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Autoencoder artificial neural network for accelerated forward and inverse design of locally resonant acoustic metamaterials
The noise issues brought about by the development of the aviation and other industries have put forward an urgent demand for the design of low-frequency noise reduction structures. An autoencoder artificial neural network (ANN) is established in this paper to achieve accelerated low-cost forward and on demand design of locally resonant metamaterials simultaneously. Inspired by the framework of the autoencoder network, the proposed ANN is composed of an in series connected inverse prediction neural network and a forward prediction neural network module to avoid program errors by multisolution problems. A theoretical model is first set up in the paper to calculate the sound transmission loss (STL) of a locally resonant metamaterial plate and then validated by finite element simulation. The autoencoder ANN is subsequently trained using the dataset constructed based on the theoretical model. The accuracy of the well-trained ANN is then evaluated by making a comparison with the theoretical calculation and originally expected STL curves. The advantages of the proposed ANN over the theoretical model and numerical simulation are analyzed, and the results indicate that the proposed autoencoder ANN takes 2 and 6 orders of magnitude less time to complete the forward design than theoretical and numerical methods. The proposed ANN also demonstrates its ability in inverse design, which is hardly achieved using theoretical and numerical methods. The proposed ANN provides a new design method for accelerated forward and inverse design of noise reduction structures.
Quantitative assessment of physical aging on dynamical heterogeneity of amorphous alloys: Insight from stress relaxation
Probing the dynamical and structural heterogeneity is a critical issue for understanding the mechanical and physical properties of amorphous alloys. Here, the stress relaxation of a (La0.6Ce0.4)65Al10Co25 amorphous alloy is investigated to probe the evolution of dynamical and structural heterogeneity due to aging. The characteristics of amorphous alloys are time-dependent during stress relaxation due to the metastability; thus, the aging effect should be considered by modifying the classical Kohlrausch–Williams–Watts (KWW) function. We find that the fixed-parameter KWW function during prolonged stress relaxation is not valid due to aging. Furthermore, the parameters of the modified KWW function indicate that physical aging leads to an increase in the characteristic time τ and decrease in the stretched exponent βKWW. The latter widens the instantaneous stress relaxation curve toward long-time end. The results demonstrated that the modified KWW function and the corresponding parameters conform to the conventional understanding of amorphous alloys.
Tuning of parity-time symmetry effect through exceptional points in non-Hermitian terahertz metasurface
In this article, we investigate parity-time (PT) symmetry in a non-Hermitian terahertz metasurface composed of two orthogonally oriented split ring resonators. Initially, one resonator is diagonally displaced relative to the other to study PT-symmetry in the system. The eigenvalues and eigenstates of the metasurface are examined at various displacement values to analyze the PT-symmetric phase transition. At a specific displacement, the metasurface design exhibits degenerate eigenvalues, marking the occurrence of an exceptional point. Beyond this point, the system transitions into a broken PT-symmetry phase. PT-symmetry is further explored by displacing the second resonator vertically and horizontally. This investigation reveals the presence of exceptional points during the transition from PT-asymmetry to PT-symmetry states. Additionally, the role of the split gap in the second resonator relative to the first is analyzed in this non-Hermitian system. It is observed that near-field coupling between the resonators plays a significant role during displacement. This is supported by the application of coupled mode theory to the metasurface. This comprehensive study on the impact of resonator displacement leading to exceptional points in a strongly coupled system holds potential for highly sensitive sensing applications and other advancements in photonics.
Terahertz non-Hermitian topological metasurface with switchable characteristics of transmission and reflection
With the increasing exploration of non-Hermitian systems, the topological properties of exceptional points (EPs) in non-Hermitian metasurfaces have garnered significant attention. This paper investigates a VO2-based non-Hermitian topological metasurface within the terahertz (THz) frequency domain. We achieve switching between transmission and reflection by actively controlling the conductivity of vanadium dioxide (VO2), thereby realizing dual-channel EPs simultaneously. Notably, there is a consistent 2π phase accumulation around the EPs, irrespective of the closure path. Furthermore, we demonstrate that EP can also be observed by varying the incidence angle. The examination of dual-channel EPs and their topological properties offers a promising avenue for the development of topological devices and future applications in optical transmission and modulation.
Effect of interface structure on solid-state amorphization of dual-phase Mg alloys
The interface and its structure have a significant impact on the mechanical properties of the magnesium (Mg) alloys. However, the role of the interface in the solid-state amorphization process of the Mg alloys is still unclear. Here, the effect of four interface structures, namely, basal/prismatic (BP), stacking fault (SF), twin boundary (TB), and high-angle grain boundary (HAGB) on the solid-state amorphization (SSA) of the amorphous/crystalline dual-phase Mg alloys is investigated using molecular dynamics simulation. The results indicate that the introduction of all four interfaces increases the SSA degree of the alloys. For the four models, the SSA degree of the alloys varies from high to low in order as the BP model, the TB model, the SF model, and the HAGB model, which means that atomic diffusion has a significant dependence on the interface structure and interface energy. The results show that the interface plays two roles in the SSA process: first, it changes the structure of the amorphous–crystalline interface in contact with the interface and second, the interface with a more open structure itself is a fast channel for atomic diffusion, both of which are beneficial for the SSA of the alloys.
A theoretical and computational model for the synergy of electrostriction and photovoltaic effect to create photostriction
The phenomenon of photostriction, which involves generating mechanical actuation in response to light, is observed in a very limited number of materials from ferroelectric, semiconductors, and organic material classes. This limited choice of materials, combined with their responsiveness to narrow light spectra, has constrained the broader adoption of photostriction in practical applications. This study introduces a novel approach by integrating photovoltaic and electrostrictive couplings in composites made of a photovoltaic matrix with ferroelectric inclusions, yielding an apparent photostrictive effect. By leveraging the simultaneous action of both photovoltaic and electrostrictive effects, the composite efficiently converts irradiated optical energy into mechanical energy, enabling mechanical actuation across a broader range of materials and light spectra. First, we develop a computational framework for intertwined multiphysics coupling of the photovoltaic effect with nonlinear electrostriction, based on a novel constitutive model. Then, to evaluate all the effective properties that define the composite's behavior, we introduce a multiphysics-coupled homogenization framework capable of computing elastic, electrostrictive, dielectric, and thermal properties. Finally, a shell finite element formulation based on the assumptions of first-order shear deformation theory is used to analyze the behavior of the homogenized photo-electrostrictive composite based actuator. This study demonstrates the feasibility of the proposed composite by examining the deflection response of structures laminated with this photostrictive composite. Several case studies are conducted to provide insights into the development and characterization of photo-electrostrictive composites, which hold great potential for applications in mechanical actuation, shape morphing, and beyond.
Electrically tunable metal–semiconductor behavior in 2H-MoTe2
In this study, we report tunable metallic and semiconducting behavior in molybdenum di-telluride (2H-MoTe2) by manipulating the Joule heating process through electrical control of the channel current. At low voltages, 2H-MoTe2 exhibits semiconducting behavior. As the current surpasses a critical threshold at higher voltages, the material transitions to a metallic-like state, confirmed by a positive temperature coefficient of resistance. Temperature- and voltage-dependent Raman studies confirm that this semiconducting to metal-like transition occurs without any accompanying structural phase transformation. This metallic behavior is likely due to enhanced phonon scattering caused by the increase in lattice temperature. In the metallic state, exposure to H2 gas results in a negative response, with increased resistance due to additional phonon scattering. Conversely, laser exposure at this state produces no noticeable photoresponse because the already high lattice temperature limits the impact of further heating. These effects were suppressed when 2H-MoTe2 was placed on a hexagonal boron nitride/multilayer graphene heat sink. This dynamic modulation of conductivity in 2H-MoTe2 through electrical stimuli highlights its potential for nanoelectronic device applications.
Identification of an upper limit to the laser pulse duration in photonic band-edge liquid crystal lasers
Photonic band-edge liquid crystal (LC) lasers are an exciting field of research, offering potential in a range of applications from medical imaging to holographic projection. Much work has been done on improving the performance of LC lasers. However, due to historical limitations in pumping techniques, very little experimental work into the temporal dynamics of LC lasers has been published. In this paper, a laser diode pump source with a variable pulse duration is used to investigate the temporal characteristics of the resultant LC laser pulses, while maintaining a constant ratio of pump pulse energy to LC laser threshold. The existence of an upper limit to the output pulse duration of stimulated emission from an LC laser is presented, with a value of 3.5 (±0.1) ns for a DCM-doped cell and 5.2 (±0.2) ns for a Coumarin504-doped cell, irrespective of the laser diode pump pulse lengths, which exceed these values. Evidence is provided to show that the remainder of the optical energy within the pump pulse results in fluorescence emission. The results are in good agreement with the theory of organic dye dynamics and may provide possible future opportunities for electronic control of laser linewidth and coherence in addition to pump parameter optimization for improved LC laser performance.
Collective excitations in active solids featuring alignment interactions
With increasing emphasis on the study of active solids, the features of these classes of nonequilibrium systems and materials beyond their mere existence shift into focus. One concept of active solids addresses them as active, self-propelled units that are elastically linked to each other. The emergence of orientationally ordered, collectively moving states in such systems has been demonstrated. We here analyze the excitability of such collectively moving elastic states. To this end, we determine corresponding fluctuation spectra. They indicate that collectively excitable modes exist in the migrating solid. Differences arise when compared to those of corresponding passive solids. We provide evidence that the modes of excitation associated with the intrinsic fluctuations are related to corresponding modes of entropy production. Overall, by our investigation, we hope to stimulate future experimental studies that focus on excitations in active solids.
Phonon dynamics in 3D quasicrystals versus amorphous solids
Quasicrystals (QCs) possess a unique long-range quasi-periodic order distinct from both crystalline and amorphous solids. The vibrational properties of QCs remain poorly unexplored. Here, we employed classical molecular dynamics simulations to investigate the phonon dynamics of a 3D single-component icosahedral QC model and a Kob–Andersen (KA) glass model. By comparing the vibrational density of state (VDOS), phonon dispersion, and phonon lifetime, we elucidate the impact of structural order on the phonon dynamics. Although QC shows similar phonon dynamics to amorphous solid rather than crystals, our findings reveal significant differences between the two systems. The VDOS of the QC exhibits a double-peak feature and a “fake” boson peak compared to the KA glass. The phonon dispersion in the QC displays more pronounced elastic anisotropy, with well-defined transverse and longitudinal modes, unlike the more isotropic dispersion in the KA glass. Moreover, the correlation between the transverse phonon linewidth and the structure factor, observed in the KA glass, is absent in the QC. Phonon lifetimes in the QC are generally longer than in the KA glass, with a faster decay rate. This comparative study highlights the distinctive vibrational properties of 3D QCs, emphasizing the role of structural order in determining phonon behaviors in solids.
Evaluating multi-state free energy profiles from splitting probability
Single molecule experiments monitor the structural transitions of biomolecules under a constant mechanical force to study their fold–unfold transitions. The activation barrier for such transitions is obtained by inverting the observed committor, which is the probability that the molecule starting from a given extension reaches the folded state before the unfolded state. This work proposes an analytical model for committor analysis of the multi-state conformational dynamics of a DNA hairpin in a complex cellular environment, within the framework of the generalized Langevin equation using a general asymmetric bistable potential with a power-law frictional memory kernel. We obtained exact analytical expressions for the probability density function, first passage time distribution, and the committor. The results are compared with those obtained from steered molecular dynamics simulation of a three-state DNA hairpin, and earlier experimental data. We investigated the dependence of the committor and the corresponding committor-inverted profiles on the linker stiffness, barrier height, and degree of asymmetry in the bistable potential. This model successfully captures the fold–unfold dynamics, reproducing the multi-state free energy profile with asymmetric energy barriers.
Tuning chiral edge state and spin-filtering in irradiated silicene nanoribbons based on side potentials
Topological edge states are critical for developing low-loss devices, making it essential to understand their properties. This study examines the influence of side potentials on the topological edge states in irradiated zigzag silicene nanoribbons. The potentials consist of three external fields, which collectively facilitate the emergence of modified chiral edge modes and spin-valley-related chiral edge states. The mechanism involves shifting the spin-polarized edge states outside the range of the Fermi level with the side potentials, thereby inducing intriguing topological edge states. Additionally, we propose a quasi-bulk state characterized by exponential decay that differs from conventional bulk modes. Based on these modified edge states, a tunable spin filter utilizing side potentials is proposed. We believe that these findings are essential for future developments in spintronic device design.
The interplay of heterogeneity and product detachment in templated polymer copying
Templated copolymerization, in which information stored in the sequence of a heteropolymer template is copied into another polymer product, is the mechanism behind all known methods of genetic information transfer. A key aspect of templated copolymerization is the eventual detachment of the product from the template. A second key feature of natural biochemical systems is that the template-binding free energies of both correctly matched and incorrect monomers are heterogeneous. Previous work has considered the thermodynamic consequences of detachment and the consequences of heterogeneity for polymerization speed and accuracy, but the interplay of both separation and heterogeneity remains unexplored. In this work, we investigate a minimal model of templated copying that simultaneously incorporates both detachment from behind the leading edge of the growing copy and heterogeneous interactions. We first extend existing coarse-graining methods for models of polymerization to allow for heterogeneous interactions. We then show that heterogeneous copying systems with explicit detachment do not exhibit the subdiffusive behavior observed in the absence of detachment when near equilibrium. Next, we show that heterogeneity in correct monomer interactions tends to result in slower, less accurate copying, while heterogeneity in incorrect monomer interactions tends to result in faster, more accurate copying, due to an increased roughness in the free energy landscape of either correct or incorrect monomer pairs. Finally, we show that heterogeneity can improve on known thermodynamic efficiencies of homogeneous copying, but these increased thermodynamic efficiencies do not always translate to increased efficiencies of information transfer.
High-temperature annealing induced electrical compensation in UID and Sn doped <i>β</i>-Ga2O3 bulk samples: The role of VGa–Sn complexes
By electron paramagnetic resonance (EPR) and photoluminescence spectroscopy, we have investigated the effect of high-temperature annealing under oxygen atmosphere on the electrical and defect properties of unintentionally doped (UID) and highly doped (Sn) n-type bulk samples of β-Ga2O3. The EPR analysis of the shallow donor concentration shows efficient electrical compensation in the Sn doped β-Ga2O3 samples but only marginal changes for the UID samples. In the Sn doped samples, we observe the formation of a Ga vacancy related acceptor defect responsible for the compensation. Its spin Hamiltonian parameters are electron spin S = 1/2, g-tensor g11 = 2.0423, g22 = 2.0160, g33 = 2.0024, and hyperfine interaction (hf) with two equivalent Ga atoms with A(69Ga) = 28 G. To identify its microscopic structure, we have performed first-principles calculations of the EPR parameters and the associated photoluminescence spectra of different Ga vacancy–Sn donor complexes, including a simple nearest neighbor pair VGa–SnGa. From these calculations, we attribute this VGa defect to a negatively charged split vacancy complex VGa1–Snib–VGa1. This VGa defect is different from the irradiation induced VGa center.
Joint approximate diagonalization approach to quasiparticle self-consistent <i>GW</i> calculations
We introduce an alternative route to quasiparticle self-consistent GW calculations (qsGW) on the basis of a joint approximate diagonalization of the one-body GW Green’s functions G(εnQP) taken at the input quasiparticle energies. Such an approach allows working with the full dynamical self-energy, without approximating the latter by a symmetrized static form as in the standard qsGW scheme. Calculations on the GW100 molecular test set lead, nevertheless, to a good agreement, at the 60 meV mean-absolute-error accuracy on the ionization potential, with respect to the conventional qsGW approach. We show further that constructing the density matrix from the full Green’s function as in the fully self-consistent scGW scheme, and not from the occupied quasiparticle one-body orbitals, allows obtaining a scheme intermediate between the qsGW and scGW approaches, closer to coupled-cluster reference values.
Shock-induced chemistry and high strain-rate viscoelastic behavior of a phenolic polymer
We use impact experiments and a finite element model (up to 1.2 GPa), and molecular dynamics simulations (up to 60 GPa), to examine the behavior of a phenolic polymer under shock compression, spanning both nonreactive and reactive regimes. In the nonreactive regime, relaxation following compression at strain rates of ∼105 s−1 can be explained by viscoelasticity observed at ordinary laboratory rates (≲1 s−1) by accounting for the temperature dependence of the phenolic β-transition. Reasonable agreement is found between the measured shock Hugoniot up to 1.2 GPa and molecular dynamics simulation for cross-linked structures of comparable density. We also observed a first-order mechanical transition near 0.36 GPa shock stress and estimated a spall strength of 0.102 GPa and Hugoniot elastic limit of 1–2 GPa. The shock stress is found to vary up to 24% among phenolics made with different resin and/or cure processes. Finally, molecular dynamics simulations are used to identify a reactive regime at shock pressures ≳20 GPa that is characterized by chemically driven, rate-dependent relaxation processes, including dehydrogenation and dehydration reactions that promote the formation of a dense, highly cross-linked carbonaceous solid and the release of light volatiles.
Bridging scales in chromatin organization: Computational models of loop formation and their implications for genome function
Chromatin loop formation plays a crucial role in 3D genome interactions, with misfolding potentially leading to irregular gene expression and various diseases. While experimental tools such as Hi-C have advanced our understanding of genome interactions, the biophysical principles underlying chromatin loop formation remain elusive. This review examines computational approaches to chromatin folding, focusing on polymer models that elucidate chromatin loop mechanics. We discuss three key models: (1) the multi-loop-subcompartment model, which investigates the structural effects of loops on chromatin conformation; (2) the strings and binders switch model, capturing thermodynamic chromatin aggregation; and (3) the loop extrusion model, revealing the role of structural maintenance of chromosome complexes. In addition, we explore advanced models that address chromatin clustering heterogeneity in biological processes and disease progression. The review concludes with an outlook on open questions and current trends in chromatin loop formation and genome interactions, emphasizing the physical and computational challenges in the field.
Investigating the effect of termination capacitor on E–H mode transition in radio frequency inductively coupled plasma
In this work, the effects of stringing termination capacitors on the external circuit parameters, plasma parameters, and mode transition in radio frequency (RF) inductively coupled Ar discharges are investigated. It has been demonstrated that at low pressure (1 Pa), in the absence of termination capacitors, the plasma parameters and external circuit parameters exhibit a continuous variation with increasing RF power. The plasma density is observed to decrease with decreasing capacitance value in the E mode when the termination capacitor is inserted, while the plasma density is increased with decreasing capacitance value in the H mode. During the E–H mode transition process, both the plasma parameters and the external circuit parameters undergo a discontinuous change characterized by a distinct “jump” in each parameter. By increasing and then decreasing RF power, the evolution of each parameter creates a significant hysteresis. As the termination capacitance decreases, the power threshold of the H–E mode transition decreases, resulting in a larger hysteresis loop. The termination capacitor, which is connected in series at the end of the coil, can alter the voltage distribution on the RF antenna. This alteration results in a reduction in the potential difference between the coil and the “common ground,” which effectively diminishes the electrostatic field. Furthermore, the electron energy probability function indicates that the addition of the termination capacitor results in a reduction in the proportion of energetic electrons in the E mode, accompanied by a reduction in the plasma potential.
Prediction of induced fluxes in reverse nonequilibrium molecular dynamics
Reverse nonequilibrium molecular dynamics (RNEMD) simulations impose a flux by swapping the velocities of two particles. This method allows for the calculation of transport coefficients, such as thermal conductivity and viscosity. The relation between the induced fluxes and the control parameters of RNEMD (such as the time interval between successive swap events) is not clear. Thus, trial-and-error is required to realize the desired fluxes in RNEMD simulations. In this study, we develop a theoretical framework using extreme value statistics to estimate the relation between the time interval and the resulting induced fluxes. Our RNEMD simulations, conducted with varying time intervals, confirm that the theoretical predictions are quantitatively consistent with the simulation results when the time interval exceeds the momentum relaxation time. Our RNEMD simulations also show that our theoretical predictions, which are valid for a large number of particles for swap candidates, work well even for a relatively small number of particles for swap candidates. These findings demonstrate that the induced fluxes can be reliably estimated, providing a valuable tool for selecting appropriate RNEMD parameters for simulations.
Controlling the Mott–Peierls transition in epitaxial VO2 (M1) film grown by PLD for near-IR photodetection
Vanadium dioxide (VO2) (M1) exhibits a unique metal–insulator transition (MIT) near room temperature, garnering considerable attention for its applications in bolometer, terahertz/infrared detectors, and microelectronic devices. Here, we explore the potential of epitaxially grown VO2 (M1) thin films for near-infrared (IR) detection by optimizing the growth conditions, followed by structural characterization and device fabrication. Alongside the VO2 (M1) phase, two other oxides from the vanadium oxide family, VO2 (A) and V2O5, were also grown on a c-cut sapphire substrate using a pulsed laser deposition (PLD) system. In-depth analysis using temperature-dependent XRD and Raman spectroscopy confirmed the crystalline structure and the quality of epitaxial thin film formation of VO2 (M1), while also unveiling structural phase transition (SPT) behavior and the critical temperature of transition. At elevated temperatures during electrical measurement, the VO2 (M1) epilayer exhibits a first-order phase transition from the metallic to the insulating state, accompanied by a significant change in resistance exceeding three orders of magnitude unveiling its potential in thermal switches, memory-based devices etc. In depth, electrical analysis on all the grown oxides shows that VO2 (M1) and V2O5 exhibit a higher temperature coefficient of resistance (TCR) (3%/K and 2%/K) and a lower 1/f noise (in the order pA/Hz at 0.1 Hz) as compared to VO2 (A), paving scope for further analysis of these two oxides toward important applications in the domain of thermal sensors. Additionally, VO2 (M1) exhibited good bolometric response (in the order of ms) to IR radiation, proving its candidature for the application in IR detectors as well.