npj Computational Materials

Rethinking phonons: The issue of disorder（对声子的重新思考：关于无序的问题)
Hamid Reza Seyf, Luke Yates, Thomas L. Bougher, Samuel Graham, Baratunde A. Cola, Theeradetch Detchprohm, Mi-Hee Ji, Jeomoh Kim, Russell Dupuis, Wei Lv&Asegun Henry
npj Computational Materials 3:44 (2017)
doi:10.1038/s41524-017-0052-9
Published online:16 November 2017
Abstract| Full Text | PDF OPEN

摘要:目前认为声子是原子振动的平面波/准粒子，能传播，可散射。问题在于，从概念上讲，当体系中引入任何水平的成分或是结构的无序时，，其振动模式的特征都会发生变化，然而几乎所有的理论处理依旧假设所有声子都是波。例如，声子对合金热导率的贡献依赖于这个假设，并且通常基于虚拟晶体近似（Virtual Crystal Approximation）来计算热导率。在某些情况下计算结果与实验结果能够较好地吻合，但在许多情况下计算结果在定性和定量上都不对。本研究表明，传统的理论需要修改，对声子的认识需要改变，因为这些都假设所有的声子/正则模都类似平面波并有明确定义的速度，而在引入无序之后，这一关键假设就不再有效。本研究惊奇地发现，声子的特征在引入杂质浓度的前几个百分比范围内变化很大，超过这个百分比，声子的模式更像非晶材料中的情形。进而，我们采用了一个新理论，其可以处理具有任意特征的模式，并通过实验确认了由该理论得出的新见解。　　

Abstract:Current understanding of phonons treats them as plane waves/quasi-particles of atomic vibration that propagate and scatter. The problem is that conceptually, when any level of disorder is introduced, whether compositional or structural, the character of vibrational modes in solids changes, yet nearly all theoretical treatments continue to assume phonons are still waves. For example, the phonon contributions to alloy thermal conductivity (TC) rely on this assumption and are most often computed from the virtual crystal approximation (VCA). Good agreement is obtained in some cases, but there are many instances where it fails—both quantitatively and qualitatively. Here, we show that the conventional theory and understanding of phonons requires revision, because the critical assumption that all phonons/normal modes resemble plane waves with well-defined velocities is no longer valid when disorder is introduced. Here we show, surprisingly, that the character of phonons changes dramatically within the first few percent of impurity concentration, beyond which phonons more closely resemble the modes found in amorphous materials. We then utilize a different theory that can treat modes with any character and experimentally confirm its new insights.

Editorial Summary

Phonon: correlation rather than scattering(声子：相关而非散射)

固体内，原子以集体运动的方式连续振动，其量子化幅度可以用所谓声子的准粒子来描述，这些准粒子可以传热和传声。声子通常被当作可传播和散射的波，但当处理包含无序的材料时，这种看法就常常与事实不符。美国佐治亚理工学院Asegun Henry教授领导的一个研究小组表明，通过关注声子相关性而不是散射，可以更准确地捕捉振动模式随无序度的变化。

In solids, atoms continuously vibrate in collective motions with quantized amplitudes that can be described in terms of quasiparticles known as phonons—which are responsible for heat transfer and sound. Phonons are usually treated as waves that propagate and scatter, but this approach can sometimes fail when dealing with materials with disorder. A team of researchers from the Georgia Institute of Technology, led by Asegun Henry, show that by focussing on phonon correlation rather than scattering, it is possible to more accurately capture the changes in vibrational mode behavior as a function of disorder.