Group Velocity Of Mode Research Articles

Impact of noble-gas filler atoms on the lattice thermal conductivity of CoSb3 skutterudites: first-principles modelling

We present a systematic first-principles modelling study of the structural dynamics and thermal transport in CoSb3 skutterudites with a series of noble-gas filler atoms. Filling with chemically-inert atoms provides an idealised model for isolating the effects of the fillers from the impact of redox changes to the host electronic structure. A range of analysis techniques are proposed to estimate the filler rattling frequencies, to quantify the separate impacts of the fillers on the phonon group velocities and lifetimes, and to show how changes to the phonon spectra and interaction strengths lead to suppressed lifetimes. The noble-gas fillers are found to reduce the thermal conductivity of the CoSb3 framework by up to 15% primarily by suppressing the group velocities of low-lying optic modes. The filler rattling frequencies are determined by a detailed balance of increasing atomic mass and stronger interactions with the framework, and are found to be a good predictor of the impact on the heat transport. Lowering the rattling frequency below ∼1.5 THz by selecting heavy fillers that interact weakly with the framework is predicted to lead to a much larger suppression of the thermal transport, by inducing avoided crossings in the acoustic-mode dispersion and facilitating enhanced scattering and a consequent large reduction in phonon lifetimes. Approximate rattling frequencies determined from the harmonic force constants may therefore provide a useful metric for selecting filler atoms to optimise the thermal transport in skutterudites and other cage compounds such as clathrates.

Mode excitability and selectivity for enhancing scanning guided wave-based characterization of subwavelength defect

The scanning guided ultrasonic waves (GUWs) have been exploited intensively for characterizing defects or material anomalies at their early stage. Yet, the evaluation of defects in a subwavelength scale, which induces indiscernible disturbance to the linear and nonlinear features of GUWs, remains a daunting task for methods using the scanning GUWs owning to their limitation in terms of sensitivity and applicability. Based on the investigation of effect of defect on the excitability of GUW modes, we establish a framework to distinguish the optimal GUW mode with which the characterization of the small defect with subwavelength size using scanning GUWs can be enhanced. In this framework, the excitation transducer is scanned across the specimen surface to generate probing GUWs. The excitability of each GUW mode is investigated analytically, and the theoretical interpretation of effect of defect on excitation of each GUW mode is interrogated using a reciprocity theorem and Born approximation. In conjunction with the analysis of group velocity of GUW modes, the candidate GUW mode, i.e. mode S1 at a frequency-thickness product of 3.58 MHz⋅mm, which features an excitability with optimal sensitivity to defect and high practical applicability is selected. A damage index that calibrates the extent of defect-induced variation in the waveform of the selected GUW mode is proposed, with which the subwavelength defects can be characterized accurately and reliably. Numerical and experimental validations are performed, in which the surface and subsurface defects of subwavelength scales are identified and visualized using the selected mode and an imaging algorithm. The results corroborate the effectiveness of the proposed framework for enhancing the characterization of subwavelength defects using modally selective scanning GUWs.

Lattice dynamics of graphene nanoribbons under twisting.

In this communication, we investigate the lattice dynamics of twisted graphene nanoribbons using the density-functional tight-binding method based on screw symmetry. The results show that the decrease in phonon group velocity induced by twisting reduces the lattice thermal conductivity. Our findings provide inspiration for the design of graphene-based phononic devices tailored by inhomogeneous strain.

Ultrasonic Measurement of Axial Stress Using High-Frequency Cylindrical Guided Wave

A new stress measurement method using single source high-frequency cylindrical guided waves is put forward to promote the efficiency and precision of the contact-type ultrasonic testing technique. The propagation patterns and acousto-elasticity of cylindrical guided waves, traveling in a loaded steel rod, are analyzed under the theoretical framework of the Pochhammer-Chree equation and nonlinear acoustics. According to the theoretical derivations, the ratio of the maximum group velocities of different modes is selected as the measurement criterion of stress. Then, a stress calibration/measurement experiment is conducted to prove the validity of the method. A single-source ultrasonic transducer driven with an impulsive excitation is introduced in the experiment and the generated multimode guided waves in the received signals are analyzed. It is observed from experimental results that the group speed ratio of two different modes is linearly proportional to the axial stress and the evaluation errors of the proposed method is less than 6%.

Omnidirectional field enhancements drive giant nonlinearities in epsilon-near-zero waveguides.

Bulk materials with a relative electric permittivity ε close to zero exhibit giant Kerr nonlinearities. However, harnessing this response in guided-wave geometries is not straightforward, due to the extreme and counterintuitive properties of such epsilon-near-zero materials. Here we investigate, through rigorous calculations of the nonlinear coefficient, how the remarkable nonlinear properties of such materials can be exploited in several structures, including bulk films, plasmonic nanowires, and metal nanoapertures. We find the largest nonlinear response when the modal area and group velocity are simultaneously minimized, leading to omnidirectional field enhancement. This insight will be key for understanding nonlinear nanophotonic systems with extreme nonlinearities and points to new design paradigms.

Tunable TM modes in a slab waveguide including a graphene-dielectric multilayer structure

TM modes in a slab waveguide with a graphene-dielectric multilayer structure as the core and different ordinary materials as the cover and the substrate are investigated. The group velocity of TM modes can be positive, negative, and zero. The standstill of electromagnetic waves at different key waveguide thickness, which depend on the gate voltage, is successfully realized. The gate voltages or mode numbers have a greater influence on the zero and negative group velocities, but have a little influence on the positive group velocity. Besides, the power flow can be positive, zero and negative values at the key waveguide thickness. Similarly, the gate voltages or mode numbers have a little influence on the zero and negative power flows when the frequency is less than 7.0 THz, but they have a big influence on both positive and negative power flows when the frequency is larger than 7.0 THz. More interestingly, as the frequency increases from 9.0THz to 10.0THz, superpower flows of TM modes reach a dynamic balance both the core and the cladding. The tunable superpower flow properties will benefit us to construct a novel full optical switch and an ultra large optical power memory.

Damage detection of composite plate based on an improved DAS algorithm by time difference due to anisotropy

This paper proposes a damage detection method based on an improved DAS imaging algorithm by introducing time difference due to anisotropy of composite material. First, the finite element characteristic frequency method is used to obtain the dispersion curve of the composite plate, and the validity of the dispersion curve is verified. Next, the average phase velocity of the Lamb wave at mixed modes in the composite plate is obtained by two-dimensional Fourier transform (2-D FFT). The mixed modal group velocity is calculated according to the corresponding phase velocity, the mean change rate of the phase velocity and the dispersion curve obtained by simulation. The time difference due to anisotropy of composite material is investigated, and the damage location is estimated by the delay-and-sum (DAS) imaging algorithm. Experiments on carbon fiber multilayer composite plates verify the effectiveness of the proposed method.

Geoacoustic parameter estimation using machine learning techniques

The Airy phase region corresponding the minimum group velocity of the dispersive acoustic normal modes in shallow water are extremely sensitive to bottom properties. The group speed minima and the associated frequency data for two lower order modes were used to train a neural network to predict the bottom parameters in a previous study (Potty et al., 2019). The geoacoustic model for the bottom consisted of a sediment layer over acoustic basement. Synthetic data generated by varying the values of the sound speeds in the sediment layer and basement were used to train and evaluate the technique. The output of the neural network was used as the background model for a linear perturbation inversion. Data collected from the Shelf break Primer experiment were used to test the algorithm and the preliminary results were promising. This study will continue the previous work by incorporating higher order modes in the training data, testing with data from other experiments and using other machine learning techniques. The performance of this hybrid approach (neural network combined with linearized inversion) will be compared with fully non-linear inversion, both in terms of accuracy and computation time. [Work supported by the Office of Naval Research.]

The dependence of lattice thermal conductivity on phonon modes in pyrochlore‐related Ln2Sn2O7 (Ln = La, Gd)

AbstractRare‐earth pyrochlore materials are promising thermal barrier coatings materials and fundamental understanding of their thermal transport is crucial for further improving its performance. In this work, using density functional theory (DFT) method, we calculated the intrinsic lattice thermal conductivities of Ln2Sn2O7 (Ln = La, Gd) and conducted a comprehensive analysis on the mode thermal conductivity, relaxation time, Grüneisen parameters, group velocity, and specific heat, respectively. It is shown that in pyrochlore‐type materials the number of the optical phonons is much larger than that of the acoustic phonon, and the thermal conductivity of acoustic phonons are suppressed, both of which increase the contribution ratio of optical phonons. Especially, through cumulative analysis, we found that the contribution of optical phonons is significant: the ratio of optical contribution is more than 50% and 64% in La2Sn2O7 and Gd2Sn2O7. This work provides a comprehensive picture illustrating the significant role of the optical phonons in the lattice thermal conduction in rare‐earth pyrochlore materials, and points out an avenue to obtain low thermal conductivity in complex structural thermal insulation materials.

Neurocomputing approach to predict 2D ferromagnetic lattices by spin dynamics

An artificial neural networks modeling, based on spin dynamics, has been proposed to predict the 2D ferromagnetic crystals. The purpose is to establish an artificial neural network model (ANNM), which is able to predict the ordered magnetic surfaces. In crystal surfaces, Wood’s notation is used to describe different configurations. From the magnons dispersion curves and group velocity data of 2D spins crystallographic waveguide, we deduce the data used as an up input–output set to feed the ANNM. The input data are being given by the magnons modes over high symmetry points, or, by the group velocity of the magnon modes. However, the basis vectors of matrix notation and the rotation angle that aligns the unit cell of the reconstructed surface are giving the output data. The proposed neurocomputing model was capable of predicting the 2D crystals for the possible arrangements that the surface might take. In addition, the magnon excitations data were a useful approach for building a neural model which was able to predict and classify the magnetic surfaces of the crystals.

"Fast" Plasmons Propagating in Graphene Plasmonic Waveguides with Negative Index Metamaterial Claddings.

We propose the monolayer graphene plasmonic waveguide (MGPW), which is composed of graphene core sandwiched by two graphene metamaterial (GMM) claddings and investigate the properties of plasmonic modes propagating in the waveguide. The effective refraction index of the GMMs claddings takes negative (or positive) at the vicinity of the Dirac-like point in the band structure. We show that when the effective refraction index of the GMMs is positive, the plasmons travel forward in the MGPW with a positive group velocity (vg > 0, vp > 0). In contrast—for the negative refraction index GMM claddings—a negative group velocity of the fundamental mode (vg < 0, vp > 0) appears in the proposed waveguide structure when the core is sufficiently narrow. A forbidden band appears between the negative and positive group velocity regions, which is enhanced gradually as the width of the core increases. On the other hand, one can overcome this limitation and even make the forbidden band disappear by increasing the chemical potential difference between the nanodisks and the ambient graphene of the GMM claddings. The proposed structure offers a novel scheme of on-chip electromagnetic field and may find significant applications in the future high density plasmonic integrated circuit technique.

Effect of biaxial strain on thermal transport in WS2 monolayer from first principles calculations

Abstract Recently, tungsten disulfide (WS2) monolayer has emerged as a two-dimensional material due to its outstanding physical properties. In this work, the thermal transports in strained WS2 monolayer are studied systematically by combining the first principles calculations and the Boltzmann transport equation. It can be found that the lattice thermal conductivity (k) decreases from 262.78 to 190.66 W m−1 K−1 for biaxial tensile strain (from 0% to 8%) and from 262.78 to 217.40 W m−1 K−1 for biaxial compressive strain (from 0% to −4%), respectively. The softened transverse/longitudinal acoustic phonon mode and the stiffened flexural acoustic phonon mode in strained WS2 monolayer change the phonon group velocities. We attribute the reduction of k in tensilely strained WS2 monolayer to the declining phonon group velocity, heat capacity and phonon lifetime of acoustic phonon modes. The comprehensive competition among the increasing phonon group velocity, heat capacity and the decreasing phonon lifetime suppresses the lattice thermal conductivity of compressively strained WS2 monolayer. Furthermore, the increasing phase space proves that the enhancing anharmonic phonon scattering, which results in the suppression of the k in tensilely strained WS2 monolayer. Additionally, the critical phonon mean free path (MFP) under different biaxial strain levels is calculated to demonstrate the contribution of the phonons with different MFPs to the total k of WS2 monolayer. This work provides a phonon behavior analysis of the thermal conduction in WS2 monolayer and paves the path for potential applications of WS2-based devices.

Ultra-low thermal conductivity and super-slow hot-carrier thermalization induced by a huge phononic gap in multifunctional nanoscale boron pnictides

Abstract Recent synthesis of thin films of boron monophosphide (BP) motivates us to report herewith the lattice thermal conductivity calculated in nanoscale boron pnictides. The lattice thermal conductivity in BX (X = P, As, Sb) monolayers are calculated to be 51.3, 20.7 and 5.4 Wm−1K−1 respectively at 300 K. The ultra-low lattice thermal conductivity in BSb, which is comparable to that in SnSe, is attributed to low elastic and bond stiffness, low Debye temperature, small group velocity and large mode Gruneisen parameter near zone center. Phonon group velocities in BSb are comparable to that in SnSe and Bi2Te3. Up to 50% reduction in lattice thermal conductivity is noticed at 300 K upon shortening the phonon mean free path, which is realizable, e.g., in 1D nanoribbons, or via defects, vacancies, nanoengineering, etc. A comparative study based on different models to compute the lattice thermal conductivity will provide useful insights into the experimentally measured ones. The widest ever phononic gap (436 & 438 cm−1) is found in BAs and BSb monolayers, where the transverse and longitudinal optical phonons meet the criterion for the prevention of Klemens decay and hence, they can be gainfully exploited in hot-carrier solar cells. BX (X = P, As, Sb) monolayers show a direct bandgap and piezoelectricity. Nanoscale boron pnictides are promising materials in futuristic thermoelectrics, bolometers, third-generation solar cells and nanopiezotronics.

Waveguided modes in a planar structure «graphene – thin semiconductor film – graphene»

We investigated optical-range waveguide modes propagating in a semiconductor film sandwiched between two graphene plates. The mode characteristics were shown to depend on the chemical potential of graphene and the film thickness. Based on the numerical analysis, we obtained dispersion relations for the first waveguide modes, frequency dependences of their group and phase velocities, and the distribution of the energy flux density in the structure. We discovered the presence of spectral bands characterized by small phase and negative group velocities of the waveguide modes. The possibility of tuning the waveguide mode by changing the chemical potential of graphene and the thickness of the semiconductor film was established.

Highly tunable thermal conductivity of C3N under tensile strain: A first-principles study

In this study, the phonon thermal transport in monolayer C3N under biaxial strains ranging from 0% to 10% has been investigated using first-principles calculations based on the Boltzmann transport equation. It is found that the thermal conductivity κ of C3N shows a nonmonotonic up-and-down behavior in response to tensile strain, and the maximum κ occurs at a strain of 6%. Interestingly, the thermal conductivity of monolayer C3N shows a remarkable high strain tunability, as its value at 6% strain is about 13.2 times higher than the value of κ in an unstrained monolayer. A mode-by-mode phonon level analysis shows that a competition between different phonon properties is responsible for such variations in the thermal conductivity. We found that the decrease in group velocity of the transverse acoustic, longitudinal acoustic, and optical modes as well as the increase in the three-phonon phase space of all the acoustic modes tend to reduce the thermal conductivity with strain. However, the group velocity of the z-direction acoustic mode and the Grüneisen parameter of all acoustic modes change in the direction of increasing the phonon lifetime and the thermal conductivity with increasing strain. Upon stretching, the change in the Grüneisen parameter and the phonon lifetime of the acoustic modes is found to be drastically higher than the change in other properties. The competition between these opposite effects leads to the up-and-down behavior of the thermal conductivity in C3N.

Lattice dynamics of Pnma Sn(S1–xSex) solid solutions: energetics, phonon spectra and thermal transport

Alloying is widely used as a means to fine-tune the properties of thermoelectric materials by reducing the lattice thermal conductivity. However, the effects of compositional variation on the lattice dynamics of alloy systems are not well understood, due in part to the difficulty of building realistic first-principles models of structurally-complex solid solutions. This work builds on our previous study of Snn(S1–xSex)m solid solutions (Gunn et al 2019 Chem. Mater. 31 3672) to explore the lattice dynamics of the Pnma Sn(S1–xSex) system, which has been widely studied for potential thermoelectric applications. We find that the vibrational internal energy and entropy have a large quantitative impact on the mixing free energy and are likely to be particularly important in alloy systems with competing phases. The thermodynamically-averaged phonon dispersions and density of states curves show that alloying preserves the structure of the low-frequency bands of modes associated with the Sn sublattice but broadens the high-frequency chalcogen bands into a near-continuous spectrum at the 50/50 mixed composition. This results in a general reduction in the phonon mode group velocities and an increase in the number of energy-conserving scattering channels for heat-carrying low-frequency modes, which is consistent with the decrease in thermal conductivity observed in experimental measurements. Finally, we discuss some of the limitations of our first-principles modelling approach and propose methods to address these in future studies.

Flexural Wave Propagation of Double-Layered Graphene Sheets Based on the Hamiltonian System

Double-layered graphene sheets (DLGSs) as a new type of nanocomponents, with special mechanical, electrical and chemical properties, have the potential of being applied in the nanoelectro-mechanical systems (NEMS) and nanoopto-mechanical systems (NOMS). In DLGSs structure, the two graphene sheets are connected by van der Waals (vdW) interaction. Thus, it can exhibit two vibration modes during the propagation of the flexural wave, i.e., in-phase mode and anti-phase mode. Based on the Kirchhoff plate theory and the nonlocal elasticity theory, Hamiltonian equations of the DLGSs are established by introducing the symplectic dual variables. By solving the Hamiltonian equation, the dispersion relation of the flexural wave propagation of the DLGSs is obtained. The numerical calculation indicates that the bending frequency, phase velocity and group velocity of the in-phase mode and anti-phase mode for the DLGSs are closely related to the nonlocal parameters, the foundation moduli and the vdW forces. The research results will provide theoretical basis for the dynamic design of DLGSs in micro-nanofunctional devices.

Refractive index and group velocity of electromagnetic modes in a relativistic Fermi gas at finite temperatures

A theoretical study of the optical properties of a relativistic Fermi gas at a finite temperature is performed. The results are obtained from Maxwell’s equations, assuming bianisotropic constitutive relations for the relativistic gas. The longitudinal and transverse spatial directions are defined according to the properties of the electric permittivity tensor. The electromagnetic modes and the refractive index of the relativistic gas are calculated along such directions. The longitudinal and transverse group velocities corresponding to the respective propagation modes are also obtained. No typical characteristics of metamaterials are observed, neither in the refractive index nor in the group velocities corresponding to the transverse modes. However, in the case of longitudinal modes, negative values of the group velocity are observed in a region of the reciprocal space where particle–antiparticle excitations can occur.

Sensitivity of longitudinal guided wave modes to pitting corrosion of rebars embedded in reinforced concrete

The detection of incipient pitting corrosion in rebars that are embedded in reinforced concrete is an important area of research currently. Similar damages in metallic and composite structures are effectively uncovered using guided waves; however, their application in the identifying of defects in concrete remains elusive. In this paper, the influence of longitudinal guided wave modes on pitting corrosion is explored in detail to identify specific phases of corrosion, namely; corrosion initiation, corrosion progression, and diameter reduction. An impressed-current-based accelerated corrosion setup is designed to induce pitting corrosion in reinforced concrete (RC) beams. These beams are continuously monitored using longitudinal guided wave modes that are generated and captured by contact transducers. Theoretical dispersion curves are generated using an approximate concrete model to recognise the guided wave modes. The effectiveness of this model is examined by comparison with a full-scale numerical model. A pitting corrosion identification strategy is then devised using a non-dimensional parameter that is termed relative amplitude (RA), and the group velocity extracted from wavelet analysis of a guided wave mode. We demonstrate that the RA values and the group velocities of the higher longitudinal wave modes, L(0,3) and L(0,4), have the potential to decipher the various phases of pitting corrosion.

Group dynamics and wave resonances in a narrow gap: modes and reduced group velocity

The spatial and temporal structure of the resonant fluid response in a narrow gap (the so-called gap resonance) between two identical fixed boxes is investigated experimentally. Transient wave groups are used to excite the gap resonance from different wave approach directions. This shows a strong beating pattern and a very long duration, reflecting that gap resonance is a multi-mode resonant and weakly damped phenomenon. For head sea excitation the linear transfer function of the gap mode is as significant as that of the mode. Gap resonance can be driven through different mechanisms, e.g. linear excitation and a nonlinear frequency-doubling process. Significant wave group structure is shown in the gap, and the group structure is more distinct in the case with frequency doubling, i.e. long wave, excitation. Then it is clearer visually that the groups originate at the end of the gap, propagate along the gap and are then partially reflected from the other end. The groups within the gap are very clear because the group velocity is close to constant for the first few gap resonance modes, and much smaller than that for free waves on the open sea. In contrast, the phase speed of waves in the gap is larger than that for free waves outside. Only in the limit of short waves do the group velocity and phase speed of the gap modes tend to those of deep-water free waves. The group and phase speeds from these experiments match well the theoretical forms given by Molin et al. (Appl. Ocean Res., vol. 24 (5), 2002, pp. 247–260), albeit for a slightly different box cross-sectional shape.