The heavy-fermion metal ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ was the first discovered unconventional, non-phonon-mediated superconductor and, for a long time, was believed to exhibit single-band $d$-wave superconductivity, as inferred from various measurements hinting at a nodal gap structure. More recently, however, measurements using a range of techniques at low temperatures ($T⪅0.1\mathrm{K}$) provided evidence for a fully gapped superconducting order parameter. In this Colloquium, after a historical overview the apparently conflicting results of numerous experimental studies on this compound are surveyed. The different theoretical scenarios that have been applied to understanding the particular gap structure are then addressed, including both isotropic (sign-preserving) and anisotropic two-band $s$-wave superconductivity, as well as an effective two-band $d$-wave model, where the latter can explain the currently available experimental data on ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. The lessons from ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ are expected to help uncover the Cooper-pair states in other unconventional, fully gapped superconductors with strongly correlated carriers, and, in particular, highlight the rich variety of such states enabled by orbital degrees of freedom.

中文翻译：

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座谈会：重费米子金属CeCu2Si2中的非常规全带隙超导性

重费米子金属${\mathrm{铈铜}}_2{硅}_2$是第一个发现的非常规、非声子介导的超导体，长期以来，人们认为它表现出单能带$d$-波超导性，从暗示节点间隙结构的各种测量中推断出来。然而，最近，在低温下使用一系列技术进行测量（$\mathrm{时间}⪅0.1\mathrm{K}$）为完全有隙超导有序参数提供了证据。在本次研讨会上，在回顾了历史之后，对该化合物的大量实验研究的明显相互矛盾的结果进行了调查。然后讨论用于理解特定间隙结构的不同理论场景，包括各向同性（符号保持）和各向异性双频带$s$波超导性，以及有效的二能带$d$-波模型，后者可以解释当前可用的实验数据${\mathrm{铈铜}}_2{硅}_2$。教训来自${\mathrm{铈铜}}_2{硅}_2$预计将有助于揭示其他具有强相关载流子的非常规、全能隙超导体中的库珀对态，特别是强调轨道自由度所实现的此类态的丰富多样性。