Experiments have shown that several materials, including MgB₂, iron-based superconductors and monolayer NbSe₂, are multiband superconductors. Superconducting pairing in multiple bands can give rise to phenomena not available in a single band, including Leggett modes. A Leggett mode is the collective periodic oscillation of the relative phase between the phases of the superconducting condensates formed in the different bands. The experimental observation of Leggett modes is challenging because multiband superconductors are rare and because these modes describe charge fluctuations between bands and therefore are hard to probe directly. Also, the excitation energy of a Leggett mode is often larger than the superconducting gaps, and therefore they are strongly overdamped via relaxation processes into the quasiparticle continuum. Here, we show that Leggett modes and their frequency can be detected in a.c. driven superconducting quantum interference devices. We then use the results to analyse the measurements of such a quantum device, one based on a Dirac semimetal Cd₃As₂, in which superconductivity is induced by proximity to superconducting Al. These results show the theoretically predicted signatures of Leggett modes, and therefore we conclude that a Leggett mode is present in the two-band superconducting state of Cd₃As₂.

实验表明，MgB ₂、铁基超导体和单层NbSe ₂等多种材料都是多带超导体。多个频带中的超导配对可能会产生单个频带中不存在的现象，包括 Leggett 模式。 Leggett模式是在不同能带中形成的超导凝聚体的相之间的相对相位的集体周期性振荡。莱格特模式的实验观察具有挑战性，因为多带超导体很少见，而且这些模式描述了带之间的电荷波动，因此很难直接探测。此外，莱格特模式的激发能通常大于超导能隙，因此它们通过弛豫过程进入准粒子连续体而被强烈过阻尼。在这里，我们展示了可以在交流驱动的超导量子干涉装置中检测到 Leggett 模式及其频率。然后，我们使用这些结果来分析这种量子设备的测量结果，该设备基于狄拉克半金属 Cd ₃ As ₂，其中超导性是通过接近超导 Al 引起的。这些结果显示了理论上预测的 Leggett 模式的特征，因此我们得出结论，Leggett 模式存在于 Cd ₃ As ₂的双能带超导态中。