Abstract The concept of photoacoustic (PA) group velocity (v g ) is largely missed in the related transport theory. Generally it is assumed that the PA signal propagation occurs at a constant velocity, and thus overlooking the attenuation and dispersion effects. In practice, the PA signals propagate as material waves coupled to the sample, and thus inherently experiencing wave dispersion. In this work, such limitation is bypassed using a modified wave-function formulation in which both amplitude attenuation and wave dispersion arise naturally from a Heaviside-type (Cattaneo) transport equation operating on the velocity potential. Thus the approach, is valid for homogeneous media, characterised by a frequency-dependent PA absorption coefficient and an explicit expression for the group velocity. The resulting model accurately predicts the experimentally observed amplitude decay, bandwidth reduction, and peak-frequency shifts of PA spectra as functions of propagation distance. Furthermore, it is consistent with local thermal equilibrium and energy conservation requirements, and provides the necessary framework to satisfy Biot’s group-velocity theorem and, consequently, the macroscopic Debye kinetic theory. This analysis also supports the interpretation of the PA signal as a dispersive wave packet propagating with an average group velocity, whose velocity distribution is governed by its spectral content.

Abstract The two-dimensional (2D) semiconductor AlP 3 single-layer having similar structure to graphene exhibits higher thermoelectric (TE) power factor and lower lattice thermal conductivity, which is expected to be an excellent TE material. To further improve its TE performance, we obtained stable AlAsP 2 and AlAs 2 P single-layers by doping As elements in AlP 3 single-layer. By substitute arsenic atoms for phosphorus atoms, the TE conversion efficiency has been largely enhanced. Using first-principles calculation combined with the nonequilibrium Green function (NEGF) method and Landauer-Buttiker theory, we studied the electronic structures, the TE parameters and the anisotropy behavior of 2D AlAs n P 3-n (n = 0,1,2) single-layer. The calculation results showed that along the armchair direction the TE performance is superior to that along the zigzag direction, and the ZT values of AlAs n P 3-n (n = 0,1,2) single-layer by p-doping are larger than those by n-type doping. At 300 K, the armchair direction of AlAs 2 P single-layer has the highest ZT value 2.94. The doping of As atoms lead to a lower lattice thermal conductivity. This is because that the lower truncation frequency suppresses and softens the phonon mode, reducing the phonon group velocity. This study predicts potential application of AlAs n P 3-n (n = 0,1,2) single-layers in the field of TE conversion and provides a significant reference for the design of novel 2D TE devices.

This paper presents the formulation of two-dimensional (2D) square elastic lattices consisting of point-like material particles and incorporating central and angular (non-central) short- and long-range interactions of arbitrary order p . Each particle is assumed to interact with all other particles of the discrete medium along arbitrary directions. At the first-order asymptotic limit, these lattices converge to simple linear elastic continua. The lattice parameters are calibrated such that the asymptotic continuum behaves as a homogeneous, linear elastic, isotropic medium with a free Poisson’s ratio under both plane stress and plane strain conditions. When higher-order terms are retained in the asymptotic expansion, the discrete medium is shown to correspond to higher-order gradient elasticity or, equivalently, at the desired order, to a nonlocal elastic continuum. The exact wave dispersion properties of the generalized lattice with long-range interactions are investigated, with particular emphasis on the role of the discrete kernel. It is demonstrated that, for kernels with monotonically decreasing influence functions, the wave dispersion curves in the first Brillouin zone are monotonic for any interaction order p . We give a first proof from the analytical determination of the roots of the group velocity function, up to p = 5 interactions. Another proof is presented, whatever the number p of interactions, by expressing this gradient function through the Dirichlet kernel. Finally, the paper provides a calibration of nonlocal elasticity models to reproduce the wave dispersion characteristics of the higher-order lattice with long-range interactions. For both isotropic and anisotropic nonlocal models, the characteristic length scales are shown to depend on the interaction order and the shape of the discrete kernel.

Topological insulator originally found in electronic systems has inspired an analogue in phononic crystals, which has revolutionized fundamental concepts of elastic and acoustic transmission, offering one-way propagation edge modes immune to backscattering. Nevertheless, for traditional topological phononic crystal (TPC), once the structure is made, only a particular bandgap frequency can occur. Realization of reconfigurability and frequency tuning of topological one-way transmission is challenging. To solve the problem, the TPC is made active on penetration with thermosensitive acoustic hydrogels to produce a reconfigurable bandgap of acoustic wave. We explore that at a certain frequency range, the topological edge state (TES) within the TPC can be reversibly turned on/off as changing the state of (vinyl alcohol) poly-N-isopropylacrylamide (PVA-PNI-PAm) hydrogel between hydrophilic and hydrophobic. In addition, the topology of the TPC does not vary as the TES was tuned by varying the sound group velocity of PVA-PNI-PAm rather than the structural geometry. Such a feature enables a continuous tuning of the central frequency of phononic bandgap and TES by gradually changing the state of PVA-PNI-Pam. The state transition of PVA-PNI-Pam thermal-acoustic hydrogel from hydrophilic to hydrophobic and vice versa has been experimentally realized. Our proposed proof of concept may provide a platform for intelligent acoustic devices with dynamically programmable and reconfigurable functionalities, leading to various potential applications in acoustic wave guiding and control, integrated acoustics, acoustic security, and information processing etc.

The structural vibration of industrial droplet dispensers can be modeled by telegraph-like equations to a good approximation. We reinterpret the telegraph equation from the standpoint of an electric–circuit system consisting of an inductor and a resistor, which is in interaction with an environment, say, a substrate. This interaction takes place through a capacitor and a shunt resistor. Such interactions serve as leakage. We have performed an analytical investigation of the frequency dispersion of telegraph equations over an unbounded one-dimensional domain. By varying newly identified key parameters, we have not only recovered the well-known characteristics but also discovered crossover phenomena regarding phase and group velocities. We have examined frequency responses of the electric circuit underlying telegraph equations, thereby confirming the role as low-pass filters. By identifying a set of physically meaningful reduced cases, we have laid the foundations on which we could further explore wave propagations over a finite domain with appropriate side conditions.

With the natural abundance, low cost, and compatibility with sustainable technologies, Mg3Sb2 has emerged as a promising mid-temperature thermoelectric material. Intrinsic point defects, particularly vacancies, are common in Mg3Sb2 and play a crucial role in shaping its thermoelectric properties, guiding experimental design and performance optimization. However, their impact on lattice thermal conductivity (κL) remains insufficiently understood. This work investigates the effects of Mg and Sb vacancies on the κL of Mg3Sb2 using a neural network potential (NNP). Our results show that both types of vacancies significantly reduce κL, primarily due to enhanced phonon-defect scattering. Comprehensive analyses of phonon dispersion, group velocities, mean square displacement (MSD), the phonon participation ratio (PPR), and elastic properties demonstrate that vacancies trigger pronounced phonon softening, slow down phonon transport, and promote strong localization, while simultaneously amplifying atomic vibrations and weakening interatomic bonding. This work clarifies the microscopic mechanisms by which point defects affect phonon transport and identifies defect engineering as an effective strategy for controlling thermal properties in thermoelectric materials.

Cavity optomechanical systems have become a topic of great interest in recent years, and the coupled-cavity model is also a classic theoretical framework. This paper aims to construct a coupledcavity optomechanical system to study induced transparency, Fano resonance, and fast-slow light effects in such a system. By transferring phenomena typically studied in a single optical cavity to a coupled-cavity system, we analyze specific phenomena detected in optical and microwave cavities, such as transmission and absorption spectra, to investigate induced transparency. We also examine Fano resonance in the model by varying detuning, and study fast-slow light effects through group velocity. This paper first constructs the corresponding physical model, as shown in Figure 1. Based on the theoretical model, a reasonable Hamiltonian is proposed. By introducing appropriate dissipation and fluctuation noise terms, the Langevin equations of motion are derived. Next, the Langevin equations are linearized, and the resonant terms are retained to obtain <i>O</i><sub>+</sub> . The amplitude of the field modes is then derived using the input-output relations. Following the experimental data from referenced literature, a numerical simulation program is implemented in Mathematica. By substituting the relevant parameters and performing calculations, the results are obtained through simulation. For the first time, the interactions among photons, magnons, microwaves, and phonons— as well as the interplay between photons in the two cavities—are investigated in a coupled cavity optomagnomechanical system. Electromagnetically induced transparency (EIT), Fano resonance, and fast-slow light effects are studied in this coupled-cavity optomagnomechanical framework. Phenomena typically examined in a single optical cavity are extended to the coupled-cavity system, with specific observations analyzed separately in the optical and microwave cavities. When <i>δ</i>=<i>ω</i><i><sub>b</sub></i>, the absorption spectrum splits, and the absorption peak decreases from its maximum to its minimum. This phenomenon arises from the disruption of quantum interference effects. The resonance condition suppresses the generation of Fano resonance. At the resonant frequency <i>ω</i><i><sub>0</sub></i>, the group delay is greater than zero, indicating slow-light propagation, and this effect is enhanced with increasing coupling strength. Additionally, a group delay of τ is achieved. Meanwhile, on either side of the resonant frequency, the group delay peaks exhibit a decreasing positive value and an increasing negative value, respectively, signifying a gradual weakening of the slow-light effect and a corresponding enhancement of the fast-light effect. This paper investigates the MIT, MMIT, and OMIT windows in a coupled-cavity optomagnomechanical (OMM) system under a strong control field and weak probe field. The MMIT phenomenon is observed through nonlinear phonon-magnon interactions. Additionally, the photon-magnon interaction in the microwave cavity leads to MIT, while OMIT is achieved via the radiation pressure interaction between photons and nonlinear phonons in the optical cavity. The frequency of the probe field is tuned to interact with both the microwave and optical cavities. When the probe field couples with the microwave cavity, its absorption at the resonant frequency is significantly suppressed under optomechanical coupling, resulting in a pronounced optical switching effect on transmission. We analyze the asymmetric Fano resonance phenomenon, which reflects the existence of quantum interference mechanisms within the system and influences the fast- and slow-light conversion processes. Furthermore, by selecting appropriate coupling parameters, not only can the fast- and slow-light effects be enhanced, but dynamic switching between them can also be achieved.

Measurement of dynamic stress using linear and nonlinear acoustoelastic effects due to colinear wave mixing.

Mid-infrared quantum light sources hold broad application prospects in fields such as gas sensing and infrared thermal imaging. However, currently used mid-infrared quantum entanglement light sources primarily rely on bulk periodically poled lithium niobate (PPLN) crystals, which suffer from limitations in both brightness and integration. This paper proposes a theoretical scheme based on lithium niobate thin films utilizing a 1556.9 nm pump to generate entangled photon pairs with a central wavelength of 3113.8 nm. Through optimized waveguide structure and periodic polarization design, Type-II phase matching and group velocity matching are achieved. This enables transverse electric (TE)-polarized pump input to downconvert to generate photon pairs with TE and transverse magnetic (TM) polarization. Furthermore, by combining a domain arrangement algorithm for customized design of the PPLN waveguide’s polarization direction, precise phase matching is achieved, yielding a quantum light source with a purity as high as 0.999 and a brightness reaching 6.18 × 10<sup>6</sup> cps/mW, representing a three-order-of-magnitude enhancement over bulk PPLN crystal sources. This work offers a promising solution for realizing high-brightness, high-purity on-chip quantum light sources in the mid-infrared band.

Abstract Inspired by the structure of natural spider webs, a bio-inspired spider web metamaterial is proposed in this study, which closely replicates the structural features of spider webs. Based on the physical properties of natural spider web structures, the material configuration of the overall structure was investigated, achieving not only high toughness (elasticity) but also ensuring high strength and lightweight characteristics of the fundamental framework. The optimized unit cell can open eight band gaps within the 0–600 Hz range, with the first band gap occurring as low as 57–79 Hz. Acceleration response results and vibration power transmission curves confirm that the finite-period spider web metamaterial effectively controls the propagation of elastic waves within the band gap frequency ranges. Notably, the spiderweb metamaterial demonstrates comprehensive multimodal capabilities across the full frequency range, showcasing its robust performance in controlling coupled elastic waves under complex operational scenarios. Based on the dispersion relation of the unit cell, the group velocity and phase velocity of the spider-web metamaterial under ideal conditions were further calculated. The results indicate that the spider-web metamaterial possesses outstanding waveguiding properties across multiple frequency bands. The effects of controlled damage defects on the band structure of the spider-web metamaterial were also examined. The material demonstrates exceptional defect tolerance, retaining superior band gap characteristics even in defective conditions. This research offers a groundbreaking design philosophy for biomimetic metamaterials and low-frequency broadband vibration mitigation.

The ability to tune thermal transport through strain engineering offers transformative potential for advanced nanodevices, yet the impact of deep elastic strains (>5%) remains largely unexplored due to challenges in experimental implementation. Here we address this gap by developing a MEMS-based platform to probe strain-thermal transport coupling in suspended silicon nanowires. Through applying uniaxial tensile strains up to 5.65%, we observed three distinct regimes: thermal conductivity remains stable below 1% strain, shows slight enhancement before ∼3% strain, then undergoes a dramatic 55% suppression at 5.65% strain, the largest reversible modulation reported in silicon nanostructures. First-principles calculations reveal that the complex interplay between strain-induced phonon group velocity enhancement, scattering rates, and phonon-phonon interactions modulation is the main contributing factor to the nonmonotonic behaviors. This work establishes deep elastic strain as a powerful knob for dynamically controlling thermal transport, with immediate implications for adaptive thermal management in nanoelectronics and high-efficiency thermoelectrics.

In this work, we analytically derive a semi-classical equation of motion describing the Zitterbewegung effects arising in the dynamics of wavepackets in non-Hermitian systems. In Hermitian non-relativistic quantum systems, the Zitterbewegung effects can arise due to the spin precession and spin–orbit coupling. Interestingly, the spin dynamics in non-Hermitian systems is qualitatively different because of the effective nonlinear terms induced by the non-Hermitian part of the Hamiltonian. In this work, we show the effects from the non-Hermitian spin dynamics by generalizing the description of Zitterbewegung effects to non-Hermitian systems. We also uncover non-Hermitian correction to the group velocity, which can be expressed in terms of the non-Hermitian quantum metric tensor in the absence of out-of-plane effective field.

ABSTRACT Laser ultrasonic testing is an advanced non-destructive testing technology for defect detection. However, the generation and propagation of laser ultrasonic guided wave (LUGW) is influenced by surface roughness, causing an impact on defect detection. To study the influence of surface roughness on LUGW propagation and defect detection, numerical simulation models are established for aluminum alloy plates. The simulation results show that the group velocity of A0 and S0 modes gradually decreases with increasing surface roughness, the amplitude of the S0 mode decreases in the roughness range of 0 μm to 3.2 μm, and the amplitude of the A0 mode decreases in the roughness range of 0 μm to 12.8 μm, and then increases. To verify that the established numerical simulation model can effectively predict the effect of surface roughness on LUGW propagation. A laser ultrasonic experimental detection system was built, and the scanning experiments were conducted on aluminum alloy specimens. The surface defects were imaged by the time-domain synthetic aperture focusing technique (T-SAFT). The experimental results show that the group velocity and amplitude of the A0 mode decrease with increasing surface roughness in the range of 0.61 μm to 3.82 μm, leading to reduced detection and localization accuracy.

Engineering structures in reservoir regions are often exposed to seismic risks, where surface waves such as Love waves can induce severe structural damage. This study presents a theoretical investigation of Love waves scattering in a double-porous composite structure with surface irregularity, a configuration representative of layered reservoir rocks. Employing the first-order perturbation technique, generalized expressions for the dispersion relation, phase velocity and group velocity are derived. The reflected displacement is also deduced for different irregularity geometries. The double-porosity framework, incorporating both storage and transport pores, provides a more realistic characterization of fluid-saturated geological media. Verification against existing results and limiting cases demonstrates excellent agreement, confirming the validity of the formulation. The analysis reveals that surface irregularity and double porosity strongly affect the dispersion behavior and reflection of scattered Love waves. These findings offer new insights into seismic response prediction, subsurface characterization, and the design of stable engineering structures in complex geological structures.

Three-wave scattering is a fascinating phenomenon with many applications in various technologies. Reducing the system symmetry greatly affects three-wave scattering, which, in this case, goes beyond the simple momentum conservation law. In this study, we examine three-magnon scattering at the edge of a thin ferromagnetic film, when a bulk spin wave interacts with an edge-localized propagating spin wave upon the reflection. This creates new bulk spin waves at mixed frequencies by means of three-magnon confluence or stimulated splitting processes. Using our developed analytical theory, which has been confirmed by full micromagnetic simulations, we demonstrate that the amplitude of the wave generated in the stimulated splitting process is several times larger than that generated in the confluence process. This is primarily due to the lower group velocity of the wave formed in the splitting process than in confluence. Furthermore, the intensity of inelastically scattered waves exhibits a pronounced dependence on the incidence angle and frequency of the edge spin wave that goes beyond existing qualitative models. We show that the observed behaviors can only be explained by taking into account that the scattered waves are created by several elementary three-magnon processes involving the incident and reflected waves. The complex nature of the scattered wave creation results in a strong sensitivity of its amplitude to the phase accumulation of spin waves upon reflection.

Abstract Threaded pipe structures are critical components in equipment used in various fields. The threaded sections are prone to induce defects due to stress concentration, threatening the safe operation of the equipment. Consequently, the inspection of these structures is essential. While the ultrasonic guided wave (UGW) method has been applied to inspect threaded pipes, the influence of thread parameters on UGW propagation characteristics is yet to be examined. This study analyzed the dispersion characteristics of UGWs. It reveals that the group velocity dispersion curve of the L(0,2) mode initially increases and then decreases over frequency range of 60–140 kHz. The group velocity is higher in trapezoidal threads than in rectangular threads. Dispersion curves for different thread heights exhibit a crossover near 85 kHz. Dispersion curves for different pitches intersect around 83 kHz. The effect of thread parameter variations on the reflection characteristics was investigated. It was found that the trapezoidal thread exhibits a higher reflection coefficient than the rectangular thread. The reflection coefficient increases with thread height and decreases with pitch. The influence of thread parameters on defect detection sensitivity was examined. Results demonstrate that the L(0,2) mode offers high sensitivity for defect detection in threaded pipes featuring trapezoidal threads, a thread height of 1 mm, and a pitch of 4.5 mm. Finally, the effectiveness of the L(0,2) mode in detecting defects of varying depths within threaded pipes was validated. This research provides a novel method for the inspection of threaded pipe structures.

Spatiotemporal (ST) wave packets constitute a broad class of optical pulses whose spatial and temporal degrees of freedom cannot be treated independently. Such space-time non-separability can induce exotic physical effects such as non-diffraction, non-transverse waves, and sub or superluminal propagation. Here, a higher-order generalised family of ST modes is presented, where modal orders are proposed to enrich their ST structural complexity, analogous to spatial higher-order Gaussian modes. This framework also incorporates spatial eigenmodes and typical ST pulses (e.g., toroidal light pulses) as elementary members. The modal orders are strongly coupled to the Gouy phase, which can unveil anomalous ST Gouy-phase dynamics, including ultrafast cycle-switching evolution, ST self-healing, and sub/super-luminal propagation. We further introduce a stretch parameter that stretches the temporal envelope while keeping the Gouy-phase coefficient unchanged. This stretch invariance decouples pulse duration from modal order, allowing us to tune the few-cycle width without shifting temporal-revival positions or altering the phase/group-velocity laws. Moreover, an approach to analysing the phase velocity and group velocity of the higher-order ST modes is proposed to quantitatively characterise the sub/super-luminal effects. The method is universal for a larger group of complex structured pulses, laying the basis for both fundamental physics and advanced applications in ultrafast optics and structured light.

Antiferromagnetic magnons with intrinsic handedness provide a unique degree of freedom enabling spin-selective control, as left-handed (LH) and right-handed (RH) magnons carry opposite spin angular momenta. While previous studies have mainly focused on antiferromagnetic resonance modes, the coherent transport of nondegenerate handed magnons has remained unexplored in antiferromagnets. In this work, the excitation and detection of LH and RH propagating magnons in the easy-axis antiferromagnet α-Fe2O3 is demonstrated. Below the Morin transition temperature (TM), an external magnetic field along the easy axis lifts the degeneracy between two modes without altering their group velocities, establishing field-insensitive transport as a distinct characteristic of antiferromagnetic magnons. In addition, the Dzyaloshinskii-Moriya interaction and magnetic anisotropy strongly modulate the frequency and group velocity, as well as the amplitude and damping of LH and RH magnons, particularly near TM. These results identify internal magnetic interactions as key factors controlling handed magnon propagation. The findings advance the fundamental understanding of handed magnon dynamics and open new pathways for spin-resolved, energy-efficient, and ultrafast magnonic devices.

Abstract Our present contribution sets out to investigate a scenario based on the effects of the Loop Quantum Gravity (LQG) on the electromagnetic sector of the Standard Model of Fundamental Interactions and Particle Physics (SM). Starting then from a post-Maxwellian version of Electromagnetism that includes LQG effects, we work out and discuss the influence of LQG parameters on classical quantities, such as the components of the stress-tensor. Furthermore, we inspect the propagation of electromagnetic waves and study optical properties of the QED vacuum in this scenario. Among these, we contemplate the combined effect between the LQG parameters and a homogeneous background magnetic field on the propagation of electromagnetic waves, considering in detail issues like group velocities and refractive indices of the QED vacuum. Finally, with the help of the LQG-extended photonic dispersion relations previously analyzed, we re-discuss the kinematics of the Compton effect and conclude that there emerges an interesting nonlinear profile in the wavelengths of both the incoming and the deflected photons. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

We investigate a short-wave instability mode recently identified via temporal stability analysis in weakly inclined falling liquid films sheared by a confined turbulent counter-current gas flow (Ishimura et al. J. Fluid Mech. vol. 971, 2023, p. A37). We perform spatio-temporal linear stability calculations based on the Navier–Stokes equations in the liquid film and the Reynolds-averaged Navier–Stokes equations in the gas, and compare these with our own experiments. We find that the short-wave instability mode is always upward-convective. The range of unstable group velocities is very wide and largely coincides with negative values of the wave velocity. Turbulence affects this mode both through the level of gas shear stress imparted and via the shape of the primary-flow gas velocity profile. Beyond a critical value of the counter-current gas flow rate, the short-wave mode merges with the long-wave Kapitza instability mode. The thus obtained merged mode is unstable for group velocities spanning from large negative to large positive values, i.e. it is absolute. The onset of the short-wave mode is precipitated by decreasing the channel height and inclination angle, and by increasing the liquid Reynolds number or the gas-to-liquid dynamic viscosity ratio. For vertically falling liquid films, merging occurs before the short-wave mode can become unstable on its own. Nonetheless, the ability to generate upward-travelling ripples is endowed to the merged mode. Preliminary calculations neglecting the linear perturbation of the turbulent viscosity suggest that three-dimensional perturbations could be more unstable than two-dimensional ones.