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The in-plane thermal conductivity of hexagonal boron nitride (h-BN) with varying thicknesses is a key property that affects the performance of various applications from electronics to optoelectronics. However, the transition of the thermal conductivity from two-dimensional (2D) to three-dimensional (3D) h-BN remains elusive. To answer this question, we have developed a machine learning interatomic potential within the neuroevolution potential (NEP) framework for h-BN, achieving a high accuracy akin to ab initio calculations in predicting its thermal conductivity and phonon transport from monolayer to multilayers and bulk. Utilizing molecular dynamics simulations based on the NEP, we predict the thermal conductivity of h-BN with a thickness up to ∼100 nm, demonstrating that its thermal conductivity quickly decreases from the monolayer and saturates to the bulk value above four layers. The saturation of its thermal conductivity is attributed to the little change in phonon group velocity and lifetime as the thickness increases beyond four layers. In particular, the weak thickness dependence of phonon lifetime in h-BN with a nanoscale thickness results from its extremely high phonon focusing along the in-plane direction. This research bridges the knowledge gap of phonon transport between 2D and 3D h-BN and will benefit the thermal design and performance optimization of relevant applications. [ABSTRACT FROM AUTHOR]

We present experimental demonstration and modeling of mechanical wave propagation in a quasi-one-dimensional (quasi-1D) phononic crystal (PnC) waveguide (WG) constructed from a periodic array of single-crystal cubic-silicon carbide (3C-SiC) coupled micromechanical resonators, with an exceptional dynamic range exceeding 92 dB. The PnC design comprises 50 periodic cells, enabling the propagation of flexural mechanical waves in high-frequency and very-high-frequency bands, featuring a broad PnC bandgap spanning approximately 24–27.5 MHz. Furthermore, the 3C-SiC PnC WG exhibits excellent characteristics, including a high group velocity of 350 m/s and a low transmission loss of 0.69 dB/mm, enabling efficient guidance and support for mechanical waves across extended distances before reaching the noise level of the device. These attributes of the PnC WG, as demonstrated in this study, may open possibilities for the development of device platforms with applications in on-chip signal processing, sensing, and quantum transducer technologies. [ABSTRACT FROM AUTHOR]

The propagation characteristics of magneto-quasistatic waves, more commonly, known as magnetostatic waves in a long, metallic rectangular waveguide filled with a metamaterial slab are comprehensively investigated. The metamaterial slab consists of split-ring resonators as an anisotropic uniaxial medium with transversal negative effective permeability. Some analytical relations and numerical validations on the characteristics of these waves are presented. The results include the dispersion relations, mode patterns (field distributions) that can be supported by such media, and their corresponding cutoff frequencies, group velocities, power flows, and storage energies of magnetostatic waves. The findings from the present research study can be advantageous to advance the synthesis and development of negative permeability materials with peculiar features in guiding structures. [ABSTRACT FROM AUTHOR]

The band-anticrossing (BAC) model provides the basis for the self-consistent Green's function method that we have previously developed to calculate the density of states of GaN x As 1 − x dilute nitride alloys. In this paper, we extend this Green's function method to include the complex energy states and to find the poles of the Green's function, thereby allowing one to calculate the dispersion relation, group velocity, and the carrier decay rate in disordered dilute nitride alloys. Two different models of the N states have been studied to investigate the band structure of these materials: (1) the conventional two-band BAC model, which assumes that all N states are located at the same energy, and (2) a model which includes N states distributed over a range of energies, as expected in actual dilute nitride samples. Our results for the second model show a much shorter carrier mean-free path, and lower carrier mobility for GaN x As 1 − x , with the magnitude of the calculated mobility in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

In this work, we calculated the lattice thermal conductivity of monolayer InTe by means of phonon Boltzmann transport theory with first-principles calculated inter-atomic force constants. The higher-order phonon anharmonicity was found to play a strong impact on thermal transport in InTe. With the involvement of the phonon–phonon scattering process up to the fourth-order, the in-plane lattice thermal conductivity of monolayer InTe is 5.1 W m−1 K−1 at room temperature, which is 35% of that considering only third-order force constants. Furthermore, strain was found to be an effective way to manipulate the thermal transport in InTe, which reduces to one half when applying 5% in-plane tensile strain. The strain adjustment is due to the decreases in the phonon group velocity as well as the increase in the phonon scattering rates. These findings can enrich thermal transport properties of group-III monochalcogenides and benefit the material design of thermoelectrics and thermal management electronic devices. [ABSTRACT FROM AUTHOR]

Using nonequilibrium molecular dynamics (NEMD) based direct method and spectral energy density (SED) method, we calculate the size-dependent thermal conductivities (TCs) of single layer graphene (SLG), AB-stacked bilayer graphene (AB-BLG), and 21.78 ° twisted BLG (tBLG) in a robust and consistent manner. Our NEMD analysis reveals discrepancies in high TC reported for graphene systems in some of the earlier studies. Similarly, some of the previous SED based studies were done with unreliable SED Φ ′ approach. We conduct size-dependent analysis of the graphene systems by the SED method for the first time and report that bulk TCs for SLG and tBLG systems are nearly the same when calculated by either the direct or the SED method. Contrary to studies that claim that phonon group velocities of AB-BLG and tBLG samples do not change, we find that although average group velocities in SLG and AB-BLG are almost the same, they are around 30% higher when compared to tBLG samples with different twist angles. On the other hand, average phonon lifetimes are almost similar for AB-BLG and 21.78 ° tBLG samples but around 43% lower than the average phonon lifetime of SLG. Together these trends suggest the reason behind the decreasing order of TCs across three systems. We also systematically study the basic phonon mode contributions to TCs and their properties and find that the high-symmetry modes contribute the most in all three systems. [ABSTRACT FROM AUTHOR]

The properties of bulk contribution to sum-frequency generation reflected and transmitted by a finite layer in a multilayer system are described. The leading term is essentially due to the processes emitting in the transmission geometry, in particular for macroscopic layers. For such transmission processes, phase mismatch leads to the production of interference fringes when the layer thickness or a wavelength is tuned, which may be mistaken for resonant processes inside or at the surface of the material. Experimental evidence of such fringes measured from centrosymmetric bulks is provided for a diamond window in the far infrared and suggested for other materials in previously published data. The existence of a stationary point in the phase mismatch, related to the group velocity mismatch, is shown to be the source of another pseudo-resonant process in centrosymmetric calcium and barium fluorides, for which theoretical predictions reproduce the experimental observations. [ABSTRACT FROM AUTHOR]

Observations of a high spin selectivity in various helical structures, which is called the chirality-induced spin selectivity, suggest a common mechanism originating from the helical geometry. In this paper, we consider a helical chain of atomic p± orbitals having the tangential angular momentum l = ±1. We show in this model that the coupling of l and the spin gives rise to spin-velocity locking, i.e., directions of spin and group velocity are parallel or antiparallel depending on the chirality of the helix, and consequently, an almost perfect spin selectivity in a specific energy region in a wide range of the curvature and the torsion of the helix. We find that the present spin-velocity locking originates from the helical symmetry in which the Hamiltonian is invariant with respect to a combined operation of the rotation around the helix axis and the translation along the helix axis. Therefore, we expect that spin-velocity locking occurs in a wide variety of helical structures. [ABSTRACT FROM AUTHOR]

In this work we present a novel, experimentally efficient set-up for performing non-contacting laser vibrometry on geologic materials and their analogues. We show it is possible to acoustically monitor a granular material experiment in real time compared to the typical timescale of analogue modelling experiments. We acquire non-contacting waveform data with consistently high signal-to-noise ratio. Compared to previously used standard contacting transducers, the novel joint use of sources and receivers that are both laser-based resulted in measured signals with improved waveforms and temporal bandwidths. These data acquisition improvements, in our case where surface waves are prominent in the data, enable enhanced multichannel surface wave processing, for example, in terms of reliable dispersion curve estimates. We find, given the high waveform fidelity of our acquisition system, that the observed surface waves are highly sensitive to relatively small changes in the medium's elastic properties, making them a demonstrably reliable to monitor any processes that affect elasticity in these models in near real time. As a demonstration, we continuously monitor a scaled analogue model containing granular glass beads. By continuously monitoring—that is, performing repeatable active-source acousto-seismic surveys at short time-lapse intervals—over a period of 10 hr, we find that an increase of relative humidity of 10 per cent can lead to as much as a factor of two increase in surface wave group velocities. Finally, we discuss future applications of the developed method by considering surface wave inversion for fault and stress monitoring during the deformation of a model. [ABSTRACT FROM AUTHOR]

This study was conducted to investigate the effect of core stability training program on ball-kicking velocity, running speed, and agility in adolescent male football players. To this end, 36 male football players aged 12–14 were divided into the training group and control group. Before implementing the core stability training program, participants' ball-kicking velocity, sprint speed, and agility performance were measured with a Bushnell velocity radar gun, 20 m sprint test, and 505 agility test, respectively, in both training group and control group. After the measurements, the training group followed core stability training program three days a week, for eight weeks in addition to the routine training program, while the control group followed the routine training program only. Ball-kicking velocity, sprint speed, and agility performance were re-measured in both groups after the completion of the core stability training program. Significant improvements were detected in all parameters, i.e., ball-kicking velocity, sprint speed, and agility performance in the training group (p<0.05). On the other hand, in the control group, no significant change was detected in the ball-kicking velocity and sprint speed (p>0.05), whereas a significant improvement was observed in agility performance (p<0.05). Inter-group comparisons revealed statistically significant differences between the groups in ball-kicking velocity and sprint speed in favor of the training group (p<0.05), while no significant difference was found between the groups in agility performance (p>0.05). In conclusion, this study's findings suggest that core stability exercises can be incorporated into the routine training program of adolescent male football players. [ABSTRACT FROM AUTHOR]

We perform Rayleigh‐wave group‐velocity dispersion measurements from 14,706 regional‐waveforms at periods of 10–120 s, followed by ray‐based tomography and inversion to obtain 3D‐Vs structure of the crust and upper mantle. The group‐velocity maps have 3–5° lateral resolution, and Vs models have ∼3%–7% average‐Vs uncertainty. The Moho depth is assigned to the bottom of the steepest‐gradient layer with Vs between 4.1 and 4.5 km s−1, and the sedimentary‐layers have Vs ≤ 2.9 km s−1. Indian cratons have high average‐crustal‐Vs of 3.6–3.9 km s−1 and thickness of 40–50 km. The intervening rift‐basins are filled with low‐Vs sedimentary‐rocks. The Himalayan Foreland Basin has along‐arc variation in sedimentary thickness with the thickest layer (8–10 km) beneath the Eastern Ganga Basin. The Indian lithospheric mantle has high‐Vs (>4.4 km s−1), and along with high‐Vs crust attest to a cold, rigid lithosphere. This lithosphere underthrust entire Western Tibet and up to the Qiangtang Terrane in Central‐Eastern Tibet. The top of the underthrusting Indian‐crust is marked by lower‐Vs and thrust‐fault earthquakes. The shallow crust beneath Tibet (0–10 km) has high‐Vs and is mechanically strong; whereas, the mid‐crust (20–40 km) has ∼5%–10% low‐Vs anomalies due to radiogenic/shear heating within the thickened crust. This layer is weak and decouples the deformation of the shallow and deep layers. Low‐Vs upper‐mantle with deeper high‐Vs layer is present beneath the Deccan and Raj‐Mahal Traps, suggesting plume‐volcanism related thermal anomaly and refertilization of the upper mantle. The intra‐cratonic basins with circular geometry, high‐Vs lithosphere and no basement earthquakes, possibly formed by thermal subsidence of isostatically‐balanced cratonic lithosphere. Plain Language Summary: We perform 1D‐path‐average surface‐wave dispersion measurements for 14,706 regional ray‐paths (10 and 120 s period), and combine these into 2D variation maps, followed by modeling to obtain 3D shear‐wave velocity structure beneath India, Himalaya and Tibet. The 2D maps have 3–5° lateral resolution and the 3D structure has 5%–7% average‐velocity uncertainty. The Indian Cratons have high average crustal‐velocity and crustal thickness of 40–50 km, and are underlain by high‐velocity upper mantle. The intervening rift‐basins are filled with lower‐velocity sedimentary‐rocks. The sedimentary layer thickness in the Himalayan Foreland Basin varies along‐arc and is thickest (8–10 km) beneath the Eastern Ganga Basin. The high‐velocity (cold, rigid) Indian plate underthrust entire Western Tibet and up to the Qiangtang Terrane in Central‐Eastern Tibet. A mid‐crustal low‐velocity‐layer is present beneath Tibet, which is weak and decouples the deformation of the shallow and deep layers. The upper‐mantle beneath Deccan and Raj‐Mahal Traps has low‐velocity overlying a high‐velocity layer suggesting plume‐volcanism related thermal anomaly. From the velocity‐structure and geometry we speculate about the origin of the intra‐cratonic Tarim and Eastern Tajik Basins. Key Points: Indian Cratonic lithosphere (ICL) has high average‐crustal Vs 3.6–3.9 km s−1, upper mantle Vs > 4.4 km s−1 and crustal thickness of 40–50 kmLow Vs upper‐mantle with a deeper high Vs layer beneath Deccan Traps suggest plume‐volcanism related thermal anomaly/refertilizationICL underthrusts entire W Tibet and up to the Qiangtang Terrane beneath Central‐E Tibet with a mid‐crustal low‐velocity anomaly of 5%–10% [ABSTRACT FROM AUTHOR]

Anti-plane shear wave propagation in a functionally graded piezoelectric substrate and a piezoelectric substrate guided by a slit is analyzed theoretically. The equations of motion are taken from the linear theory of piezoelectricity for polarized ceramics. The dispersion relation is obtained by applying appropriate boundary conditions on the derived solutions. PZT-5 and ZnO are taken as numerical examples to manifest the effect of different material parameters (gradient factor, slit width etc.) graphically on the phase and group velocity of the considered wave. $ \textrm{LiI}{\textrm{O}_\textrm{3}} $ LiI O 3 and $ \textrm{BaTi}{\textrm{O}_\textrm{3}} $ BaTi O 3 materials are taken separately to compare the results. Also, graphs are plotted for different modes of the wave as well as for different frequency ranges and for the case when slit is absent. As a special case, it is remarked that the present study has resemblance with one of the existing works. The highlights of the study are to characterize the anti-plane wave propagation within a functionally graded piezoelectric substrate/gap/piezoelectric substrate coupled structure, to show the effect of different parameters, modes, frequency, etc. on the velocity profile of the considered wave and to show the effect of gap layer. The outcomes of this study may be used to enhance the sensitivity of acoustic wave devices and sensors. [ABSTRACT FROM AUTHOR]

An ab initio characterization of the impact of tert‐Butyl isocyanate (C5H9NO${\rm C}_5{\rm H}_9{\rm NO}$) functionalization on TaS2${\rm TaS}_2$ lattice thermal conductivity is presented. Such a system has been previously synthetized and experimentally charachterized, showing that the incorporation of covalently bonded C5H9NO${\rm C}_5{\rm H}_9{\rm NO}$ on bulk TaS2${\rm TaS}_2$ leads to a dramatic in‐plane lattice thermal conductivity decrease by more than two orders of magnitudes. To elucidate these experimental findings, detailed calculations of the phonon dispersion relations and scattering rates in TaS2${\rm TaS}_2$ are performed. The analysis is addressed to discern the impact of inter‐layer covalently bonded C5H9NO${\rm C}_5{\rm H}_9{\rm NO}$ molecules on these phonon properties, providing insights into the underlying mechanisms of the observed thermal conductivity decrease. Present calculations provide evidence that the observed lattice thermal conductivity reduction is attributed to two effects: i)$i)$ the increase of inter‐layer separation and ii)$ii)$ the presence of low‐frequency molecular optical modes. The first inhibits specific Van der Waals quasi‐acoustic inter‐layer vibrational modes contributing as much as ≈55%$\approx \!\! 55\%$ in bulk TaS2${\rm TaS}_2$. The second effect dramatically decreases the phonon group velocities as a consequence of phonon‐crossing phenomena among low‐frequency molecular modes and TaS2${\rm TaS}_2$ acoustic modes, eventually leading to a strong reduction of all phonon lifetimes. [ABSTRACT FROM AUTHOR]

The propagation velocities of geomagnetic Pc5 pulsations in the azimuthal and meridional directions were analyzed for a series of events. Two methods were used: based on the phase delays of the signal between stations and the displacement of vortex centers of their equivalent current systems. The analysis showed that the propagation of pulsations and vortices coincides in direction—along the meridian, they predominantly propagate northward. In most cases, the propagation velocity is 5 km/s for pulsations and 2 km/s for vortices. In the azimuthal direction, pulsations and vortices propagate westward, with pulsation propagation velocity of 10 km/s and vortex propagation velocity of 3 km/s. However, the distributions of azimuthal velocities for both pulsations and vortices exhibit comparable maxima corresponding to eastward propagation: pulsations at a velocity of 10 km/s and vortices at 5 km/s. It is concluded that the measured phase velocities of pulsations at the ionospheric level are approximately twice the group velocities of vortices. [ABSTRACT FROM AUTHOR]

Reservoir monitoring is essential to guarantee safe operations for all activities involving the production and injection of fluids into the subsurface, such as hydrocarbon production, gas storage and the exploitation of geothermal reservoirs. For this purpose, microseismic monitoring networks are operated in real time in order to identify and locate any possible seismic events in the vicinity of the reservoir. The goal of this study is to investigate whether the large amount of ambient seismic noise recorded by seismic reservoir monitoring networks can be used to infer a one‐dimensional shear‐wave velocity profile representative of the area covered by the network. Shear‐wave velocities are generally difficult to characterize and constrain, yet they are key to precisely locate seismic events. The adopted workflow consists of three steps: first, the cross‐correlation functions between all station pairs are retrieved, using 1 year of continuous data; second, the average group‐ and phase velocity dispersion curves are extracted; third, a joint group and phase velocity inversion is done. For validation, the obtained average shear‐wave velocity profile is compared with a regional model of the area as well as with local shear‐wave velocity measurements from a sonic log. [ABSTRACT FROM AUTHOR]

We deal with dynamics of the β -Fermi–Pasta–Ulam–Tsingou chain with one free end, subjected to the sudden sinusoidal force. We examine the evolution of the total energy supplied into the chain at large times. In the harmonic case ( β = 0 ), the energy grows in time linearly at the driving frequencies, corresponding to nonzero group velocities. The rate of energy supply is shown to decrease with increasing excitation frequency. Loading with the cut-off frequency, corresponding to zero group velocity, results in the energy growth proportionally to t . Explanation of behavior in time of the energy is proposed by analysis of the obtained approximate closed-form expressions for the particle velocity. In the weak anharmonic case, large-time asymptotic approximation for the total energy is obtained by using the renormalized dispersion relation. The approximation allows one to estimate rate of the energy supply at the driving frequencies, which belong both in the pass-band and in the stop-band of the harmonic chain. Consistency of the asymptotic estimates with the results of numerical simulations is discussed. [ABSTRACT FROM AUTHOR]

Debye–Callaway model in combination with the Murnaghan and Clapeyron equations was used to calculate the hydrostatic pressure effects on lattice thermal conductivity (LTC) of wurtzite gallium nitride. The calculations are for the longitudinal and transverse phonon modes. The results are efficiently fitted with the whole temperature (1–400) of the experimental data. The peak value of LTC declines with the applied pressure from 0 to 14 GPa. This result is due to the decreasing Debye temperature, group velocity and lattice volume. Furthermore, pressure affected the number of dislocations, sample size and Gruneisen parameter (longitudinal and transverse) modes. Consequently, the values of above parameters at zero GPa are 2.5 × 10 13 m - 2 , 1.8 mm , 0.93 and 0.52 , whilst the values at 14 GPa are 15 × 10 15 m - 2 , 1.68 mm , 0.818 and 0.469 , respectively. The results show that hydrostatic pressure does not affect the number of impurities and electron concentrations. [ABSTRACT FROM AUTHOR]

In this article, the design and simulation of an optical gyroscope based on a hybrid ring resonator using the Sagnac effect are presented. In this design, by using a transfer matrix, numerical analysis, and tight-binding methods, as well as managing control factors in the sensor, such as the transfer coefficient, ring radius of each resonator, operating wavelength, and coupling coefficient, the sensor output parameters are investigated. The output parameters include sensitivity, angular random walk (ARW), minimum angular rotation, minimum group velocity, and the phase shift resulting from the power ratio changes. In this gyroscope, the maximum group velocity in terms of angular rotation with the increase of the number of rings is an exponential function in the range of 750–2500 m/s. The output phase changes as a linear function with an upward slope as the radius of the rigs increases in the range of 2130–3430 nm. Also, with the increase in the radius of the rings, the phase change relative to the operating wavelength decreases. By increasing the imaginary part of the coupling and transfer coefficients in this sensor, the changes in output power decrease compared to the output phase. The transmission peaks in the gyroscope output increase due to the rise in the number of rings. The sensitivity lowers with an increasing number of rings. The temperature affects the gyroscope output, and an increase in temperature causes a rise in ARW and a decrease in the signal-to-noise ratio. Also, the signal-to-noise ratio increases with the rise of the output power due to the transfer and coupling coefficients. [ABSTRACT FROM AUTHOR]

Health monitoring of structures is a fertile field of research. Different techniques are being developed through research to perform the health monitoring on different types of structures. Acoustic emission (AE) technique is considered to be promising, as this technique can be used for damage detection in real time. However, AE technique has some limitations as far as geometry of structure is concerned. Here in this study, modal AE (MAE) technique has been explored through experimental investigation in laboratory for its possible, effective applications in the health monitoring of structures with different geometries, namely, plates and truss. MAE technique is adopted as it facilitates an analysis of structural behaviour under the application of an AE source along with source localisation. Further, a methodology is proposed to minimise the effects of geometry and boundaries/edges. The methodology is based on identifying the front part of the time-frequency representation (TFR) diagram, i.e. first few energetic zones in the TFR, of an AE signal, followed by the alignment of these zones with respect to the contour of the group velocity curves of Lamb modes. The efficacy of the proposed method for damage localisation has been verified and found to be promising. [ABSTRACT FROM AUTHOR]

In this work, we study a model in nonlinear electrodynamics in the presence of a CPT-even term that violates Lorentz symmetry. The Lorentz-breaking vector, in addition to the usual background magnetic field, produces interesting effects in the dispersion relations. The consequences on the vacuum refractive index and the group velocity are studied. Vacuum birefringence is discussed in the case the nonlinear electrodynamics is a Euler–Heisenberg model. [ABSTRACT FROM AUTHOR]

Icosahedral B12 serves as a fundamental building block for a variety of boron allotropes and plays a crucial role in enhancing the stability of fullerene-like boron nanoclusters. However, progress in the development of two-dimensional (2D) functional assemblies made from icosahedral B12 has been limited. In this context, we propose a 2D semiconducting boron allotrope (h-B12) using icosahedral B12 as a building block. This structure features a stable honeycomb lattice, with its unit cell comprising two mirror-symmetric icosahedral B12 clusters. The results of first-principles calculations indicate that the band structure exhibits Dirac cones and a relatively high group velocity, similar to graphene. Additionally, it is found to be a material with a negative Poisson's ratio and a relatively low Young's modulus. Finally, our calculations reveal that the zigzag nanoribbon constructed from h-B12 displays prominent edge states at both edges, indicating that this structure represents a potential topological electronic material. Our findings shed light on the design and construction of nano-electronic materials via cluster assembly. [ABSTRACT FROM AUTHOR]

We conducted a systematic investigation into the spectral and pulse characteristics of C and L-band Nonlinear Polarization Rotation (NPR) mode-locked fiber lasers effectively employing nonlinear polarization rotation technology. In our experimental setup, we achieved a stable mode-locked state at 1560.076 nm, exhibiting a 3 dB spectral bandwidth of 9.1 nm. As the pump power increased, we observed spectral shifts accompanied by shifts in the first Kelly sideband and the generation of new Kelly sidebands. In this paper, the phenomenon of spectral deviation is elucidated through the interplay of self-phase modulation, group velocity drift, and polarization-dependent isolator (PD-ISO) filter effect, with an analysis of the formation and deviation of Kelly sidebands. Notably, spectral shift persisted even when the pump power exceeded 200 mW. However, continuous pump power escalation led to soliton splitting, resulting in the formation of new soliton beams. Based on the simultaneous generation of spectral shift and pulse splitting, our study contributes to an enhanced understanding of soliton dynamics in ultrafast fiber lasers and lays a foundation for the application of high-repetition-frequency harmonic mode-locked lasers with tunable wavelengths. [ABSTRACT FROM AUTHOR]

The acoustic black hole (ABH) effect is investigated within the framework of thin shell theory. Asymptotic solutions to the dispersion equation for the thin cylindrical shell are obtained, and the ABH effect is examined using analytical formulas for group velocities and anti-derivatives of the asymptotic expansions of wave numbers. It is shown that the ABH effect is achievable in thin cylindrical shells with variable thickness, in a similar manner as for beams and plates. However, it should not be expected to exist in the low-frequency range where the flexural wave motion in the wall of a shell is strongly coupled with uniform longitudinal wave motion. [ABSTRACT FROM AUTHOR]

Attenuation is the most difficult seafloor acoustic property to get, particularly at low to mid frequencies. For low velocity bottoms (LVB), it becomes even more challenging, due to its small attenuation and lower velocity (relative to the velocity of the adjacent water). The latter one causes a fatal "seafloor velocity-attenuation couplings" in geo-acoustic inversions. Thus, attenuation inversions for the LVB require an accurate seafloor velocity profile, especially the velocity in the LVB layer. The propagation of explosive sound in the Yellow Sea with a strong thermocline and a top LVB layer exhibits many prominent characteristics: modal dispersion (the ground wave, water wave, Airy phase), two groups of water waves at high frequencies, and the siphon effect which causes abnormally large sound transmission loss at selected frequencies, etc. These observations are used to precisely measure the critical frequency, the Airy frequency, Airy wave velocity, 1st mode group velocity, and to derive the velocities in the LVB layer and in the basement. Using inverted seafloor parameters, the source level-normalized transmission loss and the first mode decay rate in ranges up to 27.66 km, the sound attenuations in the LVB are derived for a frequency range of 13–5000 Hz. [ABSTRACT FROM AUTHOR]

The Radon and Hilbert transform and their applications to convectively coupled waves (CCWs) are reviewed. The Hilbert Transform is used to compute the wave envelope, whereas the Radon transform is used to estimate the phase and group velocities of CCWs. Together, they provide an objective method to understand CCW propagation. Results reveal phase speeds and group velocities for fast waves (mixed Rossby‐gravity, westward and eastward inertio‐gravity, and Kelvin) that are consistent with previous studies and with Matsuno's equatorial wave dispersion curves. However, slowly‐propagating tropical depression‐like systems and equatorial Rossby waves exhibit wave envelopes that propagate faster than the individual wave crests, which is not predicted by dry shallow water theory. [ABSTRACT FROM AUTHOR]

We computed a 3-D shear wave velocity model of the Marmara Sea region from ambient noise tomography. The correlations of up to 8 yr of vertical-component seismic recordings from 80 broad-band stations provided Rayleigh wave group velocity measurements in the period band 6–21 s at more than 1400 selected virtual source–receiver pairs. Rayleigh wave group velocity maps were used to derive a shear wave velocity model through simulated annealing inversion. The resulting crustal model provides coverage of the Marmara Sea along with its surrounding regional tectonic features. This allows for an investigation of the spatial extents of the Marmara Sea on a scale larger than that of basins. The low-velocity structures of the Marmara Sea and the Thrace Basins are coeval to a depth of approximately 9 km. The crustal velocities beneath the Marmara Sea basins exhibit a low vertical gradient and smooth horizontal variations. The regional tectonic structures, such as Istranca Massif, Istanbul and Sakarya Zones, display sharp velocity contrasts with the lower velocity crust beneath the Marmara Sea. The observed low crustal velocities, along with depth variations of the velocity isosurfaces (i.e. 3.4 km s−1) indicate that the Marmara region is a structural depression much deeper and larger than the three basins of the North Marmara Trough. The North Anatolian Fault Zone is unlikely to be the primary factor contributing to the origin of this significant depression, as the basin's development appears to have occurred before the fault propagated into the region. [ABSTRACT FROM AUTHOR]

Borneo and Sulawesi are two large islands separated by the Makassar Strait that lie within the complex tectonic setting of central Indonesia. The seismic structure beneath this region is poorly understood due to the limited data availability. In this study, we present Rayleigh wave tomography results that illuminate the underlying crustal structure. Group velocity is retrieved from dispersion analysis of Rayleigh waves extracted from the ambient noise field by cross-correlating long-term recordings from 108 seismic stations over a period of 8 months. We then produce a 3-D shear wave velocity model via a two-stage process in which group velocity maps are computed across a range of periods and then sampled over a dense grid of points to produce pseudo-dispersion curves; these dispersion curves are then separately inverted for 1-D shear wave velocity (Vs), with the resultant models combined and interpolated to form a 3-D model. In this model, we observed up to ± 1.2 km s−1 lateral Vs heterogeneities as a function of depth. Our models illuminate a strong low shear wave velocity (Vs) anomaly at shallow depth (≤ 14 km) and a strong high Vs anomaly at depths of 20–30 km beneath the North Makassar Strait. We inferred the sediment basement and Moho depth from our 3-D Vs model based on iso-velocity constrained by the positive vertical gradient of the Vs models. The broad and deep sedimentary basement at ∼14 ± 2 km depth beneath the North Makassar Strait is floored by a shallow Moho at ∼22 ± 2 km depth, which is the thinnest crust in the study area. To the east of this region, our model reveals a Moho depth of ∼45 ± 2 km beneath Central Sulawesi, the thickest crust in our study area, which suggests crustal thickening since the late Oligocene. Moreover, the presence of high near-surface Vs anomalies with only slight changes of velocity with increasing depth in southwest Borneo close to Schwaner Mountain confirm the existence of a crustal root beneath this region. [ABSTRACT FROM AUTHOR]

In a four-level quantum dot system connected with a Sagnac interferometer (Mach–Zehnder-type), photon drag (PD) under the influence of tunnelling-dependent cross-Kerr nonlinearity is explored. The PD, electric susceptibility, group index, group relativistic velocities are studied as functions of the tunnelling frequency. Additionally, in the presence of tunnelling-based Kerr effect in the system, improved positive group index and minimum group relativistic velocities are attained. Large drag angles are consequently observed in the system. The highest reached values for the group index are n g = 1.60 × 10 6 and 6 × 10 6 . Likewise, the highest values for PD are reported as θ d = ± 0.080 , ± 0.112 and ± 0.120 radians. The outcomes shown here will advance quantum information, image coding/design, and rotation sensing technology. [ABSTRACT FROM AUTHOR]

Multimodal is the prominent characteristic of Lamb waves, which means a variety of Lamb wave modes propagate in the structure and each of them has unique propagation behaviour. For a wideband multimodal Lamb wave signal, it is possible that many different modes share the same group velocity at some frequency points, while their dispersion behaviours change differently to thickness variation. In this study, the diversity and difference of the interaction between the corrosion and Lamb wave modes are investigated. On this basis, a corrosion index (CI) is established to evaluate the time gap between two modes at their identical group velocity points. The simulation results show that the CI value of a corroded path is obviously larger than that of a healthy path, indicating the presence of the damage, which is independent of any base-line signal. Furthermore, the new reconstruction algorithm for the probabilistic inspection of damage (RAPID) was proposed, where the signal difference coefficient was replaced by the CI to visualise damage location. The experimental results demonstrate the effectiveness of the proposed multimodal Lamb wave baseline-free damage detection method. [ABSTRACT FROM AUTHOR]

Performing propagating spin-wave spectroscopy of thin films at millikelvin temperatures is the next step toward the realization of large-scale integrated magnonic circuits for quantum applications. Here, we demonstrate spin-wave propagation in a 100 nm -thick yttrium-iron-garnet (YIG) film at temperatures down to 45 mK , using stripline nanoantennas deposited on YIG surface for electrical excitation and detection. The clear transmission characteristics over the distance of 10 μ m are measured and the extracted spin-wave group velocity and the YIG saturation magnetization agree well with the theoretical values. We show that the gadolinium-gallium-garnet (GGG) substrate influences the spin-wave propagation characteristics only for the applied magnetic fields beyond 75 mT , originating from a GGG magnetization up to 62 kA / m at 45 mK. Our results show that the developed fabrication and measurement methodologies enable the realization of integrated magnonic quantum nanotechnologies at millikelvin temperatures. [ABSTRACT FROM AUTHOR]

This letter presents a thin plate structure with an acoustic black hole (ABH) sub-unit to reorient the flexural wave. Different from the previously reported flexural wave metasurface, ABH sub-units are introduced into thin plates in this work, which can control the group velocity of flexural waves and realize their efficient transmission. According to generalized Snell's law, the mechanism of phase shift of transmitted waves across subwavelength sub-units is theoretically revealed. An analysis of the ABH sub-units is established by the finite element method. The deflection and focusing effect of flexural waves are demonstrated. Furthermore, adjusting the black hole section can quickly obtain the transmission phase response in the range of 2π, and it can accurately predict the phase shift and amplitude of the transmitted wave. The results show that the ABH beam-plate structure can effectively control the propagation direction of flexural waves, which provides a modern design idea and method for the manipulation and energy harvesting of the flexural wave. [ABSTRACT FROM AUTHOR]

In search of novel phononic crystals to effectively control the propagation of elastic waves, we propose a new single-material phononic crystal (PnC) with unit cells containing tapered resonators (TRs). The thickness of the circular taper radially decreases outward from the center. The device modulates dispersion of the wave by a local resonance mechanism and by slowly varying the group velocity of elastic waves. The TRs are layered on the top of a conventional PnC slab with a square arrangement of air holes. The band structure of the PnC is theoretically studied and a comparison is drawn between the avoided level crossings and the symmetry-protected ordinary degeneracies. In the absence of a bandgap, the zero group velocity at the band maximum restricts the waves from propagating. Moreover, the design shows anomalous dispersion phenomena such as self-collimation and bi-refringence, which are rare in conventional PnCs. We trace the origins of these phenomena by analyzing equifrequency contours associated with relevant frequencies. We show that the self-collimation effect persists even with a small variation in the angle of incidence and a perturbative hole at the center of each of the TRs. Within the classical limit, the scale invariance of the elastic wave equation makes the device useful in both the low frequency ultrasonic and the high frequency phononic regime. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of RbBaX (X = Sb, Bi), an anisotropic material with strong anharmonicity, are systematically studied by first-principles calculations, combined with the self-consistent phonon theory and the Boltzmann transport equation. A reasonable lattice thermal conductivity κL is captured by fully handling the phonon frequency shift and four-phonon scattering caused by the quartic anharmonicity. The κL of RbBaSb and RbBaBi along the a-axis is only 0.60 and 0.36 W m−1 K−1 at 300 K, respectively, which is much lower than that of most thermoelectric materials. The low phonon group velocity resulting from the unusually weak atomic bonding strengths along the a-axis is the origin of the ultralow κL. Furthermore, the high dispersion near the conduction band minimum enables n-type doping with a higher electrical conductivity. The results show that orthorhombic RbbaBi has a ZT as high as 1.04 at 700 K along the a-axis direction, indicating its great application potential in the thermoelectric field. [ABSTRACT FROM AUTHOR]

Using the high‐time‐resolution data from the Magnetospheric Multiscale mission, precursor waves upstream of foreshock transient (FT) shocks are statistically investigated using the four‐spacecraft timing method. The wave frequencies and wave vectors determined in the plasma rest frame (PRF) are shown to follow the cold plasma dispersion relation for whistler waves. Combining with the feature of the right‐hand polarization in the PRF, the precursors are identified as whistler‐mode waves around the lower hybrid frequency. The occurrence of whistler precursors is independent of the Alfvén Mach number and the FT shock normal angle. More importantly, all the whistler precursors have group velocities pointing upstream in the shock frame, suggesting the dispersive radiation to be a possible generation mechanism. The study improves the understanding of not only the whistler precursors but also the overall FT shock dynamics. Plain Language Summary: The characteristics of the precursor waves upstream of foreshock transient (FT) shocks are determined in the plasma rest frame using the multi‐point measurements from the Magnetospheric Multiscale mission with appropriate separation scales. The statistical results demonstrate for the first time that the precursors upstream of FT shocks are lower hybrid frequency whistler‐mode waves. The presence or absence of large amplitude whistler precursors does not depend on the FT shock normal angle and the Alfvén Mach number. These results have important implications on the nature of the whistler precursors and the dynamics of the FT shocks. Key Points: Precursor waves upstream of foreshock transient shocks are found to follow the whistler wave dispersion relation in the plasma rest frameThe occurrence of whistler precursors is independent of the Alfvén Mach numbers and normal angles of the foreshock transient shocksThe observed wave characteristics are consistent with that the precursors are generated through the dispersive radiation mechanism [ABSTRACT FROM AUTHOR]

The deep-water sound multipath arrivals and interference striation patterns not only vary with range and depth but also with frequency. The first two cases can be solved using a ray-mode method in the high-frequency limit. However, this method may cause unsatisfactory results in treating low-frequency sounds (as low as tens of Hertz) because of the Wentzel-Kramers-Brillouin (WKB) high-frequency approximation assumption. To overcome this problem, the authors exploited phase information to modify the ray parameters, such as the propagation range, the time delay, and the group velocity. Following that, a waveguide invariant (WI) expression was derived to address low-frequency sound interference problems. Numerical investigations demonstrated that the physical mechanism of the frequency dependence of the multipath arrivals and interference striation patterns could be better understood using this modified ray-mode method. In addition, the influence of bottom sound speed on multipath interference structures is also considered. [ABSTRACT FROM AUTHOR]

We report the results of the investigation of the acoustic and optical phonons in quasi-two-dimensional antiferromagnetic semiconductors of the transition metal phosphorus trisulfide family with Mn, Fe, Co, Ni, and Cd as metal atoms. The Brillouin–Mandelstam and Raman light scattering spectroscopies were conducted at room temperature to measure the acoustic and optical phonon frequencies close to the Brillouin zone center and the Γ − A high symmetry direction. The absorption and index of refraction were measured in the visible and infrared ranges using the reflectometry technique. We found an intriguing large variation, over ∼28%, in the acoustic phonon group velocities in this group of materials with similar crystal structures. Our data indicate that the full-width-at-half-maximum of the acoustic phonon peaks is strongly affected by the optical properties and the electronic bandgap. The acoustic phonon lifetime extracted for some of the materials was correlated with their thermal properties. The results are important for understanding the layered van der Waals semiconductors and assessing their potential for optoelectronic and spintronic device applications. [ABSTRACT FROM AUTHOR]

Background: The elastic modulus of polyethylene terephthalate (PET) sheets is typically measured through destructive tests that require specific sample preparation and time-consuming testing procedures. Objective: To improve the efficiency of measuring the elastic modulus of PET sheets, research on a non-destructive measurement approach using guided Lamb waves was conducted. Methods: In this approach, the group velocity of the zero-order symmetric Lamb wave mode (S0 mode) at a single frequency is first measured from PET sheets. The semi-analytical finite element method (SAFEM) is used as the forward model to calculate the corresponding numerical group velocity. Particle swarm optimisation (PSO) is used to update the elastic modulus in the SAFEM model until the numerical group velocity from the model matches the experimental results. Results: The results show that measuring the group velocity data at a single frequency is sufficient for elastic modulus measurement while the material thickness can be assumed as a constant, which improves the efficiency of the measurement. The identified modulus differs from the tensile modulus of the material due to the frequency dependence of the elastic modulus. However, this discrepancy could be eliminated by using a linear regression model. Conclusions: The method mentioned above can achieve non-destructive and efficient measurement of the elastic modulus of PET sheets, which can potentially be applied for in-line quality inspection in PET bottle production processes. [ABSTRACT FROM AUTHOR]

We develop the concept of covariance for waves and show that it plays a fundamental role in understanding the evolution of a propagating pulse. The concept clarifies several issues regarding the spread of a pulse and the motion of the mean. Exact results are obtained for the time dependence of the covariance between position and wavenumber and the covariance between position and group velocity. We also derive relevant uncertainty principles for waves. [ABSTRACT FROM AUTHOR]

Gyromonotrons are typically believed to rely on the convective interaction between the co-propagating beam and wave, with the extended energy-exchanging process stemming from the external feedback. However, numerous studies focusing on both transverse electric (TE) mode and transverse magnetic (TM) mode gyrotrons have consistently shown that beam–wave interactions in weak-feedback systems or even in uniform tubes without any structural feedback can yield a theoretical beam efficiency of more than 30% with major forward-wave output during near-cutoff operation, which is the typical operating condition for gyromonotrons. These intriguing findings raise questions about the actual feedback mechanism of gyromonotrons. In this article, comparative studies on the linear and nonlinear behaviors of uniform-tube gyrotron are investigated. The forward and backward waves are observed to co-generate and exhibit similar characteristics of ultra-slow group velocity under near-cutoff operation. This situation allows the as-generated forward wave to modulate the fresh beam, establishing a new backward-wave-like internal feedback loop. Additionally, the quasi-degenerate nature of the bi-directional propagating waves ensures their intrinsic in-phase relationship. The consequent constructive interference enables the uniform tube to function as a high-Q resonator. These findings are found to be independent of the choices of TE or TM modes, providing valuable insights into the underlying interaction mechanism of gyrotron devices. [ABSTRACT FROM AUTHOR]

This paper presents an air-coupled impact echo (IE) technique that relies on the phase spectrum of the collected data to find the frequencies corresponding to the reflections from delaminations. The proposed technique takes advantage of the fact that the IE compression wave is not a propagating wave, but it is the 1st order symmetrical (S1) mode Lamb wave at zero group velocity (S1-ZGV). Therefore, it searches the phase spectra of the data collected by multiple sensors to locate the frequency corresponding to the lowest phase difference. As a result, the technique reduces the effect of propagating waves, including the direct acoustic wave and ambient noise. It is named the Constant Phase IE (CPIE). The performance of the CPIE is experimentally compared with the regular amplitude spectrum-based IE technique and two other multisensor IE techniques. The CPIE shows a performance advantage, especially in a noisy environment. [ABSTRACT FROM AUTHOR]

Vehicle‐induced seismic waves, generated as vehicles traverse the ground surface, carry valuable information for imaging the underlying near‐surface structure. These waves propagate differently in the subsurface depending on soil properties at various spatial locations. By leveraging wave propagation characteristics, such as surface‐wave velocity and attenuation, this study presents a novel method for near‐surface monitoring. Our method employs passing vehicles as active, non‐dedicated seismic sources and leverages pre‐existing telecommunication fibers as large‐scale and cost‐effective roadside sensors empowered by Distributed Acoustic Sensing (DAS) technology. A specialized Kalman filter algorithm is integrated for automated DAS‐based traffic monitoring to accurately determine vehicles' location and speed. Then, our approach uniquely leverages vehicle trajectories to isolate space‐time windows containing high‐quality surface waves. With known vehicle (i.e., seismic source) locations, we can effectively mitigate artifacts associated with suboptimal distribution of sources in conventional ambient noise interferometry. Compared to ambient noise interferometry, our approach enables the synthesis of virtual shot gathers with a high signal‐to‐noise ratio and spatiotemporal resolution at reduced computational costs. We validate the effectiveness of our method using the Stanford DAS‐2 array, with a focus on capturing spatial heterogeneity and monitoring temporal variations in soil seismic properties during rainfall events. Specifically, in non‐built‐up areas, we observed an evident decrease in phase velocity and group velocity and an increase in attenuation due to the rainfall. Our findings illustrate our method's sensitivity and resolution in discerning variations across different spatial locations and demonstrate that our method is a promising advancement for high‐resolution near‐surface imaging in urban settings. Plain Language Summary: Continuous monitoring of near‐surface soil properties is important to various applications, including identifying subsidence and monitoring groundwater levels. Vehicles transiting in city streets excite seismic waves propagating in the subsurface, which can be analyzed to gain insights into the structures beneath the surface. In this study, we develop a novel near‐surface monitoring method utilizing traffic recordings of Distributed Acoustic Sensing (DAS) technology, transforming existing telecommunication cables into extensive ground motion sensors. To harness the vibrations generated by vehicles, our method employs a specialized algorithm, determining vehicle locations and speeds based on the ground deformation they induce. The identified vehicle location is precisely utilized to select specific traffic‐induced seismic waves to explore the subsurface. Our approach allows for the acquisition of clearer and more precise insights into underground structures in urban areas and is more computationally efficient than traditional methods based on ambient noises. Tested at Stanford University in 2023, our method adeptly identified variations in the seismic properties of the subsurface, particularly in non‐built‐up areas, during rainfall events. Such accurate detection is valuable for water resource management and urban development, enabling enhanced understanding and management of urban subsurface environments. Key Points: We utilize a large number of vehicles transiting on city streets as non‐dedicated seismic sources for continuous near‐surface monitoringOur method, tailored for environments with urban and suburban traffic, outperforms conventional interferometry in efficiency and resolution, leveraging selective cross‐correlation of surface wavesWe observed rain‐induced changes in seismic properties using time‐lapse analysis of kinematics and attenuation [ABSTRACT FROM AUTHOR]

In this study, Γ-shaped rectangular resonators, which are created in a metal insulator metal (MIM) structure, have been studied analytically and numerically. The metal is silver and rectangular resonators are filled by Si and the peak of transparency profile is tuned in the communication band with proper geometrical parameters. By employing 2-D finite difference time domain method (FDTD), simulations show a plasmonic induced transparency (PIT) window in the transmission diagram. The PIT is created by instructive and destructive interference between bright and dark resonator modes. The presented structure demonstrates tunable transparency window by variation of symmetry parameter, s. The maximum group velocity is obtained 0.1 ps in communication band corresponded to 30 μm traveling of light in the vacuum which is comparable to previous articles. The proposed structure may have potential applications in optical memory and delay blocks in designing integrated optical circuits. [ABSTRACT FROM AUTHOR]

The asymptotic Dirichlet‐to‐Neumann (D‐N) map is constructed for a class of scalar, constant coefficient, linear, third‐order, dispersive equations with asymptotically time/periodic Dirichlet boundary data and zero initial data on the half‐line, modeling a wavemaker acting upon an initially quiescent medium. The large time t$t$ asymptotics for the special cases of the linear Korteweg‐de Vries and linear Benjamin–Bona–Mahony (BBM) equations are obtained. The D‐N map is proven to be unique if and only if the radiation condition that selects the unique wave number branch of the dispersion relation for a sinusoidal, time‐dependent boundary condition holds: (i) for frequencies in a finite interval, the wave number is real and corresponds to positive group velocity, and (ii) for frequencies outside the interval, the wave number is complex with positive imaginary part. For fixed spatial location x$x$, the corresponding asymptotic solution is (i) a traveling wave or (ii) a spatially decaying, time‐periodic wave. The linearized BBM asymptotics are found to quantitatively agree with viscous core‐annular fluid experiments. [ABSTRACT FROM AUTHOR]

Seasonal variations of gravity‐wave parameters over the northern and southern Tibetan Plateau are studied using high vertical‐resolution L‐band radiosonde data during the period 2014 to 2019. The wave parameters include wavelength, frequency, phase speed, group velocity, perturbation energies and momentum fluxes. The synoptic effects on the gravity‐wave parameters are discussed and assessed with the Pearson correlation. The tropospheric vertical wavelengths as estimated from the zonal wind perturbation were found to show a pronounced decrease over the southern Tibetan Plateau in the warm season. The tropospheric (stratospheric) horizontal wavelength is shortest (longest) in summer, and the stratospheric horizontal wavelength is longer than the tropospheric horizontal wavelength. The ratio of the intrinsic frequency to the Coriolis parameter is 2.5–5 (2–4.5) with a summer (winter) maximum in the troposphere (lower stratosphere). The ratio is generally higher in the north than in the south. The typical value of the mean tropospheric (stratospheric) intrinsic‐phase speed is 3.5–6 (2–11) m·s−1. The mean intrinsic‐phase speed in the south is mostly larger than that in the north. The tropospheric zonal ground‐based group velocities are dominantly eastward in all seasons. The stratospheric zonal ground‐based group velocity shifts from eastward in summer to westward in winter. In the troposphere, the seasonal cycles of the kinetic energy (KE) and potential energy (PE) exhibit regional differences. The tropospheric vertical fluctuation energy (VE) is independent of both season and region. The stratospheric KE, PE and VE are smaller in the warm season. The negative zonal momentum flux ρu′w′‾ and positive meridional momentum flux ρv′w′‾ are strongest in winter and summer. [ABSTRACT FROM AUTHOR]

Spin-wave (SW) propagation close to the hybridization-induced transmission stop band is investigated within a trapezoid-shaped 200 nm thick yttrium iron garnet film using time-resolved magneto-optic Kerr effect microscopy and broadband spin-wave spectroscopy, supported by micromagnetic simulations. The gradual reduction of the effective field within the structure leads to local variations of the SW dispersion relation and results in a SW hybridization at a fixed position in the trapezoid where the propagation vanishes, since the SW group velocity approaches zero. By tuning external field or frequency, spatial control of the spatial stop band position and spin-wave propagation is demonstrated and utilized to gain transmission control over several microstrip lines. [ABSTRACT FROM AUTHOR]

In conventional thin materials, the diffraction limit of light constrains the number of waveguide modes that can exist at a given frequency. However, layered van der Waals (vdW) materials, such as hexagonal boron nitride (hBN), can surpass this limitation due to their dielectric anisotropy, exhibiting positive permittivity along one optic axis and negativity along the other. This enables the propagation of hyperbolic rays within the material bulk and an unlimited number of subdiffractional modes characterized by hyperbolic dispersion. By employing time-domain near-field interferometry to analyze ultrafast hyperbolic ray pulses in thin hBN, we showed that their zigzag reflection trajectories bound within the hBN layer create an illusion of backward-moving and leaping behavior of pulse fringes. These rays result from the coherent beating of hyperbolic waveguide modes but could be mistakenly interpreted as negative group velocities and backward energy flow. Moreover, the zigzag reflections produce nanoscale (60 nm) and ultrafast (40 fs) spatiotemporal optical vortices along the trajectory, presenting opportunities to chiral spatiotemporal control of light-matter interactions. Supported by experimental evidence, our simulations highlight the potential of hyperbolic ray reflections for molecular vibrational absorption nanospectroscopy. The results pave the way for miniaturized, on-chip optical spectrometers, and ultrafast optical manipulation. [ABSTRACT FROM AUTHOR]

The dynamics of magnetic Rossby waves are investigated by applying a quasigeostrophic shallow water magnetohydrodynamic system, which is linearized with respect to both uniform background flow and uniform magnetic field. Due to the influence of the free surface divergence, the phase speed for magnetic Rossby waves can be either a monotonically increasing or a monotonically decreasing function, and the resulting difference between the group velocity and the phase speed can be either positive or negative. This is determined by whether the corresponding Alfvén wave speed is the upper limit or not. Differently, the phase speed is always a monotonically increasing function and the difference between the group velocity and the phase speed is always positive for incompressible magnetic Rossby waves. Multiplying a factor, the wavenumber vector shares the same endpoint with the group velocity vector. The endpoint moves on a cycle that has a center at the k -axis and is tangent to the l -axis in the wavenumber space. The circle is quite similar to the Longuet-Higgins circle for Rossby waves on Earth's atmosphere and ocean. The fundamental dynamics is the theoretical basis for deeply understanding the meridional energy transport by waves and the interaction between waves and the background states. [ABSTRACT FROM AUTHOR]

Anomalous thermal transport of Cs2NaYbCl6 double‐halide perovskite above room temperature is reported and rationalized. Calculations of phonon dispersion relations and scattering rates up to the fourth order in lattice anharmonicity have been conducted to determine their effective dependence on temperature. These findings show that specific phonon group velocities and lifetimes increase if the temperature is raised above 500 K. This, in combination with anharmonicity, provides the microscopic mechanism responsible for the increase in lattice thermal conductivity at high temperatures, contrary to the predictions of phonon transport theories based on solely cubic anharmonicity. The model accurately and quantitatively reproduces the experimental thermal conductivity data as a function of temperature. [ABSTRACT FROM AUTHOR]

The metal-insulator-metal (MIM) plasmonic waveguide has been highly anticipated for confining and guiding surface plasmon polaritons (SPPs) on the subwavelength scale. However, perennial drawbacks such as a short propagation length and an unbounded transverse field have set limits on the use of the MIM waveguide in various applications. Herein, diffraction- and dispersion-free MIM modes are synthesized by using space–time wave packets (STWPs) and are therefore referred to as space–time MIM (ST-MIM) waveguide modes. Compared to a Gaussian pulse of the same duration and spectral bandwidth, the ST-MIM demonstrates enhanced propagation lengths of about 2.4 times for the symmetric mode and about 6.3 times for the antisymmetric mode. In the simulations, the ST-MIMs are confined in all transverse dimensions, thereby overriding the diffraction limits. In addition, the group velocities of the ST-MIMs can be arbitrarily designed, which makes it possible to synchronize the pulse propagation speeds of the symmetric and antisymmetric MIM modes. [ABSTRACT FROM AUTHOR]

Past work has shown that interannual California coastal sea level variability is mostly of equatorial origin, and decades of satellite sea surface height (SSH) and in situ dynamic height observations indicate that this interannual signal propagates westward from the California coast as nondispersive Rossby waves (RWs). These observations agree with standard linear vertical mode theory except that even when mean flow and bottom topography are considered, the fastest baroclinic vertical mode RW in each case is always much slower (1.6–2.3 cm s−1) than the observed 4.2 cm s−1. This order-1 disagreement is only resolved if the standard bottom boundary condition that the vertical velocity w′ = 0 is replaced by perturbation pressure p′ = 0. Zero p′ is an appropriate bottom boundary condition because south of San Francisco the northeastern Pacific Ocean boundary acts approximately like an impermeable vertical wall to the interannual equatorial wave signal, and therefore equatorial quasigeostrophic p′ is horizontally constant along the boundary. Thus, if equatorial p′ = 0 at the bottom, then this condition also applies off California. The large-scale equatorial ocean boundary signal is due to wind-forced eastward group velocity equatorial Kelvin waves, which at interannual and lower frequencies propagate at such a shallow angle to the horizontal that none of the baroclinic equatorial Kelvin wave signal reaches the ocean floor before striking the eastern Pacific boundary. Off California this signal can thus be approximated by a first baroclinic mode with p′ = 0 at the bottom, and hence the long RW speed there agrees with that observed (both approximately 4.2 cm s−1). Significance Statement: The California Current System is one of the most biologically rich and best-documented coastal regions in the world. In this region coastal sea level propagates westward from the coast at about 110 km month−1, slow enough to enable us to make large-scale ocean climate forecasts of the California Current ecosystem using coastal sea level. Although the westward speed seems slow, theoretically it is about double what we would expect. Offered here is an explanation of why this speed is "too fast" by linking the California wave signal to the equator, El Niño, and the shallow equatorial ocean response to the wind. [ABSTRACT FROM AUTHOR]