Possible coexistence of nodal and nodeless pairing symmetry in superconducting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mn>6</mml:mn> <mml:mi>R</mml:mi> <mml:mo>−</mml:mo> <mml:msub> <mml:mi mathvariant="normal">SnNbSe</mml:mi>

A possible route to realize fault-tolerant quantum computing is to find unconventional superconductors, where superconductivity coexists with nontrivial band topology. In this work, we report such a possibility in $6R\ensuremath{-}{\mathrm{SnNbSe}}_{2}$ obtained by Sn intercalation into ${\mathrm{NbSe}}_{2}$. Rietveld refinement confirms a centrosymmetric crystal structure with space group $R\overline{3}m$. Structural, electronic, and magnetic measurements confirm the emergence of a superconducting phase with a transition temperature ${T}_{c}\ensuremath{\approx}4$ K. The temperature dependence of the upper critical field ${H}_{c2}(T)$ is well described by a two-band model. Temperature-dependent magnetic penetration depth and superfluid density measurements are performed using the tunnel diode oscillator technique. Our analysis implies the mixing of nodal and nodeless order-parameter symmetry in $6R\ensuremath{-}{\mathrm{SnNbSe}}_{2}$. Such results are surprising for a centrosymmetric superconductor but are consistent with theoretical predictions of topological nodal-line features in ${\mathrm{SnNbSe}}_{2}$. Thus, $6R\ensuremath{-}{\mathrm{SnNbSe}}_{2}$ emerges as an interesting platform for exploring unconventional pairing mechanism in transition metal dichalcogenide superconductors.