Superconductivity in the Topological Nodal-line Semimetal NaAlSi

TL;DR

This study reports the successful synthesis and characterization of NaAlSi single crystals, revealing superconductivity at 6.8 K with evidence of electron correlations and potential unconventional mechanisms linked to topological nodal lines.

Contribution

First demonstration of single-crystal NaAlSi with detailed superconducting and normal-state properties, highlighting electron correlations and possible unconventional superconductivity related to topological features.

Findings

Superconducting transition at 6.8 K observed.

Normal state exhibits electron correlations indicated by T2 resistivity.

Superconductivity may involve unconventional mechanisms beyond phonons.

Abstract

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.