Violation of the Wiedemann-Franz law through reduction of thermal conductivity in gold thin films
Violation of the Wiedemann-Franz law through reduction of thermal conductivity in gold thin films
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DOI：
10.1103/physrevmaterials.4.065003
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发表时间：
2020-06-24
影响因子：
3.4
通讯作者：
Zink, B. L.
中科院分区：
文献类型：
--
作者：
Mason, S. J.;Wesenberg, D. J.;Zink, B. L.
我们使用微机械氮化硅膜热隔离平台测量了热蒸发金薄膜样品的平面内导热性和电导率，厚度从大约20到bbb300 nm不等。在单次金沉积中生长的大约300纳米厚的薄膜和由许多连续沉积的较薄层形成的样品中，我们观察到与电导率和热导率相关的Wiedemann-Franz定律的强烈“违反”。导电性与预期基本一致，但导热性首先随着薄膜总厚度的增加而上升，然后随着薄膜变厚而出人意料地下降。在整个78-300 k温度范围内，热导率的急剧降低使约300 nm厚样品的洛伦兹数L降低到不到索默菲尔德值的一半。在接近室温的金属薄膜中，即使存在由晶界和粗糙表面引入的无序，在费米-液体理论应该很好地描述电子传递的情况下，这种破坏以前也没有报道过。我们认为，基于这些薄膜中晶粒尺寸的详细表征，这表明非弹性散射过程可能是由较厚薄膜中强柱状颗粒结构引入的状态的修改声子密度和结构各向异性的组合驱动的。在保持高导电性的同时，这种非常不寻常的热导率降低对提高纳米级系统的热电性能有潜在的希望。
We present measurements of in-plane thermal and electrical conductivity in thermally evaporated gold thin-film samples ranging in thickness from approximate to 20 to >300 nm, performed using a micromachined silicon-nitride membrane thermal isolation platform. In both approximate to 300-nm-thick films grown in a single Au deposition and a sample built up to >300 nm by many sequential depositions of thinner layers, we observe strong "violations" of the Wiedemann-Franz law that relates electrical and thermal conductivities. While electrical conductivity behaves essentially as expected, thermal conductivity first rises with growing total film thickness, and then surprisingly drops as the film becomes thicker. The sharp reduction of thermal conductivity decreases the Lorenz number L for approximate to 300-nm-thick samples to less than half the Sommerfeld value over the entire 78-300-K temperature range studied. Such violation near room temperature, in a metal film where electron transport should be well described by Fermi-liquid theory, is previously unreported, even in the presence of disorder introduced by grain boundaries and rough surfaces. This indicates an inelastic-scattering process that we argue, based on detailed characterization of grain size in these films, is likely driven by a combination of modified phonon density of states and structural anisotropy introduced from the strongly columnar grain structure in thicker films. This highly unusual reduction of thermal conductivity while maintaining high electrical conductivity is potentially promising for increasing thermoelectric performance of nanoscale systems.