NaAlSi crystallizes in an anti-PbFCl-type layered structure [1] and exhibits superconductivity below T_c = 6.8 K [2,7]. The superconductivity seemed to be of the conventional s-wave type based on phonon-mediated Cooper pairing [3-6]. However, recent heat capacity measurements using single crystals of NaAlSi showed the possibility of a complex superconducting gap [7]. Interestingly, electronic state calculations showed that NaAlSi is a nodal-line semimetal [5,8,9], in which nontrivial topological surface states exist: a drumhead band at around the point in the (0 0 1) surface and a flat band along the line in the (1 0 0) surface [9].

We studied the resistive transitions under magnetic fields using plate-like single crystals of NaAlSi such as shown in Fig. 1. A significant reduction in resistivity was observed at a pre-transitional region (B_c2^on--- – B^*) above the bulk superconducting transition at B_c2^off--- – B_c2^on only in the case that the magnetic field was perpendicular to the plane, not parallel to the plane (Fig. 1). No anomaly was observed at T^*(B^*) in the heat capacity measurement, suggesting the existence of a fractional superconductivity different from the bulk superconductivity, although the details have not yet been clarified [10]. Meanwhile, the field dependence of the resistivity on the magnetic field parallel to the plate shows a sharp superconducting transition (Fig. 1), indicating that there is a field-angle dependence in resistive transition. In order to understand the origin of the fractional superconductivity, we measured the field dependence of the superconducting transition in electrical resistivity as a function of the magnetic field angle. We show the possibility that the fractional superconductivity has a two-dimensional character and suggest that it occurs at the crystal surface by measuring the current dependence of the transition.

Figure 1: Magnetic field dependence of the resistivity of a NaAlSi crystal at 1.8 K [7]. The red and blue curves are for B || c and a, respectively.

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[9] X. Yi et al., J. Mater. Chem. C 7, 15375 (2019).
[10] D. Hirai et al., J. Phys. Soc. Jpn, 91, 024702 (2022).

Keywords: Superconductivity, Topological material, nodal-line semimetal