PdSe2 under high pressure using pump-probe spectroscopy. We observed the electronic intraband and interband transitions occurring in the d orbitals of Pd, revealing the disappearance of the Jahn-Teller effect under high pressure. Furthermore, we found that the decay rates of interband recombination and intraband relaxation lifetimes change at 3 and 7 GPa, respectively. First-principles calculations suggest that the bandgap closure slows the decay rate of interband recombination after 3 GPa, while the saturation of phonon-phonon scattering is the main reason for the relatively constant intraband relaxation lifetime. Our work provides a novel perspective for understanding the evolution of the electron and modulation of the carrier dynamics by phonons under pressure.

Band gap shifting of halide perovskite KSrCl3 from ultra-violet to visible region under pressure for photovoltaic applications

This research delves into the influence of hydrostatic pressure on the structural, electronic, and optical characteristics of the cubic halide perovskite KSrCl3. The reduction in interatomic distance caused by pressure has a considerable effect on the unit cell volume and lattice constant of this perovskite. As pressure increases, the electronic band gap closes, transferring from the ultraviolet to the visible spectrum. This phenomenon increases the efficiency of optoelectronic devices by simplifying the transition of electrons from the valence band to the conduction band. In addition, the material becomes more appropriate for use in a variety of optoelectronic applications when the band gap changes from indirect to direct at pressures of about 50 GPa. A comprehensive optical analysis suggests that KSrCl3 holds potential applications in surgical tools, integrated circuits, QLED, OLED, solar cells, waveguides, and materials designed for solar heat reduction. The tolerance factor “t" confirms the stability of the KSrCl3 phase across the applied pressure range, and the formation energy values with negative values show the attained thermodynamic stability.

Topological phases and edge modes of an uneven ladder

Abstract We investigate the topological properties of a two-chain quantum ladder with uneven legs, i.e., the two chains differ in their periods by a factor of 2. Such an uneven ladder presents rich band structures classified by the closure of either direct or indirect bandgaps. It also provides opportunities to explore fundamental concepts concerning band topology and edge modes, including the difference of intracellular and intercellular Zak phases, and the role of the inversion symmetry (IS). We calculate the Zak phases of the two kinds and find excellent agreement with the dipole moment and extra charge accumulation. We also find that configurations with IS feature a pair of degenerate two-side edge modes emerging as the closure of the direct bandgap, while configurations without IS feature one-side edge modes emerging as not only the closure of both direct and indirect bandgaps but also within the band continuum. Furthermore, by projecting to the two sublattices, we find that the effective Bloch Hamiltonian corresponds to that of a generalized Su–Schrieffer–Heeger model or the Rice–Mele model whose hopping amplitudes depend on the quasimomentum. In this way, the topological phases can be efficiently extracted through winding numbers. We propose that uneven ladders can be realized by spin-dependent optical lattices and their rich topological characteristics can be examined by near future experiments.

Band flipping and bandgap closing in a photonic crystal ring and its applications

The size of the bandgap in a photonic crystal ring is typically intuitively considered to monotonically grow as the modulation amplitude of the grating increases, causing increasingly large frequency splittings between the "dielectric" and "air" bands. In contrast, here we report that as the modulation amplitude in a photonic crystal ring increases, the bandgap does not simply increase monotonically. Instead, after the initial increase, the bandgap closes and then reopens again with the two bands flipped in energy. The air and dielectric band edges are degenerate at the bandgap closing point. We demonstrate this behavior experimentally in silicon nitride photonic crystal microrings, where we show that the bandgap is closed to within the linewidth of the optical cavity mode, whose intrinsic quality factor remains unperturbed with a value ≈ 1×106. Moreover, through finite-element simulations, we show that such bandgap closing and band flipping phenomena exist in a variety of photonic crystal rings with varying unit cell geometries and cladding layers. At the bandgap closing point, the two standing wave modes with a degenerate frequency are particularly promising for single-frequency lasing applications. Along this line, we propose a compact self-injection locking scheme that integrates many core functionalities in one photonic crystal ring. Additionally, the single-frequency lasing might be applicable to distributed-feedback (DFB) lasers to increase their manufacturing yield.

Observation of Non-Hermitian Edge Burst Effect in One-Dimensional Photonic Quantum Walk.

Non-Hermitian systems can exhibit unique quantum phases without any Hermitian counterparts. For example, the latest theoretical studies predict a new surprising phenomenon that bulk bands can localize and dissipate prominently at the system boundary, which is dubbed the non-Hermitian edge burst effect. Here we realize a one-dimensional non-Hermitian Su-Schrieffer-Heeger lattice with bulk translation symmetry implemented with a photonic quantum walk. Employing time-resolved single-photon detection to characterize the chiral motion and boundary localization of bulk bands, we determine experimentally that the dynamics underlying the non-Hermitian edge burst effect is due to the interplay of non-Hermitian skin effect and imaginary band gap closing. This new non-Hermitian physical effect deepens our understanding of quantum dynamics in open quantum systems.

Polarization Saturation in Multilayered Interfacial Ferroelectrics.

Van der Waals polytypes of broken inversion and mirror symmetries have been recently shown to exhibit switchable electric polarization even at the ultimate two-layer thin limit. Their out-of-plane polarization has been found to accumulate in a ladder-like fashion with each successive layer, offering 2D building blocks for the bottom-up construction of 3D ferroelectrics. Here, it is demonstrated experimentally that beyond a critical stack thickness, the accumulated polarization in rhombohedral polytypes of molybdenum disulfide saturates. The underlying saturation mechanism, deciphered via density functional theory and self-consistent Poisson-Schrödinger calculations, point to a purely electronic redistribution involving: 1.Polarization-induced bandgap closure that allows for cross-stack charge transfer and the emergence of free surface charge; 2. Reduction of the polarization saturation value, as well as the critical thickness at which it is obtained, by the presence of free carriers. The resilience of polar layered structures to atomic surface reconstruction, which is essentially unavoidable in polar 3D crystals, potentially allows for the design of new devices with mobile surface charges. The findings, which are of general nature, should be accounted for when designing switching and/or conductive devices based on ferroelectric layered materials.

Ab-initio investigation of the structural, electronic and optical properties of CsPbBr3 perovskite under pressure using the DFT-1/2 method

Pressure-induced phase transitions in metal halide perovskites lead to significantly different material properties and offer great potential for diverse applications. The electronic, structural, and optical properties of Cesium lead bromide (CsPbBr3) crystals are investigated using density functional theory (DFT) to draw an integrated picture of orbital morphology, absorption spectra, and band gap values. Spin-orbit coupling and DFT-1/2 correction are used to gain a better insight into electronic properties and band splitting in the cubic phase. Calculations of pressure-induced phase transitions, electronic band gap engineering, and manipulation of optical absorption edges of CsPbBr3 are conducted. In the cubic and tetragonal crystal lattices, a band gap closure and opening occurs demonstrating a Dirac cone upon application and increase of pressure. Indeed, in the tetragonal lattice of CsPbBr3, a topological band occurs. Orthorhombic perovskite crystal lattice undergoes successive direct-indirect and indirect-direct band gap transitions, followed by a space group change at extreme pressures. However, under pressure, orthorhombic non-perovskite lattice experiences a sudden band gap breakdown. Phase and band gap transitions are explained by orbital hybridizations and structural symmetry evolution (e.g., bond length contraction/dilation and octahedral cage rotation/distortion). These results provide comprehensive guidelines for further experimental studies on pressure engineering, advanced photovoltaic and optoelectronic applications, discovering and understanding new topological band structures of perovskite materials.

Electronic signal for mechanical failure in two-dimensional g-SiC

It is non-trivial to identify mechanical failure using first-principles calculations as only long-wave phonons are used in these models due to size limitations. Here, we propose a new criterion to predict the mechanical failure by electronic bandgap closure in graphene-like two-dimensional silicon carbide (g-SiC) monolayer. The electronic bandgap decreases with strain and closes beyond the ultimate strain. This mechano-electronic coupling suggests that the onset of the zero bandgap and the correlation between electronic bandgap and ultimate strain could be used to predict the ideal mechanical failure of g-SiC monolayers.

Edge State, Band Topology, and Time Boundary Effect in the Fine-Grained Categorization of Chern Insulators.

We predict novel topological phases with broken time-reversal symmetry supporting the coexistence of opposite chiral edge states, which are fundamentally different from the photonic spin-Hall, valley-Hall, and higher-order topological phases. We find a fine-grained categorization of Chern insulators, their band topologies characterized by identical Chern numbers are completely different. Furthermore, we prove that different topologies cause zeros in their Bloch wave function overlaps, which imprint the band gap closing and appear at the degenerate points of topological phase transition. The Bloch wave function overlaps predict the reflection and refraction at a topological time boundary, and the overlap zeros ensure the existence of vanishing revival amplitude at critical times even though different topologies before and after the time boundary have identical Chern numbers. Our findings create new opportunities for topological metamaterials, uncover the topological feature hidden in the time boundary effect as a probe of topology, and open a venue for the exploration of the rich physics originating from the long-range couplings.