Unusual Resistive Transitions in the Nodal-Line Semimetallic Superconductor NaAlSi

NaAlSi is a quasi-two-dimensional semimetal with superconductivity below Tc =\n6.8 K and a band structure characterized by nodal lines near the Fermi level\nand potential topological surface states. Electrical resistivity measurements\non its superconducting transitions in magnetic fields were made using\nplate-like single crystals. In the magnetic field-temperature phase diagram, we\nobserved a substantial reduction in resistivity in a pre-transitional zone\nabove the bulk superconducting regime only when the magnetic fields were\nperpendicular to the plane, rather than parallel to it. Significant sample\n(thickness) dependence, reentrant behavior, and sensitivity to electrode\nconfigurations all indicate that a portion of the crystal has an upper critical\nfield greater than the bulk superconductivity in the pre-transitional region.\nThis fractional superconductivity may occur on the side surface of the crystal.\n