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because the critical assumption that all phonons/normal modes resemble plane waves with well-defined velocities is no longer valid when disorder is introduced.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.

当前对声子的理解将它们视为传播和散射的平面波/原子振动的准粒子。问题是，从概念上讲，当引入任何水平的无序状态时，无论是组成上的还是结构上的，固体中振动模式的特性都会发生变化，但是几乎所有的理论处理都继续假设声子仍然是波。例如，声子对合金热导率（TC）的贡献依赖于此假设，并且通常是根据虚拟晶体近似（VCA）计算得出的。在某些情况下可以达成良好的协议，但是在很多情况下，无论是从数量上还是从质量上，它都失败了。在这里，我们表明传统的声子理论和对声子的理解需要修正，因为引入扰动后，所有声子/法向模式都类似于具有明确速度的平面波的关键假设不再成立。令人惊讶的是，这里我们显示，在杂质浓度的最初百分之几内，声子的特性发生了巨大变化，在此之后，声子更类似于非晶态材料中的模式。然后，我们使用一种可以处理任何特征的模式的不同理论，并通过实验证实其新的见解。