Double Superconductivity in Nodal Line Material NaAlSi; Coexistence of Bulk and 2D Superconductivities

　Topological materials have attracted great interest because of unique electronic states resulting from their linear energy dispersion. A large number of topological materials have been synthesized and discovered. Among them, some topological materials have unique band structures. The crossing points of the bands with linear dispersion, called Dirac points, extend in k-space and form nodal line loops. One of the most intriguing properties of such topological nodal line materials is the formation of surface states on certain crystal surfaces. Till now, extensive efforts have been made to elucidate the surface states of topological materials.

　The nodal line material NaAlSi has a tetragonal unit cell composed of Al-Si tetrahedral layers and Na in between. Conduction bands are formed by the s and p orbitals of Al and Si. In spite of the low charge carriers (small Fermi surfaces), NaAlSi shows bulk superconductivity with a rather high T_c of 7 K.

　Magnetic torque is defined as the outer product of the magnetic field and magnetization. We have performed magnetic torque experiments on single crystals of NaAlSi using a micro-cantilever, which is a powerful technique for detecting anisotropic magnetic properties. Owing to its layered structure, NaAlSi has anisotropic critical magnetic fields. Thus, it is possible to precisely measure the diamagnetic signal arising from the superconductivity even for a tiny single crystal.

　In torque measurements below T_c, very unique diamagnetic torque signals have been observed in NaAlSi, i.e., a large broad signal arising from the bulk superconductivity and a small sharp signal observed in magnetic fields nearly parallel to the layers. Owing to the high anisotropy of the critical field, the sharp signal is ascribed to the 2D superconductivity in the Na-Si layers (perpendicular to the c-axis). The thickness of the 2D superconductivity is only several times the crystal c-axis (0.736 nm). As band calculation predicts the presence of a peculiar surface state on the crystal (001) plane, a possible explanation for the torque data is the coexistence of the bulk superconductivity and surface conductivity on the (001) plane. Further studies are necessary to understand the relationship between both superconductivities and the mechanism of 2D superconductivity.

(Written by S. Uji on behalf of all authors)