Double Superconductivity in Topological Nodal-line Semimetal NaAlSi

In topological materials, peculiar surface states protected by crystal symmetries have attracted great interest because of unprecedented correlation with superconductivity. A topological nodal-line semimetal with a layered structure, NaAlSi shows bulk superconductivity at T_c = 6.8 K. The magnetic torque curves as a function of the field angle in the superconducting state of NaAlSi are found to show double superconductivity; a large broad diamagnetic signal arising from the bulk superconductivity and a small sharp signal observed only in magnetic fields nearly parallel to the layers. The sharp signal is ascribed to highly two-dimensional superconductivity with the similar T_c, whose thickness is only several times of the c-axis lattice constant 0.736 nm. The band structure calculations show the presence of large Fermi lines on the (001) plane, mainly formed by the surface Si p bands. The results show that the sharp signal is likely ascribed to the (001) surface superconductivity, which will open new perspectives in topological materials.