We present measurements of in-plane thermal and electrical conductivity in thermally evaporated gold thin-film samples ranging in thickness from $\approx 20$ to $>300\mathrm{nm}$, performed using a micromachined silicon-nitride membrane thermal isolation platform. In both $\approx 300$-nm-thick films grown in a single Au deposition and a sample built up to $>300\mathrm{nm}$ 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 $\approx 300\text{−}\mathrm{nm}$-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.

中文翻译：

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通过降低金薄膜的热导率来违反Wiedemann-Franz定律

我们介绍了热蒸发金薄膜样品中面内热导率和电导率的测量值，厚度范围从 $\approx 20$ 至 $>300\mathrm{纳米}$，使用微机械氮化硅膜热隔离平台进行。同时$\approx 300$单个Au沉积中生长的纳米级厚膜和样品 $>300\mathrm{纳米}$通过多次连续沉积较薄的层，我们观察到了与电导率和导热率有关的Wiedemann-Franz定律的强烈“违反”。虽然电导率的行为基本符合预期，但热导率会随着总膜厚的增加而先上升，然后随着膜的厚度增加而出人意料地下降。导热系数的急剧降低降低了洛伦兹数$\mathrm{大号}$ 对于 $\approx 300\text{-}\mathrm{纳米}$在整个研究的78–300-K温度范围内，样品的厚度要小于Sommerfeld值的一半。以前没有报道过这种接近室温的破坏现象，即使在存在由晶界和粗糙表面引入的无序情况的情况下，金属膜中的电子传输应该用费米-液体理论很好地描述。这表明我们认为，基于这些薄膜中晶粒尺寸的详细表征，可以进行非弹性散射过程，这可能是由于改性声子态密度和较厚薄膜中强柱状晶粒结构引入的结构各向异性共同作用的结果。在保持高电导率的同时，这种极不寻常的热导率降低可能有望提高纳米级系统的热电性能。