Coexistence of Nodal and Nodeless Pairing Symmetry in Superconducting 6R-SnNbSe₂

The material realization of topological superconductivity accrues great current relevance because of its potential usage as qubits in fault-tolerant quantum computation. Towards this end, two approaches have been prescribed; one is the proximitized s-wave superconductivity on spin helical states, and the other is to realize an intrinsic topological superconductor with odd-parity spin-triplet pairing. Analogous to a topological insulator, in a three-dimensional topological superconductor (TSC), the superconducting bulk gap of the interior coexists with gapless surface states. The quasiparticle excitations of these surface states are theorized as Majorana fermions, and the experimental manifestation of such an exotic field-theoretical perspective is considered an essential component for the characterization of TSC. We report such a possibility in superconducting 6R-SnNbSe₂ by determining its order parameter symmetry from the penetration depth measurements.This compound crystallizes in the centrosymmetric space group R-3m. Structural, electronic, and magnetic measurements confirm the emergence of a superconducting phase with a transition temperature T_c ≈ 4 K. Temperature-dependent magnetic penetration depth and superfluid density measurements down to 1.5 K were performed using the tunnel diode oscillator technique. The findings suggest the mixing of nodal and nodeless order parameter symmetry in 6R-SnNbSe₂. Such results are surprising for a centrosymmetric superconductor but are consistent with theoretical predictions of topological nodal-line features in SnNbSe₂. Thus, 6R-SnNbSe₂ emerges as an interesting platform for exploring unconventional pairing mechanism.