Abstract At nanometer to micron length scales, there exists a strong competition between intrinsic and extrinsic scattering mechanisms that can curtail the free flight of phonons and ultimately affect the thermal transport. Despite significant progress in showing the ability to reach behaviors significantly below the Casimir limit, little appears to be understood about the competition between these scattering sources. In this investigation, we propose a simple one-parameter geometry that simultaneously modulates backscattering and trapping effects to enable directed study of these different means of controlling phonons. The geometry is a simple sequence of chambers offset from one another by a defined distance. We use the geometry to study the effects of phonon backscatter, trapping, and corner-turning on the thermal conductance in Si nanowires (NWs). We employ a full Brillouin zone Boltzmann Transport Equation (BTE) method to determine spatially-varying phonon densities in the geometry. Significantly greater impact is seen due to backscatter than any other means of arresting phonon flow. By creating a geometry that maximizes backscatter, a roughly 8-fold reduction in thermal conductance below the Casimir limit can be achieved at room temperature which is a factor of four smaller than the nearest reported value in the literature. The geometry is also useful for systematic investigation of other means of controlling phonons and affecting thermal transport; particularly, we investigate diffuse versus specular boundary scattering and the induced misalignment between the phonon flow and thermal flux due to the shape of the geometry. These effects combine to offer new insights into fundamental phonon behaviors and possible routes to phonon control.

中文翻译：

---

声子反向散射、捕获和未对准对低于卡西米尔极限的微尺度热导的影响

摘要 在纳米到微米的长度尺度上，内在和外在散射机制之间存在着强烈的竞争，可以限制声子的自由飞行并最终影响热传输。尽管在显示达到显着低于卡西米尔极限的行为的能力方面取得了重大进展，但人们似乎对这些散射源之间的竞争知之甚少。在这项研究中，我们提出了一种简单的单参数几何结构，它可以同时调节反向散射和捕获效应，从而能够对这些不同的声子控制方法进行直接研究。几何形状是一个简单的腔室序列，彼此偏移一个定义的距离。我们使用几何学来研究声子反向散射、捕获、硅纳米线（NWs）的热导率的拐角转向。我们采用完整的布里渊区玻尔兹曼传输方程 (BTE) 方法来确定几何中随空间变化的声子密度。由于反向散射，可以看到比任何其他阻止声子流的手段更大的影响。通过创建最大化反向散射的几何形状，在室温下可以实现低于 Casimir 极限的大约 8 倍的热导降低，这比文献中最接近的报告值小四倍。该几何结构对于系统研究控制声子和影响热传输的其他方法也很有用。特别，我们研究了漫反射与镜面反射边界散射以及由于几何形状导致的声子流和热通量之间的未对准。这些效应相结合，为基本声子行为和声子控制的可能途径提供了新的见解。