A brief review on the recent development of phonon engineering and manipulation at nanoscales

Phonons are the quantum mechanical descriptions of vibrational modes that manifest themselves in many physical properties of condensed matter systems. As the size of electronic devices continues to decrease below mean free paths of acoustic phonons, the engineering of phonon spectra at the nanoscale becomes an important topic. Phonon manipulation allows for active control and management of heat fow, enabling functions such as regulated heat transport. At the same time, phonon transmission, as a novel signal transmission method, holds great potential to revolutionize modern industry like microelectronics technology, and boasts wide-ranging applications. Unlike fermions such as electrons, polarity regulation is difficult to act on phonons as bosons, making the development of effective phonon modulation methods a daunting task.This work reviews the development of phonon engineering and strategies of phonon manipulation at different scales, reports the latest research progress of nanophononic devices such as thermal rectifiers, thermal transistors, thermal memories, and thermoelectric devices,and analyzes the phonon transport mechanisms involved. Lastly, we survey feasible perspectives and research directions of phonon engineering. Thermoelectric analogies, external field regulation, and acousto-optic co-optimization are expected to become future research hotspots.

Wide frequency phonons manipulation in Si nanowire by introducing nanopillars and nanoparticles

The combination of different nanostructures can hinder phonons transmission in a wide frequency range and further reduce the thermal conductivity(TC).This will benefit the improvement and application of thermoelectric conversion,insulating materials and thermal barrier coatings,etc.In this work,the effects of nanopillars and Ge nanoparticles(GNPs)on the thermal transport of Si nanowire(SN)are investigated by nonequilibrium molecular dynamics(NEMD)simulation.By analyzing phonons transport behaviors,it is confirmed that the introduction of nanopillars leads to the occurrence of lowfrequency phonons resonance,and nanoparticles enhance high-frequency phonons interface scattering and localization.The results show that phonons transport in the whole frequency range can be strongly hindered by the simultaneous introduction of nanopillars and nanoparticles.In addition,the effects of system length,temperature,sizes and numbers of nanoparticles on the TC are investigated.Our work provides useful insights into the effective regulation of the TC of nanomaterials.

Phonon transport properties of Janus Pb_(2)XAs(X=P,Sb,and Bi)monolayers:A DFT study

Grasping the underlying mechanisms behind the low lattice thermal conductivity of materials is essential for the efficient design and development of high-performance thermoelectric materials and thermal barrier coating materials.In this paper,we present a first-principles calculations of the phonon transport properties of Janus Pb_(2)PAs and Pb_(2)SbAs monolayers.Both materials possess low lattice thermal conductivity,at least two orders of magnitude lower than graphene and h-BN.The room temperature thermal conductivity of Pb_(2)SbAs(0.91 W/m K)is only a quarter of that of Pb_(2)PAs(3.88 W/m K).We analyze in depth the bonding,lattice dynamics,and phonon mode level information of these materials.Ultimately,it is determined that the synergistic effect of low group velocity due to weak bonding and strong phonon anharmonicity is the fundamental cause of the intrinsic low thermal conductivity in these Janus structures.Relative regular residual analysis further indicates that the four-phonon processes are limited in Pb_(2)PAs and Pb_(2)SbAs,and the three-phonon scattering is sufficient to describe their anharmonicity.In this study,the thermal transport properties of Janus Pb_(2)PAs and Pb_(2)SbAs monolayers are illuminated based on fundamental physical mechanisms,and the low lattice thermal conductivity endows them with the potential applications in the field of thermal barriers and thermoelectrics.

Phonon resonance modulation in weak van der Waals heterostructures:Controlling thermal transport in graphene-silicon nanoparticle systems

The drive for efficient thermal management has intensified with the miniaturization of electronic devices.This study explores the modulation of phonon transport within graphene by introducing silicon nanoparticles influenced by van der Waals forces.Our approach involves the application of non-equilibrium molecular dynamics to assess thermal conductivity while varying the interaction strength,leading to a noteworthy reduction in thermal conductivity.Furthermore,we observe a distinct attenuation in length-dependent behavior within the graphene-nanoparticles system.Our exploration combines wave packet simulations with phonon transmission calculations,aligning with a comprehensive analysis of the phonon transport regime to unveil the underlying physical mechanisms at play.Lastly,we conduct transient molecular dynamics simulations to investigate interfacial thermal conductance between the nanoparticles and the graphene,revealing an enhanced thermal boundary conductance.This research not only contributes to our understanding of phonon transport but also opens a new degree of freedom for utilizing van der Waals nanoparticle-induced resonance,offering promising avenues for the modulation of thermal properties in advanced materials and enhancing their performance in various technological applications.

Surface phonon resonance:A new mechanism for enhancing photonic spin Hall effect and refractive index sensor

Metal-based surface plasmon resonance(SPR)plays an important role in enhancing the photonic spin Hall effect(SHE)and developing sensitive optical sensors.However,the very large negative permittivities of metals limit their applications beyond the near-infrared regime.In this work,we theoretically present a new mechanism to enhance the photonic SHE by taking advantage of SiC-supported surface phonon resonance(SPhR)in the mid-infrared regime.The transverse displacement of photonic SHE is very sensitive to the wavelength of incident light and the thickness of SiC layer.Under the optimal parameter setup,the calculated largest transverse displacement of SiC-based SPhR structure reaches up to 163.8 ym,which is much larger than the condition of SPR.Moreover,an NO_(2) gas sensor based on the SPhR-enhanced photonic SHE is theoretically proposed with the superior sensing performance.Both the intensity and angle sensitivity of this sensor can be effectively manipulated by varying the damping rate of SiC.The results may provide a promising paradigm to enhance the photonic SHE in the mid-infrared region and open up new opportunity of highly sensitive refractive index sensors.

Sensing the heavy water concentration in an H_(2)O-D_(2)O mixture by solid-solid phononic crystals

A new method based on phononic crystals is presented to detect the concentration of heavy water(D_(2)O)in an H_(2)O-D_(2)O mixture.Results have been obtained and analyzed in the concentration range of 0%-10%and 90%-100%D_(2)O.A proposed structure of tungsten scatterers in an aluminum host is studied.In order to detect the target material,a cavity region is considered as a sound wave resonator in which the target material with different concentrations of D_(2)O is embedded.By changing the concentration of D_(2)O in the H_(2)O-D_(2)O mixture,the resonance frequency undergoes a frequency shift.Each 1%change in D_(2)O concentration in the H_(2)O-D_(2)O mixture causes a frequency change of about 120 Hz.The finite element method is used as the numerical method to calculate and analyze the natural frequencies and transmission spectra of the proposed sensor.The performance evaluation index shows a high Q factor up to 1475758 and a high sensitivity up to 13075,which are acceptable values for sensing purposes.The other figures of merit related to the detection performance also indicate high-quality performance of the designed sensor.

Phonon dichroism in proximitized graphene

We systematically investigate the phonon dichroism in proximitized graphene with broken time-reversal symmetry.We find that in the absence of any type of spin–orbit coupling,phonon dichroism vanishes.Linear and circular phonon dichroism occur in the presence of uniform(staggered)intrinsic spin–orbit coupling and ferromagnetic(antiferromagnetic)exchange coupling.All these situations can be distinguished by their specific behaviors of phonon absorption at the transition point.Our finding provides new possibilities to use phonon dichroism to identify the form of spin–orbit coupling and exchange coupling in proximitized graphene on various magnetic substrates.

Review of phonons in moiré superlattices

Moirépatterns in physics are interference fringes produced when a periodic template is stacked on another similar one with different displacement and twist angles.The phonon in two-dimensional(2D)material affected by moirépatterns in the lattice shows various novel physical phenomena,such as frequency shift,different linewidth,and mediation to the superconductivity.This review gives a brief overview of phonons in 2D moirésuperlattice.First,we introduce the theory of the moiréphonon modes based on a continuum approach using the elastic theory and discuss the effect of the moirépattern on phonons in 2D materials such as graphene and MoS_(2).Then,we discuss the electron-phonon coupling(EPC)modulated by moirépatterns,which can be detected by the spectroscopy methods.Furthermore,the phonon-mediated unconventional superconductivity in 2D moirésuperlattice is introduced.The theory of phonon-mediated superconductivity in moirésuperlattice sets up a general framework,which promises to predict the response of superconductivity to various perturbations,such as disorder,magnetic field,and electric displacement field.

Effects of phonon bandgap on phonon-phonon scattering in ultrahigh thermal conductivityθ-phase TaN

Phonon bandgap typically has a significant effect on phonon-phonon scattering process.In this work,the effects of mass modified phonon bandgap inθ-phase Ta N are systemically investigated by the means of first-principles calculations with linearized Boltzmann transport equation.Through detailed calculations,we find that phonon bandgap has a significant effect on three-phonon process while exhibits a much weaker effect on four-phonon process.The reason for the ultrahigh thermal conductivity ofθ-phase Ta N is the long lifetime of phonons including both three-phonon and four-phonon processes,which originates from the weak phonon anharmonicity and large phonon bandgap-induced small phonon-phonon scattering phase space.This work advances the understanding of phonon bandgap effects on phonon transport.

Flexural-wave-generation using a phononic crystal with a piezoelectric defect

This paper proposes a method to amplify the performance of a flexural-wave-generation system by utilizing the energy-localization characteristics of a phononic crystal(PnC)with a piezoelectric defect and an analytical approach that accelerates the predictions of such wave-generation performance.The proposed analytical model is based on the Euler-Bernoulli beam theory.The proposed analytical approach,inspired by the transfer matrix and S-parameter methods,is used to perform band-structure and time-harmonic analyses.A comparison of the results of the proposed approach with those of the finite element method validates the high predictive capability and time efficiency of the proposed model.A case study is explored;the results demonstrate an almost ten-fold amplification of the velocity amplitudes of flexural waves leaving at a defect-band frequency,compared with a system without the PnC.Moreover,design guidelines for piezoelectric-defect-introduced PnCs are provided by analyzing the changes in wave-generation performance that arise depending on the defect location.

Terahertz phononic crystal in plasmonic nanocavity

Interaction between photons and phonons in cavity optomechanical systems provides a new toolbox for quantum information technologies.A GaAs/AlAs pillar multi-optical mode microcavity optomechanical structure can obtain phonons with ultra-high frequency(~THz).However,the optical field cannot be effectively restricted when the diameter of the GaAs/AlAs pillar microcavity decreases below the diffraction limit of light.Here,we design a system that combines Ag nanocav-ity with GaAs/AlAs phononic superlattices,where phonons with the frequency of 4.2 THz can be confined in a pillar with~4 nm diameter.The Q_(c)/V reaches 0.22 nm^(-3),which is~80 times that of the photonic crystal(PhC)nanobeam and~100 times that of the hybrid point-defect PhC bowtie plasmonic nanocavity,where Q_(c) is optical quality factor and V is mode volume.The optome-chanical single-photon coupling strength can reach 12 MHz,which is an order of magnitude larger than that of the PhC nanobeam.In addition,the mechanical zero-point fluctuation amplitude is 85 fm and the efficient mass is 0.27 zg,which is much smaller than the PhC nanobeam.The phononic superlattice-Ag nanocavity optomechanical devices hold great potential for applications in the field of integrated quantum optomechanics,quantum information,and terahertz-light transducer.

Impeded thermal transport in aperiodic BN/C nanotube superlattices due to phonon Anderson localization

Anderson localization of phonons is a kind of phonon wave effect,which has been proved to occur in many structures with disorders.In this work,we introduced aperiodicity to boron nitride/carbon nanotube superlattices(BN/C NT SLs),and used molecular dynamics to calculate the thermal conductivity and the phonon transmission spectrum of the models.The existence of phonon Anderson localization was proved in this quasi one-dimensional structure by analyzing the phonon transmission spectra.Moreover,we introduced interfacial mixing to the aperiodic BN/C NT SLs and found that the coexistence of the two disorder entities(aperiodicity and interfacial mixing)can further decrease the thermal conductivity.In addition,we also showed that anharmonicity can destroy phonon localization at high temperatures.This work provides a reference for designing thermoelectric materials with low thermal conductivity by taking advantage of phonon localization.

Inverse Design of Phononic Crystal with Desired Transmission via a Gradient-Descent Approach

We propose a general approach based on the gradient descent method to study the inverse problem,making it possible to reversely engineer the microscopic configurations of materials that exhibit desired macroscopic properties.Particularly,we demonstrate its application by identifying the microscopic configurations within any given frequency range to achieve transparent phonon transport through one-dimensional harmonic lattices.Furthermore,we obtain the phonon transmission in terms of normal modes and find that the key to achieving phonon transparency or phonon blocking state lies in the ratio of the mode amplitudes at ends.

Unlocking the Potential of Two-Dimensional Janus Superlattices:Directly Visualizing Phonon Transitions

Recent research has focused on using Anderson's localization concept to modulate coherent phonon transport by introducing disorder into periodic structures.However,designing and identifying the disorder's strength remain challenging,and visual evidence characterizing phonon localization is lacking.Here,we investigate the effect of disorder on coherent phonon transport in a two-dimensional Janus MoSSe/WSSe superlattice with a defined disorder strength.Using non-equilibrium molecular dynamics,we demonstrate that strong disorder can lead to strong phonon localization,as evidenced by smaller thermal conductivity and significantly different dependence on defect ratio in strongly disordered structures.Furthermore,we propose a novel defect engineering method to determine whether phonon localization occurs.Our work provides a unique platform for modulating coherent phonon transport and presents visual evidence of the phonon transition from localization to nonlocalization.These findings will contribute to development of phonon transport and even phononics,which are essential for thermoelectric and phononic applications.

Phonon Focusing Effect in an Atomic Level Triangular Structure

The rise of artificial microstructures has made it possible to modulate propagation of various kinds of waves,such as light,sound and heat.Among them,the focusing effect is a modulation function of particular interest.We propose an atomic level triangular structure to realize the phonon focusing effect in single-layer graphene.In the positive incident direction,our phonon wave packet simulation results confirm that multiple features related to the phonon focusing effect can be controlled by adjusting the height of the triangular structure.More interestingly,a completed different focusing pattern and an enhanced energy transmission coefficient are found in the reverse incident direction.The detailed mode conversion physics is discussed based on the Fourier transform analysis on the spatial distribution of the phonon wave packet.Our study provides physical insights to achieving phonon focusing effect by designing atomic level microstructures.

Effects of Localized Interface Phonons on Heat Conductivity in Ingredient Heterogeneous Solids

Phonons are the primary heat carriers in non-metallic solids.In compositionally heterogeneous materials,the thermal properties are believed to be mainly governed by the disrupted phonon transport due to mass disorder and strain fluctuations,while the effects of compositional fluctuation induced local phonon states are usually ignored.Here,by scanning transmission electron microscopy electron energy loss spectroscopy and sophisticated calculations,we identify the vibrational properties of ingredient-dependent interface phonon modes in Alx Ga1-x N and quantify their various contributions to the local interface thermal conductance.We demonstrate that atomic-scale compositional fluctuation has significant influence on the vibrational thermodynamic properties,highly affecting the mode ratio and vibrational amplitude of interface phonon modes and subsequently redistributing their modal contribution to the interface thermal conductance.Our work provides fundamental insights into understanding of local phonon-boundary interactions in nanoscale inhomogeneities,which reveal new opportunities for optimization of thermal properties via engineering ingredient distribution.

Unveiling phonon frequency-dependent mechanism of heat transport across stacking fault in silicon carbide

Stacking faults(SFs)are often present in silicon carbide(SiC)and affect its thermal and heat-transport properties.However,it is unclear how SFs influence thermal transport.Using non-equilibrium molecular dynamics and lattice dynamics simulations,we studied phonon transport in SiC materials with an SF.Compared to perfect SiC materials,the SF can reduce thermal conductivity.This is caused by the additional interface thermal resistance(ITR)of SF,which is difficult to capture by the previous phenomenological models.By analyzing the spectral heat flux,we find that SF reduces the contribution of low-frequency(7.5 THz-12 THz)phonons to the heat flux,which can be attributed to SF reducing the phonon lifetime and group velocity,especially in the low-frequency range.The SF hinders phonon transport and results in an effective interface thermal resistance around the SF.Our results provide insight into the microscopic mechanism of the effect of defects on heat transport and have guiding significance for the regulation of the thermal conductivity of materials.

Straight and twisted Weyl nodal line phonons in Ho2CF2 material

Based on first-principles calculations, symmetry analysis and model construction, we predict that Ho2CF2hosts both straight and twisted Weyl nodal lines in its bulk phonon spectrum. We identify that the top two phonon bands entangle with each other, forming two straight Weyl nodal lines on the K–H and K′–H′paths at the Brillouin zone(BZ) boundary,and six twisted Weyl nodal lines within the BZ. All the Weyl nodal lines along the kz direction and across the entire BZ.The symmetry analysis indicates that these Weyl nodal lines are protected by the PT symmetry and crystal symmetry. The Berry phase and drumhead-like nontrivial surface states are calculated. We also construct a tight-binding model to describe these nodal lines. Our work provides an excellent material platform for exploring the fascinating physics associated with straight and twisted Weyl nodal line phonons.

Size effects on the mixed modes and defect modes for a nano-scale phononic crystal slab

The size-dependent band structure of an Si phononic crystal(PnC)slab with an air hole is studied by utilizing the non-classic wave equations of the nonlocal strain gradient theory(NSGT).The three-dimensional(3D)non-classic wave equations for the anisotropic material are derived according to the differential form of the NSGT.Based on the the general form of partial differential equation modules in COMSOL,a method is proposed to solve the non-classic wave equations.The bands of the in-plane modes and mixed modes are identified.The in-plane size effect and thickness effect on the band structure of the PnC slab are compared.It is found that the thickness effect only acts on the mixed modes.The relative width of the band gap is widened by the thickness effect.The effects of the geometric parameters on the thickness effect of the mixed modes are further studied,and a defect is introduced to the PnC supercell to reveal the influence of the size effects with stiffness-softening and stiffness-hardening on the defect modes.This study paves the way for studying and designing PnC slabs at nano-scale.

Flexural wave bandgap properties of phononic crystal beams with interval parameters

Uncertainties are unavoidable in practical engineering,and phononic crystals are no exception.In this paper,the uncertainties are treated as the interval parameters,and an interval phononic crystal beam model is established.A perturbation-based interval finite element method(P-IFEM)and an affine-based interval finite element method(A-IFEM)are proposed to study the dynamic response of this interval phononic crystal beam,based on which an interval vibration transmission analysis can be easily implemented and the safe bandgap can be defined.Finally,two numerical examples are investigated to demonstrate the effectiveness and accuracy of the P-IFEM and A-IFEM.Results show that the safe bandgap range may even decrease by 10%compared with the deterministic bandgap without considering the uncertainties.