Phonon Propagation Research Articles

Coherent Acoustic Phonon Dynamics and Coupling in Metal-van der Waals Heterostructure Nanocavities.

The development of acoustic nanocavities with resonant frequencies in the gigahertz to terahertz range has enabled advancements in quantum information processing, acoustic sensing, and advanced optoacoustic devices. Here, we demonstrate the generation and strong coupling of coherent acoustic phonons within metal-van der Waals (vdWs) heterostructure nanocavities, constructed from semiconductor MoS2 and insulating h-BN thin films, integrated with chemically synthesized Au nanosheets. Both heterostructures exhibit extended coherent phonon spectra, as observed through ultrafast femtosecond pump-probe spectroscopy. The inhomogeneous broadening features of these spectra are accurately reproduced using finite element method simulations and continuum mechanics calculations. A detailed analysis of the phonon coupling mechanism using a spring model reveals distinct coupling strengths of 78 and 55 GHz for the MoS2/Au and h-BN/Au nanocavities, respectively. Notably, the presence of a thin polymer (PVP) spacer layer at the metal-vdWs interface significantly influences the interfacial coupling strength and phonon lifetime. These findings provide insights into phonon coupling optimization in metal-vdWs nanocavities, contributing to the design of high-performance phononic devices.

Point Defects Enhance Cross-Plane Thermal Conductivity In Graphite.

Point defects typically reduce the thermal conductivity (κ) of a crystal due to increased scattering of heat-carrying phonons, a mechanism that is well understood and widely used to enhance or impede heat transfer in the material for different applications. Here an opposite effect is reported where the introduction of point defects in graphite with energetic particle irradiation increases its cross-plane κ by nearly a factor of two, from 10.8 to 18.9 WmK-1 at room temperature. Integrated differential phase contrast imaging with scanning transmission electron microscopy revealed the creation of spiro interstitials in graphite by the irradiation. The enhancement in κ is attributed to a remarkable mechanism that works to the benefit of phonon propagation in both the harmonic and anharmonic terms: these spiro interstitial defects covalently bridge neighboring basal planes, simultaneously enhancing acoustic phonon group velocity and reducing phonon-phonon scattering in the graphite structure. The enhancement of κ reveals an unconventional role of lattice defects in heat conduction, i.e., easing the propagation of heat-carrying phonons rather than impeding them in layered materials, inspiring their applications for thermal management in heavily radiative environments.

Heat transport in crystalline organic semiconductors: coexistence of phonon propagation and tunneling

Understanding heat transport in organic semiconductors is of fundamental and practical relevance. Therefore, we study the lattice thermal conductivities of a series of (oligo)acenes, where an increasing number of rings per molecule leads to a systematic increase of the crystals’ complexity. Temperature-dependent thermal conductivity experiments in these systems disagree with predictions based on the traditional Peierls–Boltzmann framework, which describes heat transport in terms of particle-like phonon propagation. We demonstrate that accounting for additional phonon-tunneling conduction mechanisms through the Wigner Transport Equation resolves this disagreement and quantitatively rationalizes experiments. The pronounced increase of tunneling transport with temperature explains several unusual experimental observations, such as a weak temperature dependence in naphthalene’s thermal conductivity and an essentially temperature-invariant conductivity in pentacene. While the anisotropic thermal conductivities within the acene planes are essentially material-independent, the tunneling contributions (and hence the total conductivities) significantly increase with molecular length in the molecular backbone direction. This, for pentacene results in a surprising minimum of the thermal conductivity at 300 K.

Ultrafast movies of the photoinduced phonon and magnon propagation using dual frequency-comb technology.

The interaction between surface acoustic waves (SAWs) and magnons is an interesting phenomenon that can be used to control spins in spintronic devices, and thus the visualization of the related spatiotemporal dynamics can be useful. In this study, we used dual-frequency-comb-based asynchronous optical sampling to record the spatiotemporal evolution of the surface vibration associated with an optically excited SAW and that of the spin precession associated with the magnon under the same excitation condition at effectively 160 frames/ns. Isotropic and anisotropic propagation were observed for phonons and magnons, respectively, demonstrating the capability of measuring such dynamics with high resolution.

Molecular dynamics work on thermal conductivity of SiGe nanotubes.

SiGe nanotubes (SiGeNTs) hold significant promise for applications in nanosolar cells, optoelectronic systems, and interconnects, where thermal conductivity is critical to performance. This study investigates the effects of length, diameter, temperature, and axial strain on the thermal conductivity of armchair and zigzag SiGeNTs through molecular dynamics simulations. Results indicate that thermal conductivity increases with sample length due to ballistic heat transport and decreases with temperature as phonon scattering intensifies. Axial strain transitions from compression to tension enhance phonon propagation, improving conductivity. Chirality affects conductivity, with zigzag SiGeNTs consistently outperforming armchair structures, while diameter exhibits negligible impact. Non-equilibrium molecular dynamics simulations were conducted using the LAMMPS package with the Tersoff potential to model Si-Ge interactions. Thermal conductivity was computed via Fourier's law, with the system divided into regions for controlled heat input and dissipation. Lengths, diameters, temperatures (100-500K), and axial strains (- 6% to + 9%) were varied systematically. Phonon spectrum analysis was performed using Fourier transforms of velocity autocorrelation functions to compute.

Lattice Anisotropy-Driven Reduction of Phonon Velocities in Black Phosphorus.

Phonon dynamics and transport determine how heat is utilized and dissipated in materials. In 2D systems for optoelectronics and thermoelectrics, the impact of nanoscale material structure on phonon propagation is central to controlling thermal conduction. Here, we directly observe in-plane coherent acoustic phonon propagation in black phosphorus (BP) using ultrafast electron microscopy. We identify a significant reduction of the group velocities in directions intermediate to the armchair and zigzag lattice directions. Using a machine learning-based model with an >8000 atom supercell, we find that this slowing results from the mixing of in-plane transverse and longitudinal acoustic phonons and is independent of broken symmetries of edge reconstructions. This work demonstrates how coherent phonon transport is sensitive to propagation direction in the lattice plane.

All-Optical Generation and Detection of Coherent Acoustic Vibrations in Single Gallium Phosphide Nanoantennas Probed near the Anapole Excitation.

Nanostructured high-index dielectrics have shown great promise as low-loss photonic platforms for wavefront control and enhancing optical nonlinearities. However, their potential as optomechanical resonators has remained unexplored. In this work, we investigate the generation and detection of coherent acoustic phonons in individual crystalline gallium phosphide nanodisks on silica in a pump-probe configuration. By pumping the dielectric above its bandgap energy and probing over its transparent region, we observe the radial breathing mode of the disk with an oscillation frequency around 10 GHz. We find that the detection efficiency peaks near the fundamental anapole state, matching numerical simulations. By comparing to reference gold plasmonic resonators, the dielectric nanoantennas display a modulation amplitude up to ∼5 times larger. We further demonstrate the launching of acoustic waves through the underlaying substrate and the mechanical coupling between two nanostructures placed 3 μm apart, laying the basis for photonic-phononic signal processing using dielectric nanoantennas.

Revisiting Cobalt Dopability in GeTe System to Design Modulation‐Doped Thermoelectrics

AbstractDopability plays a pivotal role in determining the limit of carrier concentration and the chemical potential of semiconductor thermoelectric materials, which are directly related to the figure of merit. Here, the doping behavior and mechanism of cobalt (Co) in GeTe‐based thermoelectric materials are first investigated. According to theoretical calculations and tentative experiments, the extremely hard Co dopability in GeTe system is ascribed to the formation of an insoluble intermetallic phase in eutectics, even though the point defect formation energy and charge transition level indicate a 3at.% doping limit. A two‐step method is developed to synthesize (CoGe2)xGe0.85Sb0.10Te composited thermoelectric materials and use synchrotron technologies to investigate both average and local structures. CoGe2 nanoprecipitates are observed endotaxially and uniformly embedded in Ge0.85Sb0.10Te matrix, which acts as an electron reservoir to optimize carrier concentration without deteriorating carrier mobility, conceiving an ideal modulation doping scheme. Moreover, the phonon mismatch at the semi‐coherent CoGe2/Ge0.85Sb0.10Te interfaces gives rise to the Kapitza resistance to impede phonon propagation. The synergistic manipulation of electronic and thermal transport leads to a desirable figure of merit of 2.2 at 775 K and a conversion efficiency of 8.2% under a temperature difference of 420 K, representing a promising performance in this field and providing a benchmark workflow to design composited thermoelectrics.

Molecular simulation of thermoelectric materials: polyaniline/graphyne/polypyrrole ternary composites

ABSTRACT In the previous work, PANI-n/GY systems have been studied for thermal conductivity (TC). In order to improve the thermoelectric performance of the composites, polypyrrole (PPY) were introduced into the PANI-n/GY composites to obtain the PANI-n/GY/PPY ternary composites. The thermal conductivity was investigated using the NEMD method. The electrostatic potential and the frontier orbitals were studied using DFT. It is found that the introduction of PPY effectively reduces the TCs of the PANI-n/GY/PPY composites and promotes the electrons transfer to obtain better thermoelectric performance. This is due to the fact that (1) PPY makes the polymer chains more curved and twisted reducing intrachain thermal transport in the matrix; (2) PPY supports the formation of H-bonding and the strong interaction between PPY and PANI promotes the interchain thermal transport in the matrix; (3) PPY increases the phonon vibrational mismatch at the interface, which decreases the interface TC by hindering phonon propagation; (4) PPY provides the HOMO with an energy that is close to the LUMO of the GY. It contributes the flow of electrons from the PPY to the GY, and improves the electrical conductivity of the system. This research provides a theoretical design for the multifunctionality composites.

Generation of Phonons with Out-of-Plane Angular Momentum by Superposition of Longitudinal Surface Acoustic Phonons

Generation of Phonons with Out-of-Plane Angular Momentum by Superposition of Longitudinal Surface Acoustic Phonons

Heat transfer in conductors of cylindrical cross-section with diffusive boundary scattering

We present a theoretical study of heat transfer properties in three-dimensional cylindrical conductors of finite lengths in the ballistic regime of phonon propagation. The phonon emission is considered to be radiative and its interactions with the lateral boundary is assumed to be completely diffusive. The main goal of this study is to determine the temperature profile and the features of its formation for the generalized case of a conductor of finite length. A matrix numerical solution of the integral equation for the particle energy flux is obtained and an approximate analytical solution of this equation is proposed. The heat flux and thermal conductivity coefficient are determined; and the significant difference between our results and those of the generally accepted formulation in the Knudsen regime and the Casimir model is discussed.

Direct Observation of All-Flat Bands Phononic Metamaterials.

Flat lines within a band structure represent constant frequency bands for all momentum values (i.e., they maintain zero group velocity for all wave numbers). These bands lead to energy localization near the source with very high intensity compared to regular pass bands. An all-flat photonic band structure was observed very recently. However, to date, due to the limitations on coupling stiffness ratios and the vectorial nature of phonon propagation, no existing metamaterial exhibits an all-flat band (AFB) for phonons. In this Letter, we utilize disks with magnetic couplings as a platform to realize an all-flat phononic band structure. We elucidate the required design methodology to obtain an AFB phononic band structure and report on the first direct observation of such metamaterials. Our design methodology can enable the realization of advanced materials with exotic wave control capabilities.

Long-Range Anion Correlations Mediating Dynamic Anharmonicity and Contributing to Glassy Thermal Conductivity in Well-Ordered K2Ag4Se3.

Herein an extremely low (0.32‒0.25 Wm-1K-1) and glassy temperature-dependence (300-600K) of lattice thermal conductivity (κlat)in a monoclinic K2Ag4Se3 is reported. It is found that the effective carrier delocalization, contributed by the perfect p-d* hybridization paradigm, can efficiently facilitate the spatial transfer of electron cloud perturbations induced by the anisotropic thermal vibrations of Ag4 atoms, thereby favoring long-range Se‒Se correlations. The localized rattling-like vibration of Ag4 atoms induce short phonon lifetimes, large scattering phase space, and then a low particle-like propagation. While the correlated interactions mediated competitive expressions between bubble diagrams and loop diagrams can suppress the generation of wavelike phonons from off-diagonal coupling. Ultimately, the AgSe3 structural units can enable the dual confinement of both the particle-like propagation of phonons and wavelike tunneling of coherence. The study highlights that the correlated AgSe3 coordination units can simultaneously target particle-like and wavelike phonons and then reduce their contribution to the κlat by mediating long-range transfer of charge polarization. These fundamental advances will advance the design of crystalline materials with tailored thermal properties.

Transient Structural Transition in Ovonic Threshold Switching Glass

AbstractOvonic threshold switching (OTS), characterized by a rapid resistance drop in chalcogenide glass, has enabled the realization of memory and selectors. Despite over five decades of development, the challenges in characterizing the transient switching in amorphous materials upon reaching the threshold voltage have hindered the establishment of its underlying mechanism. This study uses femtosecond terahertz spectroscopy and ab initio simulation to elucidate the dynamics of the free‐carrier and IR‐active phonons involved in OTS. Specifically, in Te‐rich amorphous GeTe, the generation of transient phonons is observed within picoseconds, a phenomenon associated with an increased Born effective charge due to the alignment of Te‐centered bonds. The findings demonstrate a correlation between the enhancement of polarizability, due to orbital alignment during the disorder–order structural transition while maintaining a macroscopic amorphous structure, and the switching behavior. These results provide valuable insights into the enigmatic OTS phenomenon.

Single-shot ultrafast imaging of plasma dynamics induced by dual-angle ultrashort laser pulses with subpicosecond delay

Spatiotemporal manipulation of ultrashort laser pulses is crucial for enhancing laser processing and phonon generation. Optimization of these applications requires ultrafast visualization of the underlying processes. In this study, we induced laser ablation using spatiotemporally manipulated double pulses focused from two angles onto a glass surface with a 0.7 ps interval, and captured the images of its dynamics with 5 sequential frames at a frame interval of 0.8 ps. The observed dynamics suggest that the laser profile reflected on the glass surface is influenced by its topography, which in turn affects the behavior of air breakdown plasmas.

Switching dynamics in Al/InAs nanowire-based gate-controlled superconducting switch

The observation of the gate-controlled supercurrent (GCS) effect in superconducting nanostructures increased the hopes for realizing a superconducting equivalent of semiconductor field-effect transistors. However, recent works attribute this effect to various leakage-based scenarios, giving rise to a debate on its origin. A proper understanding of the microscopic process underlying the GCS effect and the relevant time scales would be beneficial to evaluate the possible applications. In this work, we observed gate-induced two-level fluctuations between the superconducting state and normal state in Al/InAs nanowires (NWs). Noise correlation measurements show a strong correlation with leakage current fluctuations. The time-domain measurements show that these fluctuations have Poissonian statistics. Our detailed analysis of the leakage current measurements reveals that it is consistent with the stress-induced leakage current (SILC), in which inelastic tunneling with phonon generation is the predominant transport mechanism. Our findings shed light on the microscopic origin of the GCS effect and give deeper insight into the switching dynamics of the superconducting NW under the influence of the strong gate voltage.

Interplay between metavalent bonds and dopant orbitals enables the design of SnTe thermoelectrics

Engineering the electronic band structures upon doping is crucial to improve the thermoelectric performance of materials. Understanding how dopants influence the electronic states near the Fermi level is thus a prerequisite to precisely tune band structures. Here, we demonstrate that the Sn-s states in SnTe contribute to the density of states at the top of the valence band. This is a consequence of the half-filled p-p σ-bond (metavalent bonding) and its resulting symmetry of the orbital phases at the valence band maximum (L point of the Brillouin zone). This insight provides a recipe for identifying superior dopants. The overlap between the dopant s- and the Te p-state is maximized, if the spatial overlap of both orbitals is maximized and their energetic difference is minimized. This simple design rule has enabled us to screen out Al as a very efficient dopant to enhance the local density of states for SnTe. In conjunction with doping Sb to tune the carrier concentration and alloying with AgBiTe2 to promote band convergence, as well as introducing dislocations to impede phonon propagation, a record-high average ZT of 1.15 between 300 and 873 K and a large ZT of 0.36 at 300 K is achieved in Sn0.8Al0.08Sb0.15Te-4%AgBiTe2.

Strong Surface-Enhanced Coherent Phonon Generation in van der Waals Materials.

Terahertz (THz) coherent phonons have emerged as promising candidates for the next generation of high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. However, effectively manipulating THz coherent phonons remains a significant challenge. In this study, we investigated THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of Fe3GeTe2, Fe5GeTe2, and FePS3. We successfully generated the THz A1g coherent phonon mode in these vdW flakes. An innovative approach involved partially exfoliating vdW flakes on a gold substrate and partially on a silicon (Si) substrate to compare the THz coherent phonon generation between both sides. Interestingly, we observed a significantly enhanced THz coherent phonon in the vdW/gold area compared with that in the vdW/Si area. Frequency-domain Raman mapping across the vdW flakes corroborated these findings. Numerical simulations further indicated a stronger enhanced surface field in vdW/gold structures than in vdW/Si structures. Consequently, we attribute the observed enhancement in THz coherent phonon generation to the increased surface field on the gold substrate. This enhancement was consistent across the three different vdW materials studied, suggesting the universality of this strategy. Our results hold promise for advancing the design of THz phononic and phonon-integrated devices.

Nanometer-Scale Acoustic Wave Packets Generated by Stochastic Core-Level Photoionization Events

We demonstrate that the absorption of femtosecond hard x-ray pulses excites quasispherical, high-amplitude, and high-wave-vector coherent acoustic phonon wave packets using an all hard-x-ray pump-probe scattering experiment. The time- and momentum-resolved diffuse scattering signal is consistent with an ensemble of 3D strain wave packets induced by the rapid electron cascade dynamics following photoionization at uncorrelated excitation centers. We quantify key parameters of this process, including the localization size of the stress field and the photon energy conversion efficiency into elastic energy. The parameters are determined by the photoelectron and Auger electron cascade dynamics, as well as the electron-phonon interaction. In particular, we obtain the localization size of the observed strain wave packet to be 1.5 and 2.5 nm for bulk SrTiO3 and KTaO3 single crystals, respectively. The results provide crucial information on the mechanism of x-ray energy deposition into matter and shed light on the shortest collective length scales accessible to coherent acoustic phonon generation using x-ray excitation. Published by the American Physical Society 2024

Colloidal Ag2SbBiSe4 nanocrystals as n‑type thermoelectric materials

Colloidal Ag2SbBiSe4 nanocrystals as n‑type thermoelectric materials