Anomalous Diamagnetic Torque Signals in Topological Nodal-Line Semimetal NaAlSi

In topological materials, peculiar surface states protected by crystal symmetries have attracted great interest because of the possibility of unprecedented correlation with superconductivity. A topological nodal-line semimetal with a layered structure, NaAlSi shows moderately anisotropic bulk superconductivity with Tc approximately equal to 7.0 K. The magnetic torque curves as a function of the field angle in the superconducting state are found to show two distinct diamagnetic signals; a large broad signal arising from bulk superconductivity and a small sharp signal observed only in magnetic fields nearly parallel to the layers. The upper critical field of the bulk superconductivity follows an anisotropic 3D model with a small anisotropic factor of -4. A possible scenario is that the sharp signal is due to highly 2D superconductivity with a similar Tc, 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 sharp diamagnetic torque signal could arise from the (001) surface superconductivity.