In-plane Modes Research Articles

Phonon drag thermopower and energy loss rate in single and bilayer graphene due to piezoelectric surface acoustic phonons in Bloch-Gruneisen regime

In this report, we have both numerically and analytically calculated the phonon drag thermopower, Spz and, energy loss rate, Fpz due to piezoelectric surface acoustic phonons present in the underlying piezoelectric substrate at a distance, d apart from single and bilayer graphene as a function of carrier's temperature, concentration and phonon mean free path. The numerical results are then reduced to analytical results in the Bloch-Gruneisen regime and the corresponding power dependency of the quantities described above are obtained. We have also included the effect of Thomas-Fermi and Random Phase Approximation screening into account in these calculations. The obtained unscreened results are further reduced in much simpler form for a particular case having no distance between the graphene samples and the underlying substrate or at larger values of distances. The temperature dependency in Spz and Fpz for both SLG and BLG systems is found to be one order less than the in-plane acoustic phonons dependency, which is directly related to the matrix element of electron-in plane acoustic phonon interaction having a linear dependence on the phonon wave vector, q. Furthermore, a comparison of these calculations is made between the in-plane and piezoelectric surface acoustic phonon modes. One finds a crossover between these phonons modes depicting these are important in providing effective scattering channels for energy relaxation in typical experimental situations at different temperatures.

A Disc-Type High Speed Rotary Ultrasonic Motor with Internal Contact Teeth

This paper presents a disc-type ultrasonic piezoelectric motor, which is designed for micro flying vehicles. It provides a high output rotation speed under low operating voltage, compared with common piezoelectric devices, by employing a “contact teeth” wave transmission structure. The ultrasonic motor (USM) consists of a trimorph disc stator, with triple internal contact teeth, a shaft and two hemispheric hard-wearing rotors. The operating principle of the USM is based on the superposition of the in-plane B03 vibration mode of the trimorph disc, and the first longitudinal vibration of the contact teeth. An optimization method of the stator structure parameters was proposed and validated by numerical modeling. The diameter and thickness of the stator are 20 mm and 1 mm, respectively. A prototype with the weight of 2 g was made for this experimental test. The optimal frequency of the excitation signal and the preload force are 98.5 kHz and 0.5 N, respectively. The minimum operating voltage was tested under 7.5 V and reached the speed of 225 rpm, and the maximum unloaded rotational speed of the USM reached 5172 rpm when 30 V driving voltage was applied. The maximum lifting force generated by this USM was measured as 46.1 mN, which is 2.35 times bigger than its weight.

Strong coupling between in-plane photonic modes and out-of-plane plasmonic modes in 2D metal–dielectric–metal nanocylinder arrays

Arrays of metal–dielectric–metal (MDM) multilayer nanostructure can support various kinds of plasmonic modes including in-plane (IP) localized/delocalized surface plasmons or lattice resonances, photonic waves, as well as out-of-plane (OP) plasmon modes. Hybridization between different modes offers rich physics such as lasing, sub- and super-radiant modes, and efficient light absorption. While interplays between different sets of in-plane modes have been intensively investigated, the interaction between in-plane and out-of-plane modes is rarely explored. Here, we propose a MDM nanostructure with 2D nanocylinder array and study in detail the strong coupling between the in-plane photonic modes at the dielectric layer and the out-of-plane plasmon modes excited at the cylinder sidewalls. We demonstrate the strong coupling between the localized second-order IP mode and the first-order OP mode with a Rabi splitting up to 250 meV in the near-infrared region. A new IP mode arising from the radiative field can induce another kind of strong coupling with OP modes as the cylinder radius is larger than 98 nm. The underlying physics within this plasmon–photon system is quantitatively revealed by the coupled oscillator model and calculated E-field distribution. The proposed MDM system provides a good platform for investigating multimode coupling and nano-optics devices.

Evolution of Low‐Frequency Vibrational Modes in Ultrathin GeSbTe Films

In article number 2000434, Eugenio Zallo and co-workers report a methodology based on the Raman technique for the measurement of the layer number in ultrathin GeSbTe films obtained by molecular beam epitaxy. The experimental and theoretical evolution of the in-plane low-frequency vibrational modes of Ge2Sb2Te5 from bulk down to a single lamella is demonstrated. The energy shift is mainly related to the weak van der Waals interactions between the layers.

Dimension reduction for thin films prestrained by shallow curvature

We are concerned with the dimension reduction analysis for thin three-dimensional elastic films, prestrained via Riemannian metrics with weak curvatures. For the prestrain inducing the incompatible version of the Föppl–von Kármán equations, we find the Γ -limits of the rescaled energies, identify the optimal energy scaling laws, and display the equivalent conditions for optimality in terms of both the prestrain components and the curvatures of the related Riemannian metrics. When the stretching-inducing prestrain carries no in-plane modes, we discover similarities with the previously described shallow shell models. In higher prestrain regimes, we prove new energy upper bounds by constructing deformations as the Kirchhoff–Love extensions of the highly perturbative, Hölder-regular solutions to the Monge–Ampere equation obtained by means of convex integration.

Intrinsic DMI-free skyrmion formation and robust dynamic behaviors in magnetic hemispherical shells

We performed finite-element micromagnetic simulations to examine the formation of skyrmions without intrinsic Dzyaloshinskii–Moriya interaction (DMI) in magnetic hemispherical shells. We found that curvature-induced DM-like interaction allows for further stabilization of skyrmions without the DMI in curved-geometry hemispherical shells for a specific range of uniaxial perpendicular magnetic anisotropy (PMA) constant Ku. The larger the curvature of the shell, the higher the Ku value required for the formation of the skyrmions. With well-stabilized skyrmions, we also found in-plane gyration modes and azimuthal spin-wave modes as well as an out-of-plane breathing mode, similarly to previously found modes for planar geometries. Furthermore, additional higher-frequency hybrid modes were observed due to coupling between the gyration and azimuthal modes. This work provides further physical insight into the static and dynamic properties of intrinsic DMI-free skyrmions formed in curved-geometry systems.

In-plane dynamic crushing behaviors of joint-based hierarchical honeycombs with different topologies

Introducing the hierarchy into cellular materials has attracted increasing attention in the effort to pursue improved absorbed-energy abilities and impact resistance. In this paper, the dynamic crushing properties and energy absorption capacities of joint-based hierarchical honeycombs with different topologies were explored by means of explicit dynamic finite element (FE) analysis using ANSYS/LS-DYNA. Four types of joint-based hierarchical honeycombs with uniform cell-wall thickness were firstly constructed by substituting each vertex of regular honeycombs with a smaller self-similar cell (hexagon or square). The respective influences of hierarchical parameters and impact velocities on in-plane dynamic deformation modes, mechanical characteristic and energy absorption of joint-based hierarchical honeycombs were discussed. Research results showed that the hierarchy had a far greater influence on the in-plane deformation modes of honeycombs. Compared with regular honeycombs, the dynamic plateau stress and specific energy absorption of joint-based hierarchical honeycombs can be improved if the proper hierarchical parameters were chosen. Adding the joint-based hierarchy into regular honeycombs can enhance the crushing stress efficiency (CSE) of the specimens. In addition, by introducing a non-dimensional dynamic sensitivity index, the dynamic shock enhancement of hierarchical honeycombs was also investigated. These researches are useful for the multi-objective dynamic optimization design and controllable properties of cellular materials.

Hierarchical derivation of orthogonal cross-section modes for thin-walled beams with arbitrary sections

A new systematic approach is presented to derive the cross-section deformation modes of thin-walled beams with arbitrary sections within the framework of a higher-order beam theory (HoBT). New sets of higher-order modes, in this case warping and distortion, are derived hierarchically from the lowest mode set by considering the consistency between the strain field and the stress field generated by the modes in lower sets. Warping modes are derived by the shear stress of in-plane modes while distortion modes are induced by out-of-plane deformations via Poisson's effect. Higher-order modes are shown to be built as a linear combination of the integrated functions of lower-order modes, where the combination coefficients are determined by the orthogonality condition among the higher-order modes. Because the proposed method does not require any approximation when determining sectional mode shapes, no cross-section discretization, commonly used in existing studies, is needed. The effectiveness of the proposed mode derivation process is verified by comparing the static and modal analysis results of thin-walled beams with open, closed, and flanged cross-sections obtained by the proposed method, other HoBTs, and shell finite elements.

Non-linear Terahertz driving of plasma waves in layered cuprates

The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect. The strength of the phase stiffness is set by the Josephson coupling, which is strongly anisotropic in layered cuprates. So far, THz light pulses have been used to achieve non-linear control of the out-of-plane Josephson plasma mode, whose frequency lies in the THz range. However, the high-energy in-plane plasma mode has been considered insensitive to THz pumping. Here, we show that THz driving of both low-frequency and high-frequency plasma waves is possible via a general two-plasmon excitation mechanism. The anisotropy of the Josephson couplings leads to markedly different thermal effects for the out-of-plane and in-plane response, linking in both cases the emergence of non-linear photonics across Tc to the superfluid stiffness. Our results show that THz light pulses represent a preferential knob to selectively drive phase excitations in unconventional superconductors.

Hydration and Ion-Pair Formation of NaNO3(aq): A Vibrational Spectroscopic and Density Functional Theory Study.

Qualitative and quantitative Raman and infrared measurements on sodium nitrate (NaNO3) solutions have been carried out over a wide concentration range (5.56 × 10-6-7.946 mol/L) in water and heavy water. The Raman spectra were measured from 4000 cm-1 to low wavenumbers at 45 cm-1. Band fit analysis on the profile of the 1047 cm-1 band, ν1(a) measured at high resolution at 0.90 cm-1 produced a small contribution at 1027 cm-1 of the isotopomer N16O218O(aq). The effect of solute concentration on the Raman and infrared bands has been systematically recorded. Extrapolation of the experimental data resulted in values for all the nitrate bands of the "free", i.e., fully hydrated (aq). However, even in dilute solutions, the vibrational symmetry of the hydrated (aq) is broken and the antisymmetric N-O stretch, which is degenerate for the isolated anion, is split by 56 cm-1. At concentrations >2.5 mol/L, direct contact between Na+ and was observed and accompanied by large band parameter changes. DFT calculations on (H2O)n (n = 1-3) led to optimized geometries and vibrational frequencies which reproduced the measured ones within an accuracy of 1%. A hydrated gas phase species Na+(H2O)10 was optimized resulting in the geometry and symmetry of the nitrate, which is bound in an antisymmetric bidentate fashion with the nitrate possessing C1. The ν1 Na+(OH2) breathing mode in aqueous solution appears at 189 cm-1, whereas in heavy water, ν1 Na+(OD2) is shifted to 175.6 cm-1 due to the isotope effect. DFT calculations on hydrated Na+(OH2)n gas phase clusters provided realistic Na+ hydrate structures with n = 4 and 5, which resembled the measured frequency of ν1 Na+ OH2 mode quite well. Quantitative Raman analysis employing the symmetric stretching band, ν1(a) , has been carried out down to concentrations as low as 5.56 × 10-6 mol/L. The in-plane deformation mode ν4(e') in the Raman scattering at higher concentrations has been used as an indicator band for directly coordinated .

Stronger Interlayer Interactions Contribute to Faster Hot Carrier Cooling of Bilayer Graphene under Pressure.

We perform femtosecond pump-probe spectroscopy to insitu investigate the ultrafast photocarrier dynamics in bilayer graphene and observe an acceleration of energy relaxation under pressure. In combination with insitu Raman spectroscopy and abinitio molecular dynamics simulations, we reveal that interlayer shear and breathing modes have significant contributions to the faster hot-carrier relaxations by coupling with the in-plane vibration modes under pressure. Our work suggests that further understanding the effect of interlayer interaction on the behaviors of electrons and phonons would be critical to tailor the photocarrier dynamic properties of bilayer graphene.

Electron-vibrational coupling on the absorption spectrum of dithienyl-diketopyrrolopyrrole dye

The S1←S0 electronic spectrum of DPP2T in a chloroform solution was investigated using the vertical gradient (VG) and adiabatic shift (AS) models, which were compared with experimental results. The solvent effects were accounted through the polarizable continuum model (PCM) in non-equilibrium regime. Using the Condon approximation, linear coupling contribution for DPP2T's spectrum is mainly composed of zero-phonon and fundamental transitions. The most intense fundamental transition is related to symmetrical and in-plane normal mode stretching of carbon-carbon bonds belonging π-conjugated skeleton of DPP2T, which is quinoidized upon S1←S0 electronic transition. DPP2T's low-frequency torsional modes couple quadratically to the transition. Their thermal excitation at room temperature broadens the spectrum, shifting it slightly to higher energies.

Direct Detection of Highly Localized Metal-Metal Interface Plasmons from Bimetallic Nanoparticles

Tailoring of photon-matter interaction in solid material is critical for surface plasmon resonance-based sensing. This can be achieved from suitable material with interface engineering. The modified plasma oscillations in metal-metal interfaces are highly sought-after phenomena in plasmonics; however, such a localized nature of this oscillation has never been reported. Here we present the first evidence of localized interface plasmons from CoAg bimetallic nanoparticles by employing scanning transmission electron microscopy-electron energy-loss spectroscopy. We found that the localized interface plasmons oscillate with a frequency in between in-plane dipole localized surface plasmon resonance (LSPR) mode and quasiplanar mode. Moreover, we observed that the localized interface plasmon resonance is stronger than in-plane dipole LSPR which was characterized by comparing the quality factor of the energy-loss peaks. Such interface plasmon resonance was not distinctly observed from ensembles of CoAg nanoparticles by optical excitation incident normally; however, a broader in-plane dipole mode was observed compared to similar pure Ag nanoparticles. This direct detection of plasmons confined to the interface region could drive to future engineering of bimetallic interfaces with improved plasmonic activity.

Ballistic Penetration Analysis of Soft Laminated Composites Using Sublaminate Mesoscale Modeling

Ballistic impact mitigation requires the development of protective armor applications from composite material systems with good energy absorption and penetration resistance against threats, e.g., metallic projectiles. In this aim, high-strength and high-stiffness soft fibrous composite materials (such as ultra-high molecular weight polyethylene—UHMWPE) are often used. The high specific strength feature is one of the main reasons for using these soft composite systems in ballistic impact applications. In the present investigation, experimental and computational finite element (FE) studies were carried out to investigate the ballistic behaviors of these soft layered composite targets. To this end, a new FE multi-scale analysis framework for ballistic simulations is offered. The proposed analysis presents a new meso-scale sublaminate material model, which is applied to Dyneema® cross-ply laminate in order to predict its behavior under ballistic impact. The sublaminate model is implemented within an explicit dynamic FE code to simulate the continuum response in each element. The sublaminate model assumes a through-thickness periodic stacking of repeated cross-ply configuration. In addition, a cohesive layer is introduced in the sublaminate model in order to simulate the delamination effect leading to the subsequent degradation and deletion of the elements. This new approach eliminates the widely used costly computational approach of using explicit cohesive elements installed at pre-specified potential delamination paths between the layers. Furthermore, in-plane damage modes (such as fiber tensile, and out-of-plane shearing) are also accounted for by employing failure criteria and strain-softening. The obtained quantitative results of ballistic impact simulations show good correlation when compared to a relatively wide range of experiments. Moreover, the simulations include evidence of capturing the main energy absorption mechanisms under high-velocity impact. The proposed modeling approach can be used as a useful armor design tool.

Experimental study of seismic in-plane elliptical damper with empirical design formulae

A seismic steel damper designed to deform inelastically in an in-plane flexural bending mode for more efficient utilisation of the material was investigated. This proposed device, referred to as an in-plane elliptical damper (i-PED), consists of two circular arches with intermediate straight arms connected in a closed form to eliminate end rotation at the symmetric axis in order to enhance the ultimate capacity. Test results obtained from a series of component tests on i-PEDs with various dimensions indicated that the energy-dissipation capacities of the dampers under cyclic loads were rich and consistently stable. The i-PED with a shorter arm length and smaller radius of curvature of arch exhibited higher strength and energy-dissipation capacity. Empirical formulae for estimating the mechanical properties of the dampers were developed for preliminary design purposes. The numerical predictions were found to correlate well with the experimental results, demonstrating that using the Bouc–Wen model is sufficient for simulating the inelastic mechanical behaviour of steel dampers. Furthermore, shaking table tests of a five-storey modal frame implemented with i-PEDs were conducted, with non-linear time history analyses carried out for comparison. Encouraging results on seismic response control with good numerical prediction were obtained, suggesting the feasibility of the proposed damper and design scheme for practical application.

Simplified approach to assess the vulnerability of masonry buildings under tsunami loads

Several catastrophic tsunami events in recent years have engaged scientific awareness to assess the tsunami risk. Fragility curves are required to analyse the vulnerability of masonry buildings and analytical fragility curves should be based on local building typologies. The aim of this study was to assess the vulnerability of masonry buildings under tsunami loads by means of the minimum inundation depth to trigger a failure mechanism. This was done using a large-scale approach and with a low knowledge level of the properties of structures. The activation of out-of-plane and in-plane mechanisms was investigated by modelling an external tsunami load pattern using simplified assumptions. Shear and flexural failure were analysed as the main in-plane failure modes while horizontal and vertical bending mechanisms were considered as the main out-of-plane failure modes. Masonry walls were modelled as frame elements and the masonry was modelled as a non-linear material. Japanese guidelines were assumed for modelling tsunami loads instead of more refined models such as those in ASCE 7-16 and Fema P-646, due to the reduced achievable knowledge level. The proposed mechanical models provide important information about the vulnerability of masonry buildings under tsunami loads and represent a basic step to provide vulnerability information on masonry buildings on a regional scale.

Intrinsic effect of interfacial coupling on the high-frequency intralayer modes in twisted multilayer MoTe2.

The interfacial coupling at the interface makes the van der Waals heterostructures (vdWHs) exhibit many unique properties that cannot be realized in its constituents. Such a study usually starts with a twisted stack of two flakes exfoliated from the same layered materials to form twisted multilayers, in which the impact of interfacial coupling on the low-frequency interlayer modes had been well understood. However, it is not clear how interfacial coupling affects the high-frequency intralayer modes of twisted multilayers. Herein, we perform high-resolution resonance Raman spectroscopy of the high-frequency intralayer modes in twisted multilayer MoTe2 (tMLM). All the Davydov entities of the out-of-plane intralayer mode are observed and distinguished at 4 K. It is found that the out-of-plane intralayer modes in tMLM are sensitive to its interfacial layer-breathing coupling so that the out-of-plane intralayer modes in tMLM do not show a direct relationship with those of the two constituents. However, the case is quite different for the in-plane intralayer modes in tMLM, whose spectral profile can be fitted by those of the corresponding modes of its constituents. This indicates that the in-plane intralayer modes are localized within the constituents in tMLM because of its negligible interfacial shear coupling at the interface. All the results can be well understood using the vdW model in which only the nearest neighbor interlayer/interfacial interaction is taken into account. This work directly builds the relationship between the Davydov splitting of the high-frequency intralayer vibrations and the low-frequency interlayer vibrations in tMLM, which can be further extended to other twisted materials and the related vdWHs.

Surface roughness and substrate induced symmetry-breaking: influence on the plasmonic properties of aluminum nanostructure arrays.

The surface topography is known to play an important role on the near- and far-field optical properties of metallic nanoparticles. In particular, aluminum (Al) nanoparticles are commonly fabricated through evaporation techniques, therefore exhibiting elevated surface roughness additionally to their native oxide layer. In this study, the mode-dependent influence of surface roughness on the plasmonic properties sustained by Al nanodisks (NDs) is first numerically investigated using a realistic model taking into account the thin native oxide layer. Due to the symmetry-breaking induced by the supporting dielectric substrate to Al ND, it appears that the roughness affects differently the substrate-induced out-of-plane quadrupolar mode (below 300 nm) and the in-plane dipolar mode sustained by the Al ND. By increasing the top surface roughness of the Al ND, the substrate-induced quadrupolar mode is significantly damped especially in the ultraviolet regime, while the dipolar resonance is broadened and redshifted. The explanation of these effects relies in the decoherence and dissipation of the collective electronic oscillations as a result of the top surface roughness to the different near-field distribution of the out-of-plane quadrupolar mode and in-plane dipolar mode. Moreover, the influences of the diameter of Al ND, dielectric substrate with different refractive index, and the oxidation of Al ND on these two modes are also investigated. Particularly, the quadrupolar mode disappears with surface roughness and oxidation, explaining why this mode is very weak and sometimes barely visible on evaporated Al nanostructures reported in the literature. Finally, these results are experimentally confirmed by characterizing the optical properties of periodic Al ND arrays.

Doping controlled Fano resonance in bilayer 1T'-ReS2: Raman experiments and first-principles theoretical analysis.

In the bilayer ReS2 channel of a field-effect transistor (FET), we demonstrate using Raman spectroscopy that electron doping (n) results in softening of frequency and broadening of linewidth for the in-plane vibrational modes, leaving the out-of-plane vibrational modes unaffected. The largest change is observed for the in-plane Raman mode at ∼151 cm-1, which also shows doping induced Fano resonance with the Fano parameter 1/q = -0.17 at a doping concentration of ∼3.7 × 1013 cm-2. A quantitative understanding of our results is provided by first-principles density functional theory (DFT), showing that the electron-phonon coupling (EPC) of in-plane modes is stronger than that of out-of-plane modes, and its variation with doping is independent of the layer stacking. The origin of large EPC is traced to 1T to 1T' structural phase transition of ReS2 involving in-plane displacement of atoms whose instability is driven by the nested Fermi surface of the 1T structure. Results are compared with those of the isostructural trilayer ReSe2.

Smart Cathodic Protection System for Real-Time Quantitative Assessment of Corrosion of Sacrificial Anode Based on Electro-Mechanical Impedance (EMI)

Corrosion of metal structures is often prevented using cathodic protection systems, that employ sacrificial anodes that corrode more preferentially relative to the metal to be protected. In-situ monitoring of these sacrificial anodes during early stages of their useful life could offer several insights into deterioration of the material surrounding the infrastructure as well as serve as early warning indicator for preventive maintenance of critical infrastructure. In this article, we present an Electro-Mechanical Impedance (EMI) measurement-based technique to quantify extent of corrosion of a zinc sacrificial anode without manual intervention. The detection apparatus consists of a lead zirconate titanate (PZT) transducer affixed onto a circular zinc disc, with waterproofing epoxy protecting the transducer element when the assembly is submerged in liquid electrolyte (salt solution) for accelerated corrosion by means of impressed current. We develop an analytical model for discerning the extent of corrosion by monitoring shift in resonance frequency for in-plane radial expansion mode of the disc. The model presented here accurately captures the nonlinearity introduced by partial delamination of the corrosion product (zinc oxide) from the disc, and shows excellent agreement with experimental results. Our work establishes the efficacy of the proposed technique for monitoring the state of health of sacrificial anodes in their early stage of deterioration and could thus be widely adopted for structural health monitoring applications within the internet of things.