More Like this

1. A Comprehensive Investigation of the Two-Phonon Characteristics of Heat Conduction in Superlattices

   https://doi.org/10.3390/cryst15070654  

   Chakraborty, Pranay; Nasiri, Milad; Cui, Haoran; Maranets, Theodore; Wang, Yan (July 2025, Crystals)

   The Anderson localization of phonons in disordered superlattices has been proposed as a route to suppress thermal conductivity beyond the limits imposed by conventional scattering mechanisms. A commonly used signature of phonon localization is the emergence of the nonmonotonic dependence of thermal conductivity κ on system length L, i.e., a κ-L maximum. However, such behavior has rarely been observed. In this work, we conduct extensive non-equilibrium molecular dynamics (NEMD) simulations, using the LAMMPS package, on both periodic superlattices (SLs) and aperiodic random multilayers (RMLs) constructed from Si/Ge and Lennard-Jones materials. By systematically varying acoustic contrast, interatomic bond strength, and average layer thickness, we examine the interplay between coherent and incoherent phonon transport in these systems. Our two-phonon model decomposition reveals that coherent phonons alone consistently exhibit a strong nonmonotonic κ-L. This localization signature is often masked by the diffusive, monotonically increasing contribution from incoherent phonons. We further extract the ballistic-limit mean free paths for both phonon types, and demonstrate that incoherent transport often dominates, thereby concealing localization effects. Our findings highlight the importance of decoupling coherent and incoherent phonon contributions in both simulations and experiments. This work provides new insights and design principles for achieving phonon Anderson localization in superlattice structures. 

   more » « less
2. Ballistic phonon lensing by the non-planar interfaces of embedded nanoparticles

   https://doi.org/10.1088/1367-2630/ad025a  

   Maranets, Theodore; Wang, Yan (October 2023, New Journal of Physics)

   Abstract In this work, we investigate the scattering behavior of a ballistic phonon wave incident on a dopant spherical nanoparticle embedded within a pure crystal through molecular dynamics simulations. Unique to this work, we also conduct conjugate simulations of ballistic phonon scattering on a dopant thin slab to compare and contrast scattering by non-planar heterogeneous interfaces (nanoparticle) and planar heterogeneous interfaces (thin slab). Analysis of the wave dynamics in real and reciprocal spaces reveal phonon mode-conversion in the nanoparticle scattering system is due to an unreported ‘phonon lensing’ effect where the phonon wave propagation is altered by refraction and reflection through the non-planar interfaces of the nanoparticle. The specific states of mode-conversion is shown to change with the character of the lensing that varies with nanoparticle size. Most significantly, the lensing phenomenon is absent in the phonon scattering by the thin slab and consequentially, results in differences in the scattering behaviors between the planar and non-planar interfaces. 

   more » « less
3. Thermal resistance across Si–SiGe alloy interface from phonon distribution mismatch

   https://doi.org/10.1063/5.0202880  

   Han, Jinchen; Lee, Sangyeop (April 2024, Applied Physics Letters)

   Interfacial thermal resistance has often been attributed to the mismatch of phonon spectra between two materials and resulting phonon-interface scattering. However, we use the solution of Peierls–Boltzmann transport equation to reveal a substantial nonequilibrium thermal resistance across the interfaces of Si and SiGe alloys at room temperature, despite their nearly identical phonon dispersion and negligible phonon-interface scattering. The Kapitza length of the Si–Si0.99Ge0.01 interface is approximately 600 nm of Si. This originates from the mismatch in phonon distribution between Si and SiGe alloys due to their distinct scattering rates. The mismatch is relaxed by phonon scattering over a region of 1 μm around the interface, corresponding to the upper bound of mean free path Λx of heat-carrying phonons. The relaxation process leads to the significant entropy generation and increased thermal resistance. Introducing a gradual variation in Ge concentration near the interface markedly reduces thermal resistance when implemented over the 1 μm period. Our finding demonstrates that the interfacial thermal resistance can be significant due to the nonequilibrium phonon distribution, even in the absence of phonon-interface scattering. In addition, among various phonon modes with a wide range of Λx, the relaxation of the nonequilibrium is predominantly governed by the phonons with long Λx. 

   more » « less
4. Architecture Controls Phonon Propagation in All‐Solid Brush Colloid Metamaterials

   https://doi.org/10.1002/smll.202304157  

   Cang, Yu; Sainidou, Rebecca; Rembert, Pascal; Matyjaszewski, Krzysztof; Bockstaller, Michael; Graczykowski, Bartlomiej; Fytas, George (March 2024, Small)

   Abstract Brillouin light scattering and elastodynamic theory are concurrently used to determine and interpret the hypersonic phonon dispersion relations in brush particle solids as a function of the grafting density with perspectives in optomechanics, heat management, and materials metrology. In the limit of sparse grafting density, the phonon dispersion relations bear similarity to polymer‐embedded colloidal assembly structures in which phonon dispersion can be rationalized on the basis of perfect boundary conditions, i.e., isotropic stiffness transitions across the particle interface. In contrast, for dense brush assemblies, more complex dispersion characteristics are observed that imply anisotropic stiffness transition across the particle/polymer interface. This provides direct experimental validation of phonon propagation changes associated with chain conformational transitions in dense particle brush materials. A scaling relation between interface tangential stiffness and crowding of polymer tethers is derived that provides a guideline for chemists to design brush particle materials with tailored phononic dispersion characteristics. The results emphasize the role of interfaces in composite materials systems. Given the fundamental relevance of phonon dispersion to material properties such as thermal transport or mechanical properties, it is also envisioned that the results will spur the development of novel functional hybrid materials. 

   more » « less
5. Two stage decoherence of optical phonons in long oligomers

   https://doi.org/10.1063/5.0222580  

   Burin, Alexander L; Rubtsov, Igor V (September 2024, The Journal of Chemical Physics)

   Molecular vibrations are generally responsible for chemical energy transport and dissipation in molecular systems. This transport is fast and efficient if energy is transferred by optical phonons in periodic oligomers, but its efficiency is limited by decoherence emerging due to anharmonic interactions with acoustic phonons. Using a general theoretical model, we show that in the most common case of the optical phonon band being narrower than the acoustic bands, decoherence takes place in two stages. The faster stage involves optical phonon multiple forward scattering due to absorption and emission of transverse acoustic phonons, i.e., collective bending modes with a quadratic spectrum; the transport remains ballistic and the speed can be altered. The subsequent slower stage involves phonon backscattering in multiphonon processes involving two or more acoustic phonons resulting in a switch to diffusive transport. If the initially excited optical phonon possesses a relatively small group velocity, then it is accelerated in the first stage due to its transitions to states propagating faster. This theoretical expectation is consistent with the recent measurements of optical phonon transport velocity in alkane chains, increasing with increasing the chain length. 

   more » « less