  NaAlSi is an sp electron superconductor crystallizing in a layered structure
of the anti-PbFCl type with a relatively high transition temperature Tc 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 Tc 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 px and py 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.
