The motion of electrons in the vast majority of conductors is diffusive, obeying Ohm’s law. However, the recent discovery and growth of high-purity materials with extremely long electronic mean free paths has sparked interest in non-Ohmic alternatives, including viscous and ballistic flow. Although non-Ohmic transport regimes have been discovered across a range of materials—including two-dimensional electron gases, graphene, topological semimetals, and the delafossite metals—determining their nature has proved to be challenging. Here, we report on a new approach to the problem, employing broadband microwave spectroscopy of the delafossite metal ${\mathrm{PdCoO}}_2$ in three distinct sample geometries that would be identical for diffusive transport. The observed differences, which go as far as differing power laws, take advantage of the hexagonal symmetry of the conducting Pd planes of ${\mathrm{PdCoO}}_2$. This permits a particularly elegant symmetry-based diagnostic for nonlocal electrodynamics, with the result favoring predominantly ballistic over strictly hydrodynamic flow. Furthermore, it uncovers a new effect for ballistic electron flow, owing to the highly faceted shape of the hexagonal Fermi surface. We combine our extensive data set with an analysis of the Boltzmann equation to characterize the nonlocal regime in ${\mathrm{PdCoO}}_2$, and we include out-of-plane impurity scattering as a source of apparent momentum-conserving scattering at low temperatures. More broadly, our results highlight the potential of broadband microwave spectroscopy to play a central role in investigating exotic transport regimes in the new generation of ultrahigh-conductivity materials.

中文翻译：

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超纯PdCoO2 中的非局域电动力学

绝大多数导体中电子的运动是扩散的，遵守欧姆定律。然而，最近发现和发展了具有极长电子平均自由程的高纯度材料，引发了人们对非欧姆替代品的兴趣，包括粘性流和弹道流。尽管已经在一系列材料（包括二维电子气、石墨烯、拓扑半金属和铜铁矿金属）中发现了非欧姆输运机制，但事实证明确定它们的性质具有挑战性。在这里，我们报告了解决该问题的一种新方法，即采用铜铁矿金属的宽带微波光谱${\mathrm{钴酸钯}}_2$在三个不同的样本几何形状中，这对于扩散传输来说是相同的。观察到的差异，甚至不同的幂律，利用了导电 Pd 平面的六角对称性。${\mathrm{钴酸钯}}_2$。这允许对非局部电动力学进行特别优雅的基于对称性的诊断，其结果主要是弹道流而不是严格的流体动力流。此外，由于六边形费米表面的高度多面形状，它揭示了弹道电子流的新效应。我们将广泛的数据集与玻尔兹曼方程的分析相结合，以描述非局部状态${\mathrm{钴酸钯}}_2$，并且我们将面外杂质散射作为低温下表观动量守恒散射的来源。更广泛地说，我们的结果凸显了宽带微波光谱在研究新一代超高电导率材料的奇异输运机制方面发挥核心作用的潜力。