Experimental Investigation of Phonon Localization in Nanostructures

摘要：声子是固体中原子的集体振动，是半导体等材料中主要的热载体。声子输运物理很重要，因为它决定了材料的热学和其他相关性质。通常情况下，声子要么扩散传播，从晶格杂质中随机散射，要么弹道传播，沿直线传播。该项目旨在研究一种新的声子传输机制，称为“声子局部化”，其中声子的波性质阻止它们传播。这项研究可能会带来新的方法来设计超越当前范例的热性能，特别是实现超低导热系数，这对于广泛的热技术（如隔热和热能到电能转换）非常重要。该项目通过为研究生和本科生提供跨学科教育和培训机会，同时吸引代表性不足的群体进入科学领域，扩大了其影响。综合推广活动为圣地亚哥地区的第一代弱势高中生提供纳米和能源技术方面的教育和研究机会。技术摘要安德森局域化已经在费米子和玻色子中得到了实验证明，包括电子、光子（光），以及最近的超声波。理论研究已经预测了热声子的定位，但直到现在还没有实验验证这些预测，大概是由于与感知实验相关的艰巨挑战。本项目旨在克服这些实验挑战，首次系统地研究低维系统中的声子定位。通过精确控制表面形貌和杂质浓度的半导体纳米线的热输运测量，对声子局部化进行了实验探测。要研究的声子定位特征包括热导率对纳米线几何形状、表面粗糙度、缺陷浓度以及温度的依赖。

Non-Technical Abstract:Phonons are collective vibrations of the atoms in a solid and are the dominant heat carriers in materials such as semiconductors. Phonon transport physics is important as it dictates thermal and other related properties of materials. Normally phonons either travel diffusively, randomly scattering from lattice impurities, or ballistically, in a straight line. This project aims to investigate a new regime of phonon transport termed "phonon localization" where the wave nature of the phonons prevents them from propogating. The study could lead to new approaches to engineer thermal properties beyond the current paradigm, in particular achieving ultra-low thermal conductivity, which is important for a wide range of thermal technologies such as thermal insulation and thermal to electrical energy conversion. The project broadens its impacts by providing interdisciplinary education and training opportunities to graduate and undergraduate students while attracting underrepresented groups into science. The integrated outreach activities provide education and research opportunities in nano- and energy technologies for disadvantaged, first generation high school students in the greater San Diego area. Technical AbstractAnderson localization has been experimentally demonstrated in both fermions and bosons, including electrons, photons (light), and recently with ultrasound. Theoretical studies have already predicted localization of thermal phonons, but until now there has been no experimental verification of these predictions, presumably due to the daunting challenges associated with the perceived experiments. This project aims to overcome these experimental challenges and carry out the first systematic study of phonon localization in low dimensional systems with disorders. The phonon localization regime is experimentally probed by thermal transport measurements of semiconductor nanowires with precisely controlled surface morphology and impurity concentration. Phonon localization signatures to be investigated include the dependence of thermal conductance on the nanowire geometry, surface roughness, and defect concentration as well as the temperature.