NaAlSi is an sp electron superconductor crystallizing in a layered structure of the anti-PbFCl type with a relatively high transition temperature T_c of ~7 K. Recent electronic state calculations revealed the presence of topological nodal lines in the semimetallic band structure, which attracted much attention owing to the superconductivity. However, experimental investigation remained limited because of the lack of single crystals. Here, we successfully prepared single crystals of NaAlSi by a Na–Ga flux method and characterized their superconducting and normal-state properties through electrical resistivity, magnetization, and heat capacity measurements. A sharp superconducting transition with a T_c of 6.8 K is clearly observed, and heat capacity data suggest an anisotropic superconducting gap. Surprisingly, despite the sp electron system, the normal state is governed by the electron correlations, which is indicated by a T² resistivity and a Wilson ratio of 2.0. The origin of the electron correlation may be related to the orthogonal saddle-shaped Fermi surfaces derived from the Si p_x and p_y states, which intersect with the light Al s bands to form the nodal lines near the Fermi level. These results strongly suggest that the superconductivity of NaAlSi is not caused by a simple phonon mechanism but involves a certain unconventional aspect, although its relevance to the nodal lines is unclear.

中文翻译：

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拓扑结线半金属NaAlSi中的超导性

NaAlSi是sp电子超导体，在抗PbFCl型层状结构中结晶，具有较高的跃迁温度T _c约为〜7K 。最近的电子态计算表明，在半金属能带结构中存在拓扑结线，这吸引了很多人由于超导电性而引起关注。然而，由于缺乏单晶，实验研究仍然受到限制。在这里，我们成功地通过Na-Ga通量法制备了NaAlSi单晶，并通过电阻率，磁化强度和热容量测量表征了它们的超导和常态性能。T _c的急剧超导转变可以清楚地观察到6.8 K的热导率，热容量数据表明存在各向异性的超导间隙。出乎意料的是，尽管有sp电子系统，但正常状态仍受电子相关性的控制，这由T ²电阻率和2.0的Wilson比率表示。电子相关性的起源可能与从Si p _x和p _y态衍生的正交鞍形费米表面有关，它们与光Al s相交带形成费米能级附近的节点线。这些结果强烈表明，NaAlSi的超导电性不是由简单的声子机制引起的，而是涉及某些非常规的方面，尽管尚不清楚其与结线的相关性。