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中国物理学会期刊

基于微扰跟踪分子动力学模拟方法的晶格热非平衡暂态过程中声子性质的探测

Probing phonon properties during a transient process at non-equilibrium using the perturbation-tracking method by molecular dynamics simulations

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  • 在器件高频运行或材料受高功率光激发等特定情况下,声子的分布可较大偏离于理想平衡分布并出现暂态过程。目前用于分子动力学(molecular dynamics,MD)模拟,常见的几种获取声子性质的方法均基于统计平均,因其无法有效捕获瞬态声子信息而不适用于非平衡暂态过程的研究。本文使用新型的微扰跟踪(perturbation-tracking,PT)MD模拟方法,对基于能量均分定理的经典分布和玻色爱因斯坦分布建立的平衡与非平衡状态下硅晶格模型系统,在弛豫过程中的非简谐声子间本征散射散射率进行动态探测。研究发现,仅部分频段中声子能量密度分布的相对较小变化,即可使非平衡系统全频段各模式声子散射率均发生明显改变,其中部分低频声学模式声子散射率相比于平衡状态可出现较大差异,并随着暂态弛豫过程动态发生变化。对于两种不同分布下声子散射率也进行了非简谐晶格动力学计算作为对比,与MD的PT探测所得结果均较为一致。PT方法非基于统计平均,可保留完整的动态过程信息,是一种适用于研究暂态非平衡系统中声子散射输运性质及变化的有效方法。

     

    Molecular dynamics (MD) simulation has been widely used to study phonon-mediated thermal transport problems. The most common way of extracting the spectral phonon properties from MD simulations is through statistics-based methods, such as the normal mode analysis and spectral energy density analysis. However, these methods cannot effectively recognize the instantaneous changes in the decay of a phonon state after individual scattering events, meaning that they are unsuitable for use with systems undergoing transient processes at non-equilibrium. Previously, we have introduced a different approach called the perturbation-tracking (PT) method, and demonstrated its efficiency and accuracy in acquiring phonon properties. In this work, the PT method is employed to probe phonon scattering rates (inverse of relaxation times) in a non-equilibrium system modeled by EDIP crystalline silicon. Here we focus on anharmonic phonon-phonon scattering only so do not blend in other scattering sources. Since quantum effects are not included in classical MD simulations, the Bose-Einstein (B-E) distribution can be thought of and used as a non-equilibrium distribution. The non-equilibrium system is created with its initial phonon energy distribution complying with the B-E distribution and allowed to evolve freely after MD timesteps start; it finally reaches thermodynamic equilibrium after sufficient relaxation. We find that a non-equilibrium distribution with relatively small deviations from equilibrium can have considerable impacts on phonon-phonon interactions and markedly alter the scattering rates. And changes to the spectral energy distribution of portion frequencies affect scattering rates extensively over the whole spectrum of phonon modes. Since the PT method does not require statistical averaging for data processing, it can be used to track the variations in a phonon state and extract scattering rates in real time; this makes it a powerful tool for studying transient processes.

     

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